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1. re NE ViTime Sub VisTime Total Time Runs Average Shortest Longest Benchmark Remove Backspace Wis 300 4 2424 9 24585 4 10 1 300 160 2 Terenity Five Modes vi 1516 4 0 0 123516 4 12 8 2 0 20 0 Twrenty Two Nodes 1 s 5668 4 5 9 0 0 10 0 Thirteen Mees 4 VSG i n t LabVIEW contains several built in tools that help evaluate style In the previous sections you observed the Profile Performance and Memory window used to evaluate data memory use and execution speed The Memory Usage VI properties page determines the memory allocated to each of the four memory components of a single VI These tools provide metrics for efficiency see Section 1 1 2 and performance see Section 1 1 7 The VI Metrics window determines simplicity see Section 1 1 6 and is used to calculate the modularity index to help evaluate robustness see Section 1 1 5 Additionally code reviews provide a more direct method of evaluating style Code reviews are performed utilizing a combination of self reviews automated inspection and peer reviews The Style Rules Summary in Appendix B can be used as a style checklist for performing self reviews Additionally many of the rules presented in The LabVIEW Style Book can be evaluated using the LabVIEW VI Analyzer Toolkit from National Instruments Chapter 10 Code Reviews discusses these tools in greater detail 1 2 Style Versus Time Tradeof
2. Shi registers mot grouped mear hop Wire crossovers Lowdend routine Right dotett data how In Figure 4 21B the shift registers are labeled and grouped near the top forming a nonintrusive data highway and increasing the space available for nodes in each of the Case structure s cases The queue functions are moved to the bottom of the diagram eliminating crossovers of the error cluster wire A subVI replaces the low level routine for computing the number of wavelength steps reducing clutter and eliminating the right to left data flow Additionally the wavelength and power data are modularized into clusters Consequently the diagram of Figure 4 21B is much more readable than the one in Figure 4 21A Figure 4 21B The shift registers are grouped near the top the wires exiting the left shift register terminals are labeled wire crossovers are reduced the array size computation is modularized into a subVI the power and wavelength arrays are modularized into a cluster and right to left data flow is eliminated View full size image 131 Endnotes 1 NI s Developer Zone contains a Code Sharing area and an Instrument Driver Network containing free downloads including drivers for more than 5 000 instruments The URL is WWW ni com devzone 132 5 Icon and Connector Imagine for a moment the appearance and development techniques of traditiona
3. Figure 9 4D shows the revised panel and diagram including the Help button Help Value Change event case and Context Help window The user clicks the Help button to trigger the Help Value Change event The Control Help Window function opens the Context Help window The Context Help window displays the VI and control descriptions as the user moves the cursor over the objects on the panel Figure 9 4D The Help button fires the Event structure s Help Value Change event The Context Help window opens allowing the user to view the VI and control descriptions View full size image 307 e Configure Teni nsir rumeni Ops the Context Help mafic Mere Ube Gui rir font parel cbyacts to views Chet Cerri A he 9 3 Icon and VI Description Icons and VI descriptions comprise the most essential forms of documentation throughout LabVIEW These critical elements reinforce the purpose of each VI using graphics and text The icon graphically represents the VI as a subVI call on a diagram For best results create an icon that combines a universally recognizable glyph with color and text The descriptions appear in the Context Help window when the user moves the cursor over the subVI s icon Create a two or three sentence description at a minimum that summarizes the purpose of each subVI Rule 4 10 Create a meaningful icon and cohesive description for every subVI Rule 9 8 Create the icons and descriptions as you dev
4. CDr edge abe E F Rule 8 4 249 Avoid polling GUI objects Polling a menu of Booleans within a continuous loop as illustrated in the previous example primarily applies to developers that do not have access to Event structures This includes users of the LabVIEW Base package or versions of LabVIEW prior to 6 1 which introduced the Event structure In general GUI objects should not be polled A more efficient alternative is presented next The Event Handling Loop design pattern facilitates event driven programming in LabVIEW It consists of an Event structure within a While Loop The Event structure contains an event case for each configured event that executes its subdiagram when the event fires This is analogous to a Case structure with an event driven selector The Event structure maximizes the efficiency with which an application detects and responds to events on the graphical user interface as well as any user defined events It goes to sleep freeing the processor to work on other tasks until a registered event fires The Event Handling Loop design pattern is extremely useful for applications that involve user interface events of any variety Almost any type of user interface activity can be captured and translated into an event Additionally user defined events such as events related to a test or measurement sequence are handled by an Event structure using dynamically registered user events M
5. Checks the total number of coercion dots onthe A biock diagram and the number of coercion dats on indevidual wires and compares them to user specified brits iF as Under Front Panel SubVI the Control Alignment test has a Pixel Tolerance parameter with a default value of 10 If you use the alignment tools as recommended in Chapter 3 you can reduce this to a value of 1 Also the VI Analyzer has a Dialog Controls test within the User Interface category that is selected by default This test checks all the VP s controls to determine whether they are from the System palette Dialog controls apply only to the GUI VIs of a desktop application that is designed to resemble the appearance of the operating system Be sure to disable this test for all subVIs and industrial GUI VIs that utilize Modern or Classic style controls Additionally the VI Analyzer has a Comment Usage test within the Documentation Developer category Select Ensure subdiagrams contain at least one comment each to enforce Rule 9 3 Label every subdiagram of every multidiagram structure Finally under General Connector Pane Pattern choose the 4x2x2x4 connector pattern and the single terminal connector pattern corresponding to pattern numbers 32 and 0 respectively Number 32 enforces Rule 5 21 Use the 4x2x2x4 connector pattern for most VIs Connector pattern number 0 prevents this test from flagging a failure when you test a
6. Library Function Nodes developed prior to LabVIEW version 8 2 The presence or absence of the error out or error code terminal in anode s connector pane is one indicator of the risk associated with that node The basic mathematics functions available on the Numeric palette for example do not have error terminals and do not return errors This is because LabVIEW gracefully handles all combinations of problems that may arise and misbehavior is almost impossible Division by zero for example returns the value Inf which is a valid constant understood by all of LabVIEW s numeric functions controls and indicators Therefore simple mathematics functions do not require error handling Likewise all functions on the remaining palettes that do not contain error terminals do not generate errors This includes many functions available from the Structures Array Cluster amp Variant Boolean String Comparison and Timing palettes as well as many other palettes At the opposite end of the risk spectrum are functions and VIs that perform input and output 1 0 operations These nodes make calls to device drivers DLLs or shared libraries the operating system or any application or resource that is external to the LabVIEW environment including remote instances of LabVIEW I O operations include all of the nodes available on the following palettes File I 0 Measurements I 0 Instrument I 0 and Data Communication These nodes are risky
7. system dialog VIs GUI related subVIs that open their front panels and prompt the user for information Dialog VIs have far fewer subVIs and much less overall functionality than top level VIs Their purpose is to perform a specific transaction with the user Disk Operating System DOS The dominant operating system utilized by the IBM PC compatible throughout the 1980s preceding Microsoft Windows documentation Any text based description of the structure components or operation of a system application or source code Documentation may include specifications design documents source code documentation user manuals and online help DOS See Disk Operating System dynamic A universal data type used with Express VIs Designed for novice developers dynamic is a very flexible data type that can store many types of data without learning about data types data storage and type conversion Dynamic Framework 342 E Application framework that utilizes VI server functions to dynamically load and run its high level VIs known as components ease of use The ease with which the end user operates an application to accomplish his or her objectives This involves interacting with the application s graphical user interface GUI efficiency The manner in which an application utilizes processor memory and input output 1 0 resources An efficient LabVIEW application executes quickly without performing
8. C Call By Reference VI Server function that is used to execute a dynamically loaded subVI The Dynamic Framework utilizes the Call By Reference to execute the high level components of an application child panel The front panel of a component VI that is loaded into a subpanel control CHM 338 See Microsoft Compressed HTML Help Classic State Machine Design pattern consisting of a Case structure within a While Loop that utilizes an enumerated data type for the case selector and a shift register for passing the next case selection between loop iterations The cases are labeled intuitively based on the text items of the enumerated data type The next case selection is determined programmatically Code and Fix Software lifecycle development model in which the source code is iteratively implemented tested and debugged without any formal planning or design phase This model is characterized by misunderstood requirements inadequate architecture sloppy source code resulting in poor quality and prolonged development time and effort code memory or code One of the four VI memory components code is the portion that contains the compiled source code Code memory usage is provided by the Memory Usage category of the VI Properties window selected by choosing File VI Properties code review Systematic method of reviewing source code quality for readability maintainability and robustness utilizing
9. Figure 5 15A Icon contains text drawn freehand and connector assignments are unconventional View full size image 150 Haphazard Sub VLvi Connector Assignments Digital Inputs port 0 Pattern 0 kak amp arrir ir Digital Input Port 1 error out no error Figure 5 15B The subVI icon has been improved using 8 point small fonts and a glyph of a pair of glasses The DAQmx task passes through the top left and right terminals of the 4x2x2x4 connector pattern View full size image Haphazard Sub I Improved vi Connector Assignments Pattern 0 Digital Task In manapa Digital Task Out Go E lport 0 Data error in no error LINFUTS I kapart 1 Data tm error out mo error Pattern 1 Figure 5 16A is a VI icon containing a custom font that the engineering staff at Bloomy Controls affectionately refers to as Bob s Bold after its creator Bob Hamburger Bob is a highly regarded Business Development Manager at Bloomy Controls who also developed the Find Screw Ends VI icon in Figure 5 4 which exemplifies good style I would like to thank Bob for contributing his greatest as well as his most obnoxious icons for public display The characters of Bob s Bold are so big and bold that the icons in an application appear to be shouting at you as seen in Figure 5 16B Bob s Bold is a great way to get your point across but it should be used sparingly to keep your diagram s decibel level within legal limits Fi
10. Figure 5 6A The icon template for the Suss Interface Toolkit contains a glyph of a probe tip contacting a semiconductor device and an acronym describing the instrument manufacturer and model P Icon Editor File Edit Help Black amp White 16 Colors 256 Colors Show Terminals Figure 5 6B All VIs contained in the toolkit are based on the template as shown in the VI tree View full size image 140 Initialize Configuration Is Rule 5 16 Copy graphics Within legal limits copy your graphics instead of drawing them from scratch Copy and paste portions of other LabVIEW icons or import images from a variety of applications and image files When you copy portions of other icons make your icons distinctively different so that your subVIs are not confused with the standard VIs in LabVIEW s vi lib folder For example the OpenG File Tools contain icons that resemble LabVIEW s File I 0 palette as shown in Figure 5 7 These icons are formed by simply copying the images from the related LabVIEW functions However the OpenG File Tools are shaded green for a distinctively different appearance As another example when I create file logging VIs I like to copy and paste the floppy disk and pencil symbols that appear on other VIs and functions in the File I 0 palette as shown in Save Scan VI in Figure 5 8 The text background color and border help distinguish my VI from the LabVIEW shipping VIs Figure 5 7 The
11. Rule 6 7 171 Create arrays and clusters that associate related data Consider the Torque Hysteresis VI shown in Figure 6 3 First the application prompts the user to enter some information about the unit under test UUT The application queries a database and if the UUT is recognized it prompts the user to enter a set of motion control parameters Next the VI runs a test consisting of a motion profile that acquires torque versus angle data Upon test completion the VI performs a statistical analysis to the data and generates a report Finally the application saves the data to file Following the guidelines in Chapter 4 Block Diagram these requirements are implemented using a modular diagram containing separate subVIs for each task as follows UUT Information Dialog 1 Motion Parameters Dialog 2 Run Test VI 3 Compute Statistics VI 4 Generate Report VI 5 and Save Data VI 6 A simplified version of the top level diagram containing these six subVIs is shown in Figure 6 3A prior to the assignment of data structures The data that is generated by the first four subVIs and processed by the latter two subVIs is shown in Figure 6 3B Figure 6 3A Top level diagram of Torque Hysteresis VI prior to assignment of data constructs View full size image Figure 6 3B Data generated by the first four subVIs and processed by Generate Report VI and Save Data VI View full size image Figure 6 4A s
12. Use the Event Driven State Machine for the top level VI of most applications that do not contain lengthy states or parallel processes The Event structure need not be limited to user interface event handling Instead you can configure custom user defined events known as user events that are created registered and fired programmatically using the functions on the Events palette located under Programming Dialog amp User Interface Events The Event Machine design pattern merges the functionality of the Case structure and Event structure of the Event Driven State Machine into a new design pattern consisting of an Event structure within a While Loop User events are defined for each state and event cases are configured for any combination of user and GUI events The Event structure handles user events in a similar manner to GUI events Any registered user and GUI events that fire are buffered in first in first out order until processed by the corresponding event case The Torque Hysteresis VI is implemented as an Event Machine in Figure 8 15 Figure 8 15 The Torque Hysteresis VI is implemented as an Event Machine design pattern User events are used to merge the functionality of the Case and Event structures into an alternative design pattern with fewer constructs View full size image z UUT hia gt E b Sates F T I cl gt io Compie gt H a i T Mobo Panamn a i gt Tjee gt Data gt ii Dia v Pe
13. controls a semiconductor wafer probing system The icon convention in Figure 5 18A consists of a banner containing the instrument prefix SussPA and a demonstrative glyph of a probe tip contacting a semiconductor device The bright yellow background clearly distinguishes these icons on the caller application s diagrams The palette contains submenu icons that use the same convention but have a darker shade of yellow to distinguish them from VI icons and to enhance the three dimensional appearance These icons have the professional look and feel required for a commercial toolkit The glyph was created by first envisioning a symbol that best represents wafer probing searching to see if that symbol can be copied from a public archive and then laboriously editing that graphic for best appearance on a 32x32 pixel icon It is important to note that the same glyph appears on the icons of all VIs of the toolkit and the toolkit is a commercial product Therefore a high quality icon convention is required and it is worthwhile to invest the necessary time and effort Figure 5 18A The Functions palette for the Suss Interface Toolkit for LabVIEW a commercial product that controls a semiconductor wafer probe system All icons share a common glyph describing the instrument DoProberCmd VI is a subVI from the Action Status subpalette containing an artful silhouette of a runner and common glyph View full size image ei SussPA SussPA lvlib
14. i a g i i light Le m E Swe W aerie F epl MF er ern ant Station 1 font a M Stati on 2 4 tipa E o Tirili P tiri Firre pt Tretia T ir an pre Z z 2 ai B5 a ETEL Faw F tits core lm i T ii Cong eee Meticulous VI in Figure 1 1 is a highly complex application used in the medical industry for patient physiologic data monitoring It has a very professional and highly organized appearance The front panel is densely functional yet appears neat and intuitive The diagram is impressive containing multiple parallel loops networks of labeled wires advanced constructs and liberal documentation Nested VI in Figure 1 2 is a relatively simple application that controls a scientific instrument It has a clean uncluttered front panel and block diagram The most noteworthy characteristic is the diagram s multiple layers of structures nested within structures Spaghetti VI in Figure 1 3 is an automated test application with medium complexity The front panel contains many different colors and font styles which tend to clash and distract the user The diagram is a labyrinth of wires and nodes compacted together within a single loop So dissimilar are these three programming styles that it is worth noting that all three examples are commercial applications developed by experienced LabVIEW professionals Style is the main differentiator Let us break down Theorem 1 1 into seven different parts
15. 49 categories of files similar to Figure 2 7B After you have created one such folder hierarchy use it as a model to create a reusable folder hierarchy template Furthermore the folder hierarchy template can contain any number of source file templates including a LabVIEW project file a top level VI template documentation templates and more Indeed the folder hierarchy integrated with source file templates constitutes an accelerated starting point for application development using good style On a final note it is important to realize that the folder hierarchy template is intended as a starting point not a one size fits all repository For the folder hierarchy to reflect the application s architecture you must customize the template for your project by adding and removing folders as necessary To the extent possible customize the template before you begin coding to avoid moving source files on disk later After an organized repository has been established proceed to the LabVIEW project Use Project Explorer to organize the project files into a hierarchy of folders that reflects the application s architecture similar to the project repository If you began with an organized folder hierarchy on disk it is extremely simple to create an organized LabVIEW project In Project Explorer start with a new LabVIEW project or template right click on My Computer or another target choose Add Folder from the shortcut menu navigate to t
16. E 5 error mai LEi i Le A user event is created and registered for each state of the Torque Hysteresis application using the Create amp Register User Events VI shown on the far left of the diagram in Figure 8 15 The subVI s front panel and diagram are shown in Figure 8 16 User Event Data is a cluster of variant controls containing one element per state labeled using the state name This cluster is saved as a type definition and is analogous to the enumeration of a conventional state machine except that the data type of each of the cluster s elements defines the data type that is used to pass data into the event Event data is useful if the routine that fires the event needs to pass data into the event In particular the event generation code can exist in locations where the data cannot be passed using wires The implementation of Figure 8 15 uses shift registers to share data between event cases similar to the previous state machine implementations Shift registers provide the flexibility to share the data among all event cases not just between the event generation routine 264 and the corresponding event case However shift registers cannot be used if the event firing routine exists outside of the loop Therefore I recommend shift registers when the event data can be passed using wires and event data otherwise Figure 8 16 Create amp Register User Events VI is a subVI that creates and registers multiple user events bas
17. Enumeration Provides a pick list of text items that map to 8 bit 16 bit or 32 bit unsigned integers 16 bit unsigned integer Represents non negative whole numbers in the range of 0 to 65 535 16 bit signed integer Represents positive and negative whole numbers in the range of 32 768 to too Od El BEEBE 32 bit unsigned integer Represents non negative whole numbers in the range of 0 to 4 294 967 295 162 Table 6 2 LabVIEW Data Types and Descriptions Listed in Top Down Order of Memory Efficiency BIE Boolean Stores a TRUE or FALSE value as a byte integer 32 bit signed integer Represents positive and negative whole numbers in the range of 147 483 648 to 2 147 483 647 SEL Single precision floating point Uses 4 bytes to represent fractional numbers conforming to the ANSI IEFE Standard 754 1985 El Refnum Unique identifier for an object such as a VI file NET control or network connection Ton Ii I O name Unique identifier for I O resources such as DAQmx and VISA to communicate with an instrument or a device 64 bit unsigned integer Represents non negative whole numbers in the range of 0 to 18 446 744 073 709 551 615 64 bit signed integer Represents positive and negative whole numbers in the range of 9 223 372 036 854 775 808 to 9 223 372 036 854 775 807 E E DBL Double precision floating point tUses 8 bytes to represent fr
18. Figure 4 9B A Stacked Sequence structure requires sequence local terminals to pass data between frames View full size image 113 ith Fi 4 p34 p p grpyrp n Figure 4 9C A three frame Flat Sequence structure facilitates data flow between frames via wires and tunnels and all frames are visible This is the preferred implementation View full size image Pressure Zero H Fiw Iere m Most local and global variables and Sequence structures used in practice are not necessary Most often they are overused by developers who have not learned efficient dataflow principles The best way to master data flow is to force oneself to avoid variables and Sequence structures unless absolutely required Indeed mastering data flow is synonymous with minimizing variables and Sequence structures In fact even the example in Figure 4 9 has more efficient dataflow implementations Figure 4 10A is a copy of Figure 4 9C with several unnecessary local and global variables circled The Acquire global variable triggers the data acquisition task from the Run Test Value Change event case within the 114 Event Loop The DAQ Loop monitors the Acquire global variable until its value becomes TRUE and then executes the data acquisition task within the True frame of the Case structure Figure 4 10A Avoid polling variables in loops such as the Acquire read global variable in the DAQ Loop Additionally the Sensor Scaling local variables ca
19. Table 7 1 LabVIEW Error Codes Range Error Code Table 1800 through 1809 and 1814 Waveform 4800 through 4806 4810 4811 and 4820 through 4823 General 16211 through 16554 SMTP 20001 through 20030 20307 20334 and 20351 through 20353 Signal Processing 06000 through 56005 General 180121602 through 180121604 Shared Variable 1073479937 through 1073479940 Instrument Driver 1073676290 through 1073676457 VISA Additionally LabVIEW reserves codes in the range of 8999 through 8000 and 5000 through 9999 for custom user defined errors If your VIs need to flag a misbehavior that is not adequately described by one of the predefined error codes define a new code in this range The General Error Handler VI can be used to report user defined errors Simply specify the error codes and descriptions as arrays of integers and strings and wire them to the associated terminals This approach is very easy to implement if the application contains only one or a limited number of error reporting routines However it becomes more difficult if there are multiple instances of the General Error Handler VI requiring multiple user defined error codes Specifically the maintenance of the error codes and descriptions as arrays along with propagating the data using wires can be cumbersome Rule 7 11 Maintain user defined error codes within an XML file Instead use an XML file to maintain multiple user defined error codes and descript
20. s Instrument Driver Guidelines recommends the vertical slide switch for all Boolean controls within an instrument driver Most applications can be completed using just two types of Boolean controls command buttons for action on GUI VI panels and vertical slide switches for configuration parameters Rule 6 11 Label the TRUE and FALSE states of slide and toggle switches Always label the TRUE and FALSE states of toggle and slide switches with names corresponding to the behavior of each state and position the labels next to the physical switch positions Specifically from the control s shortcut menu choose Advanced Customize to open the Control Editor window Drop free labels adjacent to each of the control s switch positions and enter the text describing the TRUE and FALSE states Edit the font positions and orientations of the labels so that they line up with the TRUE and FALSE switch positions Close the Control Editor window and apply the custom changes This method makes the label a permanent part of the control that you cannot accidentally move or delete while editing the VI A faster alternative is to display the control s Boolean text Boolean text is made visible by selecting Visible Items Boolean Text from the control s shortcut menu and it can be repositioned and edited However the Boolean text shows only one state at a time and is not as intuitive as the TRUE and FALSE state labels Figure 6 6 provides a vertical
21. s selector terminal the text items of the enumeration appear as the labels in the Case structure s selector area Choose succinct and intuitive names for each state and enter them as items of the enumeration Any constants created froma type definition maintain synchronization of the item names with the type definition Therefore enumerated 255 constants are used to specify the next state selection in each case As items are added and removed from the enumerated type definition all of the corresponding constants update themselves automatically Enumerated type definitions are recommended for all variations of the state machine design pattern that use a Case structure Rule 8 13 Minimize code external to the Case structure Rule 8 14 Include states for Initialize Idle Shutdown and Blank Most applications require some general house cleaning code such as routines for initialization and shutdown and perhaps a routine that runs when nothing else is running For example some control values and properties are initialized prior to the execution of any other states Also an event handler or resource polling routine may need to run when no other states are being processed A shutdown routine is required to gracefully shut down any open connections to hardware devices data files and remote applications Create corresponding states for Initialize Idle and Shutdown These routines may or may not have been considered during the desi
22. 31 Conversely LabVIEW developers support the scientific method when we record our activities in a project journal In some cases the data that is recorded might even supplement the proof of discovery or invention when a company applies for a patent Recording the raw specifications ina project journal constitutes a good specification development practice Per Theorem 2 1 specifications positively influence LabVIEW style The developer can refer to her notes as needed throughout the development and maintenance of the application as long as she remains available to do so and does not lose or destroy the notepad Moreover project journals form the basis of a more formal requirements specification Simplicity and speed are the primary advantages of project journals yet there are several limitations 1 Bound notepads provide minimal organization Project notes are entered chronologically Inactive requirements ideas and notes are often mixed with active ones It is often difficult to distinguish active requirements from inactive ones 2 Many people do not have clear legible handwriting Indeed it is very difficult for even the neatest writers to maintain legibility throughout a rapid brainstorming session the kind of environment in which many LabVIEW projects are specified 3 Notepads are not transferable It is inefficient at best for multiple project team members to share someone s handwritten notes as project refere
23. 4 00 S04 608k 608k 608k 18 Set Chart Pots Yiibis for Data Collection LEZ 0 0 1 2 EEr i 11 20k 11 0 Li Cen Plain Loop 4 PEL 130 1 6 4 6 ok 4K 19 Ew Mukipe Eemats Canda DAS Forsa Lene 31 1 aL 4 4lk 44k 418 9 Format Cardi DAS Data For Charts 70 1 Wi Mik pik Pik 3 De Date Dirplay Loop vi 04 DAL 6 0 ik bk 1g Populete Charts wih Bini of Data 60 1 J EIL BESK Bek B 44 Asad Cate of Permanent Date Fie 50 1 oL 4 77 4 77 47TH io End Mubipln Elements Main Loog wi 501 aT Bl aSk ee es Reed Al Contents of Seeded Pie a BRL 3 164 7 16k iik il Cade DUS Parse Data Packet 40 1 1 4 42 2 BZ 14 77 10 Cadar DAS Pla c grad a daba 40 1 1 3 4a T E 340k i5 Pary Block of sta From Pile 20 0 100 1 UR Hak LE x Cardiac DAS Parse Daba Paeis vi 20 0 GL iik 41lik 19 6 Eqns Depay Da WDD i 10 0 4 2a 45k 488k if Mealy Eror Source Siring and Engi boo D 15 0 5 J k a Ek 3 E Cote E E ow _ Note that the efficiency of distinctly different applications such as Meticulous VI Nested VI and Spaghetti VI cannot be compared in any meaningful way This is because execution time memory consumption and I O depend on the application s requirements as well as the developer s programming style For example metrics from the Profile Performance and Memory window might indicate that Nested VI executes the fastest and uses the least memory However Nested VI is the least resource demanding of the three applications and migh
24. Combine bold text labels with plain text parentheses for control labels of commercial subVIs Maximize the contrast between text color and background color Use large text size for command buttons and critical data Allow extra space between labels and objects for multiplatform applications Apply color judiciously Create a color theme Follow universal conventions for green yellow and red Leave the panel and objects of subVIs gray Keep the color schemes simple and time limited Limit the number of controls that are visible and enabled at any one time Restrict the range of all controls to values that are relevant to the application Set the Data Range property of numeric controls 355 Os Group related controls using decorations spacing tabs and clusters 3 36 Use ring or enumerated controls over string controls when feasible 3 37 Set the Tabbing Order and use the Key Focus property to aid navigation 3 38 Customize the runtime menus for top level VIs 3 39 Always include a Help menu or button 3 40 Be consistent Chapter 4 4 1 Use 1280 x 1024 display resolution Ay Leave the background color white Ad Use a high object density 4 4 Limit the diagram size to one visible screen or limit scrolling to one direction 4 5 Create a multilayer hierarchy of subVIs 4 6 Modularize top level diagrams with subVIs A 7 Modularize the high level subVIs with lower level subVIs 4 8 Do not create subVIs just to save space 4 9 Avoid
25. DAQmx VIs and File I 0 nodes form two separate parallel error chains that are combined using Merge Errors VI View full size image a aaa error in no enirar error out Te Bal a a Da eee Most functions and VIs that LabVIEW provides are designed to check the error status from the error in terminal and if an error exists skip their code and pass the same error information straight through their error out terminal Hence once an error occurs the successive nodes of an error chain propagate the error information throughout the chain This is desirable because the nodes of an error chain are generally related and interdependent In the File I 0 example of Figure 7 1 the file writing operation is dependent on a valid file reference number returned by Open Create Replace File which itself is dependent on a valid file path returned by the File Dialog Express VI An error generated by one prevents the subsequent nodes from succeeding As long as all error terminals are properly wired the first error that occurs is trapped within the chain of nodes and wires As discussed in Chapter 4 propagating the error cluster creates data dependency which determines the order in which the nodes execute on the diagram Data dependency is an underlying principle of data flow Therefore it is also important to wire the error cluster to nodes to specify the desired execution order However the most common mistake
26. Erp Liewt Eeritetion Filter Bis iadepe Enabhe Mame Fi Serka Suber Fob Lint T Hisy Vollage Seii C derdi pij h hih pm po jpo feer pw e po e CE M ce TET ICEN 2 boo few foo a rr se er ms ee ee es es ee es ee ee ee ee ee ee ee ee ee 202 Figure 6 16C contains a code snippet of the application s configuration state Multiple nested data structures containing the configuration data for each module type are bundled into another more complicated data structure This larger structure s sole purpose is to reduce the number of terminals and associated wire clutter for the subVI that follows The nesting of this data structure might be a concern if the data was accessed during a time sensitive routine However this data structure contains only configuration data which is accessed and programmed only before the critical data acquisition routines begin Therefore the complexity of this data structure does not affect the performance of the critical task Figure 6 16C The configuration routine bundles data structures to form a complex data structure that is passed into the subVI Each data structure is an array of cluster containing configuration parameters for a module type similar to Figure 6 16B View full size image Sample Rate Hz ci Sensor Monitor O22 Monitor Module Number 032 i gt Accelerometer Settings Jpm Accelerometer Settings LVDT Settings esje LVOT S
27. However I generally recommend trapping errors from all error terminals to the maximum extent possible Rule 7 6 Report errors using a dialog and or log file When an error is trapped report the error using a dialog or log file The dialog is the simplest and most popular method of error reporting It consists of a call to the Simple or General Error Handler VI These VIs located on the Dialog amp User Interface palette examine the error passed to their error in terminal look up an error description from LabVIEW s error code database and generate a message describing the error By default they each open a dialog window that displays the error code source and description which the user must acknowledge An example is shown in Figure 7 7 Figure 7 7 The Simple and General Error Handler VIs generate a dialog window that displays the error code source and description which the user must acknowledge aes alin ewe BS Ba ed a OE ee ed Fas ee eb ieee Bw wk em mm he te ee wd bie tt z EJ in Error Clusters yi xX Error 43 occurred at Open Create Replace File Possible reason s LabVIEW Operation canceled by user Rule 7 7 Use General Error Handler VI over Simple Error Handler VI 210 The General Error Handler VI provides the flexibility to handle user defined errors and exceptions The Simple Error Handler VI is nothing more than a call to the General Error Handler VI with fewer input and output terminals exposed T
28. PC Computer that runs a non deterministic operating system such as Microsoft Windows Personal Digital Assistant PDA An electronic device that can include some of the functionality of a computer a cell phone a music player or a camera PDAs are an alternative to bound notebooks for recording project specifications during a meeting plug in VIs that are dynamically loaded by a Dynamic Framework including both high level components and low level subVIs polling Condition in which a loop continuously monitors a resource until it reaches a specific value or state Avoid polling front panel terminals and variables as there are more efficient alternatives 348 polymorphism Term that describes a function or VI with one or more terminals that can accept more than one data type Polymorphic nodes adapt to the input data type instead of breaking the wire or forcing a coercion to occur on the input terminal Portable Document Format PDF Format created by Adobe Systems for representing two and three dimensional documents in a fixed layout cross platform compatible file format PDF files encode the exact look of a document in a device independent manner Profile window Tool that monitors the data memory use and execution speed for a set of VIs loaded in memory The Profile window is accessed from Tools Profile Performance and Memory programmer Someone who develops custom software applications using a conventional
29. The tip strip is a label that appears for a few seconds when the user places the mouse over the control and then disappears It is ideal for storing a phrase or one line sentence that summarizes the control s purpose Figure 3 10C shows the Context Help window and tip strip when the user places the mouse over the Account button Hence the succinct Boolean menu has greater descriptive functionality with minimal static text compared to the textual menu of Figure 3 10A Figure 3 10C This menu of Booleans has an informative description and tip strip visible View full size image gt Collect Menu Context Help Account conics Key Navigation F1 gt Topgle and Focus Press here to create a new account Config Ure open and view an existing account oF to create a copy of an account with a new name Also in this screen is the Acquire One more comment regarding control labels It is generally best to leave the background color transparent This is the default setting of a front panel option called Use transparent name labels accessed from Tools Options Front Panel Older versions of LabVIEW applied raised labels to new controls by default This is less aesthetically pleasing and many of us used to manually color the background each of our labels transparent This is no longer an issue as long as Use transparent name labels is selected and we do not try to impose any unnecessary creativity with the labels Thi
30. and stick with them for all the GUI VIs in your application One technique that is relatively foolproof is to choose one highlight color one muted color and one shade of gray This is very easy using LabVIEW s color picker which contains separate rows for each category as shown in Figure 3 12 The grayscale and muted colors are appropriate for larger objects and the panel background You can select them from the top two rows of the color picker The highlight colors are the bright colors which you can select from the bottom row Sparingly apply one or at most two highlight colors to help draw attention to smaller objects and animation Figure 3 12 The LabVIEW color picker arranges colors into grayscale muted and highlight colors eed Grayscale Muted Highlights OOS Ss Oseeeeoosooos History System Hoooooooooo ONONE EEn G Classic Object Space bar boggles color selection Rule 3 29 Create a color theme e Use common colors to associate related items e Apply color consistently throughout all VI panels of an application In Section 3 1 Layout we discussed grouping related items on a panel to indicate association and spacing unrelated items to help disassociate them Similarly use color to reinforce and extend the association provided by the panel layout Create a color theme that includes color coding of related items on a panel Moreover extend the color theme consistently throughout all the GU
31. if the user cancels out of the file dialog the VI returns error code 43 This error causes the loop to terminate unless it is cleared from the error cluster before checking the error status Consequently Clear Error All or Specified VI is used to clear error code 43 and continue the application Figure 7 17 Clear Errors All or Specified VI clears error code 43 from Torque Hysteresis VI This error naturally occurs when the user cancels a file dialog 229 bH LJ stop E a Automatic error handling is an alternate method for trapping errors that is transparent to the diagram It consists of a VI property named Enable automatic error handling selected from the Execution category of the VI Properties dialog When an error occurs in any node with the error out terminal not wired LabVIEW suspends execution opens the diagram window highlights the node and displays an error dialog If the node s error out terminal is wired the error data propagates as programmed and no additional action is performed Automatic error handling is disabled by default in all versions of LabVIEW except for 7 0 the version in which the feature was initially released It is important to note that automatic error handling does not ensure that errors are reported If a node s error out terminal is wired but the error cluster terminates without a reporting routine the error data is lost Hence automatic error handling helps only when one or more nod
32. ms function Wait Until Next ms Multiple function or Time Delay Express VI to incorporate a delay within continuous While Loops Each of these nodes yields control of the execution thread back to the CPU and causes the loop to sleep for a number of milliseconds within each iteration This allows parallel tasks to execute and improves the overall performance and responsiveness of the application Note that LabVIEW is always multitasking even if the diagram executes all nodes sequentially LabVIEW contains multiple processes running in parallel including the user interface and parallel diagram elements such as parallel loops and nodes Additionally the operating system may have other applications or background tasks that it must perform in parallel with LabVIEW Including a delay in continuous While Loops allows these parallel tasks to run The Wait ms function and the Time Delay Express VI add a delay that is timed in parallel with the loop s code The total loop execution time is equal to the larger of the time required to execute the code or the specified wait or delay time If the delay time is longer than the code execution time the loop sleeps for the balance of the interval after the code completes For example if the loop code takes approximately 12ms to execute and you use a timing function with the wait or delay input set to 50ms the total execution time is approximately 50ms per iteration including 38ms of sleep If the code tak
33. the importance of creating succinct and intuitive labels for all controls and indicators is emphasized These labels are a primary source of documentation on the diagram but only if they are visible A tremendous advantage of LabVIEW s dataflow paradigm is that you can trace the data path from source to destination If you cannot identify the terminals much of this advantage is neutralized Observing the diagram entails toggling back and forth between the front panel and diagram windows in an effort to identify each terminal and commit the labels to memory On a diagram that conforms to good style this exercise is completely unnecessary One might assume maintaining visible labels to be automatic After all owned labels are visible on the diagram by default From what I have seen however many developers go out of their way to hide the labels In some cases the labels are even empty This practice is particularly perplexing because LabVIEW assigns a default name when each control is created I suspect that the developer does not want the labels taking up space on the diagram and decides to hide or empty then This is why it is important to make the labels succinct Chapter 3 contains rules and examples for creating control labels that are succinct and intuitive facilitating good documentation Rule 9 2 Apply free label comments in select locations Rule 4 18 Label long wires and wires from hidden source terminals Rule 4 37 301
34. without kinks or bends In Figure 4 4D the wiring is completed and the horizontal gaps between nodes are manually adjusted Figure 4 4A A data logging routine consists of several functions containing similar connector patterns and terminal assignments View full size image Ea 101 Figure 4 4B The functions are aligned horizontally using the Bottom Edges alignment tool View full size image t Date Log vi Block Dianna Figure 4 4C Wires for file refnum and error cluster proceed with even spacing and without kinks or bends View full size image Figure 4 4D The wiring is completed and the horizontal gaps between nodes are manually adjusted View full size image Rule 4 13 102 Tunnel wires through left and right borders of structures Rule 4 14 Do not wire through structures unnecessarily Tunnel wires into structures through their left border and out of structures through their right border Avoid tunneling wires through the top and bottom borders Also avoid passing wires through structures if they are not utilized within the structure unless their purpose is clearly labeled It is particularly annoying to flip through many frames of a multiframe structure such as a Case or an Event structure searching for places where the data in the wire is modified However sometimes it is useful to pass a few spare wires through a Case structure such as the state machine design patterns that are disc
35. 28 2 Prepare for Good Style LabVIEW style is positively influenced by preparing to develop your applications before you begin coding First and foremost write a requirements specification It 1s much easier to develop good code when working from a specification instead of deriving the requirements on the fly Second design the software Combine standard software design principles such as the State Machine Diagram with LabVIEW best practices such as the State Machine design pattern Third configure the LabVIEW environment Make use of desired preferences and previously developed code such as reusable templates instrument drivers and utilities Finally establish some conventions for organizing naming and controlling your project files An organized project spawns organized source code This chapter describes preparation techniques that are conducive to good development style It is noteworthy that several of these techniques relate equally to configuration management CM as well as style Indeed many of the Rules in this chapter as well as throughout the book may complement or extend a CM process All topics are presented in the context of style which is one element of CM Due to the wide diversity of LabVIEW applications and target industries there is no universal CM system that can work for most organizations Therefore the reader may evaluate the relevance of these materials to their own CM system 2 1 Specifi
36. Certified Training Centers Now back to the low emissions exhaust system application example Our requirements include control of several specialized gas analyzing instruments and SPC analysis of the data Hypothetically we looked on NI Developer Zone and did not locate drivers for our specific instruments We also inquired with the instrument manufacturers but came up empty We did find the SPC Toolkit for LabVIEW part of the LabVIEW Enterprise Connectivity Toolset from National Instruments It contains VIs and charts for all the SPC analysis that we need Hence we can elect to use the SPC Toolkit and save ourselves substantial development effort Also our LabVIEW style is positively influenced by incorporating the high quality VIs that are provided with an NI toolkit and reducing the scope and complexity of the source code In many cases it is useful to develop a proof of concept to evaluate a specific instrument hardware component or software design For example you might have a critical requirement for high speed acquisition and online data processing You might be uncertain that a given computer instrument or DAQ module or communications bus combined with a data processing algorithm can execute within a specific time interval Therefore you should design a test that will perform or simulate the critical acquisition and analysis algorithms and benchmark the execution speed You then could use 37 the results of your benchmark t
37. Compress tool from the Distribute Objects menu Next align the right edges of the column using the Right Edges tool from the Align Objects menu When you are happy with the appearance and spacing of one column of objects group them by selecting Group from the Reorder menu and clone the group four times to create four even columns of indicators Finally select the four groups and space them evenly by selecting any of the horizontal distribution tools from the Align Objects menu The same basic techniques are used to create three columns of two indicators for the cluster of power readings Note that the temperature indicators are more tightly compressed than the power indicators This is because the temperature labels are adjacent to the axis of compression allowing the indicators to abut each other 58 Just as grouping is used to associate related objects spacing is used to disassociate unrelated objects In Figure 3 1 a small amount of the panel background appears between the two primary decorations delineating the separate stations Additionally a fair amount of space appears between the temperature and power clusters for each station Space at the bottom and top of the panel resembles margins that help frame the panel and direct the user s attention to the objects in the center of the panel Symmetry and spacing need not be limited to GUI VIs Apply the same rules to subVIs to give your source code an organized and profession
38. D I will definitely be recommending that new VI developers on my team read this book and I m confident that I 11 be referencing it frequently in style discussions with my more experienced colleagues Another truly unique feature of the book is the impressive number of example VIs that Peter uses to illustrate good and sometimes bad style With his decade and a half of LabVIEW experience Peter has a vast library of VIs that he his employees or his customers have written He utilizes this library of code effectively to illustrate applicable points in every chapter What s more he will sometimes take the same example over the course of several chapters and refine it more and more with style rules picked up along the way This gives us a real time view of how good style can positively affect VIs as they are being developed So with LabVIEW celebrating 20 years of inspiring engineers scientists and even children worldwide I m confident that readers of the definitive book on LabVIEW style will ultimately appreciate the time and effort savings that result from developing VIs with consistent appropriate style If you re a LabVIEW beginner get ready for your VIs to become as Peter would say awe inspiring and if you re a LabVIEW expert you re going to learn a lot too I know I did Darren Nattinger Staff Software Engineer LabVIEW R amp D National Instruments Corporation Darren Nattinger has worked at National
39. EA Pi g b rt 1 in a ud ii dl i ii E LE DI bkk kari n E Ent Calter ator rs i z my Hab Arit Wiebe Sta E Tarp I piim Furie Vahm Sie 1 Pe Pron Pulsar daher Trowel eel Pues a oy PS ee Pree ask Pies Was Tr r Peer eri Pret ee L METE baer velar tik aTr Kine FE 7 SEER HA an est ston caetera irk 3 F es fe Hien ee eS Layout The diagram is sized appropriately with medium to low density Modularity There are 13 user VIs out of 396 nodes for a modularity index of 3 3 Clutter outside of the loop should be combined into one initialization subVI 125 Wiring Scheme Wires contain excessive bends and kinks Many wires enter and exit structures vertically Several wires are too long Data flow Data flows in all directions Initialization routines are not explicitly ordered to execute before the main While Loop There are excessive coercions Right to Left VI is an application that acquires waveform data from pressure and volume transducers until an event occurs and then logs the last sample of each waveform to a comma delimited text file It is illustrated in Figure 4 17 The diagram utilizes efficient data flow and contains liberal comments However it contains one wire that is routed right to left as shown near the center of the primary Case structure s Log Data case The wire is cleverly labeled to indicate the rig
40. Figure 8 21B The block diagram propagates the application s primary data structures using shift registers and passes them to the components in the Call Plugin state View full size image gt UUT rfo gt ka T Se e Ge B y usta Se eee F gt Contig PE ae gt gt Woareerit gt Eie gt Pugin Pah gt a priii ep t Rate gt error im na error te error out ee jy TTH Figure 8 21C The cluster of motion parameters is combined with other parameters into a single cluster of configuration parameters The initial configuration settings are read from a config file within the Initialize state View full size image 277 The Multiple Loop Application Framework extends the compound design pattern concept to combine multiple single loop design patterns messaging utilities and controls into an application framework Rule 8 27 Create a parallel loop for each cohesive parallel task The flexibility and performance of an application framework is optimized by dividing the common functional elements into cohesive tasks and applying parallel loops for each task Parallel loops provide the flexibility to execute multiple tasks simultaneously LabVIEW spawns a separate thread for each parallel loop and each thread can run on separate parallel processors if available Additionally parallel loops allow you to specify and fine tune the priority of each task using delays with While Loops and priorities with
41. For numeric controls configure the format precision and range For strings specify the display style single line and wrap behavior Customize the appearance and default value properties of each control or define your own custom controls using the Control Editor Rule 6 4 Configure an appropriate default value for each control To help promote proper VI use configure appropriate default values for each control Consider the common use cases determine what typical value each control may utilize and configure it as default if it exists At the very least the VI should be able to load from memory and run properly without modifying the control values Required inputs such as refnums and instrument sessions are an exception For subVIs indicate the default value in parentheses at the end of the owned label so that it is visible from the panel as well as the Context Help window of the calling VI Rule 6 5 Enter control descriptions As discussed in Chapter 3 intuitive and succinct owned labels help document the controls and indicators Additionally enter one or two sentences for each control description that further describes the control s purpose default value and range unless it is completely intuitive based on the label Control descriptions provide documentation that is visible from the Context Help window In Chapter 9 Documentation we discuss how a documentation set is generated containing the control labels and descrip
42. Handling template If you like shortcuts begin your subVIs with the SubVI with Error Handling template selected from New VI From Template Frameworks SubVI with Error Handling This template initiates a new VI that contains the error handling 221 mechanisms for a subVI described in this section This template saves a minute or two on each subVI and helps ensure consistent quality throughout your applications 7 3 Prioritizing Errors Per Rule 7 5 it is best to trap all errors returned from all nodes that have error terminals This helps maximize the application s reliability In practice however I know of few developers who rigorously wire the error terminals on every node of every diagram In fact there is a potential conflict between Rule 7 5 and some of the wiring rules in Section 4 2 In a dense diagram that contains parallel dataflow paths wiring every error terminal may lead to overlapping wires excessively large diagrams or right to left data flow Finding the best layout that optimizes these diverse requirements may become time consuming What tradeoffs are permissible in the interest of saving time while maintaining reliability and good style To answer this question consider your understanding and comfort level of each node s operation the importance of the operation s success within your application and your tolerance for risk If you are like me and you learned to use the Visible property for a front
43. Hysteresis VI by applying the manual checklist to the VI containing the VI Analyzer improvements from the previous section The front panel and block diagram of Torque Hysteresis Improved w VI Analyzer VI are copied to Figures 10 3A and 10 4A respectively We begin with a self review of the front panel Reading through the checklist in Appendix B the following style rules are violated by the front panel in Figure 10 3A Figure 10 3A Front panel window for Torque Hysteresis Improved w VI Analyzer VI Multiple style rules violations persist View full size image 328 E Torque Hysteresis inproved Vi Amelyrer vi Frani Panel Be EE Wem pect Gperate Jei pinke Hip ia ipt Apelcatien Font Rule 3 1 Group related controls using decorations spacing tabs and clusters Rule 3 2 Apply symmetry and spacing to front panel objects Rule 3 9 Enlarge and center the objects of an industrial GUI VI in proportion to their importance Rule 3 10 Limit the quantity of information displayed on a GUI VI panel Rule 3 19 Apply consistent fonts and capitalization Rule 3 33 Limit the number of controls that are visible and enabled at any one time Rule 6 4 Configure an appropriate default value for each control The legacy application s GUI in Figure 10 3A consists of one large panel containing all the controls and indicators all of which are visible all the time The groupings alternate with a graph indicator on the left clusters of controls
44. I observe in practice is incomplete error trapping It seems that many developers propagate the error cluster for data dependency but fail to fully trap and report the errors This is evident when the error cluster propagates between the functions requiring an order dependency but is not shared among the remaining functions or terminated appropriately 206 Rule 7 3 Trap errors from all iterations of loops Special considerations are required with looping structures It is important to examine the data flow and loop condition to see if the error is being trapped within all iterations In Figure 7 4A there is continuity between the error in control terminal the error connector terminals on the File I O nodes and the error out indicator terminal At first glance the error appears to be trapped but it is not Instead the loop input tunnel resets the error data within each new iteration of the loop Also the error data passed to the loop s output tunnel is overwritten in each successive iteration Only the error data from the last iteration of the loop passes through the output tunnel Hence the error chain does not remember any errors that may have occurred in any of the iterations prior to the last Two better alternatives are to either terminate the loop upon the first occurrence of an error or maintain the error data between successive iterations via a shift register These alternatives are shown in Figures 7 4B and 7 4C The former solutio
45. Instruments for eight years He is currently the primary developer for the VI Analyzer Toolkit and the Report Generation Toolkit for Microsoft Office He was also a reviewer for The LabVIEW Style Book Preface The LabVIEW Style Book is a comprehensive reference on recommended LabVIEW development practices It contains style rules designed to optimize the ease of use efficiency readability maintainability robustness simplicity and performance of LabVIEW applications The book provides thorough explanations of each rule including examples and illustrations The material leverages the work of the early pioneers of the LabVIEW community has evolved from many years of use by Bloomy Controls and has been reviewed by esteemed representatives of the LabVIEW community I invite you to learn from the experiences of myself and the staff at Bloomy Controls Inc by reading The LabVIEW Style Book I hope you enjoy reading it as much as I enjoyed writing it Intended Reader Intended readers include developers managers and organizations that develop or use LabVIEW applications You must have a working knowledge of fundamental LabVIEW principles and terminology as instructed in a LabVIEW Basics I and II hands on course and experience developing and deploying applications Experienced beginners can use this book to form good programming habits early in their LabVIEW careers Intermediate develo
46. Label wires exiting the left shift register terminal Rule 8 20 Label each loop in the top left corner Rule 9 3 Label every subdiagram of every multidiagram structure Rule 9 4 Label algorithms constants and Call Library Function Nodes Rule 9 5 Use default 13 point plain black application font for all diagram text Free labels are analogous to comments in a conventional text based programming language Apply them to select locations of your diagrams by single clicking the labeling tool or by double clicking the auto tool in an empty area of the front panel or block diagram One common rule is to apply a free label in every subdiagram of every multidiagram structure with a few exceptions such as the enumerated Case structure that is described later Additionally it is useful to have a convention on the location of the label At Bloomy Controls our convention is to place the label in the top left corner of most structures as seen in many of the illustrations in this book We use LabVIEW s default 13 point plain application font and a lightly shaded yellow background Apply free labels on long wires Use the greater than sign to indicate the direction of data flow and color the background white Label wires exiting shift register terminals in a similar manner Label mathematical algorithms and constants Note that algorithms are good candidates for sub VIs Constants may have owned labels similar to controls and indicators Simp
47. Moreover the most important rule regarding front panel style is to be consistent in whatever you do Rule 3 40 Be consistent e Maintain consistent appearance and location of similar controls Save controls with customized appearance as strict type definitions If you choose a particular layout or set of fonts colors and control types use them consistently throughout your application and perhaps multiple applications This will make it easy for the users to operate the software By learning how to operate one panel users should understand where to look for certain controls and indicators and how to navigate the GUI By using similar layouts fonts and colors you reinforce to the user that each panel behaves similarly Additionally it is beneficial to be consistent throughout an organization This can involve collaborating with your peers to set some GUI standards Hopefully this book provides a good starting point If you create a series of VIs that have some common functionality use identical controls to expose that functionality on each panel and keep those controls in identical locations for each VI For example if you have a succession of dialog VIs such as a wizard which you can navigate or exit by clicking buttons labeled Back Next and Cancel apply the same buttons at the same locations of each VI Consistency aids GUI navigation You can use LabVIEW s control editor to customize the appearance and
48. N T OE A E EN 316 10 1 Sel REVIEWS airera a E E SE EE N 316 10 2 PECK REVIEW Soini A a r N T ies mAs 332 Theorem 10 T uane E A A EN A va aaa S A AEE E 332 EUAN OES aaaea a aa a a aa a E E ai acta nga te 336 A GIOS SaN anete a a a a a e a a e a a a r 338 B Stvie Rules SUMMAN areon arinia ar nN tas tens teva canaetetciegsi cet aie naman venta cnntaw E NNS 354 Chapter 2 a dics citi asad nen aA AE EAEE N A E E EN 354 Gate 9 C S eee E OE A N O E OE A A AO OOTA 354 CHADA er T E E E E tavenond asondatrune 356 Coan EE srs eaucav ics eres v sa cunts ree cece cies acc cics tos au ave erev geocaches ine aes T GVO mAR OR HMR DIG NES 357 Chapter 6 seseriai nenna o InN INATA N E aie longuaa tant oneuaiebonasuecoueed 358 CHADE Tinere n eE E E E A E A T A E A E E A E 359 ANET OLE e EE DEEE BTI IAEA A nck N A E E A EA A EA E E E E E E caaae 360 EN e E AP OEEO E E EA A A A NA ATE E E E E NE E A niente 361 Chapter 10ra ne E E A A N EA N 361 Foreword I ve been writing VIs for a long time as a member of the LabVIEW R amp D team at National Instruments During my first week of LabVIEW training many years ago I decided to write a LabVIEW version of my favorite card game called Set It took me a while but I eventually finished it and it was probably some of the ugliest LabVIEW code you have ever seen As Peter Blume would say my code was THE definition of spaghetti Worse yet the front panel was full of neon colored controls and
49. Off Yes or No Open or Closed and Stop or Go When the two states are not opposites use a text ring or enumeration to provide text selections that more accurately describe the choices Rule 6 9 Assign names that identify the TRUE and FALSE value behavior When a parameter s states are indeed opposites it becomes easy to assign the control a name that clearly identifies its behavior in relation to the TRUE and FALSE Boolean states For example if we have a control named Valve State the TRUE FALSE behavior is unclear Instead consider a name such as Close Valve In this case we can determine from the name that the valve is closed when TRUE and open when FALSE Per Rule 3 23 always enter the default value in parentheses for subVIs This eliminates any chance of ambiguity In the latter example we have Close Valve F Open Rule 6 10 Use command buttons for action slide switches for parameter settings Use command buttons to represent Booleans that invoke immediate action on GUI VI panels such as Trigger Run Cancel Quit and Close Valve Simply start with the OK Cancel or Stop button from the Boolean Controls palette and customize it for your needs This may involve simply changing the Boolean text or editing the size fonts and colors as well Do not use command buttons for settings that do not generate immediate action such as configuration properties Instead use the control that appears most intuitive for the users NI
50. OpenG File Tools palette contains icons that resemble functions from the LabVIEW File I 0 palette The OpenG icons are shaded green to distinguish them from the LabVIEW functions 141 OpenG File Tools Figure 5 8 The floppy disk and pencil graphics are copied from an Express VI on the File I 0 palette The background color and text are customized for distinction View full size image Many free icon galleries are available on the Web Most web browsers allow you to copy or save the graphics right out of a web page Using Microsoft Internet Explorer simply right click on a graphic and choose Copy Then open the icon editor in LabVIEW and paste the image from the clipboard onto the icon Alternatively drag and drop the graphic from the web browser to the icon in the upper right corner of the target VI s front panel window If you need to rescale the image to fit into the 32x32 pixel area first save the image to file using a drawing application such as Microsoft Paint Scale or edit the image within the drawing application save the modified image file and then import the image onto the clipboard for use in LabVIEW s icon editor using Edit Import Picture to Clipboard LabVIEW supports several standard graphic formats including bitmaps BMP JPEG JPG Portable Networks Graphics PNG Multi Image Network Graphics MNG and Graphics Interchange Format GIF Additionally you can drag and drop the
51. Patterns An Introduction to Object Oriented Analysis and Design and Iterative 54 Howard M Kanare Writing the Laboratory Notebook Washington D C American Chemical Society 1985 Development second edition Upper Saddle River NJ Prentice Hall PTR 2001 McConnell Steven Code Complete second edition Redmond WA Microsoft Press 2004 McConnell Steven Software Project Survival Guide Redmond WA Microsoft Press 1997 Bloomy Controls is an NI Select Integration Partner that provides systems development and training services throughout the Northeast United States including offices in Windsor Connecticut Milford Massachusetts and Fort Lee New Jersey 55 3 Front Panel Style The LabVIEW front panel is named to emphasize its capability to emulate the panels of traditional self contained instruments Indeed when LabVIEW was first released in 1986 its primary purpose was to control laboratory instruments via GPIB and serial interface Oh how times have changed Today LabVIEW is applied to many different types of applications industries and instrumentation The complexity of applications has dramatically increased The evolution of front panels is no exception 3D controls tab controls tree controls containers Property Nodes VI Server Event structures and subpanels are modern innovations that facilitate powerful LabVIEW applications Despite these innovations many front panel design techniques used by early
52. Presented in This Chapter SubV GU Top Lev Simple Medium Advanced Deskto Real Ti Event Driv Inefficient Poll Sectio Page I I jel VI Complexi Complexi Complexi jp OS me OS en ing n Numbe VI ty ty ty B Machine Event Y Y Y Y Y 8 2 4 265 Machine Parallel Y i r Y 1 8 3 1 269 Loops Dynamic Y Y Y Y Y Y 8 4 1 272 Framework Multiple Y 1 i4 Y 8 4 2 278 Loop Framework Modular Y Y Y Y Y 8 4 3 283 Multiple Loop Framework Rule 4 6 Modularize top level diagrams with subVIs Rule 4 27 Avoid Sequence structures unless required Rule 4 28 Avoid nesting beyond three layers It is much easier to recognize a given design pattern when the diagram is modular Always look for opportunities to combine groups of nodes that work together to perform a specific task into a subVI There should be very few functions on the diagram of a top level VI outside of the functions that comprise the design pattern Sequence structures are generally avoided because they circumvent the dataflow method of execution ordering and are inflexible Ironically one of the most accepted uses of Sequence structures is to help delineate and frame a design pattern A Flat Sequence structure may be used when limited to the outermost structure of a top level VI or initializing a variable or resource but it should be avoided otherwise The design patterns in this chapter use dataflow alternatives to Sequence structures Avoiding Sequence structures helps prevent excessive nest
53. SubVIs lend themselves very well to code reuse Additionally controls can be customized and saved as custom controls or type definitions Design patterns and application frameworks can be saved as VI or project templates These tools can save the developer days of development time versus starting with blank VIs and developing each application from scratch Every developer should maintain a software reuse library containing useful LabVIEW code Moreover every organization that employs multiple developers should maintain a reuse library in a networked repository maintained under source control The potential benefits include faster development time greater commonality and higher quality applications We all want to save time The time savings element of reusing source code is very important Why reinvent the wheel if we do not have to Always look for existing design patterns VIs and controls that you can reuse in your applications Many developers use NI Example Finder to locate useful examples that are bundled with LabVIEW This is a very well integrated tool that presents examples in an organized and searchable manner Far fewer developers integrate and use their own LabVIEW source code perhaps because it might not be obvious how this is best accomplished We begin with the most commonly shared LabVIEW construct the instrument driver Although thousands of instrument drivers are available from NI Developer Zone
54. Timed Loops Hence an application framework based on multiple parallel loops helps optimize the application s performance The disadvantage of multiple parallel loops is the additional complexity of messaging required to share data between loops Therefore it is important to delineate tasks that are cohesive and not too interdependent with respect to data The implementation presented in this section uses queues for loop messaging The scope of this framework is a single PC hence RT FIFOs and multiple process shared variables are not necessary Queues are more functional and general purpose than local and global variables Implementing each task as a parallel loop and using queues for data sharing and synchronization helps maximize the breadth of applications that the framework can be applied to Rule 8 28 Include loops for Event Handling Main State Machine Hardware I 0 and Error Handling Most complex applications benefit from the following parallel loops Event Handling Main State Machine Hardware Input Output 1 0 and Error Handling The Event Handling Loop is the most efficient method for capturing GUI events maximizing the responsiveness of the GUI and enhancing the user s experience Always include an Event Handling Loop with every complex application that contains a GUI The Main State Machine facilitates state transitioning organization expandability and maintainability Implement any sequentially ordered tasks that are
55. Within the subVIs the data from each UUT and each test run is appended to the data structure The application writes the data to file after each UUT but maintains the data in the structure until the end of a lot at which point a report is generated summarizing the data for the multiple UUTs that comprise the lot In this example the nested data structure reduces wire clutter on the top level diagram by combining multiple structures into one and adds the flexibility to perform multiple test runs per UUT by storing the data for all test runs performed on multiple UUTs Figure 6 11B The nested data structure propagates among all subVIs of the top level diagram The data structure and diagram enhancements facilitate multiple test runs per UUT multiple UUTs and organized data with fewer wires View full size image Figure 6 11C illustrates one possible approach for implementing Run Test VI in which the Torque vs Angle measurement data is acquired and combined with the nested data structure However this approach is not recommended because of the continuous resizing of the arrays using Build Array and manipulation of the Torque vs Angle subcluster within a time sensitive acquisition loop Specifically the output of each Build Array function has a different array size than the inputs causing LabVIEW to periodically allocate new memory buffers Latencies resulting from the reallocation of 192 buffers may cause unacceptable jitter d
56. ahaa ala hahaa alal aaa aaa alae lalalalale tL Li CEL Li n M M Cb Lo i La Lie Figure 4 8B The Sequence structure is replaced with parallel nodes which LabVIEW executes efficiently Number 1 BLE Number 2 DELH Result 1 PDEL Result 2 110 Rule 4 28 Avoid nesting beyond three layers Nesting is the placement of structures within structures Nesting beyond two or three layers begins to obscure the underlying logic and data flow as shown in the Nested VI example from Chapter 1 It is difficult to visualize all of the possible logical branches and data paths in a highly nested diagram Excessive nesting is normally caused by faulty logic excessive use of Sequence structures and lack of standard design patterns Faulty logic is an important concern because the more nesting the more difficult it is to comprehend the diagram to identify and debug the problem Spend some time creating a flow chart truth table or Karnaugh map to understand the desired logic before implementing the source code In Chapter 8 specific architectures that minimize nesting are presented However some level of nesting is required even for the most common and useful design patterns Because the structures that comprise the design pattern are recognized as the design pattern they need not count toward the excessive nesting layers In this case always save the VI with the most important frames of the nested structures selected so that t
57. allowing simultaneous write operations to occur without affecting the read data This leads to extra memory buffers and race conditions A race condition occurs when a write variable changes the data after a read variable has copied the previous value so that the read buffer data is different from the new variable data A functional global has only one memory buffer per shift register and cannot be written to and read from simultaneously Additionally functional globals have diagrams that may incorporate expanded functionality beyond simple write and read operations Figure 8 3 shows the front panel and block diagram for a very simple functional global used for storing a numeric value It supports only two operations write and read corresponding to the two cases of the Case structure shown Notice that the functional global also meets the criteria for an immediate subVI Hence error trapping is applied as discussed in the previous section Figure 8 3A The front panel of a functional global that stores a numeric value It contains an enumeration for selecting the desired operation a control for writing data an indicator for reading data and error clusters 244 Functional Global vi Front Panel File Edit View Project Operate Tools Window Help Ec VEL ANIMA aa oan aa aid A pa METTEN AETR A L T Figure 8 3B The diagram of the functional global has a While Loop with a shift register for storing a numeric v
58. an enum is used for the case selector The selector labels contain the test names and the free labels are not necessary Additionally the Case structure is maintained simply by editing the values in the enum control Hence the enum control is a central definition of the Case structure s cases and documentation 303 Figure 9 3A This Case structure has a numeric selector forcing the developer to translate between the numeric values and the test names 0 Default Test to Run Transmitter Test i EEFI 2 TX 3 ee TT Test 4 6 Figure 9 3B The enumerated Case structure displays the test names in the selector area using intuitive text Transmitter Power Up RS 232 Rectifier Voltage Battery Charger Test to Run ee ee ee ae WICC Zone Pins LEDs Receiver Oscillator Rule 6 17 Save enums as type definitions It is common to use multiple instances of the enum on a diagram as constants as well as controls and the text items are subject to change Therefore I recommend saving all enums as type definitions This way when edits are made to the items of the enum using the Control Editor window all instances of the enum update automatically maintaining identical items as the type definition Ring controls must be saved as strict type definition to provide similar behavior Enums ring controls and type defs are discuss
59. an error occurred If an error occurred this WI ea description of the error and optionally displays a dialog Figure 7 8B The diagram of the Simple Error Handler VI contains a call to the General Error Handler VI This is a trivial subVI that violates Rule 4 9 View full size image 211 a EPID PEE Error Handler vi Block Diagram Ee e Edt view a Bees PE E Help Error code no error 0 L ne of dialog OK msa 1 3 a fourec ou Rule 7 8 Implement an error log file for application deployment After an application is deployed using the dialog error reporting method it is often left up to the application s users to relay information regarding errors to the developer Without explicit instructions users may acknowledge an error dialog without recording the error or the information they record might be incomplete or not thoroughly communicated to the developer Additionally error dialogs may confuse or annoy many users Consequently an error log file is used in addition to or in place of the dialog error reporting method The error log file method simply consists of a routine for programmatically logging error information to file There are many possible methods of implementing an error log file routine The primary considerations are readability and efficiency For maximum readability log a high level error description along with the code source date and time to a tex
60. and Idle Default Also each state is called in response to a Boolean control s value change Therefore this VI can be implemented more efficiently using an Event Handling Loop as shown in Figure 8 27D The Event structure is the most efficient method of processing user interface activity Event cases configured to respond to each Boolean control s Value Change event contain the code from the corresponding cases of the Classic State Machine The Idle Default state that polls the Boolean controls within the Classic State Machine is eliminated by the Event Handling Loop s Event structure The Initialize state containing an initialization routine is replaced by the Event Handling Loop s Timeout event case Per Rule 8 8 continuous Timeout events are to be avoided In Figure 8 27D a shift register is wired to the Event structure s Timeout terminal The shift register is initialized to 0 forcing the Timeout event to fire immediately After the Timeout event case runs the initialization routine the shift register is set to 1 disabling the Timeout event Hence the Timeout event case functions similar to the Initialize application state Figure 8 27D The VI is implemented using the Event Handling Loop design pattern The states are replaced with event cases corresponding to the Value Change event for each Boolean control The Timeout event is used to perform an initialization routine View full size image 289 5 i
61. and Read Datalog are available from the File I O Datalog palette The datalog file method saves the data in a highly efficient binary format along with limited header information describing the data This method is very simple to program However the data is not readable outside of LabVIEW The datalog reading routine must consist of the Open Create Replace Datalog function with the cluster wired to the record type input terminal and the Read Datalog function Use the datalog error log file if there is a possibility of many errors and efficiency is required or if you want the file contents encrypted in binary format However text files are much more universal and are recommended otherwise Figure 7 9B Efficiency is maximized while readability is sacrificed by logging the error data and time stamp to a datalog file The resulting file is not readable without a LabVIEW datalog file reading routine View full size image uy File reset error T error log file path Es m typa of dialog OK msg 1 aie error in Cre error Bo kem 2 i m pe e error aut An important consideration with error log files is parsing the errors into files In some circumstances such as error reporting within continuous loops that do not terminate on error appending every error to one file could result in a large or runaway file Large error log files are difficult to load using a spreadsheet or word processor application and
62. and evaluate each one individually ease of use efficiency readability maintainability robustness simplicity and performance Several of these terms are general subjective 15 ambiguous and overlapping Therefore each section begins with a definition of how the term is used here and then discusses how that term relates to development style Ease of use is the ease with which the end user operates the software and accomplishes her objectives This involves interacting with the application s graphical user interface GUI Ease of use ranges in importance from less important for a one time experiment to be run only by the application s developer who is intimately familiar with the software to very important for a production application to be used by a variety of semiskilled operators within a single organization to extremely important for a mission or safety critical application or a commercial application intended for distribution and resale If you are designing a commercial application read a GUI style reference for your application s target operating system If your application is mission or safety critical read an appropriate text on human factors The LabVIEW Style Book contains general style rules that can be applied to most LabVIEW applications from the lab to production on most operating systems from desktop PCs running Windows to embedded processors running a real time or hand held operating sys
63. and indicators on the right a path control and indicator in the vertical center Boolean command buttons along the bottom and Help and Browse Boolean command buttons mixed in with the clusters The application predates modern style 3D controls and therefore utilizes classic controls In Figure 10 3B the revised front panel utilizes a tab control to group together the controls used most often on the Main tab the controls that configure a measurement sequence on the Configuration tab and the indicators that summarize the test results on the Statistics tab Because it is an industrial application the classic style controls are replaced with modern style 3D controls The controls on the Main tab are enlarged for improved visibility and ease of use In addition to grouping related controls the tab control hides the controls until they are needed thus limiting the quantity of information displayed at one time On the Configuration tab the control labels consistently use bold 13 point application font with units in parentheses in normal font Also a default value has been added to the path control 329 Figure 10 3B The front panel is revised to incorporate additional improvements from the rules summary in Appendix B View full size image E b hoe ee ee E The block diagram for Torque Hysteresis Improved with VI Analyzer VI is copied in Figure 10 4A A self review utilizing the manual checklist reveals the following additional rul
64. applications and are highly subject to change Save all clusters as type definitions or strict type definitions and apply the type definition for each instance of the cluster This ensures that any changes made to the type definition are automatically applied to every instance within every VI that uses it This guarantees consistent cluster configurations substantially streamlining application maintenance Use the Control Editor window to create or edit the type definition and use the Type Def Status ring to specify the control type Per Rule 6 6 choose the Strict Type Def if the control has custom properties in addition to the element data types and choose the Type Def otherwise Personally I prefer strict type definitions over type definitions because I might want to edit the properties of the type definition after I have created multiple instances 185 Therefore I can customize the appearance of the cluster or any element and the properties are applied to all instances Clone an instance of the type definition by choosing Select a Control from the Controls palette or drag and drop a type definition from the Project Explorer window Also if you need to make a constant that is compatible with the cluster select Create Constant from the shortcut menu of one of the terminals of the type definition Alternatively select a type definition control file from the Project Explorer window or by using Select a VI from the functions palette and drag
65. are formed exclusively to resolve the unknown requirements before the software development commences However government regulated industries are generally not rapidly evolving In summary the LabVIEW project journal contains an informal history of the project requirements and evolution and the requirements specification document establishes the active project scope These elements are prerequisites for good programming style LabVIEW projects represent a specialized subset of the vast software industry for which the IEEE standards were created Specifically LabVIEW is primarily applied to measurement and automation applications andarelatively fast development cycle is expected Most applications perform acquisition analysis and presentation These factors can be used to form a specialized standard for writing requirements specifications for LabVIEW projects Specifically I recommend the following elements e Statement of high level objectives e Budget e Timetable e Detailed requirements for acquisition analysis and presentation e Priorities for each requirement e Test methodology Figure 2 1 shows a LabVIEW Project Requirements Specification template Figure 2 1 A template for a LabVIEW Project Requirements Specification View full size image 33 Sect hre irh Se ee ees erte bere LabVIEW Project Specification System Name Company Revision 1 0 Date Acqorirtran erei a Re et ee ee h E ee
66. because they rely on an external driver or a resource that may or may not be in a state that will respond 222 appropriately to an application s requests The external resources perform their I 0 operations independent of LabVIEW and there are many scenarios in which the operation could fail Also failed I 0 operations are more likely to cause the calling application to misbehave such as hang up waiting for a device to respond Most I O functions and VIs in LabVIEW return the error cluster Trapping and handling the returned error information is an absolute necessity Medium risk nodes have error terminals but are not calling resources external to the LabVIEW environment These include Property Nodes associated with front panel controls all functions in the Synchronization palette VI Server when controlling the local instance of LabVIEW most Express VIs with the exception of the VIs on the Express Input and Output subpalettes Scan from String and Format into String and Mathematics VIs Most of the errors generated by medium risk nodes are well behaved and will not cause long latencies or crashes Error handling is the best method of identifying problems with how these functions are being applied and is strongly recommended For example wiring the error returned from a Property Node helps determine that a programmed property value is invalid which may otherwise go unnoticed Figure 7 13 provides source code samples for functions in the
67. being tested The VI controls a signal generator while monitoring the receiver s audio output level The desktop version in Figure 3 3A is comprised of a panel with Dialog Window Appearance and controls from the System palette All the controls indicators and text have default size and appearance consistent with the GUI standards for the operating system This forces the user to be physically close to the monitor and to examine all the controls The desktop dialog VI is well suited for an R amp D lab where the user closely monitors the application and can use any combination of control values with equal priority 61 Figure 3 3A This custom dialog VI is designed for desktop use such as a typical R amp D environment View full size image gt Configure Test Paranreters aM PM ono Modulation Carrier Frequency Hr Power Lewel dhu Modulation Frequency Hz Peak Deviation Hz Pesce verily standard test parameters and adjust voluma bo 2 Yeme Loft Spakar Wri zma Nore me a0 Right Spaakar wrrr The industrial version in Figure 3 3B contains larger text 3D controls and a single OK button The Left and Right Audio Output indicators are very large and vertically centered drawing the user s attention These features make it easy for the industrial user to operate the panel quickly The user can skim the GUI identify view and operate the most important data and controls and complete the
68. common functional elements or routines or are constructed in a similar manner For example most applications that I develop contain routines for data acquisition instrument control analysis user interface event handling state transitioning and error handling I have my own preferred design patterns and utilities that I reuse extensively Instead of developing each new application from the design pattern templates and reusable utilities I have an application framework that combines the common routines into a template and further advances the starting point for complex application development An application framework is an elaborate template filled in with multiple constructs including design patterns subVIs data structures and messaging as well as common routines It serves as a blueprint for application development A crude example of an application framework is copying the entire application source code from a previous project and performing aggressive edits to satisfy the custom requirements of a new application I refer to this approach as hacking and I strongly discourage it because the time saved by starting with a code base is offset by time consumed repeatedly searching examining and editing the source code Moreover no matter how closely you examine the code it will likely contain some underpinnings of the previous application that do not apply to the new application These remnants tend to rear their ugly heads at the worst
69. completion data is supplied to its output terminals and propagates to the next node in the dataflow path data memory One of the four VI memory components data memory contains all the data that flows through the diagram as well as the diagram constants default values for front panel controls and the data that is copied when written to variables and front panel indicators Data memory usage is provided by the Memory Usage category of the VI Properties window selected by choosing File VI Properties Additionally the Profile Memory and Performance window selected from Tools Profile Performance and Memory gathers comprehensive memory statistics for the VI and its entire hierarchy data structure Any LabVIEW data type or data construct utilized by an application for storing data Data structures are defined by the developer s choice of controls arrays and clusters on the VI front panels as well as the operations performed on the diagrams data type The fundamental data elements depicted by unique terminal or wire styles Data types define the memory size and functionality of the data datalog file Binary file format that stores data as a sequence of identically structured records along with a data type descriptor Use the functions on the Datalog palette to read and write to a datalog file The record structure is defined by the data type of the cluster wired to the record type input terminal of the Open Create R
70. definitions of terms as applied within The LabVIEW Style Book The terms are generally consistent with the LabVIEW documentation and appropriate software and virtual instrumentation industry standards However some of the terms are evolutionary overlapping or ambiguous In some instances I have taken the liberty to choose forms of the definitions that best support the relevant material in the book The definitions remain consistent throughout the book A application framework An elaborate template filled in with multiple constructs including design patterns subVIs data structures and messaging application performance The effectiveness with which an application or VI completes its intended mission artificial data dependency A wiring assignment between two objects in which the receiving object does not actually utilize the data contained within the wire Instead the wire serves to specify the execution order of the objects via dataflow principles asynchronous loops Two or more parallel looping structures that execute independently without any timing or data sharing relationship that would cause one loop to wait for another B block diagram memory One of the four VI memory components block diagram memory stores the graphical objects and images that comprise the block diagram Block diagram memory usage is provided by the Memory Usage category of the VI Properties window selected by choosing File VI Properties
71. desired terminal locations In Figure 5 24 Clear Error All or Specified VI uses a custom border and the 5x3x3x5 connector pattern rotated 90 degrees This pattern provides nine terminals within the icon s border five of which are assigned The connector pattern is rotated by selecting Rotate 90 Degrees from the connector s shortcut menu This VI s function is discussed in Chapter p Figure 5 24 Clear Error All or Specified VI has a custom border providing a unique shape and a 5x3x3x5 connector pattern rotated 90 degrees View full size image Clear Error All or Specified vi F icon Editor enor In no enor Show Error Diskoa F Ho Code to Clear 0 ear Mode 0 Clear All mec Show Terminals Connector Assignments Pa The VI in Figure 5 25 also has a unique shape formed by a custom border Its purpose is to incorporate an in line delay using data flow instead of a Sequence structure Recall that good block diagram style involves using data flow over Sequence structures and variables Wait n mSec VI promotes good style Its small size allows it to be seamlessly inserted between the VIs of a dataflow sequence while allowing other wires to be connected among the input and output terminals without bends The connector pattern is 5x3x3x5 with three terminals assigned 158 Figure 5 25 Wait n mSec VI has a custom border and promotes good data flow style It has a 5x3x3x5 connector pattern with three termin
72. diagram it will appear cluttered Figure 5 17A Instrument driver VI for a fictitious medical instrument has a text heavy icon and too many terminals View full size image Philips PM 1000 Initialize vi Serial Baud Rate 57600 Serial Data Bits 8 Serial Pork COMI VISA session Serial Flow Control None frati Returned String Serial Party Mone oe Error Cat Serial Stop Bits 1 error in no emor ID Query T Don t Check Reset F Dont Reset Ba Closer inspection of the icon s text reveals that all 14 characters on the first two lines are dedicated to identifying the instrument s manufacturer and model This can be improved There exists a long established convention originating from the VXIplug amp play consortium for which a very succinct instrument prefix is used to represent the instrument s manufacturer and model number The first two or three characters of the prefix identify the manufacturer and the following four or five characters identify the instrument model or family The resulting six to eight character acronym is used in the VI or library name and appears as a banner across the top of the icons throughout the driver Additionally five of the connector terminals correspond to serial port settings in addition to a terminal for the Serial Port Because these five controls are closely related we can modularize them into a cluster and reduce the required number of terminals and als
73. distinctions between arrays and clusters beyond the allowable data types The elements of an array share the same properties whereas each element of a cluster has its own unique properties Specifically an array control has only one owned label control size color font description unit caption tip strip and other properties that all elements must share Consequently the elements of an array all share the same identity and are distinguishable only by their index As a result of this distinction the elements of an array have a stronger association than the elements of acluster Arrays are used to store multivalued or vector quantities whereas clusters are used for grouping multiple related but autonomous data items Array examples include the data samples of a waveform and the coefficients of a polynomial These items may be thought of as single entities containing multiple elements The index number satisfactorily identifies each element within the array as the sequential waveform sample number and polynomial coefficient number Cluster examples include the error in and error out clusters These clusters contain a Boolean for the status a numeric for the code and a string for the source Because the elements are related they are combined into a cluster which logically associates the elements Because the element data types are dissimilar an array is not an option However if we imagine for a moment that all elements are s
74. enters the loop 123 Data flow Data flows vertically as well as horizontally including through two tunnels on the bottom border of the While Loop Also there is one coercion dot as sound data is converted from 116 to U8 The diagram of Excessively Nested VI shown in Figure 4 15 is oversized and severely nested The top illustration is the Navigation window which unfortunately is the only way to view the whole diagram The bottom illustration is a highly nested section of the diagram Less than half of the overall diagram is visible on the screen with 1280x1024 resolution Navigation entails much scrolling and even more flipping through the frames and cases of the nested structures The quadrant shown contains 11 layers of nesting It is not possible for even the most advanced developers and blackjack players to comprehend all the logical branches represented Avoid large diagrams and avoid excessive nesting Chapter 8 presents standard diagram architectures that help prevent large and unwieldy diagrams like this one Figure 4 15 The Navigation window shown at the top is required to navigate the oversized diagram The subsection at the bottom magnifies 9 of the diagrams 11 layers of nesting View full size image Layout The diagram is oversized overly nested and unwieldy Modularity There are only 40 user VIs out of 2 040 nodes for a modularity index of just 1 9 Hence it is not modular 124 Wiring Scheme Th
75. f amp Application Virtual spectrum analyzer Conditions Multiple display types and resolutions medium object density Color scheme Blue white default gray Layout Graph is prominently sized and centered on the panel Navigation is restricted via the Disabled property Heading text 18 point normal black Application font on a light gray background Boolean text 13 point normal black Application font on default gray background Numeric amp String Control Data 13 point normal black Application font on white background Secondary Fonts 12 point normal black Application font on white or gray background Navigation Tab control and Boolean command buttons The Parafoil Guidance Interface shown in Figure 3 19 is a flight control interface that allows a pilot on the ground to interactively fly and collect parafoil performance data The GUI resembles a virtual cockpit that contains four groups of indicators Vertical and Horizontal Situation Indicators VSI HSI Status and Control Each group of indicators is delineated spatially and using decorations Simple numeric and string indicators which contain minimally thin borders 92 are used to maximize the density of indicators without sacrificing visibility Arial font is used throughout the application to maintain consistent alignment and appearance regardless of the computer operating system and user preferences Normal black text is used on a white or very l
76. flexibility and rapid development capability are advantageous in such dynamic industrial settings Alternatively if the original requirements are satisfied ahead of schedule then the project scope may expand to include greater levels of automation This correlates to greater benefits However if the specifications are not documented then it becomes very difficult to verify what the requirements are at any given point in time throughout the project life cycle whether you are succeeding whether the application scope has expanded and most importantly when the project is complete Theorem 2 1 Written specifications positively influence LabVIEW style The presence or absence of a written specification has a direct effect on LabVIEW programming style as well as the overall outcome of the application A specification is a detailed description of the project s requirements Writing a specification entails significantly more planning and attention to detail than verbal communications It is a much different thought process than source code development It forces the writers and readers to envision the entire scope of the application before the software development begins The specification may be reviewed and refined by the contributors as much as necessary to build consensus Once the coding begins the risk for misunderstandings oversights and unforeseen scope change is dramatically reduced Reviewing or contribut
77. from the instrument driver template The icon for DoProberCmd VI contains a sideways triangle a standard glyph for Run Start and Initiate This convention weakens the association with the wafer prober View full size image SussPA lvlib DoProberCmd vi Registration In Command error in ro error Figure 5 18C merges the standard glyphs from the instrument driver template with the custom icon convention for the prober into a new icon convention that provides the best of both worlds The wafer prober glyph provides direct association with the instrument type The template glyphs reinforce the consistency with LabVIEW Plug and Play standards and reduce the dependence on text The resulting driver is intuitive and fully conforms to LabVIEW Plug and Play standards Figure 5 18C The standard glyphs from the instrument driver template are merged with the instrument specific glyphs to maintain strong association to both the wafer prober and instrument driver icon conventions View full size image SussPA lvlib DoProberCmd vi Registration In Command error in no ero lt Unfortunately it is not always possible to merge multiple glyphs and a banner on a single icon that is only 32x32 pixels In fact it can be very tedious to attempt to do so If your icons contain a banner with the instrument prefix a color convention and a reasonable balance between glyphs and text and if all of the required VIs and palettes are
78. function of each button than the phrases do Also the cluster label has been moved from the panel to the window title and the menu bar has been removed further minimizing the overall quantity of text As a result Figure 3 10B appears more intuitive than Figure 3 10A Figure 3 10A This menu of Booleans contains excessive embedded text 74 e Boolean Menu vi E x File Edit View Project Operate Tools Window Help COLLECT MENU ACCOUNT DATA Enter or Edit lt Fl gt SETUP HARDWARE Edit and Configure lt Fa gt INSTRUMENT DATA Select and Monitor Feo CYCLE DATA Collect and View lt F4 gt Figure 3 10B This menu of Booleans contains succinct embedded text hides the menu bar and displays the menu name in the window s title gt Collect Menu 75 What happens when the user needs more information about the control such as a description of the action it invokes or the assigned shortcut key This type of information is more appropriately stored in the control s description and tip strip fields The description is displayed in the Context Help window when the user drags the mouse over the corresponding control The description also enables you to enter a full paragraph or more of text that describes the control s function This gives us the flexibility to provide much greater detail than the embedded text Unlike a free label or embedded text the Context Help window can be closed when it is not needed
79. given application can be solved by configuring a few Express 36 VIs or examples the design phase is not necessary Also if you can quickly implement an application using a combination of your favorite design pattern templates drivers and utilities all of which you are intimately familiar with and the application is strictly for your own use do not bother creating a design document At what point does the application s complexity become worthy of a separate design phase The requirements specification should provide some insight assuming that you wrote one Finally sometimes a conventional design does not translate well to a LabVIEW diagram or might even adversely affect your application s style For example the literal translation of a flow chart to a LabVIEW diagram might entail networks of Sequence and Case structures that do not constitute good programming style In fact the result might appear like a cross between Nested VI and Spaghetti VI from the examples in Chapter 1 while occupying the space of Meticulous VI This would be the worst of all worlds Instead it is best to understand the precepts of good LabVIEW style and to apply modeling and design techniques in a manner to complement your diagrams For example the State Machine Diagram specifies a design that gracefully translates into the most popular LabVIEW design patterns Search for resources that will expedite design and development while positively influe
80. hrt Soe eg it i Peo 945 sanded Bly ts The front panel and diagram of DAQ Loop VI are shown in Figure 8 24B The front panel contains references for the DAQ Queue Error Queue and a type defined cluster of control references The DAQ Queue buffers the enumeration that drives the subVI s case selector The Error Queue reports any errors to the Error Loop VI The control references allow the diagram to update indicators on the panel of the GUI VI including the DAQ Loop State enumeration and the Sample cluster The diagram consists of a Queued State Machine On the left of the Case structure the diagram calls a subVI that removes the next state to run from the queue and updates the DAQ Loop State indicator using a Property Node configured with the Value property just to the left of the Case structure The corresponding indicator resides on a diagnostic tab of the GUI VIP s tab control It allows a user with appropriate privileges to monitor the active state of the DAQ LoopVI In the DAQ Read Data state the subVI reads a sample of data updates the Sample cluster indicator and shared variable and adds another DAQ Read Data state to the Queue Loop timing is accomplished using the Wait Until Next mS Multiple function based on the programmed loop rate Figure 8 24B DAQ Loop VI is a loop subVI containing the data acquisition task The front panel contains queue references and a type defined cluster of control references The diagram u
81. i E z gt Dij gt pe Ci I Liar EY ia 3 Dramaj m e Ej Rule 8 15 Avoid timeout with the Enqueue and Dequeue Element Event driven performance is achieved using the Queued State Machine design pattern by avoiding the timeout terminal of the Enqueue and Dequeue Element functions Specifically when the queue is empty and no states are active the loop goes to sleep allowing background tasks to run As soon as an element is added to the queue the Dequeue Element function immediately wakes up removes the item and runs the corresponding case This is maximally efficient and represents a substantial advantage over the Classic State Machine Continuous Loop and other design patterns However whenever a timeout is specified this advantage is eliminated Therefore avoid the timeout terminal as shown throughout the examples in this chapter The Queued State Machine design pattern is appropriate for continuous routines and subVIs with medium or greater complexity Similar to the Classic State Machine the Queued State Machine is not appropriate for top level and GUI VIs unless it is combined with an Event structure or a separate Event Handling Loop These design patterns are discussed in Section 8 2 3 Event Driven State Machine and Section 8 3 1 Parallel Loops respectively As discussed in Section 8 1 4 Event Handling Loop the Event structure is the most efficient method of capturing GUI events beca
82. image file from Microsoft 142 Explorer into the icon on the front panel window Using the latter method LabVIEW automatically resizes the image as necessary to fit the 32x32 pixel area Finally note that the NI Developer Zone provides an online icon art glossary The corresponding glyphs are already sized appropriately for a LabVIEW icon LabVIEW is used internationally with localized versions available for multiple languages Because icons are images they appear the same regardless of the native language of the LabVIEW development system they are loaded into This is an important consideration if you happen to be developing VIs intended for use by developers on an international scale This may include the software reuse library for a multinational organization or a commercial toolkit intended for international distribution In these instances observe the following rule Rule 5 17 Avoid text and graphics not universally understood on international icons Although text alone makes for fast and easy icons text is language specific A glyph combined with a line of text is preferred as long as the text and glyph are chosen to be as universal as possible For example all LabVIEW Plug and Play compatible instrument drivers contain an acronym representing the instrument manufacturer and model This is known as the instrument prefix a universally recognized convention with roots from the VXIplug amp play Consortium Many LabVIEW develope
83. ina fee gt i gt jr esd Be If your diagrams commonly extend beyond one window maximized on a monitor with 1280x1024 resolution your source code is not sufficiently modular If the Navigation Window is integral to your development you definitely need more subVIs Rule 4 5 Create a multilayer hierarchy of subVIs e Strive for a modularity index greater than 3 0 Develop your applications as a multilayer hierarchy of subVIs using a combination of top down and bottom up design and development techniques The VI Hierarchy is viewed by selecting View VI Hierarchy Deselect Include VI Lib Include Globals and Include typedef from the toolbar to remove these items from the window and view only the hierarchy of user VIs that you provided Common geometries include pyramid diamond and oval Except for very simple applications the VI Hierarchy should contain multiple rows of subVIs below the top level VI In Chapter 1 the modularity index was defined as the ratio of the number of user VIs to total nodes multiplied by 100 These quantities are quickly referenced using the VI Metrics window selected from Tools Profile VI Metrics A modularity index of 3 0 or greater is recommended for a typical application Rule 4 6 Modularize top level diagrams with subVIs e Develop high level component VIs e Replace collections of Property Nodes with Control References and subVIs Depending on the design pattern most top level di
84. indicators that were blindingly bright for no good reason My code was functional but it was not at all useable and certainly not maintainable I tried to add some new functionality a few years ago after I was much more familiar with LabVIEW programming but I quickly gave up because I had no idea how my own code worked Many years later I can comfortably say that my programming style has improved leaps and bounds over those initial months with LabVIEW However I can also say without a doubt that my growing pains regarding LabVIEW programming style would have been all but eliminated had The LabVIEW Style Book been around back then This book is a fully comprehensive resource covering every aspect of VI style from the highest level project planning and organization to the smallest detail wires with too many bends If I had read this book while I was learning LabVIEW my code would have been many times more useable and maintainable from the start The LabVIEW Style Book is a must read for every LabVIEW developer Not only does it contain essential style rules for any new LabVIEW developer it is also a necessary refresher for seasoned LabVIEW veterans In particular there are some chapters like Chapter 6 Data Structures Chapter 7 Error Handling Chapter 8 Design Patterns that contain crucial programming tips and techniques not available in any other single resource As one of the VI style advocates in LabVIEW R amp
85. is further improved by applying the Multiple Loop Application Framework The separate Event Handling Loop labeled Event Handling and the Main State Machine labeled Main Loop improve the response to GUI events and performance of state transitioning versus the Event Driven State Machine design pattern Data acquisition is performed in a dedicated loop labeled DAQ Loop for continuous monitoring of torque and angle data similar to the parallel loops presented in Figure 8 18 Also the loop labeled Error Handling Loop processes errors from any of the loops This allows the application to report errors gracefully recover and continue running Note that the Main State Machine DAQ Loop and Error Handling Loops are all Queued State Machines Delays are used to prevent tight loops and messaging is accomplished using queues Figure 8 23 The top level diagram of the Torque Hysteresis application contains separate loops for Event Handling State Machine Data Acquisition and Error Handling View full size image Pig ae Pian ta l a Fire eu 3 281 The DAQ Loop is designed to add a new state to the queue after each read operation As shown the DAQ Read Data case adds a new DAQ Read Data element to the back of the queue each time this case runs This causes this state to run continuously until the application ends or an error occurs Consequently a delay must be applied to this loop to prevent a ti
86. is replaced with an occurrence the Curve local variable is eliminated and the Sensor Scaling local variables are replaced by a wire The occurrence allows the DAQ Loop to sleep not utilizing the processor until the Set Occurrence function executes inside the Event Loop or a timeout occurs The timeout input terminal of the Event Loop s Event Structure and the DAQ Loop s Wait on Occurrence function is set to 200ms allowing 115 each loop to periodically poll the Stop global variable and update the stop condition Polling the Stop global every 200ms is more efficient than polling the Acquire global at full speed but it is still a violation of Rule 4 33 Figure 4 10B An occurrence is used in place of the Acquire global variables a wire replaces the Sensor Scaling local variables and the Curve local variable is eliminated View full size image MaBe Deke hede Rae Geka eed eee ee A eed eee ee eed eA eee ee ee eee FIr alsin Shutdown i Tita bette eet tie eet ee Pe ee 2 ee et ee ee et ee ee EL ee Applying a notifier instead of an occurrence further streamlines data flow and efficiency as observed in Figure 4 11 A notifier is a synchronization construct that is similar to an occurrence However a notifier is programmatically released causing the Wait on Notification to return from an indefinite wait By comparison the Wait on Occurrence function cannot terminate programmatically Also the notifier sends data from the sen
87. it is difficult to work within this constraint such as a complex diagram containing multiple parallel loops In this case organize the large diagram so that it may be viewed by scrolling in only one direction or modularize the loops into subVIs to reduce space Loop subVIs are discussed in Chapter 8 Design Patterns Avoid large diagrams that require both horizontal and vertical scrolling because this is cumbersome to navigate Figure 4 1 contains two functionally equivalent implementations of a VI that evaluates a calibration interval The diagram of Figure 4 1A is overly dense and sloppy As you can see several of the functions and wires overlap and the diagram appears confusing In Figure 4 1B the same VI is revised for improved readability The functions are neatly spaced the wiring is clear a few comments and enumerations are included and the error cluster propagates throughout the diagram The implementation of Figure 4 1B is much more readable than Figure 4 1A We revisit this example very shortly Figure 4 l1A The diagram for a VI that evaluates a calibration interval is overly dense and confusing View full size image 96 Figure 4 1B A different implementation of the Calibration Interval VI contains appropriate spacing clear wiring and documentation View full size image fae bor ee re ee ee ee pe Naai Alle es ded ke en Eae Sni GA Ee ae gt _ a Peel fo a a Bara Parr rit eq Cal
88. latter 149 Figure 5 14A contains the icon terminals and connector assignments for SubVI from Selection VI previously discussed in Chapters 3 and 4 A subVI from selection is created automatically by selecting an area of a diagram and choosing Edit Create SubVI They are characterized by LabVIEW s default icon unconventional front panel layout counterintuitive control and indicator labels and sloppy wiring on the diagram As discussed in the previous chapters these VIs never conform to good programming style without significant cleanup Specifically the terminal labels in Figure 5 14A are inappropriate including task ID out task ID out 2 error out 2 and output chunk Input terminals should not contain the word out and duplicate terminal labels should not be distinguished using numbers Additionally it contains the default icon assigned by LabVIEW instead of a unique and meaningful icon Finally it utilizes the 3x2 connector pattern with no spare terminals These are violations of Rules 3 21 5 2 5 3 5 19 and 5 21 Figure 5 14A SubVI from Selection VI violates multiple style rules including default icon and counterintuitive terminal labels View full size image SubYI from Selection vi Connector Assignments task ID out task ID out 2 output chunk error out error out 2 SubVI from Selection w Cleanup VI shown in Figure 5 14B is an improved revision of the VI The duplicate terminal labels are distinguishe
89. left and right borders Each structure should have only one error input and output tunnel pair and they should normally be the bottom most tunnels All the examples in this chapter support this convention However do not wire the error cluster through a multiframe structure if it is not used within the structure Merge Errors VI was introduced in Section 7 1 1 Trapping Errors as a method for combining multiple error clusters into one As shown in the Context Help window of Figure 7 15A it has three error in terminals one error in array terminal and one error out terminal The VI searches the error inputs in top to bottom terminal order and returns the first error it finds If no errors are found it returns either the first warning or no error if there are no warnings This technique is used for combining multiple parallel error chains such as in Figure 7 3 or converting an array of loop indexed errors into a scalar as shown in Figure 7 5A The merged error output cluster facilitates error propagation using a single error out terminal or reporting using a single reporting VI Figure 7 15A Five parallel error chains are combined using Merge Errors VI Because of the nonstandard error in array terminal at the bottom left the subVI is not aligned with any of the other subVIs Also a Build Array function is required to merge five error chains View full size image 226 Merge Errors vi error in 1 no error error i
90. libraries First use the LLB Manager Tools LLB Manager to convert the LLB to a folder of VIs Then go into Project Explorer and add the folder of VIs to a project This creates a project folder that contains the library VIs Next choose Convert to Library from the project folder s shortcut menu Finally name and save the project library This section presents file naming conventions for the LabVIEW source files Rule 2 12 Create unique and intuitive source filenames It 1s important to use intuitive filenames that describe their primary function As a rule the source file and project library names should combine to uniquely identify each file Consider the hypothetical situation that you might need to take an unexpected leave of absence during the middle of an important project Your colleagues should be able to locate and identify your source files understand your programming style and resume project development in your absence This is a true test of good style Rule 2 13 Do not abbreviate filenames 52 Avoid conventions that rely on very short abbreviations acronyms and numbers For example the source filename IPS Osc H to Q vi might be meaningful to the original developer but not to many others Use as many characters and distinct words as necessary to uniquely identify the source file Avoid using characters that not all file systems accept such as slash ean backslash colon and tilde C LLB Ma
91. may contain many redundant errors Similarly creating a new file for each new error prevents large files but may generate large quantities of files As an alternative consider an intelligent error logging routine that limits the size and number of errors saved to the error log file Specifically the error logging routine can read the contents of the error log file and either append to file replace the oldest entry or skip its logging operation The number of logged errors file size or existence of redundant error codes can be used as criteria for appending replacing or skipping It is important to notify the user or developer when an error occurs For best results combine an error dialog or front panel indicator with a log file The dialog or indicator notifies the user that an error occurred and the log file contains a complete record of the error details An error indicator may be a Boolean that indicates a new error was logged or a numeric that counts the number of errors that have occurred or simply an error cluster displaying the 213 most recent error data Alternatively LabVIEW can send an email that includes the error log file as an attachment using the VIs on the SMTP Email palette Rule 7 9 Suppress dialog error reporting for unattended or remote operation The dialog reporting method either with or without an error log file is not practical for applications designed to run unattended or remotely When a dialog open
92. messages vi 18 Error Handler vi 13 MaxForth 3 5 VISA wi 42 Upload File vi 52 Strip Command Echo wi 23 Check For error vi i4 GetLine VISA vl i Test Line vi zi SendLine WISA vi q Cask Data File 36 16 14 lz a fe i oO 0 0 0 T 0 0 0 0 0 0 0 0 0 oo O O geqceoceocecgec G ae oe 0 eo 0O 0rer u0 genre ooogaoaqgnoeqoecqcocoadca tn For comparison purposes we can define a modularity index as follows Equation 1 1 Modularity Index user VIs total of nodes x 100 I recommend a modularity index greater than 3 0 In our examples Meticulous VI has 111 user VIs and 4 947 nodes for a modularity index of 2 2 Nested VI has 16 user VIs and 471 nodes for a modularity index of 3 3 Spaghetti VI contains 56 user VIs and 1 944 nodes for a modularity index of 2 9 Chapter 4 discusses subVI modularization in detail Error handling consists of trapping errors by propagating the error cluster and reporting errors using a dialog or log file Error handling is critical for robust application performance Meticulous VI contains very thorough error handling as can be seen by the error clusters that are propagated among all nodes that have error terminals Any errors that occur within a loop are passed to a dedicated error handling loop via queue where they are evaluated reported and logged Figure 1 9 shows the Error Handling Loop Figure 1 9 Meticulous VI contains a dedicated error handling loop that
93. multimeter has the prefix ke2000 In addition multiple required files are explicitly specified in the VXIplug amp play Instrument Driver Functional Body specification You can view the complete specification at WWW vxipnp org However modern project style instrument drivers maintain the instrument prefix in the project library instead of in the source filenames Hence the instrument prefix is no longer needed within the individual filenames Rule 2 15 Identify the top level VIs The most important source files to identify in any LabVIEW project are the top level VIs Always distinguish these files by maintaining them near the root of the folder hierarchy on disk as well as within the project and apply an appropriate naming convention such as lt project name gt Main VI or lt project name gt Top Level VI Figures 2 7B and 2 8 illustrate this convention Because the top level VI is not contained within any of the project s libraries the project name is included within the filename This ensures that the filename is unique and intuitive Source control is a process by which source files are secured shared and maintained in a multideveloper environment Source control is facilitated by a third party source control application LabVIEW integrates with multiple source control applications by enabling us to perform the most common source control operations from within the LabVIEW project As of LabVIEW version 8 2 LabVIEW sup
94. new cluster along with an array of cluster that combines the Torque vs Angle and Statistics clusters The latter array stores data from multiple test runs reusing the same UUT Information and Motion Parameters for each run Additionally the larger cluster is contained within an array storing data from multiple UUTs that comprise a lot This data structure contains five layers of nesting as follows l Torque vs Angle and Statistics subclusters A subcluster containing the latter clusters that stores the data acquired from a single run The Multiple Run Data array consisting of an array of single run subclusters S a A cluster that combines the Multiple Run Data together with the UUT Information and Motion Parameters containing all data for a single UUT 5 The Multiple UUT Data array consisting of an array of the composite data for the multiple UUTs that comprise a lot Figure 6 11A An alternative data structure for the Torque Hysteresis VI has five layers of nesting View full size image 191 Figure 6 11B presents the enhanced top level diagram that takes advantage of the nested data structure There are now two additional looping structures The inner For Loop allows multiple tests to run on a single UUT and the corresponding statistics are computed after each run The middle While Loop allows multiple UUTs to be tested consecutively Shift registers and wires are used to propagate the nested data structure between each loop
95. of Figure 6 15A with the organization and neatness of Figure 6 15B Figure 6 15A This data structure is nested and the controls are haphazardly arranged View full size image 200 Figure 6 15B The nested data structure has been divided into four data structures containing fewer layers of nesting and front panel style rules have been applied to dramatically improve the appearance of each control View full size image ee ee i LF ecw 20 00 T i 7 SR ara wu Ot ij Aii i o J Soe Hi Goo Centrifuge DAQ System VI also discussed in Sections 3 5 4 and 4 4 6 is an application that performs dynamic measurements from soil samples in the payload of a centrifuge The front panel shown in Figure 6 16A is highly configurable allowing the user to configure a wide variety of signal conditioning modules The hardware supports up to six SCXI chassis containing 201 any combination of relay accelerometer LVDT strain gage voltage and thermocouple conditioning modules Any number of these modules can reside in any slots of any chassis Each module type has a different set of programmable parameters such as excitation voltage signal amplification gain filtering bridge completion scaling factor and engineering units These parameters are configured using a set of nested data structures When the user selects a sensor type using the Sensor Selection listbox control on the left one of seven data structures be
96. of a name data type data and attributes F ActiveX container Provides an ActiveX control or document on the front panel CF TI NET container Provides a NET control on the front panel CF Rule 6 2 Choose memory efficient data types Because many of the control types can be configured to represent or contain a variety of data types memory efficiency depends on the combination of the control type and data type Therefore it is useful to consider data type efficiency in addition to control type simplicity Specifically the data types define the size and simplicity of the data in memory For example we can organize the numeric data types in order of representation for which the smallest integer representation is the most memory efficient data type and the largest floating point representation is the least efficient Table 6 2 contains the complete list of LabVIEW data types presented in approximate order of memory efficiency As shown the smallest numeric representation is an 8 bit l byte integer and the largest is an 8 to 16 byte extended precision floating point numeric Table 6 2 LabVIEW Data Types and Descriptions Listed in Top Down Order of Memory Efficiency 7 Boolean Stores a TRUE or FALSE value as a byte integer 8 bit unsigned integer Represents only non negative whole numbers in the range of 0 to 255 8 bit signed integer Represents positive and negative whole numbers in the range of 128 to 127
97. of the data elements they contain along with the values of any simple data types in contiguous memory locations The data of any arrays strings paths and subclusters are not stored within the cluster Rather LabVIEW stores a handle containing the memory location where each multivalued data element is stored Consequently nested data structures may be stored as a network of handles and data that is distributed throughout the target computer s data memory space The more layers the more complicated this network can become Because LabVIEW manages memory automatically the complication of a nested data structure s memory network is transparent to the developer However the developer should be concerned with memory efficiency Specifically each branch of the nested data structure requires additional memory operations to allocate and manipulate the data First most calls to Bundle By Name Unbundle By Name and Index Array cause a copy of the data provided at the output of each function to be created in memory Additionally as the size of the data contained in any array layer grows the corresponding memory buffer may have to be reallocated requiring more memory operations The previous memory buffers fragment their memory blocks when new buffers are allocated in new locations Hence nested data structures use memory less efficiently than simple data structures Per Theorem 6 1 memory access time is the principal latency in m
98. offices Blume established and evolved the company s LabVIEW development practices Blume has written and presented multiple LabVIEW style related presentations including Bloomy Controls Professional LabVIEW Development Guidelines at NIWeek 2002 and Five Techniques for Better LabVIEW Code at NIWeek 2003 He also has published technical articles in various trade publications including Test amp Measurement World Evaluation Engineering Electronic Design and Desktop Engineering Blume holds a Bachelor of Science degree in electrical engineering from the University of Connecticut He is a National Instruments Certified LabVIEW Developer and Certified Professional Instructor The company has offices in Connecticut Massachusetts and New Jersey For more information visit www bloomy com Readers who want to contact Blume regarding style related suggestions questions or comments may do so at the following email address lvstyle bloomy com Readers interested in contracting Bloomy Controls for a LabVIEW development project should call us directly or contact us through our website at WWW bloomy com quote 11 1 The Significance of Style LabVIEW is a graphical programming language for developing diverse applications in a multitude of industries The block diagram provides a unique form of source code expression that is dissimilar to most programming languages and development environments The data flow paradigm repres
99. om seaman Cg i Ei Aiete Resolution Raga O Fi h Resolution Dagits TR Does Measurement Lm kara ETO ETE Signal Out of Range Cece Hare bi i i B J a n 7 amar ih Ag error LPs 3 Be Dml abcahute Aus as JES error out Measurement Type ubeolube Resolution Range F Rule 5 21 Use the 4x2x2x4 connector pattern for most VIs We can take the unified pattern rule one step further and standardize on the same pattern for most VIs we create regardless of their purpose and association If all of our VIs have similar connector patterns and terminal assignments to the maximum extent possible the benefit to our diagrams is maximized As of LabVIEW version 8 2 4x2x2x4 is the default pattern applied to new VIs I recommend this pattern because it promotes better modularity and neater diagrams than patterns with more terminals while providing ample room for expansion compared to patterns with fewer terminals If you need more than 12 connector terminals you are probably performing too many tasks within one subVI or you may need to modularize some of your wires with a cluster As an example Find Screw Ends VI from Figure 5 1 uses the 4x2x2x4 pattern It propagates an array of cluster named Line Endpoints and returns a cluster named Edge Point in addition to six other inputs and outputs These parameters occupy a total of 9 of the 12 terminals leaving 3 spares As seen in Figure 5 10 the DAQmx VIs use the 5x3x3x5 pattern a
100. on a default gray background Boolean text 13 point bold black Application font on default gray background Numeric and String Control Data 13 point normal black Application font on a white background Secondary Fonts 13 point normal white Application font on a maroon background Navigation A combination of menus tab control listbox control and Boolean command buttons are used for navigation The GUI VI panel shown in Figure 3 18 is a commercial application for diagnosing mechanical wear of industrial equipment The operators are primarily maintenance technicians A tab control groups relevant controls together The FFT graph 1s prominently displayed in the middle of the screen Note that the Boolean controls in the upper left corner are always visible and are programmatically Disabled and Grayed Out when not active The company logo appears in an open area on the right side of the graph The GUI conforms to most of our rules and appears very intuitive 91 Figure 3 18 Spectralyzer is a commercial application for diagnosing mechanical wear of industrial equipment The GUI conforms to most of the rules and appears very intuitive View full size image UE Spectralyzer v1 11 wat I I J J a j 1 E I I E I I I J I I Bub MOD SW a EOD Lik LE iak LA iik IEE LIE Pi Eik LE J k IEE pik pik Ji i Frequency Cursors on off Frequency I 7 cursor 2000 0 0 0 Ed EN amp HAS cursor D curser 2 E SP curoza 2000 0 loo
101. one or more independent perspectives or reference points Code reviews have three primary benefits identifying and fixing problems exchanging knowledge and ideas that improve developer skill level and promoting and evolving standards across an organization cohesion A measure of how well the block diagram nodes within a module or subVI work together to perform a specific task A subVI is considered cohesive if its purpose is clearly described using two or three sentences commercial subVI SubVI that is part of a product such as a developer toolkit or instrument driver component VI High level subVI or dynamically loaded plug in VI that encapsulates a major portion or subsystem of the application compound design pattern 339 Design pattern created using a combination of two or more single loop design patterns and adding a messaging scheme to pass data between loops construct Any LabVIEW item that is constructed by the developer such as data structures subVIs and design patterns See also data construct Context Help LabVIEW Help window that displays the descriptions of controls and VIs as the user scrolls the mouse over them The Context Help window opens via Help Show Context Help or the lt Ctrl gt lt H gt shortcut key combination Continuous Loop Design pattern that consists of a single While Loop or Timed Loop shift registers loop timing and error trapping The design pattern is equ
102. only by the developer With the ability to control which VIs are usable you can maintain a consistent public VI interface while changing the underlying private VIs without fear of breaking the user s code Note that legacy instrument drivers toolkits and other libraries developed prior to version 8 0 are generally distributed in a compressed file format known as the LLB LLBs are a different type of entity than a project library LLBs physically contain the source files that they reference whereas project libraries are XML files that contain references and property values for the files that comprise the library Libraries distributed as LLBs have several disadvantages compared to folders and project library files These include lack of organization limited compatibility with operating system utilities longer load and save time and risk of file corruption Specifically LLBs provide only two levels of organization top level and not top level Operating system utilities such as Windows Explorer cannot search for source files within LLBs Source control tools cannot check the individual files in and out within an LLB Additionally because the source files are compressed within the LLB they take longer to load and save Finally many people tend to pack too many source files into one LLB further reducing organization and risking loss of work from file corruption Fortunately LLBs can be converted to folders of VIs with project
103. organized according to the NI guidelines the driver should satisfy the icon requirements for a LabVIEW Plug and Play instrument driver It is more important for your VIs to be intuitive and strongly associated with the instrument than it is to share the same glyphs as the template The users of your instrument driver can also identify the VIs based on the VI names and locations on the palette 155 This section presents some random examples that also may be improved upon The icon and connector from Figure 5 19A belongs to a VI that converts a data structure into a formatted string The icon consists of a clever glyph of a telephone handset and text I commend the developer s creativity or her ability to find and copy graphics whichever the case may be The connector however can use a few improvements First the labels of the control and indicator assigned to the input and output terminals are too long In fact the full labels Communication Parameters Cluster and Communication Parameters String have been cut off by the Context Help window The words Cluster and String are not necessary because the data types are identified by the design and color of the wire stubs provided by the terminals on the caller s diagram and appearing in the Context Help window Also the terms Com and Params are commonly used abbreviations for Communications and Parameters respectively However controls and indicators should always have unique names otherw
104. panel 237 Endnotes l Merge Multiple Errors VI and Clear Error All or Specified VI can be downloaded from WWW bloomy com lvstyle 238 8 Design Patterns Design patterns are standard VI architectures that solve common software design problems They consist of an arrangement of structures functions controls and error handling that form a generic construct for common tasks such as looping event handling state transitioning and data sharing and encapsulation Design patterns are available as templates for promoting reuse productivity and quality Also one or more design patterns are combined with additional subVIs controls utilities and libraries to form an application framework An application framework is a more elaborate set of tools that advances the starting point for developing an application Both the fundamental constructs and the higher level frameworks are design patterns if they are common recognizable and reusable Design patterns promote good programming style If all of the VIs in an application resemble well known design patterns it becomes easy for developers to recognize understand maintain and reuse each other s code Also design patterns are ideal candidates for templates Instead of creating every VI from scratch start with a template and modify it to your needs This saves time and helps ensure that your VIs employ common architectures consistently Moreover ina multideveloper organization desig
105. panels The following are some rules regarding front panel layout organized into sections for general rules GUI VI panels and subVI panels The general rules presented in this section apply to the layout of all types of VI front panels including both GUI VIs and subVIs Rule 3 1 Group related controls using decorations spacing tabs and clusters Identify logically related controls and group them in the same area of the front panel Consider the example shown in Figure 3 l One LabVIEW application controls two identical test fixtures labeled Station One and Station Two The GUI contains all controls and indicators for operating both test fixtures on one panel Several distinct groupings exist First the panel is divided vertically into two halves with objects associated with Station One on the left half and Station Two on the right half In addition to the divisional grouping decorations visually partition the groupings and provide the appearance of two separate and distinct panels Within each decoration are two primary subgroupings of indicators temperature and power Thirty two digital indicators are tightly spaced within clusters near the center of each decoration displaying the temperature measurements and six digital indicators are bundled within clusters at the bottom of each decoration displaying the power measurements The borders and light background shading within each cluster further delineate the contents of each F
106. pattern The diagram appears neat and orderly due to the use of clusters Each cluster is saved as a type definition for ease of maintenance It is simple to add and remove parameters by adding and removing controls to the type definitions The sections that follow contain more guidelines for simple data types and simple and complicated data constructs as well as many more examples 6 2 Simple Data Types Simple data types are a subset of the fundamental LabVIEW data types that are stored in contiguous memory locations They include Boolean numeric string path picture and several special numeric types In this section we consider style rules pertaining to the simple data types Boolean is the simplest control type in terms of both operational and data simplicity and has two possible values TRUE or FALSE Boolean data is actually stored as a byte integer Hence the Boolean data type is really no more efficient in terms of memory size than the smallest integer representation However Boolean controls are extremely intuitive They have two clearly defined states and resemble objects commonly seen in everyday life They include push buttons toggle switches command buttons slide switches LEDs and radio buttons Rule 6 8 Use Booleans if two states are logical opposites 174 The most important rule with Booleans is to use them to represent parameters that have exactly two states that are logical opposites for example On or
107. propagated throughout the server application the communications functions will not execute and any new client connection attempts will be thwarted Instead the server should process and clear the error associated with a client disconnect 228 Figure 7 16 illustrates a utility subVI named Clear Error All or Specified VI Its purpose is to programmatically reset the error cluster with no error Depending on the value of Clear Mode it resets the error code always or just when the error code equals the programmed Code to Clear Also it can optionally display the error dialog before clearing the error Notice the small icon size This allows other wires on the calling VI s diagram to flow above the subVI without the icon obstructing their path For example any reference numbers running in parallel to the error cluster can pass without bending around the icon The icon and connector for Clear Error All or Specified VI are discussed in Chapter K Figure 7 16 Clear Error All or Specified VI is a utility that selectively clears errors according to the values of Clear Mode and Code to Clear View full size image D De ier ipet gena Tek biu ie BEE OESE i Figure 7 17 contains an illustration of Clear Error All or Specified VI applied to the Torque Hysteresis VI discussed in Chapter 6 Data Structures After each test Save Data VI prompts the operator for a filename using a file dialog and the test data is logged to file However
108. providing the functional equivalent of multiple panels with one panel visible at a time The total number of visible controls and indicators is reduced to 48 per station Each tab is divided into five logical groups including three decorations one cluster of temperature indicators and one cluster of power indicators The reduced object density increases white space and makes the existing objects easier to recognize and view Figure 3 5 A tab control organizes the objects by station reducing the total number of visible objects to 48 per station View full size image 64 Power Monitor System 5g The organization of tabs and indicators in Figure 3 5 limits the user to viewing only one station at a time Because both stations operate simultaneously the user is forced to frequently flip back and forth between stations An alternative is to include an additional tab that contains a high level overview of both stations using a limited number of controls and indicators For example the Overview tab in Figure 3 6 displays the maximum and average temperatures for each station the average power a bar plot of temperatures and the Power ON OFF Boolean controls The bar plot is a very useful indicator for visualizing multiple data channels As you can see it is easier to notice the relative temperatures of each channel as well as trends and outliers than the cluster of numeric indicators This illustrates the i
109. reflect these priorities as was described in Chapter 3 Front Panel Style Rule 5 29 Specify required priority for critical inputs and outputs Rule 5 30 Specify optional priority for inputs and outputs that are normally not used If a subVI performs file I O data acquisition or instrument control with a resource that was opened prior to the subVI call a valid reference task or resource name is required or the subVI cannot operate properly Specify required priority 148 for any critical terminals that the subVI cannot run without such as reference inputs Additionally specify optional priority for any inputs or outputs that are normally not used because the default values are generally acceptable This way the user can quickly identify the important terminals by the bold appearance of the labels in the Context Help window However do not set priorities unless they are really warranted Leave the default recommended priority for most terminals Figure 5 13 shows the Context Help window containing Find Screw Ends VI Original Image in and Grayscale Image in are references to images that are required for the VI to run properly The corresponding terminal labels are boldface indicating that the priority of these two terminals is required All other terminals are recommended All input terminals on the left side are propagated through the subVI s diagram to corresponding output terminals on the right side This provides co
110. s Instrument Driver Network WWW Ni com idnet there are tens of thousands of programmable instruments out there Most of us have had to develop a few drivers on our own as well as customize some of the downloaded drivers Many organizations maintain their instrument drivers in a repository The key to using them is to make them directly accessible on the LabVIEW Functions palette of the developer s computer This is accomplished by placing copies of the desired drivers into the LabVIEW instr lib folder on the developer s machine LabVIEW reads the contents of this folder each time it launches and populates the Instrument Drivers palette with subpalettes for each driver it finds This feature facilitates seamless integration of instrument drivers copied from anywhere with the LabVIEW environment Figure 2 3 shows a Functions palette with the Instrument Drivers subpalette expanded Figure 2 3 Functions palette with the Instrument Drivers subpalette expanded Each icon corresponds to an instrument driver contained within the LabVIEW instr lib folder 41 Functions Programming Express Measurement Lo Instrument 1 0 L Instrument Drivers Vision and Motion Mathematics Signal Processing Data Communication Connectivity Favorites User Libraries Select a VI Rule 2 5 Develop reusable SubVIs e Perform specific well defined tasks o Use controls instead of constants e Apply good style Likewise
111. s preferences Because the Dynamic Framework loads runs and unloads one component at a time the subpanel is mildly preferred over separate windows Consider how the Dynamic Framework is applied to the Torque Hysteresis application The various design pattern implementations examined in the previous sections are modular with each application state implemented as a subVI Some of the subVIs are dialogs such as UUT Information Prompt VI and Define Cycle VI Dialogs are not complex and do not consume much memory Other subVIs such as Run Test VI and Compute Statistics VI are high level VIs that contain a hierarchy of subVIs and large data structures These are the high level components of the application The application s performance is improved by dynamic loading of the high level components This is easily accomplished using the VI Call Configuration dialog within any of the design patterns described in the earlier sections Figure 8 20 shows a call configuration for Run Test VI Loading and retaining on first call is the preferred selection because the Run Test VI normally runs many times Figure 8 20 The performance of the Torque Hysteresis application is improved by dynamically loading the primary components including Run Test VI This is accomplished using the VI Call Configuration dialog window 275 F VI Call Configuration Cry Torque Hysteresis SubyIs Run Test vi Loading Options Load with callers E Reload For ea
112. section covers data flow the fundamental principle of LabVIEW It contains a brief review of basic principles and style rules considerations regarding variables and Sequence structures and techniques for optimizing data flow The rules are illustrated with a combination of simple diagram snippets and a working application example In LabVIEW data flows along wires from source terminals to destination terminals A block diagram node executes when data is received at all wired input terminals Upon completion data is supplied to its output terminals and propagates to the next node in the dataflow path The dataflow principle distinguishes LabVIEW from traditional text based software development environments The following are some basic rules regarding data flow Rule 4 22 Always flow data from left to right Rule 4 23 Propagate the error cluster With only one exception always flow data through wires running from left to right This is a sacred age old convention within the LabVIEW community However Feedback Nodes and Sequence Locals are built in LabVIEW constructs that inherently violate this rule In my opinion the Feedback Node is a credible exception when the length of the right to left wire segment is very short resulting in a reduction of wire clutter compared to a shift register Sequence Locals are relevant only to Stacked Sequence structures which should be avoided Zooming in on the diagram of Spaghetti VI discusse
113. seen in the Log Data case low level functions for file path forming should be modularized into a subVI as per Rule 4 7 e Wiring Scheme Wires are straight and some overlapping is required e Data flow Most data flows left to right with the exception of one wire that flows right to left Shift registers are utilized to maintain data between loop iterations The File Path local variable can be replaced with another shift register Figure 4 18 shows the same VI from Figure 4 17 formerly known as Right to Left VI modified with several enhancements It contains a subVI for forming the file path and a shift register for maintaining the file path between loop iterations The revised diagram no longer requires the local variable tunnel on the bottom border or right to left data flow Additionally the object spacing and structure sizes have been reduced providing a higher overall object density An important lesson learned in this example is that the style rules are interrelated The more you follow on a consistent basis the easier it becomes to maintain good style throughout an application Figure 4 18 Left to Right VI is functionally equivalent to Right to Left VI but contains multiple style enhancements View full size image Layout This is the Classic State Machine design pattern with high object density Modularity Additional subVI increases the modularity index to 4 0 Wiring Scheme Wires are straight and
114. shift register The Wait n mSec VI uses data flow to order its delay before the Find Screw Ends VIs View full size image 234 Figure 7 21C A While Loop is used to terminate the loop on first error Nodes are rearranged to eliminate output tunnels below the error cluster View full size image The application in Figure 7 22 is a test executive implemented using a variation of a state machine design pattern discussed in Chapter 8 Design Patterns Two cases of the primary Case structure Initialize and Load Script are shown In the Initialize case the error terminals are unwired on all nodes including two variables nine Property Nodes and a File Dialog Express VI Also a high level VI is used for reading the test steps from file that has error handling built in Specifically Read Lines from File VI contains a call to the General Error Handler VI with the type of dialog input set to continue or stop message This is shown in Figure 7 23 An error passed to this subVI generates a dialog which may not be desirable if the test executive is intended to run unattended Figure 7 22 The Initialize and Load Script cases of a test executive are shown The Initialize case has many nodes with unwired error terminals The Load Script case propagates the error cluster between subVIs but the error out terminal of the last subVI in the error chain is unwired allowing the error to disappear 235 View full size image F
115. style Additionally consider inviting a domain expert to critique any subsystems of your application that are highly specialized For example if a colleague is an expert in GUI design ask her to participate in the review of your GUI The same holds true for network communications data acquisition image analysis real time control and more Be sure to keep the requirements specification and style rules checklist on hand These are important materials that are referenced throughout the meeting Begin with a review of the critical requirements including priorities Next discuss the top level architecture and justify the reason for it This might include a presentation of a state diagram or other design method followed by a top level source code walk through Then discuss each of the application s subsystems such as high level software components and low level device drivers Every subsystem should be covered including a discussion of the source code Discuss the data structures and messaging or communication protocols used to connect each high level component or subsystem Also be sure to discuss the error handling strategy Look for common mistakes such as incomplete error trapping missing VI descriptions and unnecessary operations within loops Rule 10 11 333 Designate a neutral party to take notes on desired changes Rule 10 12 Do not modify the functional source code during the peer review It is best to de
116. text based language Q queue Similar to a first in first out buffer used to pass data between parallel loops without variables R readability The ease with which developers can comprehend source code Readability pertains to both the front panel and the block diagram real time An application or operating system that runs with precise and reliable timing also known as deterministic Refnum Reference to a specific instance of an open resource such as a file instrument or device network connection image LabVIEW application VI or control Refnums function similarly to a pointer to a data structure describing the resource ring controls Controls that map text selections to numeric values They are very useful for presenting a discrete number of selections that are better described using text labels than numbers 349 robust Characteristic of a LabVIEW application if it performs reliable error handling and shuts down gracefully S shift registers Terminals on looping structure borders that shift data between loop iterations They are functionally and conceptually similar to terminals that extend wires from the end of one iteration to the beginning of the next simple controls Controls that are intuitive and easy to operate and that represent simple data types Simple controls possess properties that can be configured to help validate user and programmatic input simple data structures Data structures that s
117. the Boolean text to apply unique menu labels for each control Rules for front panel text including control labels and embedded text are discussed in Chapter 3 The error clusters have been assigned to the lower left and right terminals of the 4x2x2x4 connector pattern as recommended in Chapter 5 You can scroll these controls off the visible area of the panel if desired The diagram of the Continuous Loop design pattern template incorporates a While Loop with error handling shift registers delay Stop Boolean and termination conditions Error handling consists of trapping the error by propagating the error cluster and reporting the error using the General Error Handler VI outside the loop Three shift registers are positioned at the top of the loop Replace the Boolean data with any data that needs to be stored between loop iterations The Wait ms function is included for efficiency The error status and the Stop Boolean control terminal are applied as the initial criteria for loop termination The Compound Arithmetic function is used instead of the binary Or operation for flexibility of adding terminals Figure 8 4A The front panel of the Continuous Loop template contains error clusters and a customized Stop Boolean control View full size image 247 b iti Ninos Loon jempiate vi nome fare en We i 1 t Ma Ek en pot Comta poe Ahe eh SLE AD Laet Apelco f Rori Figure 8 4B The diagram of the Continuous
118. the more documentation is required to explain it The style rules presented throughout this book are designed to facilitate readability of the source code Therefore the better your development style the less additional effort is required to document your source code Moreover when you use LabVIEW s Print VI Documentation features your source code and documentation is automatically compiled into a document Readable source code good documentation practices and built in LabVIEW tools help to streamline the documentation process Print VI Documentation is discussed in Section 9 4 Documentation is directly related to readability maintainability and ease of use Hence documentation is integral to good style Documentation is often neglected in the pursuit of fast development cycles As discussed in Chapter l The Significance of Style shortcuts do not reduce development time when the entire life cycle of the application is considered Rather good development style reduces development time and effort This chapter presents style rules for source code documentation that is directly incorporated into the front panel block diagram icon and VI and control properties Additionally we review style rules from previous chapters that directly relate to readability such as appropriate use of text If you master the rules from the previous chapters your VIs are already readable and have an excellent foundation for streamlining th
119. the peer review One additional point regarding self reviews Many LabVIEW developers work by themselves Perhaps you are the lone developer within your organization or are a self employed consultant In these circumstances self reviews are probably your only option Self reviews combined with some self discipline should provide plenty of opportunity for maintaining good style The LabVIEW VI Analyzer Toolkit is an add on product from NI that automatically inspects VIs for style and performance It contains both interactive and programmatic interfaces The interactive interface enables you to configure the desired tests to run save the configuration to file run the tests and view and save the results Additionally if you click on a failure in the Results window the VI Analyzer opens the diagram and highlights the specific object or area containing the problem This enables you to iteratively view and fix each problem Additionally the programmatic interface enables you to develop an application that performs the desired tests using the VIs on the VI Analyzer palette For example you can write an application that automatically inspects all the VIs in a source code repository and generates a report You can configure the application to inspect the repository on a periodic basis and email or publish a report automatically The VI Analyzer contains more than 60 tests organized into the following categories Block Diagram Do
120. the proof of concepts that they apply to the finished application Indeed good style allows the proof of concept to scale gracefully into an application Good style is important all the time When the design phase is complete and the critical requirements have been evaluated via proof of concept revisit the specification Changes are inevitable Conflicts might exist between several of the requirements and tradeoffs and adjustments might be necessary The specification is a living document Be sure to revisit the specification on a periodic basis and make revisions Remove any obsolete requirements Add any new requirements Evaluate the impact on schedule Elaborate on any aspects of the application that are better understood than when the project began Maintain the specification ina controlled repository where your project team members can access it easily Manage changes and revisions according to your organization s applicable standards 38 Considering our exhaust system we might need to update the acquisition and analysis requirements based on our finding with the resource search and proof of concept development phases Specifically expand the SPC requirements to include more than simple Xbar amp R charts due to the VIs available in the SPC Toolkit Also update the acquisition and data processing rates based on the benchmarks acquired using the proof of concept Finally we might reduce the risk level and time budgeted for eac
121. three categories of risk Figure 7 13A contains a mathematical operation using numeric functions that do not have error terminals None of the functions on the Numeric palette contain error terminals and they have no risk of misbehaving Figure 7 13B contains a Property Node and Scan from String function with error terminals wired These functions have medium level of risk and error handling is recommended Figure 7 13C contains VISAWrite and Read functions which are examples of high risk I 0 functions Error handling is essential for I O operations including instrument communications with VISA Figure 7 13A The functions on the Numeric palette do not have error terminals cannot generate errors and do not misbehave These functions are categorized as low risk Numeric iain MENETE Ed Eker view 132 a gt i gt gt i gt E Returns the product of the inputs Detailed help 5 C y Result DEL POEL Scaling Factor DEL Figure 7 13B Property Nodes associated with front panel controls and the Scan from String function are examples of medium risk nodes Errors are usually not fatal but error trapping is highly recommended 223 Context Help Format string input string remaining string initial scan location ules offset past scan error in no error error out default 1 0 dbh Ba BLSHE 1 Scans the input string and converts the string according ta format string Detailed he
122. to all of the style rules First each control has a succinct and intuitive label with the default value in parentheses Additionally there are 313 descriptions for the cluster shell as well as each control The control descriptions all contain the control type valid range default value and purpose Figure 9 8 This cluster contains controls that configure serial communications with a pressure transducer Each of the controls contains meticulous descriptions as seen in the Context Help windows View full size image i imd ee i tant icin 7 ee aid Babe ijti Experian Tet Fling ST LE I FE PO LR A SY 1 Cefa 1070 li Ba Der eat elg Flore Contra Hore LOG UL Farga 0h More OORT 29 Pie ATSC TS 2h MOM OOrr end RTECS Defaut U0 hiore ot pr Hi Conf F Taaa ha fies crio method for both beeing and reig date Ths control ipadisi tha eral daia biki esias ha ceia port ard thee Haia OD Gers pee ia ERA oaen e h the Hae Cot Sores prera birdur at a paa Lin E oa Tps il Erare a a Teen Ar Loire Heda Latte a4 Birks Selector 1 0 15 2 0 olthers Deak jD Thir amiral Larger yeh Te portal pop baht epg eed hg Terina er aF rapea Pent Farag Crozer eles Tank Fay Selecteer Cede Even Flak Sate ood Cala Ever Sebections E Line Feed lt LF gt 1 Cariage eta oc Bre gaits NC Lre Fad c gt phe mhia Kins Le habe Core mium eGR or lien
123. toggle switch and command button for a valve control configured with three different labeling schemes The command button contains a glyph of a valve imported as a decal In Figure 6 6A the Valve State control labels are ambiguous in terms of the TRUE FALSE behavior In Figure 6 6B the Close Valve control labels clearly depict the TRUE FALSE behavior Additionally the vertical toggle switch contains visible Boolean text with default 175 ON OFF strings In Figure 6 6C the Boolean text of the vertical toggle switch has been replaced with more intuitive state position labels identifying the TRUE FALSE behavior as Closed and Open Additionally the default value for each control has been appended to the control labels in parentheses This additional text is distracting on a GUI VI but is helpful for reinforcing a subVI terminal s behavior via the Context Help window Per Rule 6 10 the command button is preferred for controls that invoke immediate action such as opening and closing an important valve However proper labeling makes the two control types approximately equivalent Figure 6 6A Two alternatives for valve control include the vertical toggle switch and the command button with valve image decal The label Valve State is ambiguous in this example Figure 6 6B The owned label Close Valve clearly identifies the TRUE and FALSE behavior The vertical switch has Boolean text visible with default TRUE FALSE state labels The command butto
124. transaction quickly and efficiently Industrial dialog VIs are preferred for a repetitious production environment Figure 3 3B This custom dialog VI is functionally equivalent to Figure 3 3A but is designed for industrial use such as a production environment View full size image 62 Configure Test Parameters Please verify standard test parameters and adjust volume to 2 Vrms eo MVI Le ONTROLS ING Audio Output Right 2 0274 Vrms Rule 3 10 Limit the quantity of information displayed on a GUI VI panel e Limit the panel to a maximum of seven groups of seven unique objects e Maintain ample white space between groups Some applications involve hundreds or even thousands of measured and calculated parameters all of which need to be accessible from the GUI Instead of creating hundreds of very small indicators in an effort to maximize the number of parameters displayed on one screen form logical groups of parameters that are reasonably displayed using legible indicators and ample spacing Organize the groups onto multiple tabs panels or subpanels that the user can readily navigate The actual size and number of permissible objects vary with indicator type labeling alignment white space monitor size and resolution One rule of thumb is seven groups of up to seven unique objects per group The Power Monitor System VI from Figure 3 1 contains seven groups of objects as shown in Figure 3 4 These include one gro
125. trivial subVIs containing few nodes 4 10 Create a meaningful icon and cohesive description for every subVI All Minimize wire bends eliminate kinks and loops A l2 Maintain even spacing of parallel wires 4 13 Tunnel wires through left and right borders of structures 4 14 Do not wire through structures unnecessarily 4 15 Never obstruct the view of wires and nodes 4 16 Limit wire lengths such that source and destination are visible on one screen 4 17 Never use local and global variables for wiring convenience 4 18 Label long wires and wires from hidden source terminals 4 19 Place unwired front panel terminals in a consistent location 4 20 Modularize wires of related data into clusters A 21 Save clusters as type definitions A 22 Always flow data from left to right 4 23 Propagate the error cluster 356 Ayd Use 1280 x 1024 display resolution 4 24 Avoid array and cluster coercions 4 25 Create controls and constants from a terminal s context menu 4 26 Disable dots at wire junctions Avoid Sequence structures unless required 4 28 Avoid nesting beyond three layers 4 29 Use write local variables for initializing control values 4 30 Use global variables for simple data sharing between parallel loops or VIs 4 l Use a Sequence structure to order operations if no data dependency exists A OZ Use only Flat Sequence structures when required 4 33 Avoid polling variables within continuous loops 4 34 Avoid variables if wires
126. type definition Instead of searching for every instance of the control edit the strict type definition all instances then are updated immediately Also because a strict type definition maintains the appearance of the control the strict type definition 1s appropriate for maintaining consistent appearance with GUI VIs Consistency aids GUI navigation while inconsistency hinders GUI navigation Chapter 6 Data Structures discusses type definitions in more detail 3 5 Examples This section presents specific examples of GUI VI and subVI front panels Color versions of these panels can be freely downloaded from the Publisher s website page for this book at www prenhal lprofessional com title 0131458353 An electronic eBook version of this book containing the full color images is available for purchase fron www prenhallprofessional com title 0132414813 A subVI from selection is created by selecting a section of the block diagram and choosing Edit Create SubVI Like magic the selected code is replaced with a new subVI This is a nifty tool but is also the world s most flagrant style offender The resulting front panel block diagram icon and connector pane require significant manual cleanup For example the front panel in Figure 3 14A violates several rules The controls are arranged horizontally instead of resembling the connector assignments Rule 3 15 Standard controls such as task IDs and error clusters are not in th
127. unnecessary operations particularly ones that are performed repeatedly within looping structures An efficient application also conserves memory by limiting the size of LabVIEW s four memory components front panel block diagram data space and code enumeration error Error error error error Also called an enum a data type that provides a user selectable list of strings that are associated with numeric values similar to a text ring except that the numeric values and string labels are part of the data type Enums are very useful for presenting a discrete number of selections that are better described using text labels than numbers Unlike ring controls enums are always represented as unsigned integers and the text selections are mapped to sequential numbers starting with 0 A failure of a function or VI to complete its programmed task Most nodes in LabVIEW propagate error data using the error in and error out clusters Case Structure Case structure with an error in cluster terminal wired to the selector resulting in cases for Error and No Error The Error Case Structure is commonly used with subVIs chain Method for trapping errors by propagating the error cluster among the nodes that have error terminals A group of data dependent nodes that execute sequentially comprises an error chain cluster The error in and error out cluster controls used extensively for error propagation code 343 T
128. wire stubs in the Context Help window The example VI in Figure 5 12A uses a densely populated 5x3x3x5 connector pattern to receive and return multiple arrays The input arrays are set points for FieldPoint analog output modules and the output arrays are measurements from analog input modules One of the array inputs is assigned to the top middle right terminal while two of the array output terminals are assigned to the top middle left terminals causing crossovers to appear in the Context Help window These assignments may have been careless mistakes by the developer or a conscious decision to avoid terminal reassignments when adding new inputs or outputs Indeed each instance of this VI would require rewiring within the caller s diagram if the connector pattern is changed or the terminals are reassigned In Figure 5 12B the terminals are reassigned with the inputs on the left and outputs on the right and no crossovers in the Context Help window In Figure 5 12C the input arrays are all assigned to the left side and bottom middle terminals and the output arrays are all assigned to the top middle and right side terminals Figures 5 12B and 5 12C are equivalent methods of eliminating crossovers However the dense population of the 5x38x3x5 connector pattern limits expandability and maintainability while promoting wire clutter In Figure 5 12D the input and output arrays have each been combined into clusters reducing the required number of t
129. with the exception of Please Select Instrument to Configure This lengthy phrase should be a caption instead of a control label Keep the labels succinct and intuitive per Rule 3 21 Note that none of the control labels contain default values Default values are 299 not necessary for the labels of GUI VIs because the controls generally contain their default values when loaded unless they are overwritten programmatically using local variables or Property Nodes However the default values are useful to have in the control descriptions All of the controls on the panel of Configure Test Instrument VI contain descriptions and tip strips However the descriptions are not available to users unless they know how to open the Context Help window via the lt Ctrl gt lt H gt shortcut Hence this dialog requires familiarity with the LabVIEW environment to obtain help Figure 9 2A This dialog prompts the user to configure a test instrument Please Select Instrument to Configure is an unnecessarily lengthy control label There is no obvious method for the user to open the Context Help window Configure Test Instrument fx Please Select Instrument to Configure NI 4071 Function DC Volks Resolution in Digits Range 6 1 2 y 02 Figure 9 2B is the front panel of the same dialog as Figure 9 2A with several improvements The succinct control label Instrument replaces the lengthy phrase Please Select Instrument to Configure Lengthy phr
130. wrong object may be haghlighted sls Uist mm Tega Hytheredig Goad Styte vi z Backwards wires w I a a a Cxorence Siham Fahre Reported Comment Lease Global are Lona 1 High Rank Test CI Sort by Tet i Low Bark Test The eie aire ure under an object on the block clageami letked Comtext Help H Locked Context Help Sanwa Serre bogia hysteresis data to a bab derited text file for ura in Ena od ered ot an image of the Trent pa boo HTML Mia For uts Bih a web brigis structure flagging a Wires Under Objects BJ aepepa fagaga p n a neee EA ile Depia L i Laborer Figures 10 2H through 10 2J are the medium ranking test failures In Figure 10 2H the VI Analyzer identifies insufficient comments A brief comment is added within each case of the Case structure using free labels Additionally two comments describing the function of the two primary structures are added as shown in Figure 10 2K In Figure 10 21 the VI 325 Analyzer s spell checker identifies the misspelling of velocity as vel in two control labels Although this abbreviation was intended by the developer abbreviating important terms within control labels on a GUI VI is not recommended Hence this problem is corrected by simply expanding the term to velocity Similarly each of the control labels within the Statistics cluster is expanded as shown in Figure 10 2K Figure 10 2J contains a violation of the Wire Bends tes
131. 14A The front panel of the Torque Hysteresis VI contains a menu of Boolean controls and a graph indicator View full size image b Torque Hysteresis Event Driven State Machine vi Graph Statistics Run Test i Hy Lilt AU airea w Figure 8 14B The diagram of the Torque Hysteresis VI is implemented using the Event Driven State Machine design pattern comprised of a Queued State Machine with an Event structure in the Idle case The Event structure is configured to buffer one or more states for each of the Boolean control s Value Change event View full size image 262 a Tie Demut T HAL Eun Tert eet Value Change Figure 8 14C A separate state is delineated for each of the application s primary tasks in addition to Initialize Idle Shutdown and Blank View full size image ee Di bd oe i i Pe The Event Driven State Machine is the most powerful design pattern presented so far However all of the states execute sequentially within one loop This means that any state that takes a long time will suspend the application Specifically a state that contains a lengthy test or measurement step will prevent the application from running any other states including Idle This prevents the application from responding to GUI events during the lengthy state If your application contains lengthy steps or parallel routines proceed to Section 8 3 Compound Design Patterns
132. 222 LA Error Handing VIDS erse a a a a a a a E 226 Dic De EXIM LES aara A A A N E A E O E 230 ENdNOTES sieniin inar A A E EO A EAN E EAEN EEEE 238 Be DSS ISIN PANES cases sn a E E AE A ote 239 8 1 Simple Design Patterns 21a cviwsideviorcrcasrotsndoicstdechideusadibout E SE e ESEE ASN 241 82 State MI CNINE S raar a E E E TE T EE E A T TROR 254 8 3 COMPOUNG Design Patterns siisseviscierxsinccser vec ceniacsssavassabanercen baa E EAT N 266 8 4 Complex Application Frameworks s isccdasesces sauces cisiecceasinaldacaleieas dalsvnsdaadiccvdetuachaaatesee tia taah asc asd ake iedevdea 271 Bee EX ANIC narr anA E nie EN E kibaties helnssdh buleiasande E N 284 ENONOTE Sas AE Rinse E Vow ila O E E est ualos en ie edad OA 294 9 DOCUMENTATION feieraike ce srhernitai aint ations Nees eee aie aie ee aes 296 MY elie E E a rains Ges alco sa in sce sraul eS eo sarge ttn 0 aida autos E ER 296 TNEOrEM la EE N acy acts osahablonoteusteo dsuantcuie E E E E ENE 296 9 1 Front Panel DOCUMENTATION ies sinsarasceortcavecatavwredaatiensss teswgneredenpaasedieney dha vauavedetoroysenniarseay resoateerpugtaueerdes 297 9 2 Blo k Diagram nnnc E E Gabodi cassie E E A A ab seen aihe tu eaauevssRonea tiem 301 9 3 icon and VI Descriptio erenn a a e a a a a aa ncdeteks 308 9 4 Online DOCUMENTA OI isi ease viernes oud ena a a e e wee ae e a ae E 309 SS EXIMD IOS ane ee A A EO TA A AAE AOO AA 312 ENdNOTES sieniniai inar A A E E A EA NNE E EAEN ONEEN 315 10 COG REVICWS comer R AA
133. 32 Parse and Transmit Utility VI is a dialog VI that prompts the user to configure a serial messaging protocol Figure 3 15A has characteristics similar to a dialog such as the window appearance layout of controls and data types However the control styles fonts decoration and colors do not conform to standard dialog conventions Specifically 13 point plain black Application font is used for all control data and 16 point plain black Trebuchet MS is used for all labels instead of using Dialog fonts for both The panel is colored dark gray and sized small to resemble a dialog The control labels and Boolean text are legible because of high contrast between black text and light gray and white backgrounds but the black control data text is not sufficiently legible on dark gray background The controls are navigated using the Tab key Figure 3 15A RS 232 Parse and Transmit Utility VI does not have a native operating system appearance View full size image RS 232 Parse and Transmit Utility vi stop bits 10 1 bit _ flow control O none Application Utility for configuring periodic messages transmitted to an instrument Conditions Not conforming to dialog conventions Color scheme Light gray decoration on dark gray panel with dark gray controls and black text Layout Related controls grouped together using a decoration Heading text 16 point plain black Trebuchet MS 87 Boolean text 13 point normal black Applica
134. AI 110 3 Measurements i dup Main RT YI Refnum Main RT VI Refnum Non PID setpoints ray FP AI 110 4 Measurements Error In no error fF P AI 110 5 Measurements FP A0 200 6 SetPoints m Error Out FP 40 200 SetPoints FP A0 210 6 SetPoints Figure 5 12D Input arrays of set points and output arrays of measurements have been combined into clusters and the 4x2x2x4 pattern has been applied The number of assigned terminals is greatly reduced decreasing wire clutter on the calling VI diagrams FP Measure amp Control with Clusters vi Main RT VI Refnum fram ent dup Main RT VI Refnum FP Set Points FP Measurements Error In no error Error Out Rule 5 24 Specify terminal assignments resembling the panel layout Rule 5 25 147 Assign error clusters to bottom left and right terminals Rule 5 26 Assign references and I O names to top left and right terminals It is a long standing convention that error clusters are always assigned to the bottom left and right terminals and references and I O names are always assigned to top left and right terminals References and I O names include file refnum DAQmx task and channel names VISA resource name VI Server application and VI references control references and similar items The remaining controls and indicators may be assigned to the remaining terminals in the same respective locations as the object positions on the front panel Therefore t
135. B the Convert Power Units VI is incorporated as a subVI within Measure VI Hence the power data is converted to appropriate units before it is returned Also the wavelength and power arrays are modularized intoacluster Finally the routine that unbundles several scan parameters and calculates the number of wavelength steps has been modularized into a cohesive subVI 106 Figure 4 6A Two separate wires for the wavelength and power arrays are used together in most places and may be modularized into acluster The routine for calculating the number of wavelength steps can be modularized into a subVI The routine for converting power units can be performed within Measure VI View full size image Modularize routine Convertunis within Modularize arrays with subi Measure Wi with cluster Figure 4 6B The routine is further modularized with Calculate Wavelength Steps VI and Power vs Wavelength cluster View full size image ET enor guk This VI calculates the number of wavelength stapes far the scan based on the specified center wavelength nL DOGOOOOO0C Rule 4 21 Save clusters as type definitions I cannot overemphasize this rule Consequently it is further discussed in Chapter 6 Data Structures Save every cluster as a type definition or strict type definition A type definition or typedef for short is a control that maintains its data type information in a CTL file The typedef is copied onto any num
136. Conceptually variant can be thought of as a wrapper that converts any data into a new format that is described in a universal manner In LabVIEW the variant data type is compatible with the standard used in Microsoft COM and NET technologies Specifically variant is used with DataSocket and ActiveX communication protocol VIs Additionally variant is commonly used as a generic data type that passes data defined dynamically during runtime instead of edit time It allows a data source or server to provide any type of data to a destination or client that can decode and use the data The functional interface such as a subVI connector terminal maintains the variant data type regardless of the variant s actual contents This allows the client and server to be revised without affecting the interface The disadvantage of variant is that it is less memory and processor efficient than the conventional fixed data types and may require additional programming to encode and decode the data within the client and server applications In LabVIEW variants are assembled using the To Variant and Set Variant Attribute functions Alternatively any LabVIEW data type is automatically converted to variant when wired to an input terminal of type variant The conversion is denoted by a coercion dot As noted previously arrays and clusters are tremendously flexible data structures For example there is no practical limit to the number of dimensions arrays can have a
137. DS Uniphase Ernie St Louis Ciencia Jim Kring James Kring Inc Ken Tumidajski Testand Corporation Alan Blankman LeCroy Corporation Daniel L Press Prime Test Corporation Danny Allard Videotron Brian Gangloff DataAct Incorporated Dave Galanis UTC Power Roger Emerick Pioneer Aerospace Corp Robert Greene NSK Robert Breidenthal Precision Optics Additionally I would like to acknowledge the early pioneers of LabVIEW style Meg Kay and Gary Johnson wrote a LabVIEW style guide that has subsequently evolved into National Instruments LabVIEW Development Guidelines part of the LabVIEW shipping documentation Randy Johnson wrote Rules to Wire By which was printed in the LabVIEW Technical Resource newsletter LTR in 1999 More recently Darren Nattinger of NI developed the LabVIEW VI Analyzer Toolkit an add on toolkit that automatically analyzes VI style Finally Noel Adorno wrote NI s Instrument Driver Development Guidelines which are available for free download from WWW N1 COM 10 About the Author Peter Blume is the founder and president of Bloomy Controls Inc a National Instruments Select Integration Partner that specializes in LabVIEW based systems development Since LabVIEW Version 2 5 Blume and his staff of engineers have solved more than a thousand industrial applications for customers throughout the northeastern United States To promote consistent quality among multiple developers in multiple
138. DoProberCmd vi Registration In Command DoProberCmd VI is one of the VIs from the Action Status palette The icon in Figure 5 18A contains the palette s icon convention with the addition of an artful silhouette of a runner The VI name and the icon could more closely resemble each other Why not name it SussPARunProberCmd VI Alternatively the runner could be replaced with the word Do Moreover the toolkit is intended for international use and the glyph of a runner is not a universal symbol for the VI s primary purpose of executing a command Finally note that Registration In and Command are input terminals with required priority Figure 5 18B contains the palette of a similar instrument driver developed from the LabVIEW instrument driver template using the Create New Instrument Driver Project utility It contains standard glyphs for common functions and subpalettes in place of the instrument specific glyph The banner is the only feature identifying the instrument on the icon Consequently the palette appears rather generic with no direct association to wafer probing DoProberCmd VI contains the green sideways triangle glyph commonly used to indicate Run Start or Initiate This glyph is more appropriate 154 than the runner in Figure 5 18A for describing the execute command function but the association with wafer probing remains weak Figure 5 18B An alternate icon convention consists of the standard glyphs
139. E ap CRemote Targets O Corespondence ir u User interface Instrument Marsis Caitie 2 Variables E21 Server a Application Name Top Level O Error Handling O File 1 O Prototype Vis 2 Remote Targets G Remote Target 1 O Remote Target 2 G Type Definitions 5 User Interface Vartabhes G VI Server Most developers agree with the usefulness of an organized repository However like many Rules we often start out with good intentions but might lose focus under the pressures of tight deadlines Unless we form good habits we risk sacrificing many Rules including the source folder hierarchy Furthermore it is problematic for the LabVIEW project as well as most source code control tools to move source files among folders on disk When the files are referenced within the project moving them on disk causes file linking errors So here is the secret to maintaining an organized repository Rule 2 10 Create the folder hierarchy before you begin coding If you initiate your project with an organized folder hierarchy from the outset maintaining an organized repository is as simple as saving each file in the appropriate folders as you create and edit the files throughout the development cycle I find that it is much more feasible if the folder hierarchy already exists Furthermore you need not create a new folder hierarchy for every project from scratch I find that most projects I develop have many of the same general
140. E F Val 3 gt Timacut gt 12 S Run WE Value g fo Ti Run WI ucing Run method 1 Menu Dialogue Box Value Change FI gt Panel State gt i 2 Menu Window has Title Bar Value Change me 3 Teru Reszable Value Changa 4 Meru Shoa Scroll Bars Value Change F 5 Tir Yr Vaja Change 6 Quit Value Change gt Error gt res een The main loop of Centrifuge DAQ VI was presented in Chapter 4 The broader view shown in Figure 8 28 reveals a compound design pattern consisting of a Queued State Machine at the top an Event Handling Loop at the bottom and a queue for messaging The Event Handling Loop violates several rules including continuous Timeout events Rule 8 8 dissimilar horizontal loop sizes Rule 8 18 and incomplete error handling see Chapter 7 Figure 8 28 Centrifuge DAQ VI is a compound design pattern comprised of a Queued State Machine at the top an Event Handling Loop at the bottom and a queue for messaging View full size image 290 U ft al 5 Epon i yi Repa nps Sci mT pen m gT pa a H ITH beet F mH Limma e Webe retin F i ee eriep ce ai a am oo Syl The application shown in Figure 8 29 is a full featured instrument control utility that is bundled together with a driver for a digital pressure transducer The control utility configures acquires and logs data and provides full interactive control of any number of transducer
141. E LabVIEW SHALL m J fz R E TRAMES Se IR S asi 0 The LabVIEW Style Book Peter A Blume EASE OF USE EFFICIENCY READABILITY SIMPLICITY PERFORMANCE MAINTAINABILITY ROBUSTNESS Copyright Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks Where those designations appear in this book and the publisher was aware of a trademark claim the designations have been printed with initial capital letters or in all capitals The author and publisher have taken care in the preparation of this book but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein The publisher offers excellent discounts on this book when ordered in quantity for bulk purchases or special sales which may include electronic versions and or custom covers and content particular to your business training goals marketing focus and branding interests For more information please contact U S Corporate and Government Sales 800 382 3419 corpsales pearsontechgroup com For sales outside the United States please contact International Sales international pearsoned com Visit us on the Web www prenhallprofessional com Library of Congress Catalogi
142. Feed LPs Thi intra Pae Batch the carting of the He COT Praa Main paj ilera Tae Demre a h iu tae in ba AN wath Hes Hens DOD erie ore vie peur the peida Thin control opened ee marked parity requires bn Gwara eth the Heis COT erie Ho 0 el eeen i tramachoor ot the specii addres Documentation need not be limited to text Sometimes an illustration is more descriptive Figure 9 9 contains the block diagram of a VI that controls a ramp soak temperature profile It contains an illustration of the temperature profile that it produces The illustration was created using a drawing application and was saved to an image file The image file is copied and pasted onto the diagram by choosing Edit Import Picture to Clipboard followed by Edit Paste Figure 9 9 The diagram of a VI that controls a ramp soak temperature profile contains an illustration of the profile it produces View full size image 314 gt Temperature Profile vi Block Diagram oao maea i oreo ro Be Endnotes l Chris Megandoff One Click Documentation Automating Your User Manual Development LabVIEW Technical Resource LOW s A205 315 10 Code Reviews The LabVIEW Style Book provides many style rules that can help an organization establish a style convention However the rules descriptions and examples contained herein fall short of requiring their adoption and use This chapter addresses how to enforce the standardization of a style con
143. Figure 4 2B each routine is modularized into a cohesive subVI The subVIs remain in the same respective locations as the code they encapsulate In Figure 4 2C the three subVIs are placed in series the error cluster propagates between each subVI and the Merge Errors VI is eliminated The resulting diagram is now a simple set of three subVI calls We can see that the diagram is much neater simpler and organized with subVls Figure 4 2A The calibration interval VI contains three routines View full size image 98 eT Pe ie a Le ai Figure 4 2B Each routine is modularized into cohesive subVlIs View full size image wrap Ez Clock Wraps Fie path BS Eo Renk error in no error one ol Er FP error out Cal Interval sec DEL Es ITAN Figure 4 2C The subVIs are rearranged into a dataflow sequence that propagates the error cluster and reduces wire clutter View full size image Rule 4 8 Do not create subVIs just to save space Rule 4 9 Avoid trivial subVIs containing few nodes SubVIs are advantageous because it is easier to develop test debug maintain and reuse software modules as subVIs versus sections of a large diagram As shown in Figure 4 2C they also provide a considerable space saving benefit In general if the collection of functions or Property Nodes or other routine is used in more than one place it is an easy decision Replace the repeated code with a subVI
144. Figure 6 7 Four different controls contain a digital pattern for writing to an 8 bit digital I O port They include an array of Boolean and three numeric controls formatted as binary decimal and hexadecimal It is important to make the radix visible for numeric controls not configured for decimal notation View full size image 178 Four numeric data types that have special properties are the time stamp reference number refnum and I O name text and menu rings ring and enumeration enum The time stamp data type stores absolute time with separate 8 byte fields for seconds and fractions of a second Hence the time stamp is extremely precise Refnums and I O names are references to a specific instance of an open resource such as a file instrument or device network connection image LabVIEW application VI or control They function similarly to a pointer to a data structure describing the resource While the purpose of the two types of references is similar the controls are substantially different Specifically I 0 names display a value in an intuitive format whereas refnums do not VISA IVI and DAQmx Name Controls for example provide information about the specific hardware that they reference ActiveX and NET Refnums provide only a data type ina label Because each type of resource has different requirements the actual data type memory use and underlying data structure is different for each The placement of refnums and I 0 nam
145. G Empty Array VI is used to determine whether the numeric array wired to its input terminal is empty The Context Help window indicates a purple wire for the array input terminal representing the variant data type The illustration contains three functionally equivalent dataflow paths The top path connects the array directly to the Empty Array VI and a coercion dot appears on the VI s input terminal The middle path inserts the To Variant function to remove the coercion dot The bottom path uses the LabVIEW Empty Array function for the same purpose The LabVIEW function is polymorphic and does not require a conversion In terms of performance the top two paths are identical The middle path is mildly preferred over the top path because it demonstrates that the type conversion is intended and eliminates the coercion dot The bottom path is the most efficient overall because the polymorphic function maintains native LabVIEW data types and the type conversion is eliminated Figure 6 13 This diagram contains three methods of determining whether the input array is empty The top dataflow path contains a coercion dot on the input terminal of Empty Array VI The middle path removes the coercion by explicitly converting to the variant data type The bottom path uses a polymorphic LabVIEW function to maintain consistent data types View full size image Context Help Empty Array Empty Array vi Array Empty Array Empty Array
146. I VIs in an application Consider providing a color legend to formally document the color theme Rule 3 30 Follow universal conventions for green yellow and red Green yellow and red have definitive meanings in most industrial environments Green indicates normal operating conditions yellow indicates caution or warning and red indicates alarm or emergency If your VI is an industrial GUI heed these universal conventions Rule 3 31 79 Leave the panel and objects of subVIs gray Panels of subVIs need not be colored As stated previously it is a convention that subVI panels maintain their default appearance to help identify them as subVIs If you are developing a commercial product such as a developer toolkit or instrument driver you might want to incorporate a modest color scheme to help associate the panels of the toolkit In this case I recommend using a single color to add some limited color markings However attractive and meaningful icons are much more important with commercial subVIs because icons are visible from the caller s diagram Therefore I recommend investing time in a nifty icon convention and leaving the panels and objects with their defaults Rule 3 32 Keep the color schemes simple and time limited Some developers might find themselves expending substantial time and effort experimenting with color schemes in search of the optimal GUI Prioritize the application s functional requirements inclu
147. I are read from or written to within subVIs using control references and Property Nodes configured to read from or write to the control s Value property However because this is a template many of the controls have not been created yet So how can we modularize the loops of the application framework Simply pass the top level VI s control references using a cluster saved as a type definition and add new control references to the type definition as new controls are added to the front panel In this manner you need to maintain only the type definition The subVI connector assignments remain constant Alternatively consider modularizing the top level VI s user interface using the subVI front panels and a subpanel control This is similar to the Dynamic Framework discussed previously Rule 8 30 Avoid subVI from selection with continuous loops Let us refer to the construct that is comprised of a high level loop modularized into a subVI as a loopsubVI One might be inclined to expedite the modularization of loops into loop subVIs using the SubVI from Selection utility This is accomplished by selecting a loop and choosing Edit Create SubVI However the SubVI from Selection utility does not handle front panel terminals gracefully By default it creates control reference constants for all control and indicator terminals within the selection and passes them into the subVI using individual connector terminals for each Additionally data wri
148. IEW development style really is significant Few would argue that clean and neat diagrams are easier to read than sloppy ones Organized user interfaces are more intuitive and easier to operate But did you know that VIs containing neat panels and diagrams normally execute more efficiently and with fewer bugs Do you consider that your applications might need to be operated and maintained by people who are unfamiliar with your coding style Ironically this person might even be you 6 months a year or several years from now after you have forgotten your own work Indeed I have seen developers confess their inability to explain source code that they developed last week never mind last year By contrast many multideveloper teams that reside in remote locations can productively work together share and apply LabVIEW code seamlessly and without explanation The difference is style Consider a few examples Figures 1 l 1 2 and 1 3 illustrate top level VIs created by different developers for very dissimilar application types and complexity levels However eachreflects the developer s distinctive programming style Which front panel would you prefer to operate Which diagram would you rather modify and maintain Which VI executes efficiently Which VI is more likely to have bugs race conditions or memory leaks Figure 1 1 Meticulous VI the front panel and block diagram for a complex application developed with meticulous a
149. Initialization code is on the left continuous read operation is within the loop and shutdown is performed on the right View full size image F m EHE Figure 8 10B The Classic State Machine implementation contains states for Initialize Read Data Shutdown and Blank This implementation requires less horizontal space contains state labels and expands more gracefully compared to the Continuous Loop View full size image Fid Coa ee alt eel of cats eed pee 1r eed era Figure 8 11 provides a template for the Classic State Machine design pattern In addition to initializing control values and properties use the Initialize case to initialize instrumentation read configuration parameters from file and anything else required to set the application to its desired initial state Use Shutdown for setting all hardware to a known state and disposing any instrument data acquisition or file references The Idle case is normally the default case that is traditionally used to poll resources such as instruments or control values The Blank case simply propagates 258 all wires from the input tunnels straight through the output tunnels Duplicate the Blank case to reduce edits when developing new states Figure 8 11 A template for the Classic State Machine design pattern contains states for Initialize Idle Shutdown and Blank View full size image ot abel ab Pathe ap Ui Dre ig regs eee i Be ei ted Epp ier J Du
150. LabVIEW developers still apply today Additionally many of the modern innovations aid our ability to organize and simplify an otherwise complex graphical user interface GUI This chapter provides guidelines that help make your LabVIEW front panels intuitive user friendly and conforming to industry standards We begin by defining some terms that I use to describe the scope of each guideline Top level VIs are VIs that reside at the highest level of the application hierarchy with front panels that comprise the primary display screens Some applications have only one top level VI which developers often refer to as the Main VI Other applications have a top level VI that dynamically loads other high level VIs that comprise the primary display screens For our purposes all high level VIs that contain user viewable panels and a hierarchy of subVI calls are considered top level VIs Dialog VIs are GUl related subVIs that open their front panels and prompt the user for information Dialog VIs have far fewer subVIs and much less overall functionality than top level VIs Their purpose is to perform a specific transaction with the user It is also common for dialog VIs to be modal for which the front panel window is locked on top of all open windows until the interaction is completed and the window closes Modal behavior is a default property setting with LabVIEW s Dialog window appearance as selected from File VI Properties Window Appearance However
151. Likewise if several nonrepetitive nodes are related to one 99 another and work together to perform a specific task modularize them into cohesive subVIs whether they are needed in multiple places or not However do not randomly select areas of the diagram and create subVIs just to save space SubVIs created in this manner are not cohesive intuitive or reusable Also do not create trivial subVIs that contain few nodes In this case the subVI icon unnecessarily masks the underlying code on the diagram For example the subVI in Figure 4 3 contains only a single Index Array function However the subVI s icon name and description disguise the function The LabVIEW functions and shipping subVIs within vi lib are universally recognizable so avoid masking them within trivial subVIs Figure 4 3 This subVI contains too few nodes Its icon name and description disguise the Index Array function on the diagram of the calling VI View full size image b Copy Wl Front Panel m E Copy RM vi Block Diszram E yj oF taet Application Fort v foo Rule 4 10 Create a meaningful icon and cohesive description for every subVI Always create a meaningful icon and cohesive description for every subVI I cannot emphasize this enough At Bloomy Controls this is one of our most sacred precepts The icon and description identify the subVI from the diagr
152. Loop template consists of a While Loop with error handling shift registers timing and several loop termination conditions View full size image L pe EF gt Uad gt C gt u F gt Umea H Fy O ee aro TE aren nH e gt Emor L E Jj m seep maa Ee et Customize the Continuous Loop template for use as a subVI or top level VI For a subVI delete the General Error Handler VI and Boolean controls and surround the diagram with an Error Case Structure For a top level VI hide the error clusters and customize the panel Figure 8 5 shows a top level implementation of the Torque Hysteresis VI using the Continuous Loop design pattern The front panel contains a Boolean menu and graph indicator on a tab control On the block diagran the functionality has been divided into three Case structures The first Case structure s True case contains dialog subVIs that prompt the operator for UUT information and motion parameters This case is called when the user clicks 248 a Boolean labeled New UUT The UUT information and motion parameters are returned by the subVIs and maintained in shift registers Also a Boolean flag is set when the UUT information is validated and the Boolean data propagates using a shift register The middle Case structure runs the test and analyzes the data when the user clicks a Boolean labeled Run Test and the UUT information is valid The acquired torque versus angle data and calcula
153. Never cross wire stubs in the Context Help window 5 24 Specify terminal assignments resembling the panel layout 5 25 Assign error clusters to bottom left and right terminals 5 26 Assign references and I O names to top left and right terminals 5 27 Choose left and right vertical edge connector terminals for high priority inputs and outputs 5 28 Choose top and bottom horizontal edge connector terminals for lower priority inputs and outputs 5 29 Specify required priority for critical inputs and outputs 5 30 Specify optional priority for inputs and outputs that are normally not used Chapter 6 6 1 Choose controls that simplify the operation of the panel 6 2 Choose memory efficient data types 6 3 Choose controls and data types that facilitate consistent data structures throughout an application 6 4 Configure an appropriate default value for each control 6 5 Enter control descriptions 6 6 Save custom controls as strict type definitions 6 7 Create arrays and clusters that associate related data 6 8 Use Booleans if two states are logical opposites 6 9 Assign names that identify the TRUE and FALSE value behavior 6 10 Use command buttons for action slide switches for parameter settings 6 11 Label the TRUE and FALSE states of slide and toggle switches 358 6 1 Choose controls that simplify the operation of the panel 6 12 Avoid using buttons or switches as indicators and LEDs as controls 6 13 Use 32 representation for integers and DBL f
154. Next State gt 254 Rule 8 9 Use a state machine design pattern in most VIs of medium or greater complexity State machines provide tremendous flexibility and expandability The state machine s shift register or messaging construct allows the code to jump from any state to any state Each state can determine the While Loop s run condition allowing it to stop after any state Also you can easily add more code by simply adding cases to the Case structure without increasing the overall size of the diagram These powerful features make the state machine invaluable For VIs of medium or greater complexity you almost cannot go wrong using a state machine Rule 8 10 Derive the application s primary states from the specification or design document To use a state machine the application must be divided into a series of states This can be done during the planning phase using a properly delineated requirements specification or design document or during the implementation phase using experience and intuition As discussed in Chapter 2 Prepare for Good Style the more planning is performed before coding the better the outcome of the application The State Machine Diagram is a fundamental component of UML and an ideal method for defining the primary states of an application Additionally the State Diagram Toolkit is a utility available from NI that provides an editor for creating state diagrams from which the co
155. Stacked Sequence structure when the sequentially ordered code is placed in cases of the Case structure instead of the frames of the Sequence structure However the execution order of the cases is controlled programmatically via the Case structure s selector terminal This is more flexible than a Sequence structure for which the execution order is strictly determined by the frame numbers Use shift registers on the loop to share data between cases of the looped Case structure Shift registers promote better data flow than the Sequence Locals of a Sequence structure because the data enters and exits the cases in a consistent location and the data flows left to right Figure 4 12 provides four different implementations of a test sequence comprised of 12 sequentially ordered test VIs Each test VI shares several common inputs and outputs including a DAQmx task a cluster of sensor scaling coefficients a cluster of high and low test limits and an error cluster Additionally each test appends a row to the Test Results table indicator for reporting the test results These updates are performed immediately after each test is completed Also the limits for each test are calculated within the previous test VI based on its test results Figure 4 12A utilizes a Stacked Sequence structure in violation of Rules 4 27 and 4 32 The error cluster is passed between frames using sequence local terminals on the Sequence structure s inner bo
156. The OpenG and Bloomy libraries contain File I O utility VIs that reside within LabVIEW user lib The LabVIEW palettes are customized to access these VIs equally from the LabVIEW User Libraries palette as shown in palettes A and B and the File I O palette as shown in palette C View full size image i AN E Z ern Ee Select a VO Most of us customize the controls and indicators available from LabVIEW s Controls palette Some of the most common edits include changing the data type or representation resizing coloring the various foreground and background elements and editing fonts of the labels or embedded text Alternatively the control editor can be applied to dramatically modify the appearance and create a brand new type of control Customized controls can be saved as custom controls type definitions or strict type definitions and can be reused throughout your applications Any such controls can be accessed from the User Controls palette simply by placing them within the LabVIEW user lib folder LabVIEW uses the file extensions to determine whether each item appears on the Controls palette or Functions palette Files with the V or Vit extension appear in the Functions palette files with the Ctl extension appear in the Controls palette 44 Templates are VIs that contain commonly used design patterns or constructs saved with the VIt extension Some common templates inclu
157. VI panels should not resemble a magic show An application is easy to use if the objects are present in consistent locations that are easy to find Consistency is the key My favorite strategy is to keep the inactive controls visible and to alternate the value of the Disabled property between 0 Enabled and 2 Disabled and Grayed Out This way the operator always knows what parameters exist and where to find them In the resistance measurement example the Resistance Type control remains visible at the same place at all times but it alternates between Enabled and Disabled and Grayed Out Rule 3 34 Restrict the range of all controls to values that are relevant to the application Rule 3 35 Set the Data Range property of numeric controls Rule 3 36 Use ring or enumerated controls over string controls when feasible Another consideration is what happens if the user enters any value into a numeric or string control Are there some values that do not make sense that will throw an error or that will cause some other problem This issue is usually easy to resolve for a numeric control Simply specify the control s Data Range settings from the Properties dialog or 81 programmatically using a Property Node For a string control you might need to create a subVI that searches for invalid characters or combinations LabVIEW has many string functions such as Match Regular Expression and Search and Replace String that enable you to create a c
158. Vision and Motion Mathematics Signal Processing H Data Communication P Connectivity Favorites Y User Libraries KAKA Select a VI SubVIs that perform low level tasks that complement or extend the capabilities of the built in LabVIEW functions are known as utility VIs It is desirable to access the utility VIs directly from the associated LabVIEW palettes in addition 43 to or instead of the User Libraries palette For example the OpenG and Bloomy reuse libraries both contain File I 0 libraries These VIs are accessed by navigating the User Libraries palette as shown in Figures 2 5A and 2 5B However it might be faster and more intuitive if these libraries were accessible from the File I 0 palette Fortunately LabVIEW provides complete flexibility in customizing the palettes In Figure 2 5C redundant subpalettes have been created for the File I 0 utility libraries on the File I O palette This is accomplished from the palette set editor by inserting new subpalettes on the associated LabVIEW palettes that are linked to the MNU files folders project libraries or LLBs that comprise the utility libraries It is important to recognize that the source files uniquely remain in the LabVIEW user lib folder The MNU files that define the palette contents are edited by the palette set editor and maintained at the following path default data directory gt lt LabVIEW version number gt palettes Figure 2 5
159. YI changed since you performed the analysis the wrong object may be highlighted Results List Subitems El Torque Hysteresis Bad Style vi 1 Backwards Wires Error Cluster Wired Unused Code I Documentation Wires Under Objects Coercion Dots Comment Usage Globals and Locals Spell Check Wire Bends Control Terminal Wiring String Constant Style HF OF Oo Pa On oo on On Pa cd hon E rs i i E F H Show Failures Only Sort by VI High Rank Test Description Sort by Test i Low Rank Test Figure 10 2C The string constant is positioned above and slightly to the right of the File Dialog function which causes right to left data flow The VI Analyzer flags a Backwards Wire failure View full size image 322 Vl Analywrer Results Window Test ands Deuble chik alaa Gedurnere a ko Piggtabgtee thay laka ohael m haw Fiha Wi chaga since you performed iha amaiga bha arang object may ba highighbed Ta a B Style B Backwards Wires Dooumernce Ee 3 E z ES 5 E 5 in a ie a z Es 3 E 5 ki Wi ryto aye S Show Pairis On Sort by i Hh C Sort by Test i Low Rank Test z erne eot toet sere toe Ooo _ Figures 10 2C through 10 2G are the high ranking test failures In Figure 10 2C the Backwards Wires test identifies a wire that flows data from right to left In Figure 10 2D the Error Cluster Wired test locates a function with unwired error o
160. a Boolean control is not appropriate because the two states are not logical opposites per Rule 6 8 Also the diagram inefficiently computes both units all the time regardless of the actual unit selected Figure 6 12B replaces the Boolean with an enum The enum improves maintenance over the Boolean because more units can be added in the future Also the diagram uses a Case structure so that only one unit is computed based on the Temp Scale selection Additionally the control and indicator are viewed as terminals and the Express VIs are viewed as icons SubVI calls for both versions are illustrated in Figure 6 12C We see that the Boolean input of Thermometer VI is ambiguous from the caller VI s diagram The enum is clearly superior to the Boolean in this example Figure 6 12A Front panel and block diagram for Thermometer VI The vertical toggle switch for selecting temperature units violates Rule 6 8 because the two states are not logical opposites View full size image 195 Thermometer vi Front Panel Figure 6 12B Thermometer with Enum VI uses an enum to select the temperature units The diagram is more efficient because the conversion to Fahrenheit is not made unless deg F is selected 196 The sensor conversion equation is Voltage 100 degrees Celsius Figure 6 12C The subVI calls for Thermometer VI and Thermometer with Enum VI illustrate that the Boolean is ambiguous from
161. a GUI panel naturally if you capitalize the first letter of the label on one indicator you should capitalize the first letter on all This sounds like common sense but it is worth mentioning because I have seen many applications that do not use consistent capitalization The best way to ensure that your applications use consistent fonts and capitalization is to decide on your scheme before you begin your development Better yet consider using consistent schemes across most of your applications This saves time when selecting fonts ensures consistent style and improves ease of use This section provides conventions for control label text Rule 3 21 Use succinct intuitive control labels and embedded text Let us extend Rule 3 17 to controls Create succinct and intuitive labels for controls and indicators as well as any text embedded within them Challenge yourself to think of one or two simple words that best describe each control indicator or designated action Figure 3 10 illustrates this principle applied to a menu of Boolean controls The menus are functionally equivalent Figure 3 10A contains a cluster labeled COLLECT MENU and three lines of text embedded in each control including a two or three word name in all capital letters a phrase that briefly describes the action and a shortcut navigation key Figure 3 10B contains maximally succinct one word embedded labels for each control The one word labels more accurately describe the
162. a project file repository with separate top level folders for Application Build Data Documentation Graphics and LV Source The Application Build folder contains the executable and install utility The Data folder contains sample data sets generated by the application for the developer s reference The Documentation folder contains reference documents such as project specifications and instrument user manuals as well as documents that ship with the product such as help files and user manuals The Graphics folder contains image files such as screen shots company logos and icons The LV Source folder contains LabVIEW source files that are uniquely developed for the project Source files from a reuse library that are used by the project but not edited within the project are located in a separate repository for use in multiple projects This includes reusable utilities and components such as instrument drivers Figure 2 7A Folder hierarchy for maintaining project files on disk with a basic set of top level folders View full size image 47 Project Name E El Eq Fle Edt View Favorites Toos Help 3 sack amp JP search gt Fetes Ei Address E Ci Repostory Project Nama x C3 Application Build F Data e O Project Name a ia coumentaton E E Application Build aaps Ps Deta CLV Source E Project Name aliases 5 Documentation r gN Seay E Graphics bd Project Name proj E C LV Sourc
163. a task at periodic intervals Moreover continuous Timeout events cause the Event structure to behave similarly to a traditional GUI handling loop that polls the controls at timed intervals The timeout code causes latencies in the response to other registered events and reduces the overall efficiency of the Event structure Figure 8 6 illustrates a template for the Event Handling Loop design pattern The front panel simply contains a tab control with a page containing several Boolean command buttons customized for an industrial GUI a spare page and a page containing error clusters The diagram consists of the Event structure within a While Loop shift registers and a shutdown routine outside of the loop The Event structure is preconfigured with cases for each Boolean command button s Value Change event and cases for the Panel Close and Application Exit filter events A two button dialog confirms the user s decision to close or exit Additionally the Event structure is customized to promote good programming style for common applications The control terminals are in their respective Value Change event cases and the Timeout event case has been removed Figure 8 6A The front panel of the Event Handling Loop design pattern contains a tab control with a page containing several command buttons customized for use as an industrial GUI a spare page and a page containing error clusters View full size image 251 Eveni Handing Loop
164. a time This is important if the loops run at different rates and you do not want to lose any data or if the application transfers data in chunks instead of a single element at a time Additionally queues and shared variables are capable of synchronizing as well as sharing data between parallel loops Queues and RT FIFOs are sized programmatically Shared variables are sized during edit time but can transfer data over a network and incorporate a timestamp Even if these features are not important for your application today they provide more overall capability in case it is required in the future Rule 8 17 Prioritize loops using delays or thread priorities Parallel loops are very important for high performance applications including applications that are intended to run in real time Each parallel loop is prioritized using adelay Timed Loop Priority property or subVI Execution properties The simplest technique is to incorporate a delay within each While Loop using Wait ms Wait Until Next ms Multiple or Time Delay Express VI The loop with the smallest delay receives the highest priority Alternatively modularize each loop into a subVI and configure the Priority and Preferred Execution System using the subVI s Execution properties selected from File VI Properties Execution This technique involves configuring threads and a basic understanding of multithreading and hyperthreading is recommended The Priority property for a Timed L
165. aced inside the existing Case structure resulting in four layers of nesting a violation of Rule 4 28 However the slave loop itself can be modified into a flexible 120 sequencer Specifically in Figure 4 13A the slave loop has been converted into a Queued State Machine design pattern for which a queue replaces the notifier for synchronization and messaging between loops an enumerated data type replaces the Boolean case selector and the Case structure contains multiple cases corresponding to the tests The queue is similar toafirst in first out buffer that stores multiple enumerations representing the slave loop s case selections or states As shown in Figure 4 13A the master loop s Run Test event case calls the Enqueue Element function in a For Loop enqueuing the 12 cases that comprise the test sequence Additionally in Figure 4 13B notice that the File I O functions that read the Sensor Scaling data from file appear within the Initialize case of the slave loop s Case structure The Enqueue Element function to the left of the slave loop enqueues the Initialize case via an enumerated constant which ensures that the Initialize case executes first Therefore the functionality of the Queued State Machine is not limited to the test sequence but also performs initialization and other routines as required This implementation is neater and more flexible than the notifier implementation from Figure 4 11 Figure 4 13A The master s
166. actional numbers conforming to the ANSI IEEE Standard 754 1985 Pri x Be Extended precision floating point wUses 8 to 16 bytes depending on the platform to represent fractional numbers with maximum precision 5 Complex single precision floating point Fractional number containing real and imaginary parts each represented with 4 bytes of precision SGL Complex double precision floating point Fractional number containing real and imaginary parts each represented with 8 bytes of precision DBL Complex extended precision floating point Fractional number containing real and imaginary parts each represented with 10 to 16 bytes of precision EXT lt 64 64 gt bit time stamp Stores absolute time with very high precision Actually stores the number of seconds that have elapsed since 12 00 a m Friday January 1 1904 Universal Time using 16 bytes Real matrix Two dimensional array of double precision floating point DBL numeric data saved as E H a type definition Used with mathematics functions that require the real matrix type definition such as linear algebra BL Complex matrix Two dimensional array of complex double precision floating point CDB numeric data saved as a type definition Used with mathematics functions that require the complex matrix type definition such as linear algebra String A sequence of ASCII characters stored as a 1D array of bytes Can contain nu
167. ager may allocate new memory causing a latency Latencies affect the reliability of software timed tasks which may result in lost data or unacceptable jitter Rule 6 30 Limit the size of arrays by initializing to maximum length Another method of preventing memory allocation latencies is to predetermine the maximum size of all arrays including the array layers of anested data structure and initialize and maintain them at their maximum length All array operations subsequent to the initialization can then be performed using the Index Array and Replace Array Subset functions In this manner the data structures maintain a constant size and memory management is limited and predictable Use a specific uncommon initial value to help distinguish valid data from initialization data The NaN constant which stands for Not a Number is a particularly useful constant for initializing floating point numeric arrays NaN can easily be identified and searched for using LabVIEW s array functions and cannot be confused with valid measurement data Additionally all of LabVIEW s graph indicators will skip drawing any data points containing NaN Figure 6 11 contains an alternative implementation of the Torque Hysteresis VI utilizing a nested data structure to provide enhanced functionality The controls that comprise the data structure are shown in Figure 6 l11A The UUT Information and Motion Parameters clusters from Figure 6 5A have been combined to form a
168. agrams should consist of structures wires component VIs and subVIs Component VIs are very high level subVIs or dynamically loaded plug in VIs that encapsulate a major portion or subsystem of the application An application s graphical user interface and data acquisition engine implemented as separate VIs are examples of component VIs The top level and high level diagrams should contain very few low level data manipulation functions such as math array manipulations string formatting and similar functions 97 Some applications require large numbers of Property Nodes for controlling GUI behavior Most Property Node read and write operations are triggered by GUI events Consequently the Event structure is an ideal construct for handling Property Nodes Because the Event structure contains separate subdiagrams for each event case it is uncommon to run out of space However it is common to have multiple event cases that require many of the same Property Nodes with different values read or written to them in each Modularize these common Property Nodes into subVIs and pass the Control References and property values to the subVI in each location Each instance of the subVI refers to the same collection of Property Nodes in memory This substantially reduces memory use and diagram complexity Rule 4 7 Modularize the high level subVIs with lower level subVIs o Modularize low level routines into cohesive subVIs e Use or develop inst
169. ains the complete list of LabVIEW control types listed in approximate order of operational and data simplicity The controls at the top are the simplest beginning with Boolean and numerics and the controls at the bottom are potentially the most complex Note that the list is both overlapping and subjective For example numeric simplicity depends on the representation Arrays and clusters are data constructs and their simplicity depends on their contents Also a string s memory use is the same as a one dimensional array of byte integers of equivalent length Therefore the control types are approximately organized by data simplicity Because arrays have the potential to contain larger and more complex data than strings the string type is ordered simpler than array Table 6 1 LabVIEW Controls and Descriptions Listed in Top Down Order of Operational and Data Simplicity alee switches and lights that are used to enter and display Boolean TRUE FALSE values Ring Provides a user selectable list of strings text or menu ring or pictures picture ring that are associated with numeric values Enumeration Provides a user selectable list of strings that are associated with numeric values similar to a text ring except that the numeric values and string labels are part of the data type Also called an enum Tab Consists of multiple pages for controls and GUI objects and a selector for choosing the foreground page Each tab i
170. al appearance Section 3 2 3 SubVI Panel Text provides an example and some additional rules that pertain to subVIs Rule 3 3 Size similar objects the same Objects that have similar function and priority should be identically sized Sometimes it is difficult to know in advance exactly what size will work for all the related controls in a group For example in Figure 3 1 the indicators in the power clusters are related but have varying levels of precision from one to three decimals On the surface it might seem that wider controls are required for three decimals instead of for one or two Instead of having indicators of varying sizes resize all of the indicators to the width of the widest indicator This is easy using the tools in the Resize Objects ring Specifically select all the controls in the group and then choose Maximum Width Additionally you can resize groups of controls to the width or height of the smallest or largest object using the respective Resize Objects tools This section presents rules pertaining to the layout of GUI VI panels Rule 3 4 Maximize the top level VI panels for industrial applications Many industrial applications serve as a dedicated interface to the industrial equipment that they monitor or control It is usually not desirable to provide visible access to the operating system tempting users to access resources unrelated to the application Consequently most industrial applications launch auto
171. al source of latencies in all modern computing devices As memory consumption grows during the 16 execution of an application LabVIEW s memory manager is called upon to allocate new and larger memory blocks This causes a delay while the memory manager runs and results in fragmented memory for which some of the previous blocks are not efficiently used The memory manager is a wonderful aspect of LabVIEW that handles memory allocation automatically However developers should be aware of the types of operations that might cause it to run and avoid these situations from occurring unnecessarily LabVIEW contains a tool called the Profile Performance and Memory window that directly measures the execution speed and data memory of all VIs loaded in memory This tool shown in Figure 1 4 is accessed by selecting Tools Profile Performance and Memory This is an excellent tool for helping to improve the efficiency of an application Use the Profile Performance and Memory window to determine which VIs consume the most time and memory and examine those more closely You can optimize your VI s efficiency by iteratively making improvements and checking the profile metrics Figure 1 4 The LabVIEW Profile Performance and Memory window displays the data memory of Meticulous VI View full size image Profile Performance and Memory 5 Prohiie marmors ubir F Hemery Lina r TEH 1071 5 E22 8 43E 80k 4 8 4556 80 666 e066 De Cardiac DAS Paria Loop
172. ally applicable to top level VIs and subVlIs custom control Control that is customized using the Control Editor window and saved to a CTL file The instances of a custom control can be edited individually on the panel without affecting the appearance properties and data type of the source file See also type definition and strict type definition D data acquisition DAQ Involves gathering signals from measurement sources and digitizing the signal for storage analysis and presentation Data acquisition is often associated with the use of plug in analog to digital converter boards data construct A collection of one or more of the fundamental data types used to form a new data type They include developer defined constructs such as array cluster variant variable and queue as well as built in data types such as matrix error waveform and dynamic data dependency The use of the data flow principle to dictate the execution order of nodes and objects connected by one or more wires Specifically any node that receives data cannot execute until all nodes that source data have completed The receiving node is data dependent on the sourcing nodes data flow The fundamental principle of LabVIEW in which data flows along wires from the output terminals of data sourcing nodes to the input terminals of data receiving nodes A block diagram node executes when data is received at 340 all wired input terminals Upon
173. als assigned View full size image P icom Editer Wait nmSec vi error in mo error error out A Connector Assignments nh Endnotes I Color versions of the example illustrations in Chapter 5 can be freely downloaded from the Publisher s website page for this book at www prenhallprofessional com title 0131458353 For full color illustrations the electronic eBook version of The LabVIEW Style Book is available for purchase from www prenhallprofessional com title 0132414813 NI Instrument Driver Guidelines online tutorial available from the NI Developer Zone The URL is www ni com devzone idnet library instrument driver guidelines htm An icon art glossary is available from the NI Developer Zone at www ni com devzone idnet library icon art glossary htm 159 6 Data Structures The data structures of an application consist of LabVIEW s built in data types and developer defined data constructs Data types are the fundamental data elements depicted by unique terminal or wire styles on the diagram Data types define the memory size and functionality of the data Data constructs are data sets that use the fundamental data types as elements such as arrays and clusters Data structures are the data types and data constructs used by an application Data structures are defined by the developer s choice of controls arrays and clusters on the VI front panels as well as the o
174. alue a Case structure with cases for reading and writing the data and error propagation View full size image Write Data 0 0 The Continuous Loop design pattern consists of a single While Loop or Timed Loop shift registers loop timing and error trapping The design pattern is equally applicable to top level VIs and subVlIs Rule 8 1 245 Use multiple criteria for the loop condition Carefully consider the stop continue condition with continuous loops Always make certain that the loop terminates gracefully without requiring the Abort Execution tool It is a good practice to use multiple criteria which may include a user interface event such as the user clicking a Stop Boolean an error occurrence and perhaps a maximum time interval or number of iterations Never use the Abort Execution tool as the means of terminating a continuous loop Instead use appropriate termination conditions and always hide the Abort Execution tool on the top level VIs of deployed applications This is accomplished by choosing File VI Properties selecting Window Appearance from the Category pull down menu and choosing Top level application Rule 4 35 Use shift registers over local and global variables Rule 4 36 Group most shift registers near the top of the loop Rule 4 37 Label wires exiting the left shift register terminal Use shift registers with the looping structure of the Continuous Loop design pattern to store data locally and sh
175. am of the calling VI through the Context Help window The description enforces the cohesion test If you cannot summarize its purpose in two or three sentences it probably contains too much code for one subVI Icons and descriptions are discussed in more detail in Chapter 5 Icon and Connector and Chapter 9 Documentation respectively 4 2 Wiring This section covers rules for wiring including clear wiring techniques and cluster modularization 100 Rule 4 11 Minimize wire bends eliminate kinks and loops The fewer bends kinks and loops your wires have the more fluid your diagrams appear There are manual and automatic methods of wire routing and cleaning I prefer manual wire routing because the automatic methods prioritize horizontal terminal entries and avoiding overlaps at the expense of creating extra bends To minimize the bends it is necessary to route wires manually Auto wire routing can be disabled temporarily or for all new wires To disable and enable auto wire routing on the fly toggle the lt A gt key at any time after you initiate a new wire To disable auto wire routing for all new wires navigate to Tools Options select Block Diagram from the Category list and deselect the Enable auto wiring check box My preference is to first connect the wires manually and then consider repositioning nodes and individual wire segments to minimize overlaps The result is fewer bends and overlaps Additionally rig
176. an enumerated data type for the case selector and a shift register for passing the next case selection between loop iterations The cases are intuitively labeled based on the text items of the enumerated data type instead of integers This eliminates the free labels within each case Also the Classic State Machine programmatically selects the next case based on an operation performed in the previous case In this manner the test sequence is formed dynamically This implementation maximizes flexibility compared to the previous implementations Figure 4 12D A flexible sequencer is implemented utilizing the Classic State Machine design pattern An enumerated data type and shift register are utilized for the case selection The sequence is formed dynamically View full size image Test Results ajii Pressure Leak Test p AANS Fo gt Test Results gt mafiaan me e aa q zi gt Task gt kd F pate gt Limits gt H germenen ennenen ry EEEE EEEE gt Scaling gt gt Next Test gt i paisana n erreerrrererrreeer ref gt Thermal Test Ehee gt Error gt emee Li Let us now apply the flexible sequencer to a larger application Consider the master slave test VI from Figure 4 11 The slave loop is modified to perform the sequence of 12 tests from Figure 4 12 instead of the single data acquisition task On the surface it appears as though the test sequence should be pl
177. and drop the type definition onto the diagram Constants formed in this manner maintain compatibility with the type definition Rule 6 26 Always use Bundle and Unbundle By Name On the diagram use the Bundle By Name and Unbundle By Name functions to access and replace the elements of a cluster These functions provide more flexibility versus Bundle and Unbundle Specifically you can access or update any element of the cluster in any order using Bundle By Name and Unbundle By Name Also these operations do not break if your cluster changes unless the specific elements that are being bundled or unbundled are affected Hence this rule works together with Rule 6 25 to streamline maintenance Most important the name labels uniquely identify the elements of the cluster and provide excellent documentation on the diagram Because the Bundle and Unbundle By Name functions are sized according to the longest label keep the control labels succinct and intuitive Hence Bundle and Unbundle By Name enhance the maintainability and readability of the diagram Rule 6 27 Avoid clusters for interactive controls with Dialog VIs Note that the strict type defined clusters of the Torque Hysteresis VI shown in Figure 6 5A are not designed for interactive GUI behavior Rather the compression default fonts and properties make them less than ideal for GUI VIs such as dialogs Additionally there are some considerations with respect to tab key navigation I
178. and indicators automatically Specifically LabVIEW displays the appropriate number of decimals and switches between decimal and exponential notation similar to a hand held calculator Sometimes it is desirable to display a specific number of decimal places all the time to indicate the precision of a physical measurement coefficient or other parameter Unless you have a good reason such as this use the automatic formatting Rule 6 15 Show radix for hex octal or binary data Numeric controls with integer representation are formatted as decimal data by default Hexadecimal Octal and Binary formats can also be selected from the Format and Precision tab of the Numeric Properties Dialog Select Advanced Editing Mode to make the Numeric Format Codes visible When configuring numeric controls to represent data in these less common formats always make the control s radix visible This is done by selecting Visible Items Radix from the shortcut menu If the radix is not visible the data is generally assumed to be decimal format Figure 6 7 shows four different controls that may be used to write a digital pattern to an 8 bit digital I O port They include an array of Boolean and three numeric controls configured for binary decimal and hexadecimal notation respectively Although each control represents the same value the different formats for each numeric control would be interpreted much differently without a visible radix
179. ange of numeric selections first consider a ring or an enumeration followed by a numeric Select the first control type that meets the operational requirements Next traverse the corresponding column of data type compatibility from top to bottom until you identify the first data type that provides the desired functionality The result is the control and data type combination that provide the simplest operation and greatest memory efficiency Table 6 3 The Simplicity and Compatibility of Each Control Type Listed in the Top Horizontal Heading with Data Types Listed in the Left Vertical Heading View full size image 164 il 3 F 33 z ela 3 3 S LE re Bea Le em em et Peete B j 0 Tl 7 Tee T is ks T hs ks k T Ls T T is T _ Real matrix Note that the use of Rules 6 1 and 6 2 and Table 6 3 help prevent the application from generating invalid data For example you would never choose a string control for data that is strictly numeric A numeric control is much easier and more reliable for a user to operate and the data is stored efficiently Also if you have a discrete number of alphanumeric selections use an enumeration or a ring control before using a string control As we can see from Table 6 3 enumeration and ring controls are much simpler than string controls because they restrict user in
180. application specific within the Main State Machine Hardware I 0 is often the most critical task for a LabVIEW application Create a dedicated Hardware 278 1 0 Loop for each independent continuous I 0 task so that they are appropriately prioritized with respect to the other tasks Finally an Error Handling Loop provides a means of reporting all significant errors throughout the system using a single routine and recovering gracefully Event driven performance is achieved by applying a queue or an Event structure for each continuous loop and avoiding timeouts With Event structures avoid continuous Timeout events or consider removing the Timeout event case as per Rule 8 8 Additionally avoid the timeout terminal of the Enqueue and Dequeue Element functions per Rule 8 15 The unwired default prevents these functions from timing out Apply this technique to each loop to ensure event driven performance On the surface prioritizing loops using delays or Timed Loop priorities becomes less important when each loop is event driven However some loops might need to run at timed intervals and may still require loop timing Also delays prevent tight loops A tight loop is a loop that monopolizes the processor due to its priority or the type of task it performs Each call to a loop timing function such as Wait ms Wait Until Next ms Multiple and the Time Delay Express VI forces the thread to yield control of the processor to allow other t
181. are it between loop iterations Shift registers are preferred over local and global variables and functional globals because shift registers utilize dataflow Specifically the data flowis readable memory efficient and immune to race conditions Group the shift registers near the top of the loop and label the wires exiting the left shift register terminal Rule 8 2 Use a Timed Loop for highly precise or complex timing and use a While Loop otherwise A Timed Loop is used for applications with highly precise or complex timing requirements that are impractical to implement with the standard While Loop These applications are typically deterministic applications intended for a real time target For example the Timed Loop can use a hardware device or high performance real time processor to achieve 1 microsecond timing resolution You can configure the period offset and other parameters using the input node right data node and VIs from the Timed Structures palette Additionally you can assign a priority to the execution of a Timed Loop that is independent of the calling VI s priority However the standard While Loop is simpler requires less memory and processing overhead and is appropriate for most desktop applications In general use a While Loop for most nondeterministic applications and a Timed Loop for more complicated and precise timing requirements Rule 8 3 Include a delay within continuous While Loops 246 Use the Wait
182. are feasible A 30 Use shift registers over local and global variables 4 36 Group most shift registers near the top of the loop 4 37 Label wires exiting the left shift register terminal 4 38 Use looped Case structures over Sequence structures Chapter 5 Dy d Have fun creating icons Dy Create a unique and meaningful icon for every VI 5 3 Never use LabVIEW s default icons 5 4 Save VIs with subVI icons visible instead of terminals 5 5 Use a black border by 6 Combine a glyph with color and text for best style R Choose universally recognized glyphs 5 8 Use 8 or 10 point small fonts for most text 5 9 Choose a unified style for related VIs 5 10 Budget your time in proportion to intended VI reuse 5 11 Use textual foreground colored background and a black border for fastest results 5 12 Contrast the text and background colors 5 13 Choose a color convention for the type of VI 357 D 1 Have fun creating icons 5 14 Create an icon template for related VIs 5 15 Reuse one glyph color scheme and font for related VIs 5 16 Copy graphics 5 17 Avoid text and graphics not universally understood on international icons 5 18 Choose connectors and terminal assignments that promote clear wiring and proper data flow 5 19 Select a pattern with extra terminals 5 20 Choose a unified pattern for related VIs 5 21 Use the 4x2x2x4 connector pattern for most VIs 5 22 Assign controls to left terminals indicators to right terminals 5 23
183. ase the efficiency within subVIs containing functions that do propagate the error cluster If your subVI is an instrument driver for example propagating the error cluster among VISA functions ensures that an error upstream will prevent VISA functions downstream from executing However most instrument drivers perform substantial command string manipulation and response parsing in addition to the instrument communications Processing efficiency is optimized by enclosing these functions within the Case structure as well In Figure 7 12A an instrument driver subVI for a digital multimeter assembles a command using several string manipulation functions and sends the command to the instrument using a call to VISA Write This subVI contains incomplete error trapping Specifically the error cluster is passed to the VISA Write function only The error terminals on three Format Into String functions are unwired Consequently these functions execute regardless of the error status of the error in cluster In Figure 7 12B an Error Case Structure encloses all the subVI s code and the error cluster is shared among all nodes that have error terminals Additionally functions that do not have error terminals such as In Range and Coerce Select and Pick Line are now enclosed by the Error Case Structure and will not execute if an error is present Because of the lack of error terminals the Case structure is the only method that prevents these functions from execut
184. ases whether labels or captions are usually not necessary with dialogs The style of the panel and controls should be enough for the user to understand that the panel is interactive and action is required Finally a Boolean labeled Help has been added This control is used to simply launch the Context Help window allowing the user to view the VI and control descriptions The diagram of Configure Test Instrument VI is presented in Section 9 2 Figure 9 2B Configure Test Instrument VI has been improved using succinct control labels and a Help button that launches the Context Help window for viewing the control descriptions View full size image 300 gt Configure Test Instrument Resolution in Digits Resolution 6 5 digits Selections 5 0 5 5 6 0 6 5 7 0 7 5 Defout 6 5 Spady the sbeolute resolution of the measurement This parameter affects the speed of the measurement The permeable values and Corresponding measurement time depend on the selected instrumert Rafer to the selected inchrumert s documertstion for details 9 2 Block Diagram Block diagram documentation consists of text such as control labels free label comments and constructs containing text This section covers block diagram documentation techniques Rule 9 1 Keep front panel terminal labels visible on the diagram The single most underrated source of documentation is the owned labels of the controls and indicators In the previous section
185. asks to run For best results always use a delay within continuous While Loops Figure 8 22 contains a multiple loop application framework developed by Greg Burroughs Senior Project Engineer at Bloomy Controls The main front panel contains a large invisible tab control and instructions for editing The graphical user interface is comprised of multiple display screens implemented on tabs that are programmatically controlled from the diagram The diagram consists of an Event Handling Loop a Main State Machine an Error Handling Loop and a queue based messaging scheme Notice that each loop is controlled by a queue or an Event structure without timeouts facilitating event driven performance Additionally the framework contains multiple constructs for initialization configuration security application control error handling documentation and more The built in features are quite extensive and beyond the scope of this chapter Multiple parallel loops are a key feature of this framework The best single loop design patterns are combined for flexibility expandability and performance Figure 8 22A The top level VI front panel for a multiple loop application framework consists of an invisible tab control and instructions for editing and use View full size image 279 amp PROC MAME Hain Fie Parent Aegae at PROJECT TITLE Ba 1 8 lt lt Tabs are vise to the left When the appia ir needy bo depkiry Che take sand be made r
186. ata type is used to minimize development effort Figure 6 14A The Motion Parameters cluster is converted to variant and written to a section of an INI file using Write Section Cluster VI View full size image Wribe Section Cluster Ti aranom varianboonfigiih Write Section Chisher vi eaction opna refr dup retmi Section Formatted Cluster error lin ne error Error oust Writes hey value pairs to a section of the configuration data identified by raf which conresoond to the named Rens and Ihar values within the Section Fornmatbed Cluster input If Tay input is mot specified the re TEENE NTE ee ee a eet eee a Taa l i ama Eie e l a Cot error in ro o erro mal Figure 6 14B The Motion Parameters section of the INI file resulting from the operation in Figure 6 14A Torque Hysteresis ini Notepad Sei va Motion Parameters Velocity Amplitude Crpmj i0 000000 Angle Amplitude deg J i160 000000 Velocity Ramp Angle degj 30 000000 Torque Units in ozj In Oz Torque Limit torque unitsjJ d 300000 Figure 6 14C A nested cluster populates multiple sections of an INI file using Write INI Cluster VI View full size image 199 Context Help Wribe INI Cluster c peneartantconfia wertantconfiedab Write INI Chuster vi refr ENI Formatted Chsber t ero in imo aroj Writes a duster of sachon formatted custers bo configuration data dentified by refnum Section custer na
187. ates to style The total number of nodes required to perform a specific task varies based on how the developer chooses to implement the task Implementations that contain fewer nodes are generally more efficient and readable than implementations that contain more nodes For example OpenG held a public LabVIEW coding contest The requirement was to develop a VI that removed all backspaces and their immediately preceding character from an input string The VIs were informally judged in terms of icon style performance and diagram style A wide variety of VIs were submitted all performing the same task Some of the submissions were implemented with only 12 or 13 nodes others contained 25 or more nodes Figure 1 11 illustrates three submissions The implementation with the fewest nodes Figure 1 11A is easier to read and understand than implementations with more nodes including Figure 1 11B and Figure 1 11C We compare the performance of these examples next Figure 1 11A This subVI implementation uses only 13 nodes to remove all backspaces from the input string String in Figure 1 11B This implementation uses 22 nodes including nested Case structures to achieve the same functionality View full size image 25 Figure 1 11C This implementation uses 25 nodes including two For Loops and nested Case structures View full size image Performance is a particularly broad term that has
188. ation contains the desired state for the case selector as normal The variant is used to pass data from one state to another using the queue functions instead of shift registers For example in Figure 8 13 the Acquire Waveform state acquires a waveform converts it to variant combines it with the state enumerated constant with Update GUI selected and adds the cluster to the queue In a subsequent loop iteration the Dequeue Element function removes the cluster and unbundles the State and Data items Update GUI state converts the variant back to a waveform and updates the waveform indicator The variant data type maximizes the flexibility of the data that is passed between states This approach involves fewer wires than the shift register method of data sharing However the data is stored only temporarily in the queue and is not accessible in subsequent states after dequeuing By comparison shift registers maintain and share the data among all states and use wires and dataflow Therefore shift registers are generally preferred over queues for data sharing between the states of a state machine 260 Figure 8 13 The queue is used to pass data between states via a cluster containing a variant and state enumeration This reduces wires and shift registers but the data is not accessible in subsequent states after dequeuing View full size image Wee ee GEREN pais Joris a oniga p ses a l SEES m EF s Uraaa
189. bably need to see the full color version to appreciate it The VI is used to build a file path Beware 157 that the relationship between a graphic of a foot path and a file path is language sensitive The 3x3 connector pattern is used to center the single input and single output terminals However a standard 4x2x2x4 connector pattern with error terminals should be used for consistency and proper subVI error handling SubVI error handling is discussed in Chapter 7 Error Handling Figure 5 23 Dynamic VI Path Builder VI is composed of two thirds demonstrative graphic and one third text View full size image Dynamic YI Path Builder vi Connector Assignments Ful Path Out The next two examples utilize some special effects to achieve unique sizes and shapes This is accomplished by erasing the default border and drawing a new border in the desired shape of the icon LabVIEW uses the outermost nonwhite pixels to define the icon s border It is important to note that all controls and indicators designed to pass data into and out of the subVI must be assigned to connector terminals that reside within the icon s border All terminals outside the border are not visible on the calling VI diagrams Select the connector pattern that provides the desired terminal configuration within the icon s border From the icon editor select Show Terminals to display the terminal partitions along with the icon image and design the icon to fit the
190. be readily visible without scrolling the diagram window However this is not always possible even if the diagram is limited to one screen For example the terminals may be hidden 103 within the frames of a multiframe structure In this situation be sure to label the wire in the frames or areas where the source terminal is not visible While limiting wire lengths is desirable long wires are preferred over no wires Never use local or global variables as a method of reducing wire clutter Variables increase processing overhead memory use and complexity Moreover variables undermine LabVIEW s dataflow principles by obscuring the actual source of the data When variables are written and read from more than one location on the diagram it becomes difficult to determine what is actually affecting the data When wires are used it is easy to trace the data to its unique source terminal If the wires are long or the data source terminal is hidden label them Indicate the name of the wire s data source so that it is readily apparent when you view the diagram Use the greater than sign gt to reinforce the direction of r data flow Considerations with respect to local and global variables are discussed in Section 4 3 Data Flow Rule 4 19 Place unwired front panel terminals in a consistent location Place any unwired terminals of front panel objects in a consistent location on the diagram so that developers can find them eas
191. be wired together among the VIs of the driver are comprised of separate type definitions for the controls and indicators Wiring different type definitions together causes a coercion Consequently the driver VIs violate Rule 6 3 However the coercions are performed on scalar data which are trivial operations that will not cause latencies 194 I would like to add one final note regarding nested data structures Some references recommend avoiding nested data structures entirely I disagree with this recommendation As seen from the previous example nested data structures can be effectively applied to organize data increase functionality reduce wire clutter and improve maintenance Performance is controlled by paying attention to where and how the data is manipulated When applied properly the organizational and functional benefits of a nested data structure offset the performance considerations More examples r are presented in Section 6 4 Examples 6 4 Examples This section presents several more examples of proper and improper use of data structures Thermometer VI shown in Figure 6 12A is a subVI that acquires a voltage sample from a thermistor and converts it to engineering units You may recognize it as a classic LabVIEW training exercise on how to create a subVI It uses a vertical toggle switch labeled Temp Scale to select between units of degrees Fahrenheit and degrees Celsius Although there are only two states
192. ber of VI panels as a control or indicator or diagrams as a constant by either dragging and dropping from the project tree or choosing either Select a Control from the Controls palette or Select a VI from the Functions palette All instances of the typedef maintain the data type specified in the typedef s CTL file Therefore multiple instances of the control are maintained from one location via the Control Editor window This is extremely beneficial for clusters because most clusters are used in multiple places and the contents are subject to change For example in Figure 4 5C new parameters are added to or removed from the Wavelength Scan Params cluster by simply adding or removing the corresponding controls to the typedef In Figure 4 5D all of the subVIs containing the typedef will automatically update to match the revised cluster A strict type definition also known as strict typedef maintains the control s properties in addition to the data type in the CTL file so that all instances of the strict typedef maintain an identical appearance and behavior I prefer the strict typedef because I often find that I need to maintain a common range default value and appearance 107 among instances To specify the type definition status choose the corresponding item from the Typedef Status ring control on the Control Editor window s toolbar Clusters and type definitions are discussed in greater detail in Chapter 6 4 3 Data Flow This
193. black text on a light blue background each containing the word GET across the top followed by the name of the configuration parameter across the bottom The report generation icons resemble a page on a brown border containing some text The unit conversion icons contain a two colored bidirectional arrow with the measured parameter in capital letters across the top and the word UNIT in the middle Hence graphics color and fonts are used to associate each set of related VIs and text is used to distinguish the specific purpose of each VI Figure 5 4 An icon convention with unified styles for several sets of related VIs View full size image 137 Pressure Instrument YIs Flow Instrument Is Configuration File VIs Se eres a Rule 5 10 Budget your time in proportion to intended VI reuse Budget the time and effort for icon development in proportion to the intended scope of VI reuse If you are creating a developer toolkit or instrument driver for distribution as a product or shareware it is worthwhile to invest significant time and effort to create a well conceived icon convention If you are developing a very large application with several hundred subVIs a tight deadline and no specific plans for reuse you might need to apply the most expedient method possible for creating unique and meaningful icons Icon shortcuts are discussed next Creating really cute icons from scratch is an art However some developers a
194. c and others are general purpose and reusable In theory one might apply a different level of development effort depending on the intended scope of subVI reuse In practice it is beneficial to design all subVIs expecting them to be reusable to help ensure consistent quality and promote reuse In a few places I differentiate rules for commercial subVIs that are part of a product such as a developer toolkit or an instrument driver 56 and standard subVIs which are assumed to be reusable from a stylistic perspective but are not commercial If the subVI scope is not specified as standard or commercial I am referring to all subVIs both standard and commercial This chapter presents front panel style rules and examples that conform as well as violate the rules However I make no attempt to discuss how to design and implement a graphical user interface I recommend LabVIEW GUI Essential Techniques by David Ritter as a comprehensive resource on GUI development Additionally if you are developing a commercial application for a specific operating system I recommend reading a style guide for the target operating system The objective of this chapter is to provide rules to make your front panels intuitive consistent and conforming to industry standards 3 1 Layout Layout pertains to how objects are arranged on the front panel Layout influences ease of use both for users who operate GUI VI panels and for developers who access subVI
195. cations Most LabVIEW applications begin witha business objective that involves an industrial measurement and automation challenge Business objectives include accelerating research and development increasing production throughput improving quality and reducing manual labor Most people use LabVIEW because it is the fastest method of developing applications for achieving their desired business objective There are many other benefits people consider such as the graphical paradigm open support for thousands of instruments platform portability and network connectivity However most industrial decisions are justified by considering the savings of time and money As such most industrial users choose LabVIEW because of the rapid software development cycle LabVIEW provides the opportunity to develop applications very quickly High level tools such as Express VIs instrument drivers templates and examples empower the user to develop software with very little planning or preparation The speed with which you can connect to one or more instruments and begin making measurements is extraordinary Additionally rapid code development provides developers and managers often under significant time pressures instant gratification As aresult many LabVIEW developers adopt fast programming habits Shortcuts are routinely taken Any development steps that are deemed unnecessary are frequently skipped Requirements specifications development co
196. ce conflict or a network directory path might change Even if the application is completely bug free at the time of deployment it will not overcome the unexpected without an effective error handling scheme Moreover how quickly the developer and the application s users can identify and correct any problems that arise depends on how well the application incorporates error handling Theorem 7 1 Error handling is essential for troubleshooting applications overcoming the unexpected and implementing good style Error handling is an essential part of all LabVIEW applications and serves multiple purposes from development through deployment First error handling is imperative for debugging an application When the application reports an error we can quickly identify and correct many problems based on the error data Error handling helps identify and describe errors long before we might otherwise observe the application misbehaving Debugging an application without error handling is analogous to graphical programming wearing a blindfold It is extraordinarily difficult to locate the source of most problems without good error handling After an application has been debugged error handling helps test the limits of the application s capabilities During testing one may experiment with the maximum number of channels fastest acquisition rate most demanding analysis routine fastest user interface update and disk streaming ra
197. ch call Load and retain on First call Torque s Angle Improve the long term maintenance and performance of the Torque Hysteresis application using the Dynamic Framework In the implementation shown in Figure 8 21 all of the primary subVIs including the dialogs as well as the high level VIs are implemented as components This maximizes flexibility in terms of upgrades The cluster of motion parameters is merged with miscellaneous parameters required by each component into a new cluster of configuration parameters These parameters are initialized from a config file during the Initialize state Each component has been modified to receive and return all data structures as inputs and outputs whether they are required by the component or not Finally one pair of variant control and indicator terminals is included for future expansion These enhancements maximize the flexibility of the functionality that exists within each component VI while maintaining standard connector assignments for all components Figure 8 21A The front panel of the Dynamic Framework applied to the Torque Hysteresis application The component Run Test VI is loaded in the subpanel control View full size image 276 Dynamic Framework for Torque Hysteresis wi Fle Edt Wew Project Operate 32 W Ey sek i CCCI m AN ili MAV
198. chain Outside the Case structure the error cluster s status is unbundled and if an error exists the Error is passed into the shift register on the right border and the Error case executes next The Error case logs the error and time stamp to a datalog file and clears the error from the error cluster Figure 7 24 The test executive has been modified to implement proper error handling Errors are trapped by propagating the error cluster to all nodes that have error terminals and are reported by logging to a datalog file in the Error Case View full size image Tre E o FT io o Jarmil Dip Cape gd beer Tesk Fie eee Fh TEDT Enp W ar cr A dEr a EE b rae A Li a acl m E iuf Poptart 2 ae ee e TSS T SoS 1ST Notice that the Initialize case now contains the intermediate level File I O functions in place of Read Lines from File VI This allows the error cluster to propagate in a manner that is consistent with the rest of the application and to avoid any dialogs from popping up during unattended operation Finally note that the error in control and error out indicator are not required in this example because it is a top level VI However it is a good practice to write all VIs assuming that they might be used as a subVI and may require propagating the error cluster For a GUI VI the error clusters can be placed outside the visible area of the
199. changes 12 Do not modify the functional source code during the peer review 362
200. ck upon which a large application is constructed Most applications of advanced complexity are best implemented using a combination of multiple single loop design patterns to take advantage of the benefits of each while optimizing the overall performance Compound design patterns are created using a combination of two or more of the single loop design patterns and adding a messaging scheme to pass data between loops The messaging scheme adds complexity Hence good development tools examples and templates are beneficial Develop applications employing compound design patterns either using compound templates or by merging the templates of the single loop design patterns and adding the messaging It is convenient and quite clever to have each of the single loop design patterns available for selection from the Functions palette for fast and easy access similar to the structures on the Structures subpalette This is accomplished by creating a custom palette view and adding an icon for each single loop design pattern template to the Structures subpalette Configure the submenu icons to merge the contents of the corresponding design pattern template into the target VI instead of placing the icon as a subVI call Developing compound design patterns becomes as easy as click drag and drop The custom palette view in Figure 8 17A contains icons representing four of the single loop design patterns Figure 8 17B shows the outline of a design pattern as it
201. code set the Block Diagram Warnings Globals and Locals Maximum Number for Write and Read Globals and Locals to 0 each This helps you examine every instance of a variable to ensure that they are all necessary If your VI is large and complex and contains multiple variables and coercions consider creating separate configuration files for examining only the variables and only the coercion dots respectively This enables you to focus on one issue at a time Figure 10 1 The VI Analyzer s interactive window is configured to run a test using customized settings The Coercion Dots test is configured for a Maximum Number of 0 to identify every instance of this style rule violation View full size image 318 gt Vi Analyzer Select Tests Select the checkbox next boa best or group to include it in the analysis Configure a selected best with the options under Selected Tests Configuration Tests Selected Test s Configuration le Ti E v Block Diagram i De TEREA Check for coercion dots on fr Arrays ond Strings in Loops gt Block Diagram Coearcon Dots Enakied Debary Single Wire Wak in While Loop fv Wired Terminals in Subdisgrams E Style E F Warnings Ignore coercion dots on Ge Documentation f F Front Pane C warisnts E F sv SE Aan E F User interface C Sealer Datatypes Clipped Text _ Diskog Controts Duplicate Control Labels fer Empty List Items we rm ee lt a
202. comes visible Each data structure is an array of cluster containing one element per SCXI module This allows each module to be configured independently Figure 6 16B shows the data structure used to configure strain gages Its appearance is controlled programmatically using Property Nodes Also on the front panel the array index control is normally not visible and the control s background color is transparent Figure 6 16A The GUI VI panel for a highly configurable centrifuge data acquisition system View full size image b Centrifuge Gi System Configure ktoenitor Acquire Arnalyre Semo Sekcion Strain Gage hiedie 1 Selling ea r LFUTE ROM Chassis Addens Pp S030 Slot Number T no ENT Pai Ser i Line fie fis _ ne teamin iem sena Scaling Eng Enput Limits ing UnA fartation Miter Preke Derida Fib ai its Hah H Taiki i ceed gaa al r 5 SL Tai m ee bru ee se a TI O A E a EE SS Se Se Makai oS M TC Ce E S E Eo oo ee fiee 6 E Tce Diber See tire Figure 6 16B This array of cluster containing strain gage configuration parameters is one of seven data structures used on the Configure tab to configure a corresponding SCXI module type On the GUI VI panel the array index control is not visible and the background is transparent View full size image SCX1 Chas address T Sted stot samber a au fF miao Faen Location crm Ler Scalia Comp Mrak Lindt
203. completed a project for a demanding customer Multiple changes in the requirements have resulted in a much longer and more difficult development cycle than you envisioned The project is well over budget and is significantly past due You are going through final checkout and acceptance and the customer asks Where is the documentation It is something that was never previously discussed as one of the requirements but is often expected as part of the software Does this sound familiar Do not panic Your source code is well documented per the style rules in this chapter You started with a specification and design document as per the rules in Chapter 2 You have detailed VI and control descriptions throughout your VIs Your diagrams have appropriate labels comments and text All you need to do is Print VI Documentation Almost Print VI Documentation is a built in tool for generating documentation for a VI It extracts various pieces of documentation from the source code depending on the user selected fields and creates a document in HTML Rich Text Format RTF or Plain Text TXT file format Alternatively it prints a hardcopy of the documentation to a printer This tool can be run interactively by selecting File Print VI Documentation or programmatically using the VIs in the VI Documentation sub palette located under Programming Report Generation Also VI Server provides programmatic access to the properties of each VI including e
204. contains the same VI with multiple improvements However the error cluster propagation is deferred until Chapter 7 Error Handling Figure 4 20A Screw Inspection VI is neat with cute icons and a staircase wiring appeal At first glance it appears to have satisfactory layout wiring and data flow View full size image Figure 4 20B Close inspection reveals multiple violations of the block diagram style rules View full size image 129 Unlabeled long wires wires exiting shift registers Right to4eft dataflow Dis connniuous error cluster Kink Figure 4 20C Multiple improvements are made to the VI Error cluster propagation is discussed in Chapter 7 View full size image e Layout The diagrams in Figures 4 20A and 4 20B are sufficiently dense and occupy less than one screen One overlapping label exists e Modularity There are 7 user VIs out of 292 nodes for a modularity index of 2 4 e Wiring Scheme Wiring appears neat overall Clusters help minimize clutter However there are unnecessary bends and kinks overlapping objects and unlabeled long wires including wires exiting shift registers e Data flow Most data flows like a staircase from left to right and top to bottom Shift registers minimize variables However error handling is discontinuous and some right to left data flow exists 130 The optical filter test application introduced in Section 4 2 2 is continu
205. cts such as shared variables occurrences notifiers and queues If one simple and unambiguous data item is being written with only one or two instances of a write global variable and on only one occasion throughout the application the global variable is a good choice It is unnecessary to set up a shared variable occurrence notifier or queue for such a limited scope In Figure 4 9A two single frame Sequence structures are placed on each side of the While Loops and wires are used to create data dependency between each structure The large Sequence structure on the left initializes the variables and passes two wires out of tunnels to the While Loops Because data will not pass through the tunnels of the Sequence structure until all its operations have completed via the principle of data flow the write local variable operations must occur before the data propagates through the tunnels Because the While Loops receive data from the initialization Sequence structure the While Loops cannot begin executing until the initialization Sequence structure has fully completed This condition is known as data dependency Notice also that the Boolean data that is passed to the border of the Event Loop is not actually used within the loop Instead the wire terminates on the While Loop s border Nonetheless this wire ensures that the Event Loop begins executing only after the initial Sequence structure completes This is known as artificial da
206. cumentation Front Panel and General The Block Diagram category tests several of the rules listed in Chapter 4 Block Diagram such as right to left data flow backward wires bends and objects overlapping wires Additionally several tests identify more subtle issues that are not directly associated with style These include unnecessary elements on the block diagram that cannot execute referred to as dead code and hidden objects within structures Documentation tests inspect several of the rules in Chapter 9 Documentation including VI and control descriptions and comments in free labels Additionally the VI Analyzer can check the spelling of text on the front panel and block diagram Front Panel tests inspect several of the rules in Chapter 3 Front Panel Style including control alignment and overlapping controls Additionally the VI Analyzer can check the default values of list items and arrays which are common sources of inefficient memory usage The General category contains tests for the icon and connector file properties and VI properties The icon and connector tests correspond to several of the rules in Chapter 5 Icon and Connector File and VI properties include tests for the VI s saved version of LabVIEW and the compatibility of properties and methods with a built application The LabVIEW VI Analyzer Toolkit was introduced with LabVIEW version 7 0 By this time Bloomy Controls had a well es
207. cycle is controlled while torque and angle data is acquired during the Run Test application state using data acquisition routines within Run Test VI When this subVI is called it opens its panel and graphically displays live torque versus angle data throughout the motion cycle When the test completes the data is saved to file and a report is generated However it is desirable to acquire and display the torque and angle data continuously throughout all states of the application Specifically live digital indicators for torque and angle on the front panel of the top level VI as shown in Figure 8 18A allow the user to monitor these important parameters without initiating a test This helps the operator set up the test fixture and unit under test manually apply a torque and monitor the response and confirm when the force is removed for safety purposes Figure 8 18A The top level front panel of Torque Hysteresis VI contains digital indicators with live display of the torque and angle data This facilitates continuous monitoring of these important parameters View full size image 269 gt Torgue Hysteresis Parallel Loops vi Front Panel A R Continuous acquisition and display is accomplished by adding a second Continuous Loop on the top level diagram adjacent to the Event Driven State Machine as shown in Figure 8 18B The data acquisition routine is extracted from the Run Test VI and runs in the parallel loop This allows co
208. d overlapping wires it is not necessary to highlight junctions My personal opinion is that dots at wire junctions appear a tad obnoxious on a diagram with clear wiring and efficient data flow Because wire junctions can be identified by triple clicking on a wire of interest I recommend disabling dots at wire junctions This selection is available from Tools Options select Block Diagram from the Category list and deselect Show dots at wire junctions Rule 4 27 Avoid Sequence structures unless required Avoid using Sequence structures unless they are required In particular never use a Sequence structure to force the execution order of functions that can execute in parallel This is a common tendency of former text based programmers that are accustomed to textual statements executing in the order in which they appear Challenge yourself to avoid Sequence structures Instead try to keep independent functions and routines parallel to each other For example the Flat Sequence shown in Figure 4 8A is unnecessary Additionally the variables are unnecessary The diagram shown in Figure 4 8B is functionally equivalent but cleaner and more efficient Specific rules on the practical and impractical use of Sequence structures and variables are discussed in the sections that follow Figure 4 8A This Flat Sequence structure is unnecessary because the contents need not execute sequentially View full size image baa alae alae alae la epa aaa ahala
209. d and overlapping Known as the Continuous Loop design pattern this is discussed in Chapter 8 If the scope of the application expands the user either increases the size of the While Loop which is already larger than what can be displayed on one screen in high resolution or adds to the confusion within the existing area Creating new wires and nodes within this framework would be tortuous The front panel also has maintenance considerations Adding a new station for example involves replicating the station cluster and the station specific controls located outside the cluster However the front panel cannot support an additional station in its current form Fortunately a tab control can be integrated to provide substantial improvements with minimal effort You can dedicate a separate tab for each station and then add and remove tabs as needed In addition you can incorporate the station specific controls outside the cluster into the station cluster and save the cluster as a type definition This way any changes to the type definition are applied to all stations that use the type definition Chapter 3 discusses tab controls and Chapter 6 covers type definitions The style rules presented throughout this book help ensure maintainability A LabVIEW application is robust if it is bug free and never crashes LabVIEW provides fundamental constructs that promote robust applications including subVIs and error handling as well as fundamental element
210. d in Chapter 1 we observe several violations of the left to right dataflow rule including the wires highlighted in Figure 4 7 We also see unnecessary bends and kinks 4 11 overlapping wires and nodes 4 15 multiple local variables 4 17 and many other issues contributing to the spaghetti effect Figure 4 7 The diagram of Spaghetti VI contains several wires that violate the left to right dataflow rule View full size image 108 Staten coro Seaton 1 contre E Le a i i TEA TENEO Left to right data flow is accomplished by first positioning the order dependent nodes from left to right so that interconnecting wires naturally flow left to right In most cases the order of execution is determined by data dependency Simply propagate one or more common data elements such as the error cluster between the functions that require a specific execution order This is similar to what is illustrated in Figure 4 4 Rule 4 24 Avoid array and cluster coercions Coercions are the unattractive dots that appear on terminals when there is a numeric representation mismatch By default they are colored gray prior to version 8 2 and red subsequent to version 8 2 but you can specify their color by selecting Tools Options choose Colors from the Category list deselect use default colors and click on the color next to Coercion Dots Coercions indicate that LabVIEW is converting the data from one type to another It is a
211. d or unbundled are affected Most important the name labels uniquely identify the elements of the cluster and provide useful documentation This is further proof that control labels are an important source of documentation on the diagram Because the Bundle and Unbundle By Name functions are sized according to the longest label keep the control labels succinct and intuitive Configure Test Instrument dialog VI is continued in Figure 9 4 The block diagram in Figure 9 4A contains no documentation whatsoever Specifically all of the front panel terminal labels are not visible and free label comments are missing from the Case structure s cases and wires exiting shift registers Additionally the Bundle function contains terminals that appear ambiguous The diagram in Figure 9 4B contains multiple improvements versus Figure 9 4A First the front panel terminal labels are visible Additionally there are free labels on wires exiting shift registers and free label comments in each case of the Case structure Notice that the diagram in Figure 9 4B is significantly wider than the diagram in Figure 9 4A This is due to the lengthy terminal labels Perhaps this is the reason some developers hide their control labels on the diagram Remember to keep the control labels succinct and intuitive In particular Please Select Instrument to Configure is an unnecessarily lengthy label Additionally notice that the terminal label style raised versus t
212. d right perimeters are Boolean controls with customized appearance and embedded text A knob is used for rapid menu and waveform navigation The main display area resembles a collage of indicators suspended on a black background It is divided into two sections a large subpanel control on the left and multiple overlapping indicators on the right Twelve preset display configurations can be programmed by dynamically loading any one of twelve subVIs into the subpanel and making a subset of the overlapping indicators visible The indicators are customized with a transparent background overlaid on a black decoration giving them the appearance of floating in space The diagram see Figure 1 1 is easy to follow because all data flows through wires from left to right with no more than one bend to wrap around structures and objects The diagram is extensively documented including visible terminal labels free labels on long wires and within each structure and enumerated case selectors Figure 1 6 The front panel of Meticulous VI in edit mode It contains customized Boolean controls along the bottom and right perimeters a knob a subpanel control and multiple overlapping indicators View full size image E Haik ura Ye es a Doe fe LE Yee mbi ieri Peet idee Geb Sl dig Rica rs Pe oof a li eh Ha Sda d 14 PAM DSATE BALA H 19 The front panel of Nested VI see Figure 1 2 is very simple and readable The objects o
213. d using the suffix in for the inputs and out for the outputs The input array label has the prefix input instead of output The VI has a unique and meaningful icon that depicts what the VI actually does It utilizes the standard 4x2x2x4 connector pattern with plenty of spare terminals available for future expansion Figure 5 14B SubVI from Selection w Cleanup VI contains a meaningful icon intuitive terminal labels and the standard 4x2x2x4 connector pattern View full size image Sub 1 from Selection w Cleanup vi P Connector Assignments task ID out error out On more than one occasion I have seen source code in which the developer creates icons by drawing the text freehand instead of using the Icon Editor s text tool Figures 5 15 and 5 16 are two such examples Figure 5 15A is a subVI extracted from the Haphazard VI from Chapter 4 We can see that it also contains an output assigned to a terminal on the left and the data acquisition task reference has been bundled with the error cluster into one input These elements are unrelated and should not be bundled together Figure 5 15B is a revision containing an improved icon connector pattern and terminal assignments The icon utilizes 8 point small fonts for the complete words DIGITAL INPUTS and a glyph of a pair of glasses copied from the DAQmx Read VI The standard 4x2x2x4 connector pattern has been applied with the DAQmx Task passed through the top left and right terminals
214. data file using Microsoft Word or Excel Note that prototyping candidate display screens and reports generally entails some design effort which falls outside the pure definition of a requirements specification In an ideal world a solid boundary exists between the specification and design phases However in practice prototype designs are instrumental in fleshing out some of the functional requirements In addition the requirements specification should be viewed as a living document that is revised as necessary throughout the design and development phases Therefore the level of detail contained within the requirements specification might evolve with time Indeed a requirements specification might eventually form the basis of a user manual Be sure to save each revision or you might lose sight of the original requirements and fall back into the Code and Fix development model Source code control tools ensure that each revision of the project documents as well as source code is automatically archived in a repository The test methodology describes how the system is tested A test scenario is conceived that verifies the correct operation of the software and hardware Ideally every required feature has a corresponding test that isolates the feature and verifies that it functions properly Additionally a series of use cases can be performed in which the most common uses of the software are defined and executed under controlled conditions For a tes
215. de the SubVI with Error Handling Dialog Using Events and Standard State Machine These templates are all available from LabVIEW s New dialog box which appears every time you select VI or Project from Template from the Getting Started window or New from the File menu The New dialog box is shown in Figure 2 6 The SubVI with Error Handling consists of a front panel with the error in and error out clusters assigned to the lower left and right connector terminals and a block diagram containing a Case structure with error in wired to the selector terminal The developer adds other controls and indicators to the front panel assigns them to appropriate connector terminals and creates and wires together diagram objects inside the No Error frame of the Case structure This template might save only a couple minutes of development effort but it can be applied repetitively reducing tedium Dialog Using Events is a VI that opens its front panel when called and prompts the user with any data or selections provided on the panel Its Window Appearance property is configured as Dialog and it contains two dialog box style Boolean controls labeled OK and Cancel The diagram consists of a While Loop that contains an Event structure that sleeps until the Value Change event fires on either control which stops the VI If all your dialog boxes originate from this template your application s graphical user interface will exhibit a style and behavior that i
216. der to the receiver In Figure 4 11A the Event Loop s Run Test event case calls the Send Notification function along with a Boolean TRUE instructing the DAQ Loop to acquire data The DAQ Loop s Wait on Notification function wakes up and returns the TRUE value from the notification output terminal to the case selector and the Acquire routine runs When any event fires that causes the Event Loop to stop the Release Notifier function is called outside the Event Loop For example if the user clicks the Quit Boolean control the Quit Value Change event case of the Event Loop fires as shown in Figure 4 11B This causes the Event Loop to stop and the Release Notifier function runs The DAQ Loop s Wait on Notification function immediately wakes up and returns a FALSE value from the notification output terminal and an error from the error out terminal The DAQ Loop executes the Case structure s False case which clears the error and stops the loop Hence the Event Loop fully controls the DAQ Loop without variables Also notice that an error in either loop stops both loops without variables Specifically the error status is unbundled and wired to each loop s conditional terminal A TRUE error status in the Event Loop stops the loop and calls the Release Notifier function terminating the DAQ Loop as previously described Additionally a TRUE error status in the DAQ Loop terminates the loop and the Property Node outside the DAQ Loop runs This Pr
217. developers edit source code within the LabVIEW environment These selections are confusing for end users who are unfamiliar with LabVIEW Instead create custom runtime menus for the top level VIs that are relevant to the application Eliminate all the developer items and provide only the specific items required for the user to operate the application At a minimum always include a Help menu with functional selections for providing the user help This might include Show Context Help as well as one or more online documents The Show Context Help menu item opens the Context Help window which displays any descriptions available from the controls indicators and VI as the user scrolls the mouse over them Compiled help files are documentation files compiled into CHM format as discussed in Section 3 2 2 Control Labels Custom runtime menus are configured by selecting Edit Run Time Menu Menus are not appropriate with dialog VIs Instead provide Boolean command buttons for any required menu selections This includes a Boolean for Help or that can launch the Context Help window or an online document Also create shortcut menus for any actions related to a specific control You can create shortcut menus by selecting Advanced Run Time Shortcut Menu Edit from a control s shortcut menu You might not apply or agree with every rule I have presented in this chapter and that is fine GUI style in particular is a subjective topic
218. diately became clear that this was impractical and a nuisance for many applications during the early stages of development prior to the first release This rule was abandoned less than two weeks after it began An example of a rule that some consider a hindrance but that I adamantly require among all of my staff is writing VI descriptions for every VI VI descriptions are a critical source of documentation I have seen employees customers and colleagues try to save time by saving this simple snippet of documentation for later because they re trying to get a piece of production machinery online or meet some critical deadline However more often than not later is really never Even with the best intentions the same time pressures that cause one to skip the descriptions in the first place usually linger around later Alternatively if we do return to our code and write the descriptions later it is much more time consuming because we have to locate the undocumented VIs one at a time and refresh our memories about their purpose It is much less time consuming and more reliable and efficient to write the description immediately after the VI is written or revised and it is fresh in our minds This practice ensures that the descriptions are written promptly accurately and efficiently Chapter 9 covers documentation including VI descriptions The style rules presented throughout this book are practical to implement after they are learned and applied
219. ding the GUI appearance and budget your time accordingly For commercial applications a professional GUI is important and effective color schemes are often a high priority However the primary purpose of most LabVIEW applications is the underlying tests measurements and control Be sure your application functions properly before you expend excessive time and effort on color schemes My preference is to identify develop and test the requirements in order of priority Also I often start with a combination of colors that I have used successfully in the past and if time permits after the critical requirements have been satisfied I add further enhancements Section 3 5 Examples presents specific examples of GUI VI and subVI panels Because this book is printed in black and white you cannot see the full effect of the color examples Color versions of those example panels can be freely downloaded from the Publisher s website page for this book at www prenhal lprofessional com title 0131458353 An electronic eBook version of this book containing full color illustrations is available for purchase from www prenhal lprofessional com title 0132414813 3 4 GUI Navigation Theorem 3 1 Reliability is the developer s responsibility GUI navigation pertains to how the user interacts with the controls and menus of a GUI VI to operate the software If you are developing an application for use by anyone other than
220. e s error out terminals are unwired such as when nodes have been prioritized by risk as discussed in the previous section However most error handling problems I have seen in practice are the result of incomplete error handling in which many of the error terminals are wired but the error is not fully trapped or reported Automatic error handling does not help in these situations Rule 7 18 Leave automatic error handling disabled If you are deploying an industrial application the last thing you want is the application to suspend itself open the diagram and highlight a portion of the source code This invites anyone who happens to be in front of the computer when the error occurs to tinker with the diagram Hence automatic error handling must be disabled before application deployment Additionally note that automatic error handling circumvents programmatic error handling Per Theorem 7 1 programmatic error handling is essential for overcoming unforeseen problems before during and after deployment Moreover programmatic error handling is a principle method of establishing data flow and a key ingredient to good style Therefore use programmatic trapping and reporting of errors and leave automatic error handling disabled 7 5 Examples This section contains more error handling examples including loops user defined error parallel error chains and a state machine Cont Acq amp Graph Voltage To File Binary VI is a shippin
221. e industry and application type Specifications written according to these standards are very thorough high quality documents In some evolutionary industries the rapid pace of changes or time to market pressures pose challenges to the process of writing formal requirements specification documents In many situations it is desirable to have the software developed in parallel with product development Many of the performance specifications that affect the application s requirements are unknown or change frequently during application development These situations do not justify abandoning the requirements specification process Instead a very aggressive specification development process should be pursued in 32 parallel with product and software development Specifically Agile project management methods such as Extreme Programming are best suited to evolutionary projects At Bloomy Controls we dedicate a resource to the specifications development and maintenance effort separate from the software to gather and maintain the requirements in an iterative manner The requirements specification need not finalize requirements that are evolving Instead try to determine most probable and worst case scenarios for the undefined requirements and include them in the specification If this is not feasible simply use TBD and come back to it later In many government regulated industries however TBDs are expressly forbidden New projects
222. e 6 3 and should be avoided Two categories of numeric data types exist integer and floating point number Integers are used to represent whole numbers while floating point numbers are required for fractional data Rule 6 13 Use 32 representation for integers and DBL for floating point numbers Section 6 1 1 Choose the Controls and Data Types discusses the significance of simplicity memory efficiency and data type consistency Per Rule 6 3 it is important to maintain consistent data structures throughout an application In most situations use 4 byte signed integer 132 representation for integer data types and 8 byte double precision DBL representation for floating point numbers Anything with physical units such as voltage power frequency length angle and similar quantities should be DBL LabVIEW s numeric functions can accept and operate on any numeric data types Polymorphism is a term that describes a function or VI with one or more terminals that can accept more than one data type Polymorphic functions and VIs adapt to the input data type instead of breaking the wire or forcing a coercion to occur on the input terminal More important the function completes its operation successfully Most of LabVIEW s built in functions VIs and constructs that are not polymorphic consistently use 132 for integers and DBL for floating point data For example the iteration and count terminals of looping structures are 32 as are
223. e 6 7 the data from Figure 6 3B is grouped into the clusters shown in Figure 6 5A The diagram of Torque Hysteresis VI is implemented using clusters as shown in Figure 6 5B Specifically the eight parameters of UUT information are bundled into a cluster named UUT Information Five motion control parameters are stored in a cluster named Motion Parameters The Motion Parameters cluster is passed to Run Test VI Upon test completion the VI stores the raw data acquired during the test into a cluster named Torque vs Angle Next the Motion Parameters and Torque vs Angle clusters are both passed to Compute Statistics VI The computed results are stored into a cluster named Statistics Also Compute Statistics VI passes the Motion Parameters and Torque vs Angle clusters unmodified to Generate Report VI This VI receives the UUT Information cluster from the UUT Information Dialog and creates the report Finally all clusters are passed to Save Data VI where the data is saved to disk Figure 6 5A The 21 parameters from Figure 6 3B are grouped into four clusters View full size image 173 Figure 6 5B Top level diagram of Torque Hysteresis VI is implemented using clusters The diagram appears neat and orderly and maintenance is simplified View full size image In this example 21 parameters have been logically grouped into four clusters All data is passed among subVIs using terminals and wires All subVIs use the standard 4x2x2x4 connector
224. e VI property category they are also programmable using VI Server s VI Class properties If your GUI is comprised of multiple panels that are designed to open adjacent to each other for simultaneous viewing create a routine that programmatically adjusts the panel positions Use VI Server functions to programmatically read the Display All Monitors application property to determine the host computer s display resolution Reposition and resize the panels and objects according to the resolution using VI Server functions and front panel window VI properties Rule 3 7 Use LabVIEW s dialogs for desktop applications avoid them for industrial applications LabVIEW s native dialogs are conveniently accessed from the Dialog amp User Interface palette They include the One Two and Three Button Dialog functions as well as Prompt for User Input and Display Message to User Express VIs The LabVIEW dialogs are simple to use and designed to resemble the dialogs native to the host computer s operating system Hence they are very useful for platform portable desktop applications However their appearance cannot be customized Also the dialog windows and objects are relatively small compared to an industrial VI panel with 3D or classic style controls and larger panel size The LabVIEW dialogs can be difficult to read unless you are sitting right in front of the monitor and are paying close attention Consequently the LabVIEW dialogs are n
225. e cluster including size and font and arrange or compress the contents Specifically arrange the elements in compact vertical form in top down order of the cluster order numbers by selecting Autosizing Arrange Vertically from the cluster s context menu Alternatively select and arrange any subset of elements in compressed vertical or horizontal order by selecting Vertical Compress or Horizontal Compress from the Distribute Objects toolbar menu These techniques help reduce the front panel real estate occupied by large clusters Figure 6 9 shows the Motion Parameters cluster that is used throughout the Torque Hysteresis VI The controls and labels within the cluster are independently aligned using the Right Edges and Left Edges alignment tools respectively The controls are evenly spaced and compressed using the Vertical Compress tool Each control contains a description including the Velocity Amplitude as shown in the Context Help window Figure 6 9 Motion Parameters cluster from Torque Hysteresis VI with the Context Help window displaying the description for Velocity Amplitude The cluster is neat compact and well documented View full size image Context Help Velocity Amplitude rpm The steady state velocity reached in each quarter of the cycle in revolutions per minute f bo G Rule 6 25 Save all clusters as type definitions This rule cannot be overstated Clusters form the data infrastructure of most
226. e documentation process 9 1 Front Panel Documentation Front panel documentation consists of control labels descriptions tip strips and free labels These items are beneficial for users as well as developers Style rules and examples pertaining to front panel text are presented in Chapters 3 and 6 The rules are repeated in this section for convenience followed by an example Rule 3 17 Minimize front panel text Rule 3 18 Delete template instructions immediately after edits are performed Avoid entering lengthy comments as permanent labels on front panels This is particularly important for GUI VIs Intuitive GUIs are graphical not textual Long paragraphs of text should be limited to the user manuals and online help SubVIs containing notes for the developer are an exception Specifically most template VIs contain instructions for editing and use within free labels Always delete the instructions after each edit is performed This helps signify that the edits have been completed Rule 3 21 Use succinct intuitive control labels and embedded text Rule 3 23 Provide default values and units in parentheses at the end of owned labels 297 Control labels are the most prominent type of text appearing throughout a LabVIEW application It is extremely important to make them succinct and intuitive Append the default value and units in parentheses for any controls representing physical parameters Apply caption
227. e hed Rule 2 9 Create a LabVIEW source folder hierarchy that reflects your application s architecture The five folders shown in the Project Name folder of Figure 2 7A form a base set of file categories If your project files are located in the roots of these five folders all of which reside within a single project folder you have successfully created a project repository However each of the folders can be further divided into subfolders for greater organization on disk In particular you might create a subfolder hierarchy to organize the LabVIEW source files in a manner that reflects the application s architecture For example some typical subfolder names include Analysis Configuration DAQ Data Manipulation File IO and User Interface I recommend placing the top level VIs at the root of LV Source and placing all subVIs and controls in the appropriate subfolders Likewise the other base folders can be further organized into subfolders Figure 2 7B provides an example of an expanded folder hierarchy for a complex application Figure 2 7B Expanded folder hierarchy that provides detailed organization and an accelerated starting point for complex application development View full size image 48 LV Source Address E C Reposkory Praject NomeJIL Source Folders x S ARR A Configuration E C Project Name til ay is Dats Manipulation Saas Diagnostic Yis ecutabbe error Harding O Dats en 53 0 Reference S
228. e of text filling one third of the icon s height per line This font is effective for icons that contain just a few large characters such as F gt C representing a Fahrenheit to Celsius unit conversion Use 8 or 10 point small fonts for whole words and acronyms because this maximizes the number of characters that will fit on the icon while maintaining legibility Using uppercase 8 point small fonts we can squeeze seven characters of text in one line occupying just one quarter of the icon Note that I am not advocating that we fill up our icons with four lines of text Graphics and some empty space are desirable as per Rule 5 6 However 8 or 10 point small fonts provide more room for graphics and white space for the same amount of text as any larger font Figure 5 2 provides a comparison of several icon fonts and capitalization Figure 5 2 This code snippet provides a comparison of several icon fonts and capitalization Eight point small font with all characters capitalized is the smallest legible font View full size image The most intuitive and attractive icons combine a glyph succinct text and two or three colors The glyph comprises about half or two thirds of the icon with one to three succinct words or meaningful acronyms Any color can be used for any portion of the icon but a high level of contrast between foreground and background is desired This is the case for Find Screw Ends VI shown in Figure 5 3 A glyph of a scre
229. e rae eS A ee ee a Anaye Ta etre m pirea br he coe Cearb Pa pind trappa ir om bra raren EET i lia birita Cee pi ip er ee Se ed je Pi fji TS ee ee ey el eee ee ee eee ee E Pa in hie Labis Fa M PD Sey PB dia oA dii bmg a G a Lja a Di l Bo a i E m Sela i ie et Pee ag a A iia Gd e E LR AH Fej eee ee oe ee a a mmiri e mi ir hai mi a iea arh ed Pra dr EL ier Ermi in Hap msya Farigi p TA E ee er ey ee r teres nAn prir HPA DP panim Te Le ale tampa arra mi dm tabh rahi pn ma ea oe palan ee eo oe Be SES eee a a ee Pee rn me ener ee et Te ed mad slg aiii Mei wl UT eS Me oa TR caer ll a ae i eee eG ie Sop PEA AH Dri E aH keir odati P Mep ipei H a Pe pe ea Be Prae ie Ph Tenja p a p p rt p e eee iy Tey EG Pe Dba pa re ae dA ee ot ed ee Ca ee e ee E Shee E ee eee j Darsa Peepers bha rii eee h he Ue euria Fa eee hera p Orewiew a pa ene ee MEE ETE ie ee ee ee I ey ope ec eee ba rha ra yee et ee minn el m re dTie e isles ele ae epee ee ee ee ee gee h i de re eT WAA SS el See es ne Se a eT B lame iii Mem RBS FB a oe law iae WE A aad ee el BS GP mel ami imini Hindan Dande hi eae pan rrp E mamba mata raa ra res epigte e fst bee Dareh tie ee r mba phan ees Sudo ber r Bpi hh a r el inant Getput hir peie i e a Bed paT ay ae a Oe ee ae ia eas a ee ee fe ee ee Pr ee BO ety ee Baer iai aes pE ei eg a re E ofterare fees Re Bee ee B ae ee ee a ee ee a ed mo i Sey eee ee Pe
230. e their corresponding Value Change event case unless they are needed elsewhere This provides a handy reference point for the developers to locate the controls Rule 8 7 Resize the Event Data Node to hide unused terminals 250 In most event cases I find that I use only a couple one or none of the terminals from the Event Data Node Conserve space and reduce clutter within the Event structure by resizing the Event Data Node to hide the unused terminals In the examples in this chapter the Event Data Node is reduced to the minimum single terminal and moved to the bottom left area of the Event structure Rule 8 8 Avoid continuous timeout events By default the Event structure has a Timeout event case that becomes enabled whenever the Timeout terminal is wired The Timeout event occurs if no other event fires within the specified timeout interval and executes the corresponding Timeout event case The Timeout event is frequently misused It can be very tempting to place a continuous software timed task inside the Timeout event because it saves the developer from creating an additional parallel loop and messaging scheme reducing diagram complexity However note that the occurrence of any registered event will suspend Timeout events until the registered event is processed The software has no control over how many statically registered events may fire or how often Therefore the Timeout event cannot be relied upon to perform
231. e them readily accessible to the developer Searching through archives that contain source code from previous projects can be cumbersome and unwieldy Instead identify reusable components as you code and place them in a dedicated repository Then you can upload copies of the reusable components to locations in your LabVIEW installation directory that will integrate with the LabVIEW environment Specifically the LabVIEW user lib folder behaves similarly to LabVIEW instr lib LabVIEW maps any folders project libraries or LLBs that it finds in this directory to subpalettes underneath the User Libraries Functions palette Figure 2 4 illustrates an example of a Functions palette with the User Libraries category expanded In this case the user lib folder contains five libraries OpenG Bloomy Utilities Bloomy Library Win Utilities and an unidentified library containing a generic icon LabVIEW automatically assigns the generic icon shown on the far right to any folders project libraries or LLBs that do not have an associated menu file The palette properties including the folder or library mapping as well as the arrangement of subpalettes menus icons and VIs are specified by selecting Tools Advanced Edit Palette Set Figure 2 4 The Functions palette with the User Libraries category expanded reveals five user libraries Functions R search view il Programming P Express P Measurement LO P Instrument Io
232. e violations of Rules 3 21 4 10 5 2 5 3 and 9 7 SubVI from Selection w Cleanup VI shown in Figure 9 6B is an improved revision of the VI containing appropriate terminal labels and a meaningful icon and description More importantly there is no way of understanding the VI s purpose from the Context Help window or front panel Always remember to finish any subVI from selections that you develop by applying the style rules Figure 9 6A The Context Help window for SubVI from Selection VI This VI violates multiple style rules including default icon inappropriate terminal labels and missing VI description Context Help Sub I from Selection yi task ID out task ID out 2 output chunk error out error out 2 312 Figure 9 6B SubVI from Selection w Cleanup VI contains a meaningful icon and VI description and appropriate terminal labels Locked Context Help Sub 1 from Selection w Cleanup yvi task ID in i task ID out input chunk error in no error error out Writes the input chunk of recorded data to the sound card For output to the speakers Uses a different thread so that this Function does not slow down parallel tasks The optical filter test application from Chapter 4 is continued in Figure 9 7 A routine that controls a laser utilizing clear wiring and good data flow but inadequate documentation is shown in Figure 9 7A Specifically a cluster is unbundled using the Unbundle function an inst
233. e yee wrt nate essen sath aes rN EEr EEEN EEEE epee NEEE EAEN EEES ESENE REESE E OEE EREE 7 EONO E esar E E EE E A E AE E E 8 PCO WSO VG INL S eane A E E EA E sears nese i aereueienemausiyeenetat 9 ADOUNC ARNO atecets apes yds a E A onan Gants beainedanaitls ynbapebicieaatarsaeuiean a 11 1 The Significance Of Style cc ecccccccsssssecccccseseecceeaeeseceeeecaeeeeccesseeseceessaeaseeceeesseaseceesaaaseceessuaaseeeesaugeeseesegeaas 12 1 1 Style SignifiCANe cc ceccccccesssececcccaessecceceesseceeesuesseeceeesaaseceeeeeeaeeceesssaaaeceesaeaseceessuanscecessauaeceesauasagaeeess 12 TACO hi lics cttiest quiyn cad E E cad eawniotesaedssconsawenpnones esa npeasenatnonsquish tad E E alsin 12 ANON ET occas aerate cs savin ct on cto A vb Gates ara ee aaa apemanne A E 22 1 2 Style Versus Time Trade of cccccscccccccssseeccecseeeeccccaeesecceeeseaeeceeseeeseceessueesceceessaeseceessuaueeessagageeeeeees 27 Definition Ls satecctactnanapeatncnmtectadnvalcasvenodatoatyoteasescaeubacanandantausabsdisadanied ianvodcaenexodarsannotnadiacendeaaniandanedumeniaiadeuns 27 PEDO Ue e E E E A ENA 28 2 Prepare for Good Styl sennen a e N E a ae aaenne pts 29 2 1 Specifications sisvevisaccunsiniuncantuncasvedansshuhewnteshacabessvansotuteninatebusacohetandswisbasseheabtasdauersssintusroderdacinbionsantoktabsiecteaends 29 Theorem 2 1 annnnnssennsseenesssensssersssrersssrrsssreresseressrersssrrssreresserressrressrtresstresstresssteesss
234. ector assignments Rule 3 16 Resize the panel for a snug fit The rules for arranging the location of controls and indicators on a subVI panel are similar to and consistent with the rules for connector patterns and terminal assignments that are discussed in Chapter 5 Icon and Connector Place controls on the left side of subVI panels and indicators on the right I O name and Refnum controls belong at the top and error clusters at the bottom For a dense subVI panel that contains more than eight controls and indicators arrange the most important controls and indicators in the farthest left and right sides of the panel respectively Put the less frequently used controls and indicators in the horizontal center Arrange the controls and indicators in these locations resize the panel to fit the objects and then use the alignment and distribution tools to snap them perfectly into place for a professional grade subVI panel When the panel layout is complete the connector assignments are performed according to the layout Figure 3 8 shows two candidate panels for Interval Timer VI a utility subVI that times an interval The Interval control is the most important parameter and its corresponding terminal priority is required The Pause and Mode controls are the least often used The panel in Figure 3 8A uses the default panel size and position and the controls are arranged haphazardly The Interval control is located in the center of the pan
235. ed Explain Error A built in utility that displays information about an error code ina dialog It is invoked by selecting Explain Error from the shortcut menu from any error cluster or by selecting Help Explain Error F Field Programmable Gate Array FPGA A device that contains a matrix of reconfigurable gate array logic circuitry When an FPGA is configured the internal circuitry is connected in a way that creates a hardware implementation of the software application Unlike processors FPGAs use dedicated hardware for processing logic and do not have an operating system flexible sequencer A construct for executing a multiframe sequence implemented as a looped Case structure in which the execution order of the cases and stop condition are formed dynamically during execution The Classic State Machine design pattern can be utilized to implement a flexible sequencer front panel memory 344 One of the four VI memory components front panel memory stores the graphical objects and images that comprise the front panel Front panel memory usage is provided by the Memory Usage category of the VI Properties window selected by choosing File VI Properties Functional Global glyph GUI VI I Design pattern utilized for sharing data using subVIs instead of variables It consists of a While Loop a Case structure an enumeration and controls for reading and writing data The While Loop contains one or more uninitiali
236. ed application Rule 3 19 Apply consistent fonts and capitalization Most applications require more than one text style to distinguish different types of data and events For example on an industrial GUI it is common to display important data using a large bold style to make it stand out prominently on the panel Additionally you might want a large plain style for headings or a high contrast colored style for important messages such as alarms and warnings However too many text styles makes the panels appear unnecessarily busy and perhaps even confusing Most applications require exactly three text styles a standard style for most text a large or bold style for emphasizing important data and a heading or contrast style for special situations It is important to choose the corresponding fonts and remain consistent throughout the GUI panels of an application 71 LabVIEW supports many fonts but not all are equal Application System and Dialog fonts are not really fonts they are symbolic mappings to fonts that are native to the host computer s operating system Application font is the standard font the operating system uses for most text Dialog font is used for dialog control labels and messages System font is used for window titles and important messages intended to grab attention The symbolic fonts maintain the look and feel of the operating system for each of these three contexts However the actual font used varies based on the op
237. ed in Figure 4 21 This illustration represents the top level VI that calls the filter measurement routine as a state within the Queued State Machine design pattern Specifically the filter measurement routine from Figure 4 6A is incorporated in the Normal Sweep case of the state machine in Figure 4 21A The application s execution order is controlled programmatically using the queue Shift registers facilitate data flow However the shift registers are randomly spaced 4 36 and not labeled 4 37 This results in wires flowing through the middle of each state dividing the diagram into multiple sections Additionally there are multiple data flow and wiring rules violations including wire crossovers 4 15 and right to left data flow 4 22 Specifically the error cluster crosses over several shift register wires to reach the Dequeue Element function on the left of the Case structure Also the low level routine that calculates the number of wavelength steps propagates the result to two instrument driver VIs below it Finally readability is hindered by several low level routines on a high level VI 4 7 in addition to the data flow and wiring issues Figure 4 21A The optical filter test application contains a Queued State Machine design pattern The shift registers are not grouped the wires exiting the shift registers are not labeled there is right to left data flow and there is clutter from low level routines View full size image
238. ed in more detail in Chapter 6 Also the enumerated Case structure is a fundamental element of the state machine design pattern that is discussed in Chapter 8 LabVIEW provides a block diagram option called Show subVI names when dropped available from Tools Options and selecting Block Diagram from the Category list I recommend disabling this selection per the default value All subVIs should be readily identified by a unique icon and the name and path are available from the Context Help window I recommend keeping the Context Help window open while editing because it contains valuable information Specifically the Context Help window contains the VI description inputs outputs priorities and data types in addition to the VI s name 304 Hiding names of functions and subVIs reduces unnecessary text from the diagram which saves space reduces clutter and improves readability of the graphics and data flow The name format of Property and Invoke Nodes affects the size of their icons on the diagram Short Names are used by default and are specified from the Property or Invoke Node s shortcut menu by selecting Name Format Short Names Similar to a subVI the long name of a property or method can be determined from the Context Help window Therefore use the short name format for Property and Invoke Nodes to reduce the space that they consume on the diagram without eliminating the name entirely Rule 6 21 Use arrays for
239. ed on the labels and data types of the elements of the User Event Data cluster View full size image k Create amp Register Usar Everts Front Panel As shown in the diagram in Figure 8 16 the elements of the User Event Data cluster are unbundled and passed to the user event data type input terminal of the Create User Event function This function defines the name and data type for each user event based on the label and data type of the data wired to the terminal Create User Event returns a user event refnum that is bundled into the User Events Out cluster These operations are repeated in the For Loop generating a user event for each state of the application The events are registered by passing the User Events Out cluster to the event source terminal of the Register for Events function This function returns an event registration refnum which is returned by the subVI and wired to the Event structure s dynamic event terminal as shown in Figure 8 15 Finally user events are fired anywhere in the application using the Generate User Event function The user event refnum corresponding to the desired state is unbundled from the User Events Out cluster and passed to the corresponding input of the Generate User Event function along with the event data In the Run Test event case the Analyze Data event is fired with an empty variant constant wired to the event data terminal The Event Machine design pattern is similar to the Event Dri
240. ed with the application All of these types of documentation apply to LabVIEW applications Documentation requirements vary widely depending on the application type developer end user organization and industry For example a commercial application may require an extensive user manual and online help whereas a one of a kind lab experiment may require no documentation at all Many organizations have a set of internal standards that all software and systems must adhere to Government regulated industries traditionally have stringent documentation requirements imposed by regulatory committees such as the Food and Drug Administration This chapter focuses on LabVIEW source code documentation that comprises good LabVIEW development style regardless of the developer end user organization or industry Myth 9 1 LabVIEW diagrams are self documenting The graphical nature of LabVIEW presents a strong advantage over text based programming languages in source code comprehension A picture is worth a thousand words However it is a myth that LabVIEW source code is self documenting Because documentation is used to describe the source code it cannot be exactly the same as the source code Rather LabVIEW diagrams are ultimately designed for graphical compilation while documentation is explicitly designed for linguistic expression Consequently documentation is primarily implemented using text Because LabVIEW source code is graphica
241. edit the library s default icon and the icon is automatically applied to all new VIs in the library For a template create a VI containing the desired default icon connector controls and indicators and save it with the VIT extension Create one glyph that depicts the type of operations that the VIs are used for and leave room for a line of text across the top Create each new VI from the library or template and edit only the text portion of each icon This way you invest your time on only one glyph and you maximize the return by applying it to all related VIs Additionally note that instrument drivers have some conventions already established for icons and connectors Refer to the NI Instrument 2 Driver Development Guidelines document for more information Figure 5 6A contains the icon of the VI template used for a commercial developer toolkit that controls a semiconductor wafer probe system It consists of a glyph of a probe tip contacting a semiconductor device and some text containing an acronym for the product manufacturer and model across the top This icon required a significant amount of time effort and creativity to create As shown in the VI Tree in Figure 5 6B all VIs in the toolkit contain the same icon convention based on the template Hence only one creative icon design was developed and the resulting template is reused throughout 7 the toolkit This example is revisited in Section 5 3 Examples
242. ee eee aa ta 1 f0 2 e eT ee E E GE T E T TEEELILELELI Lh Le Peek hee kL Pathe ttl tier k Leck k Lice Pak hPa eek Lt Ls Finally error handling is both incomplete and ineffective in Spaghetti VI As you can see in Figure 1 3 no apparent error handling scheme is used Overall Meticulous VI is the most robust in terms of error handling Proper error handling is an essential ingredient for reliable and robust applications Chapter 7 Error Handling covers this topic in detail Writing to local and global variables can cause unexpected results in an application For example a race condition occurs when two copies of a write variable are written to at the same time As a general rule the more write variables an application has the greater the potential for unexpected and undesirable behavior The quantity of local and global write variables is quickly determined from the VI Metrics window Meticulous VI contains 92 write variables Nested VI contains only 5 and Spaghetti VI contains 46 Hence Nested VI is the most robust in terms of fewest write variables Chapter 4 discusses variables in greater detail The rules presented throughout this book ensure robust applications Simplicity inversely relates to the quantity of nodes and terminals that comprise an application Simplicity is the opposite of complexity The fewer front panel objects and block diagram nodes the simpler the application Simplicity affects readability and performa
243. eee oe eee ees ee Praney Airi iisa d ipite oam ei i g a ee eii Bel a peers Pagida igm oa a je Be Fp Bii Bei et Parr aam rpm ee rip mihe pj ee ey ee n Brut Prrry Echo mhar taban mae Tet Meee ogy Papeete ey be pe ie iia ware eT Dein ay miran ek haima i el ated erige trai DELH Leal ee ay Se ce a Oe ae Le ed he ee ee F aes Pai ea mni SF Pl ag Pai Pe ee Appamdia A Gloosaey er p ep Se ee ee eee ee Appeedis B inputOutped Charu Livi Dn bran eer CD eee ja A eee eee Hiii nta gees rei ri qhi basicaly Aprilia E fompele Rago ne n pua an 2a r ae pee ee oi pprsir Product Specific asta ee ee ee ee Pe ee ee ee ee SPS ee ee ae The high level objectives should clearly describe why the project is being undertaken It is particularly useful to describe how the objectives relate to the company s core products and services This is also known as the business objective For example a high level objective might be stated as follows Evaluate a low emissions exhaust system that is being considered by the automotive industry for 2010 model automobiles The system must comprehensively test the first 100 prototypes in January 2008 The system must be scaled into high volume production in July The market potential of the product is estimated to be 1 billion over a 5 year span Budget and timetable are very important considerations that not only directly affect the system design but might also determine the overal
244. een components using another shift register As discussed in Chapter 6 Data Structures variant is a generic data type that is defined dynamically at runtime The actual data contained by the variant is specified within the components This approach maximizes flexibility for future expansion The diagram of the framework template shown in Figure 8 19B contains five shift registers for sharing data between components and one shift register for passing the path to the desired component for dynamic loading This allows all components to generate modify and propagate data All data structures in the framework s template are type variant Replace up to four of the variants with standard data structures required by the components for your specific application 274 Figure 8 19B The block diagram is an Event Driven State Machine without the queue Three of the primary states are shown Data is passed between components via shift registers View full size image Label 1 a lated To Late gt gt jah 4 3 z Label T r oe gt igi Fah gt COTE miea ep ate 1 ETOP ming ere Fhe p Boe gt Rule 8 26 Display plug in panels within subpanels The subpanel facilitates seamless integration of the component VI panels into the framework s graphical user interface However only one front panel is displayed in a subpanel at a time whereas separate front panel windows can be opened and positioned according to the user
245. ees logged cm eich erie parr E Define Waveleneth Scan Parameters Pest Tain If wren Test Ration i Piian e Cate Pie pah Cata 3 Feature pr ami Lice peii Sep Se E Corta 1 Tamy Ayant A Saai Tolerance 5 00 ren Refiecton Cr eria oF outst peewee Tiag ua Figure 4 5B An optical filter measurement routine calls Define Scan VI and propagates the laser scan parameters using individual wires The subVI connector panes are densely populated wiring is cluttered and maintenance is tedious View full size image ab bP ppm pL aie a Lio nt EN Ea 43 oor 2 oR E AmE a RETI _ ad a Figure 4 5C contains an alternative implementation of Define Scan VI with the laser scan parameters returned as a cluster In Figure 4 5D the measurement routine is revised using the cluster instead of individual wires Wire clutter and development toil are substantially reduced Additionally parameters can be added and removed from the cluster without requiring any changes to the wires and subVI connector terminal assignments Hence clusters improve the diagram s appearance while reducing overall development and maintenance effort Figure 4 5C Define Scan VI modularizes the laser scan parameters into a cluster Terminal assignments are simplified View full size image 105 Careri Help Define Biek Soon Parenter Diir Sani suet La al wen Frem Dak ato
246. eft data flow or wire crossovers Figure 4 12C contains a looped Case structure that utilizes a While Loop instead of a For Loop This implementation is similar to the For Loop with one important distinction The While Loop is programmed to terminate if a test fails or an error occurs without completing the full test sequence Figure 4 12B The test sequence is implemented using a looped Case structure with a For Loop as the looping structure Five shift registers replace the local variables and sequence local terminals View full size image F EEN wa Test Results Pressure Leak Test ey Fe gt Test Resuts gt lt ffiaananssnnsnssnsnesnsanninasn P She Fy gt Task gt we ka gt Limits gt ka gt Scaling gt seme i Tact PE FEE Pee ee ee ee es ee ee j pim m oka meme Error om f fi Figure 4 12C An alternate looped Case structure utilizes a While Loop that terminates the test sequence if a test fails or an error occurs 119 Test Results Pressure Leak Test awe gt Test Results gt af ca gt Task gt oof 99 fon gt Liity gt em Ed iao gt Scaling Sy xm g A looped Case structure need not be limited to sequentially ordered execution of numerically selected cases A flexible sequencer utilizing the Classic State Machine design pattern is shown in Figure 4 12D The Classic State Machine discussed extensively in Chapter 8 consists of a looped Case structure with
247. eir standard locations at the top and bottom of the panel Rule 3 40 The panel is not sized for a snug fit Rule 3 16 Moreover the control labels are counterintuitive Rule 3 21 All three controls which are the inputs to the subVI contain out or output in their labels A number is used to distinguish similarly named controls and indicators Consequently manual cleanup is essential Figure 3 14B addresses these issues The block diagram icon and connector pane of this example are discussed in Chapters 4 5 and 9 Figure 3 14A SubVI from Selection VI violates rules 3 15 3 16 3 21 and 3 40 View full size image 84 SubVI from Selection vi tack ID out tput chunk sarot out error out 2 SubVi from Sebection wi Front Panel Figure 3 14B SubVI from Selection w Cleanup VI conforms to the rules for subVI front panels 85 Sub I from Selection w Cleanup vi Application SubVI writes audio data to the computer s sound card Conditions Created using the subVI from selection utility and contains multiple style rules violations Color scheme Default appearance of panel and all controls Layout Controls are arranged horizontally instead of resembling the connector assignments Heading text Not applicable Boolean text Not applicable Numeric and String Control Data Default 13 point normal black Application font 86 Secondary Fonts Not applicable Navigation Not applicable for a subVI The RS 2
248. el and assigned to a middle terminal This location is well suited for an important parameter on a GUI VI panel but is incorrect for a subVI The panel in Figure 3 8B has the controls arranged neatly and the panel sized appropriately for the controls It is organized with the most important controls on the left starting with the Interval control at the top left and the least important controls in the horizontal center Error clusters have been added for proper subVI error handling The connector assignments appropriately reflect the panel layout Chapter 7 Error Handling discusses error handling and Chapter 5 covers connector assignments and priorities Figure 3 8A The controls of Interval Timer VI are haphazardly arranged and the panel is oversized View full size image 68 nterwal Diner Front Panel era ell ee I Ee Figure 3 8B The controls are aligned distributed and prioritized via locations and text style The panel is resized for a snug appearance View full size image 69 Interval Tmer vi Pause F 17 Interval 0 0 Int crys Interval Elapsed Units sec Remaining Time Reset F Elapsed Time error in no error error out Mode Interval Sable erable pat ae 2n 3 2 Text Text has a substantial bearing on the perceived user friendliness of software including the LabVIEW applications that we develop The following are some specific rules regarding front panel text We begi
249. el error clusters View full size image 232 Td Inkialize Py tt ee en oe a oe wo ao OK Button Cancel Button Ao Configuration Params a E i homed tel Glee Piisibie _ pristie teste Figure 7 20B Merge Multiple Errors VI has eight scalar error in terminals which eliminates wire kinks and promotes good style View full size image gt Contig Params gt zz ee ee ee ee ee OK Button Cancel Button stop L H Hh e f Screw Inspection VI is an image processing application that was previously discussed in Chapter 4 The diagram snippet shown in Figure 7 21A contains two incomplete parallel error chains within a For Loop The top error chain consists of three overlay VIs from the Vision Utilities palette Mysteriously the rightmost VI is omitted from the error chain Also the error is propagated among the Find Screw Ends VIs in the bottom error chain but the first VI s error in terminal and the last VI s error out terminal are unwired Hence the error is not trapped in either error chain Additionally the Case structure containing the Wait ms function has no data dependency with the remaining nodes and the order in which the delay occurs is not known Figure 7 21A Screw Inspection VI contains two incomplete parallel error chains within a For Loop and a Wait ms function with unknown execution order View full size image 233 Figure 7 21B illustrates proper error trapp
250. elop your source code Many developers consider developing VI icons and descriptions a lower priority than developing working source code Most have good intentions expecting to return to these elements at a later time but procrastinate until another activity takes priority As aresult LabVIEW s default icons are used instead of meaningful ones and VI descriptions are skipped Eventually the software satisfies the operational requirements and editing the icons and entering descriptions becomes an even lower priority compared to the next project that is in the pipeline Some developers justify using LabVIEW s default icons and skipping the descriptions because they are uncertain as to whether the VIs will be included in the final application Why waste time on icons and descriptions if they might end up in the recycle bin In practice the opposite is a much greater concern beginning with default icons and empty descriptions but not having the time to go back and edit them The readability and maintainability of such applications are sacrificed as is the reusability of the subVIs Moreover it is much more time consuming to develop meaningful icons and cohesive descriptions if you delay versus immediately after you develop them Therefore create meaningful icons and cohesive descriptions as you develop your source code while it is fresh in your mind Furthermore the icons and descriptions should be considered an integral part of your sou
251. elp Wak p meet yi ii a rec T AS Tii WI iret the uiar delay vathin an ami hain It ongso ptal bo iha Wek fire function wath ab am peopagaik aian aba k chee 20 oy Cabo ba All lapis Piere In Figure 7 14B the Wait n mSec VI is applied to an instrument communications example Specifically the Wait n mSec VI is inserted between VISA Write and a Property Node that returns the Number of Bytes at Serial Port This delay gives 225 the instrument time to respond to the query before reading the response Note that any errors returned prior to the Wait n mSec VI cause the delay to skip Therefore error handling with this subVI facilitates execution ordering via data dependency and optimizes the efficiency in the event of an error Figure 7 14B Wait n mSec VI inserts a 100ms delay between the VISA Write function and a Property Node allowing an instrument time to respond to a query View full size image EE read buffer VISA Reference T PEA te Se Fri dup VISA Reference BEO in no error EF ACELL ELLELLE 7 P ii E Fre error DLL i00 f 7 4 Error Handling Tips This section provides tips on error handling including structure wiring merging errors clearing errors and automatic error handling Rule 7 17 Tunnel the error cluster near the bottom of structures Wire the error cluster through structures including loops Case Event and Sequence via vertically aligned input and output tunnels on the
252. els 20 point bold for all indicator labels and data 13 point bold for Log Interval label and data 13 point bold for Reset Energy Boolean text 13 point plain for Revision Number label and data and 37 point bold red for Exit Boolean text The different variations might have been chosen to distinguish the importance of each control However it becomes indistinguishable and distracting to use more than three text styles Instead the size and locations of the controls can help distinguish their priorities In Figure 3 9B the same panel is revised using three text styles A 20 point bold Application font is used for all owned labels data and Boolean text A 20 point 72 plain font is used for minor labels including units and toggle switch ON OFF labels The Power Monitor System heading remains 42 point bold This front panel appears neater and more intuitive than the one in Figure 3 9A Figure 3 9A Power Monitor System VI contains eight different styles of text based on variations of point size boldness and color View full size image oS er a Fe 0 0 Figure 3 9B Power Monitor System VI is revised with three fonts applied consistently It appears neater and more intuitive than Figure 3 9A View full size image 73 Power Monitor System LEHE Jiddada s e elel eeje EE oo Me D a ce i Apply capitalization consistently as well If you have 12 indicators of similar context and significance on
253. ent Normal priority rules are recommendations that are generally considered as good practices but are either not as critical as the high priority rules or more exceptions exist They are denoted by plain italic as follows Rule 2 3 Maintain good LabVIEW style throughout the proof of concepts Endnotes 1 See the Acknowledgments section for a list of reviewers contributors and people who have helped advance the science of LabVIEW Style 2 Bloomy Controls is a National Instruments Select Integration Partner with offices in Windsor Connecticut Milford Massachusetts and Fort Lee New Jersey Information is available at www bloomy com 3 LabVIEW Basics I and II is a one week hands on course offered by NI Certified Training Centers More information is available from http www ni com training Acknowledgments I would like to thank the following reviewers and contributors of source code for illustrations Darren Nattinger National Instruments Greg Burroughs Bloomy Controls Inc John Compton Smith Dover Technology Limited Crystal Drumheller National Instruments Greg McKaskle National Instruments Brian Powell National Instruments Heather Eisenbraun National Instruments Bob Hamburger Bloomy Controls Inc James Fowler Bloomy Controls Inc Bart Craft National Instruments Alex Khazanov Harris Corporation Robert Cornwell Bloomy Controls Inc Keith Brainard Anthony Conaci CiDRA Robert Gough J
254. ently selected via the Window Appearance property page The most significant is to disable the Show Abort button for several reasons First this button would allow users to abort the application immediately and abruptly regardless of the state of your application and without notifying the executing source code No programmatic method exists for detecting or responding to the abort operation This is a potentially dangerous capability to provide your application s users Most LabVIEW applications control some type of instrumentation and many applications are networked or distributed across multiple targets Abort causes LabVIEW to suddenly stop regardless of the state of any connected instruments or applications This can cause the external resources to hang up in some awkward state possibly requiring a reboot Additionally associated memory buffers can become fragmented and open files can be corrupted Therefore always hide the Abort Execution button from your completed applications and provide a graceful method of closing your GUI VI panels and exiting the application Rule 3 12 Hide the toolbar Better yet hide the entire toolbar When the Abort Execution button is not visible the only tools remaining with runtime functionality are Run Run Continuously and Pause These tools are primarily useful for novice level training purposes If you are reading this book you have probably mastered the While Loop and debugging tools Also m
255. ents the program as wires terminals structures and nodes rich with functionality and innovation LabVIEW extends this innovation to the developer providing tremendous freedom of expression and creativity As such there are many means to an end or possible development styles with LabVIEW 1 1 Style Significance A given software task might have numerous possible implementations that appear functionally equivalent What on the surface seems a matter of personal development preference creative license or style has significant implications Developer tendencies have direct effects on the outcome of applications Throughout my career as a professional LabVIEW consultant manager and trainer I have observed many different development styles and have enjoyed debating the pros and cons of each I can tell you that style is a sensitive issue for many developers However it is extremely important to recognize that good development style is not merely a matter of personal preference Some styles lend themselves to better performance source code that is easier to read and maintain and applications that are more reliable and robust Hence I present Theorem 1 1 Theorem 1 1 A direct relationship exists between LabVIEW development style and the ease of use efficiency readability maintainability robustness simplicity and performance of the completed application Theorem 1 1 is the foundation upon which this book is based LabV
256. eplace Datalog function Datalog files cannot be read or written outside of LabVIEW design patterns Standard VI architectures that solve common software designs required in a wide variety of applications They consist of an arrangement of structures functions controls and error handling that forms a construct for common tasks such as looping event handling state transitioning and data sharing and encapsulation desktop application Any application that runs on a personal computer and is not intended for a real time or embedded target desktop GUI VI AVI with a front panel designed for personal use in an office private laboratory or other individual computing environment Desktop GUI VIs appear and behave similarly to other native applications on the computer s operating system 341 developer Someone who creates custom software applications which may entail any combination of low level programming and high level configuration development time The time required to develop document test modify and maintain an application throughout its entire life cycle Dialog function LabVIEW s native dialogs accessed from the Dialog amp User Interface palette including the One Two and Three Button Dialog functions as well as Prompt for User Input and Display Message to User Express VIs The LabVIEW dialog functions are simple to use and designed to resemble the dialogs native to the host computer s operating
257. equirements in a specification tight loop A loop that executes at the highest possible rate without any loop timing inefficiently utilizing CPU resources A tight loop may unnecessarily monopolize the processor Each call to a loop timing function such as Wait ms Wait Until Next ms Multiple and the Time Delay Express VI forces the execution thread to yield control of the processor to allow other tasks to run time stamp Data type that stores an absolute time with very high precision using separate 8 byte fields for seconds and fractions of a second top level VI VI that resides at the highest level of the application hierarchy having a front panel that comprises the primary display screen type definition Also know as typedef a custom control defined using the Control Editor window that maintains its data type information in a CTL file When the data type of a type definition changes all instances of the type definition automatically update See also custom control and Strict type definition U Unit Under Test UUT The objective of many LabVIEW applications is to validate or test UUTs UUTs are also commonly referred to as devices under test or DUTs user event 351 Custom developer defined event that is processed by an Event structure s corresponding Event case User events are created registered and fired programmatically using the functions on the Events palette located under Programming Dial
258. er s Invoke Node with Set Control Value Run and Get Control Value methods However the Call By Reference Node executes faster and requires less source code than multiple Invoke Nodes Also the Call By Reference Node resembles a traditional subVI call simplifying development and readability Note that all components must share a common connector terminal pattern and assigned data types to use the Call By Reference Node Also maintaining the components in an appropriately named directory on disk helps simplify upgrades and maintenance All of the examples in this section use a folder named Plugins Rule 8 24 Pass data between components with shift registers Design the plug in components as independent stand alone VIs that can execute in almost any order and without much support from the framework Limited data is passed between components using shift registers The limitation is imposed by the developer s choice of connector pattern and assigned data structures Careful consideration is required to specify data structures that are consistent among all components including future add ons This is accomplished by examining the input and output requirements of each component grouping related data into type defined clusters and specifying a maximum of four data structures that combine the known data requirements of all components Duplicate these data structures as both controls and indicators on the front panel of each compone
259. er a floating point value that invalid operations produce such as taking the square root of anegative number The NaN constant is particularly useful for initializing floating point numeric arrays because the NaN values are easily identified in a search algorithm and are not visible on a graph indicator nested data structure Data construct containing multiple layers of arrays and clusters NI National Instruments the company that provides LabVIEW The URL is www ni com NI Developer Zone A forum for developers in need of LabVIEW resources such as articles example code and support Thousands of instrument drivers are available from NI Developer Zone s Instrument Driver Network www ni com idnet node 347 Any object on the block diagram that has inputs and or outputs and performs operations when a VI runs Nodes include functions subVIs and structures 0 OpenG An organized community committed to the development and use of open source LabVIEW tools The URL is WWW Openg org P PDA See Personal Digital Assistant PDF See Portable Document Format peer review Code review performed with the assistance of one or more peers Many bugs and coding issues are discovered by the developer as she explains her challenges to a colleague performance Very broad term that describes software effectiveness see also application performance or execution speed see also SUDVI performance Personal computer
260. er on the right contains error code 43 indicating that the user canceled the operation This is a very common error that occurs when a file dialog is canceled Most VIs have one error in control and one error out indicator assigned to the lower left and right connector terminals respectively Figure 7 l The error out clusters indicate no error or warning on the left a warning in the middle and an error on the right View full size image Rule 7 1 All VIs must trap and report the errors returned from error terminals Error handling involves trapping and reporting the errors returned from the functions and VIs that have error terminals Trapping is capturing the error returned from each node s error out terminal Reporting involves displaying or logging the error information using a dialog or log file Thorough trapping and reporting of errors is the best means by which errors are handled This section presents error handling basics including rules techniques and illustrations for trapping and reporting errors Error codes and ranges are also discussed Rule 7 2 Trap errors by propagating the error cluster among the error terminals Errors are trapped by propagating the error cluster among the nodes that have error terminals throughout every VI in an application Begin by placing the error in control and error out indicator on the panel of each VI or simply use 205 a template that has been prepopulated with these cont
261. er to wire the corresponding terminals or leave as default 298 Figure 9 1A niDMM Configure Measurement Digits VI is a subVI that configures a modular instrument The controls have succinct intuitive labels The default values depend on the selected Function and are absent from the labels View full size image t nh Confi ore Hessum mi Diw Froni Parl m e Ele Ede Hey Gperate Tools ince wz lz Cenbert tlelp Canfiguret the common progeeties of Bhy maacuremernt These properties dude Funcion Range and Digs Figure 9 1B contains the front panel for niDMM Configure Measurement Digits VI with the Context Help window displaying the control description for Resolution in Digits The Context Help window updates with the description for the control that the cursor moves over All of the controls of this subVI have descriptions This helps developers understand the programmable functions of the instrument Figure 9 1B The description of Resolution in Digits is shown in the Context Help window All of the controls have descriptions View full size image P hus i onlinure Meaturoment Lipit wi Front Panel Leanbext Help kaerad nhan Range k mk IS AEAEE r i def alk Figure 9 2A contains the front panel of Configure Test Instrument VI a dialog that prompts the user to configure a digital multimeter The labels of the front panel controls are succinct and intuitive
262. erating system s supported fonts and the user s configured preferences Moreover Windows currently uses only one font for all three contexts Specifically in the United States all three symbolic fonts default to Tahoma under Windows XP and Segoe UI under Windows Vista The advantage of the symbolic fonts is that they maintain platform compatibility and portability Hence if you port your application to a new operating system or even a new version of the same operating system your application maintains the native operating system look and feel We do not know which fonts Windows and other operating systems might utilize in the future but the symbolic fonts will map accordingly Alternatively most operating systems support universal fonts such as Arial Times New Roman and Tahoma Choose a universal font instead of a symbolic font if you want to specify the exact font that is used These fonts generally appear the same on each platform Hence a tradeoff exists between native operating system look and feel and consistent application look and feel Use the symbolic fonts for the former and universal fonts for the latter Sometimes a specialized font is required to achieve a special effect For example I use the Symbol font to apply Greek characters to labels indicating engineering units I use monospace fonts such as Courier New to limit the visible characters ina string control or indicator Beware that special fonts might behave different
263. erator to enter some motion control parameters including the velocity angle and torque limit Because the data is related it is stored in its own cluster named Motion Parameters Note that the Motion Parameters are weakly associated with the UUT Information Specifically the motion parameters are generally chosen to reflect the physical characteristics of the UUT However this is not a firm requirement In Figure 6 5 the UUT Information and Motion Parameters are stored in separate clusters If the application was larger and more complex we could merge the elements from these two clusters into one cluster to minimize wire clutter and subVI terminals The larger cluster could be renamed to indicate its more general scope such as Config Data Next the test runs and torque and angle data is returned as a cluster containing two separate one dimensional 1D arrays Arrays are used for torque and angle because they are each waveforms containing multiple samples acquired from two separate input channels Note that a single two dimensional 2D array could have been chosen instead of the cluster of arrays If the torque and angle data had been returned from the data acquisition VIs as a 2D array I would continue the 2D array for data type consistency as per Rule 6 3 However in this case the torque and angle are acquired from different subVIs within Run Test VI and are returned as separate 1D arrays Bundling them into a cluster helps preserve
264. ere are many long wires due to a large diagram along with several unnecessary bends Data flow Nesting obscures data flow There are several instances of right to left data flow as well as data entering structures through vertical tunnels The diagram of Haphazard VI shown in Figure 4 16 contains haphazard wiring and data flow More wires enter and exit structures vertically than horizontally There are many wire bends and kinks Data is flowing left to right right to left up down and all around Additionally several local variables are improperly initialized outside the While Loop in parallel with code that is reading the values from the corresponding control terminals inside the loop Because there is no data dependency between the initialization operations and the While Loop the order of execution is not specified Therefore these controls may not be correctly initialized before they are accessed within the looping structure This is a Situation in which a single frame Sequence structure with artificial data dependency or a multiframe Flat Sequence structure is required similar to Figures 4 9A and 4 9C respectively The unusual icon convention is another matter that is discussed in Chapter 5 Figure 4 16 This diagram has excessive wire bends haphazard data flow and unusual icon convention The order of execution between the local variable write operations and the While Loop is not specified View full size image or
265. erminals while increasing expandability and maintainability Additional arrays can simply be added to the clusters without adding or rearranging terminals Also the connector pattern is reduced to the preferred 4x2x2x4 pattern Figure 5 12A This VI uses the 5x3x3x5 connector pattern The Context Help window reveals wire stub crossovers FP Measure amp Control with Crossover vi FP Al 110 5 Measurements FP 4I 110 1 Measurements dup Main RT VI Refnum FP AI 110 2 Measurements FP AI 110 3 Measurements Error Out FP AI 110 4 Measurements FP A0 210 8 SetPoints Main RT VI Refnum FP 40 200 6 SetPoints FP 40 200 SetPoints Error In no error Non PID setpoints 146 Figure 5 12B Connector assignments with middle terminals divided with controls on the left and indicators on the right and no crossovers FP Measure amp Control without Crossover vi Non PID setpoints FP Al 110 4 Measurements Main RT YI Refnum dup Main RT YI Refnum FP 40 200 6 SetPoints FP 41 110 1 Measurements FP AO 200 7 SetPoints FP Al 110 2 Measurements FP 40 210 SetPoints FP AI 110 3 Measurements Error In no error Error Out FP Al 110 5 Measurements Figure 5 12C Connector assignment with controls assigned to bottom middle terminals and indicators assigned to top middle terminals and no crossovers FP Measure amp Control without Crossover vi FP AI 110 1 Measurements FP AI 110 2 Measurements FP
266. error out fibe Waveform Array Figure 6 1E The top level VI bundles the scaled array and other waveform components into a cluster and updates a waveform graph 168 Waveform Start Waveform End 10000 Each VI in this call chain has different data requirements and the control and data types have been chosen for maximum simplicity and memory efficiency within each specific VI However the dissimilar data types in this call chain increase the memory consumption by a factor of 9 versus the original array of 16 bit signed integers More important the waveform s data type is converted five times throughout the call chain adding unnecessary buffer allocations that reduce execution speed Therefore using the simplest and most memory efficient data types within each individual VI reduces the overall performance of the application because of the inconsistent data structures Figure 6 2 contains the same example except consistent data types are used throughout the call chain In Figure 6 2A every 2 successive bytes of the binary string are converted directly to an array of double precision floating point numbers Note that this selection requires four times more memory than the 16 bit signed integer used in Figure 6 l1A but it improves consistency throughout the call chain Also notice that the intermediate conversion of the binary string to 8 bit unsigned integers has been eliminated partially offsetting some of the extra memory and p
267. es The improved diagram shown in Figure 10 4B utilizes a proper Event Driven State Machine design pattern including an enumerated type definition for the case selector and an Event structure within the Idle case The state machine contains the standard states for Initialize Idle Shutdown and Blank The Event structure replaces the polling of the Boolean menu and Torque cluster A local variable and shift register for propagating the Units are eliminated The control initialization code is moved into the state machine s Initialize state and the Sequence structure is eliminated The state labels from the enumerated type definition are intuitive All wires exiting the left shift register terminals are labeled Finally General Error Handler VI is used in place of Simple Error Handler VI The resulting diagram is compact readable neat and intuitive Figure 10 4B The block diagram is revised to incorporate additional improvements from the rules summary in Appendix B including a proper Event Driven State Machine design pattern View full size image 331 j gt Complex Data gt Fie Path i an her gt Nad Stabe gt fi memel E Cyde Parameters HAME Eper Number Crescrigticn Self reviews provide you with a specific opportunity to review inspect and refine your source code s style This enables you to reacquaint yourself with code that yo
268. es plug ins a multiple loop framework and a modular multiple loop framework that uses loop subVIs Chapter 9 presents rules for documenting your source code including the front panel block diagram and icon and VI description Additionally the generation and integration of online documents is discussed Chapter 10 Code Reviews presents several methods of reviewing source code and enforcing style rules including self reviews utilizing a manual checklist automated self reviews utilizing the LabVIEW VI Analyzer Toolkit and peer reviews An application is evaluated using each of these techniques Appendixes include a glossary anda style rules summary Appendix A Glossary provides a list of terms and definitions many LabVIEW and software industry terms are evolutionary and context sensitive Any term that seems specialized or ambiguous is defined where it first appears within the book and used consistently in successive chapters The definitions are repeated in the glossary for ease of reference Appendix B Style Rules Summary lists the style rules presented in each chapter Style Rules Priority Convention Throughout The LabVIEW Style Book two priority levels are applied to the rules and distinguished as bold or plain italic High priority rules are laws that should almost always be followed with very few exceptions They are denoted by bold italics as follows Rule 2 2 Write a requirements specification docum
269. es and ranges of controls and indicators on the front panel and then save the VIs under a new name Some but not all of the variations are backed up to a network server CDs and various media Any significant upgrade would entail first taking inventory of every instance of the application examining the differences and possibly making redundant edits to multiple variations of the application Instead establish some conventions for file organization naming and control Rule 2 8 Maintain an organized repository on disk Because the LabVIEW project allows the project s files to reside almost anywhere a project that appears well organized within the Project Explorer window might have source files scattered throughout your hard drive network servers peers storage devices and miscellaneous targets It can be a CM nightmare if the files are accessible to other projects and users without source control or if any of the files are moved independent of the project Also it becomes difficult to manage and maintain the files using conventional operating system utilities such as Windows Explorer when the project associations are transparent to the operating system and the files are scattered in many places Therefore create an organized file repository on disk for every project to group the project files and place them under source control Create a folder hierarchy to maintain organization of the files within the repository Figure 2 7A illustrates
270. es in Tables 6 2 and 6 3 which list the data types ordered according to memory efficiency only considers the reference portion of these data structures It is important to note that this is not representative of the actual resource The ring and enum controls map text selections to numeric values They are very useful for presenting a discrete number of selections that are intuitively described using text labels on the front panel but the numeric is more functional on the diagram Enum controls are always represented as unsigned integers and the text selections are mapped to sequential numbers starting with 0 Ring controls may have any numeric representation and the text selections are mapped to any value within the range allowed by the representation including nonsequential negative and floating point numeric values The advantage of using enum and ring controls on the panel is that text selections can be more descriptive and meaningful than numbers and selecting one item from a discrete list is very intuitive for the user These controls play an important part in good programming style because they promote readability on both the front panel and the diagram On the diagram enum and ring constants can display the text labels instead of or in addition to the numeric values they represent Specifically create a constant from the shortcut menu of a ring or enum terminal by selecting Create Constant from the terminal s shortcut menu The con
271. es more than 50ms to execute the loop does not sleep However the wait or delay timing function forces the diagram s thread to yield control of the CPU regardless of the delay time Therefore including the delay has benefits even if the delay time is very small The Wait Until Next ms Multiple function waits until the value of the system clock becomes a multiple of the specified millisecond multiple You can use this function to synchronize the loop timing of multiple loops Be sure that your code does not exceed the time interval that you are trying to maintain or the loop timing may actually double If a loop polls another resource such as a communications port instrument or user interface object and is paced by the resource instead of a time interval use the Wait ms function Set the milliseconds to wait input to the largest value that the loop can tolerate and still operate responsively Note that delays are not necessary with event driven constructs including the Timed Loop and the Event structure These structures sleep whenever no events are active The Continuous Loop design pattern template is shown in Figure 8 4 The front panel simply consists of the error clusters and a Stop Boolean control The Stop Boolean contains a color and font scheme that makes it more legible for an industrial application than the default controls on the Boolean controls palette Copy and paste the Stop Boolean to create a menu of Boolean controls Edit
272. es violations Figure 10 4A Multiple style rule violations persist within the block diagram for Torque Hysteresis Improved w VI Analyzer VI View full size image p bert Sates fire cay aL er ar Sce Der 330 Rule 4 11 Minimize wire bends eliminate kinks and loops Rule 4 13 Tunnel wires through left and right borders of structures Rule 4 27 Avoid Sequence structures unless required Rule 4 34 Avoid variables if wires are feasible Rule 4 37 Label wires exiting the left shift register terminal Rule 7 7 Use General Error Handler VI over Simple Error Handler VI Rule 8 4 Avoid polling GUI objects Rule 8 12 Use an enumerated type definition for the case selector Rule 8 13 Minimize code external to the Case structure Rule 8 14 Include states for Initialize Idle Shutdown and Blank The legacy diagram utilizes a design pattern that combines a Sequence structure and While Loop The Sequence structure initializes several front panel controls and indicators The While Loop contains a Case structure and is similar to the Classic State Machine design pattern except that it utilizes an integer data type for the case selector Also the Default case polls the menu of Boolean controls and checks the Units element of the Cycle cluster for a value change These features violate the rules for a proper state machine design pattern discussed in Chapter 8 Design r Patterns Section 8 2 State Machin
273. ests to validate your design seek alternatives such as higher performing instrumentation or faster data processing algorithms or relax the requirements in your specification if necessary In regard to the exhaust system we need to be able to send set points to mass flow controllers acquire data from several gas analyzers synchronized within 10ms of each other and perform SPC analysis algorithms and chart updates in 100ms intervals These are critical requirements There is no use working on any of the other features until we are certain that these requirements can be satisfied Therefore we develop a proof of concept consisting of VIs that perform these functions on a small scale and we set up some experiments to monitor the performance When the proof of concept is complete make sure you save it You might be able to reuse all or portions of it in the final application Alternatively you might find that the performance of your completed application differs from the proof of concept In this case you should return to your proof of concept as a troubleshooting tool If it still functions properly you can begin to isolate performance issues in other areas If the proof of concept suddenly stops working the hardware system configuration or other conditions might have changed since the last successful run Reverting to a previously tested software and hardware configuration is sometimes referred to as a sanity check If nothing has c
274. ettings Strain Gage Settings Es djm Strain Gage Settings _ Load Cell Settings Spm Load Cell Settings Pore Pressure Settings a Pore Sett tings Thermocouple Settings zape Temperatures Relay Settings Sl ee Relay channel Settings Other DAQ Settings F Other DAG Settings F ror Fi n 0 error j SE Pree T TCSP CTOCPTTTOetr rer rer tree Endnotes 1 The LabVIEW online help topic VI Memory Usage contains useful information concerning memory usage 2 NI Instrument Driver Guidelines Control Indicator Naming and Data Representation 3 OpenG is an organized community committed to the development and use of open source LabVIEW tools The URL is WWW Openg org 203 7 Error Handling A typical LabVIEW application controls instrumentation while acquiring analyzing displaying and logging data hence the marketing slogan Acquire Analyze and Present Things can go wrong however regardless of the graphical programming methodology application complexity level and developer skill level LabVIEW applications execute exactly as programmed and are not immune to bugs The complexity of the applications LabVIEW is applied to is ever increasing Also in the dynamic world we live in applications can begin misbehaving long after they are tested and deployed For example an instrument might hang up or become disconnected or changes to the target computing device might cause a resour
275. eventeasaresintdiyesaeutemnebrigsonmncniemnvontsemer omenrucastctaerad centacnbetansnebunadnencestousicnrseanaeneneatys 108 FEE MANN SS oiiaaie et ateeysedes ee ea a EE EE e aana a a aa 122 EMCO CSS a E E E AE 132 Di COU ANIC Connector inedia a a a E A ceapeans 133 Ei P COUN EEE E D A E E T A ANE AE A A A N N sav E A sachs 135 5 2 Connector Pane oranana On ee nee AA een een nn nae Sena ae OTE me arte Teen mer ee ieee ey 143 Si Sic EMAMIDICS orner neniesan as A AET ouaicaleuasiace Sein A ba cease longisnalouesa E ERE 149 E VNC OCS esses neg E hc Sa cae E arse dea diane alo sna ao ea ost E aed oneaaed aa AORE 159 De ANA SUCUS ii i RA E AEE EE E RRE A mabe tem vanonaeceeeebe even 160 CONS Kaa EA E E E ITON REE A E E EAEE NO A ETS 160 6 1 Data Structure Design Methodology cccccccsssscccccssscccsensccccensccesesecccseuscceseuceesauecesssauseesuaeeessaes 160 6 2 Simpie Data Types essere hacia eaea aeae a a a tiasee lata deca aes 174 6 3 Data CONS UCTS erinrar mn Py EP e E AE EE 183 GAEI SS aa N T A EEEN A A 195 EVIE sinant aA AT wale cones SE E A S 203 PENO HaNOUDE senere TR E E E E AEE E OE 204 ENEO ENTZ dearer E N E E E A E E teow itondacenn 204 7 1 Error Handling Basics ccatceartineanenosarcipasvevalt eneoeuiehensad taneansa se caion avatevesdyaa va aettshwundcaleswuatncesouatadseanwnintacseetet 204 7 2 SUDVT Error FAM GUNG ennnen e E AE O aab aie Neate Rea asauaite 218 J3 PrNOrtIZiNE ENOS rossa aaa a a a a a ald a aa a a
276. evised block diagram Several data structures are propagated using shift registers near the top of the While Loop Specifically the UUT Information and Motion Parameters occupy separate shift registers instead of bundling the two together This reduces the complexity of the data structure Additionally the Torque vs Angle and Statistics clusters are passed using shift registers This reduces local variables and increases flexibility by having the data available in each case The data structures and shift registers maximize data flow and memory efficiency 335 Figure 10 5B Block diagram for Torque Hysteresis Improved w Peer Review VI Shift registers eliminate variables and promote data flow Also several routines from the organization s reuse library have been added for quit confirmation error logging and closing LabVIEW View full size image Event cases have been added for New UUT and Quit Value Change events The New UUT Value Change event case calls the dialogs for Prompt UUT Information and Prompt Motion Parameters The Quit Value Change event replaces the polling of the Quit Boolean that was previously wired to the condition terminal at the bottom of the While Loop Events are more efficient than polling Also a dialog VI for confirming before quitting is standard within the organization Additionally the General Error Handler VI is replaced with an error logging routine This replaces the error dialog with an error log file t
277. f Good LabVIEW developers are often in high demand within our organizations The applications we develop are critical to our companies research development or production endeavors Intense time to market pressures encourage fast application development cycles As such LabVIEW developers often gravitate toward shortcut and fast development techniques for speeding their development time After all that is why we all chose LabVIEW in the first place it substantially accelerates our development time versus traditional text based programming languages Definition 1 1 Development time includes the hours required to develop document test modify and maintain an application throughout its entire life cycle On the surface the pursuit of fast development time appears at odds with good programming style The former might entail shortcuts that bypass the latter At Bloomy Controls we are familiar with this apparent paradox Many of our customers 27 contract us as ameans of achieving tight deadlines Any style rules that increase development time without an offsetting productivity benefit must be eliminated However it is important to look at the entire life cycle of the application when considering development time Many developers mistakenly consider the time from conception until the first version of the software is released as the development time and strive to minimize this time In actuality development time includes the hours consu
278. f string controls and indicators include entering descriptions and displaying instrument responses If an operator needs the capability to enter free form comments such as a description of a one of a kind test or the cause of an alarm limit violation a string control is appropriate Instead of having a string control on the main GUI VI panel create a dialog VI that prompts the user for the description and closes immediately afterward String indicators are invaluable for troubleshooting instrument communications Instrument control is performed at a low level using calls to VISA Write and VISA Read These functions write instrument commands and read responses as strings Instrument drivers provide a layer of abstraction between the developer and the command and response strings which are generally cryptic However when developing or troubleshooting an instrument driver it is usually necessary to view these low level messages in string indicators Indeed string indicators have some very useful display modes including backslash and hex In backslash mode we can view nondisplayable characters such as tabs carriage returns and line feeds via their backslash codes such as t r and n In hex mode we can view binary data in hexadecimal format Rule 6 19 Use enum ring and path controls in place of string controls where possible Rule 6 20 Keep the Browse button visible for path controls on GUI VI panels Use Table 6 1 to
279. first out manner and all buffered events are executed until the While Loop stops Therefore I recommend the Event Machine only for complex GUI applications where multiple Event structures are required Finally a limitation common to all state machine design patterns discussed to this point is that each is capable of processing only one application state at a time This causes any states containing time consuming subdiagrams to prevent the application from responding quickly to other states or events For example the GUI will appear unresponsive to user interface activity during a time consuming state or user event case These limitations are addressed using the compound design patterns discussed next 8 3 Compound Design Patterns The design patterns discussed in the previous sections all have limitations when applied as the principle architecture for a large application Immediate SubVI and Functional Global design patterns are strictly limited to subVIs The Continuous Loop design pattern does not expand in a graceful organized manner The Event Handling Loop and Classic State Machine design patterns are readily expandable through cases but neither can perform the function of the other The Event Driven State Machine and Event Machine design patterns combine GUI event handling and state transitioning but their performance is limited by the serial processing of a single loop Instead consider each single loop design pattern as a building blo
280. for proper wiring and data flow that facilitate readability A LabVIEW application is maintainable if other LabVIEW developers besides the author understand the source code and if it can be easily modified and expanded to change or add new functionality Hence the source code must be readable to be maintainable Additionally the source code must use constructs that are modular and scalable thereby allowing for future expansion of functionality The front panel of Meticulous VI contains the subpanel control and multiple overlapping indicators as shown in Figure 1 6 The subpanel control promotes maintenance because new displays are created and existing displays are modified via component subVIs that are loaded into the subpanel control This provides tremendous flexibility in the display appearance along with the capability to modularize different display configurations as subVI panels Hence editing a display configuration involves editing a specific component subVI panel However changes to the overlapping indicators on the right require showing hiding and careful sizing and positioning of the indicators Maintaining this portion of the front panel is tedious The diagram of Meticulous VI see Figure 1 1 uses a complex application framework that consists of multiple parallel loops Most loops including the loop labeled INSTR COMM LOOP use a variation of the State Machine design pattern This consists of a Case structure with a separa
281. g LabVIEW example that continuously acquires data and logs it to file using the DAQmx VIs and File I 0 functions The DAQmx VIs share a common task and the File I 0 functions share 230 a common reference number and are placed in parallel to facilitate propagation of these data elements Because the nodes are interdependent a common error chain is shared between the VIs for error dependency and execution ordering The loop terminates if an error occurs in any node and an error message is reported with a dialog The implementation shown in Figure 7 18A is located using NI Example Finder as follows Hardware Input and Output DAQmx Analog Measurements Voltage Cont Acq amp Graph Voltage To File Binary VI Figure 7 18A The shipping example continuously acquires data and logs it to file The error cluster is shared by the DAQmx VIs and File I O functions because the nodes are interdependent The loop terminates if an error occurs in any node and it is reported in a dialog View full size image Fi path dinky ff empty A Sauer Ls Epa Figure 7 18B contains a few minor enhancements from the shipping example First the error cluster enters and exits the While Loop through vertically aligned tunnels located near the bottom of the loop Additionally the loop condition is placed in the bottom right corner of the loop along with the code that unbundles the error status and evaluates the loop condition Finally the General Err
282. gh 800 and 824 Timed Loop 620 through 600 Windows Registry Access 215 Table 7 1 LabVIEW Error Codes Range through 20 through 92 30 through 32 40 through 41 D3 through 66 61 through 60 67 through 91 92 through 96 97 through 100 102 ana 103 108 through 121 116 through 118 ana 122 1000 through 1045 1046 through 1050 ana 1053 1051 through 1091 1101 1114 1115 1132 through 1134 1139 through 1143 ana 1178 through 1185 1094 through 1157 1158 through 1169 1318 1404 and 1437 1172 1173 1189 ana 1199 1174 through 1188 1190 through 1194 and 1196 through 1198 1301 through 1321 1357 through 1366 1370 1376 through 1378 1380 through 1391 and 1395 through 1399 1325 1375 1386 and 1387 1367 1368 and 1379 1337 through 1356 1369 and 1383 1371 1373 1392 through 1394 1400 through 1403 1448 and 1486 1430 through 1455 1468 through 1470 1483 through 1485 1487 through 1493 and 1497 through 1500 1456 through 1467 and 1471 through 1482 216 Error Code Table Signal Processing GPIB Instrument Driver Formula Parsing VISA GPIB General GPIB Networking Serial General Windows Connectivity General Instrument Driver Networking General General Script Nodes General Networking General Run Time Menu Windows Connectivity General General Windows Connectivity Security Networking LabVIEW Object Oriented Programming General 3D Picture Control
283. ght loop Likewise delays are applied to the remaining While Loops as a precautionary measure On the surface the Multiple Loop Application Framework seems scalable without limitations Adding more functionality is as easy as adding cases to each single loop construct or adding more loops and queues for each new parallel task A complex application appears quite impressive as with Meticulous VI described in Chapter 1 However the Multiple Loop Application Framework violates several rules for good LabVIEW programming style In Chapter 4 we presented rules for modularizing top level diagrams with subVIs and limiting the diagram to one visible screen using 1280x1024 display resolution see Rules 4 1 4 4 4 5 4 6 Although the framework contains many subVIs the top level diagram vertically Spans two or more screens and the modularity index is often below 3 0 The Modular Multiple Loop Application Framework is amore modular alternative to the Multiple Loop Application Framework in which the loops are modularized into subVlIs Rule 8 29 Pass control references into subVIs via a type defined cluster Because each of the Multiple Loop Application Framework s loops delineates a cohesive task each loop is a candidate for subVI modularization This would improve the modularity and reduce the size of the top level diagram However within each loop are front panel terminals that need to be considered Controls and indicators of the top level V
284. gn phase but from my experience they are always necessary Maximize the utilization of the state machine by incorporating these routines as states within the state machine instead of external code Finally create a Blank state containing propagation of shift register wires for duplication purposes Creating new states by duplicating the Blank state saves time For example the diagram of Figure 8 9A contains initialization and shutdown code in sequence frames to the left and right of the state machine Also the state machine polls a Stop Boolean on the left of the Case structure and checks the error status on the right This is an inefficient use of space Figure 8 9B contains the equivalent code utilizing a state machine with states for Initialize Idle Shutdown and Blank The two terminal shift register provides a buffer that stores the next two states Each time the Idle state is called it adds another Idle state to the end of the buffer ensuring that Idle alternates with the other states Additionally the Idle state checks the error status and stops the While Loop if an error occurs Consequently the diagram of Figure 8 9B improves space efficiency by using states instead of external code Figure 8 9A A state machine design pattern is combined with a Flat Sequence structure to perform initialization control polling check the error status and shutdown The diagram uses excessive horizontal space View full size image i ee fo ET ee
285. gure 5 16A This icon illustrates Bob s bold font Find Columns with Data vi Sweep Data Table in Columns with Data in ItemNames Columns with Data out Figure 5 16B Bob s bold font on several icons in a diagram section appears to shout 151 moos MEE Falsa eo Cancel button not pressed ee eee DO ee ee ooeeeee eee Write DR Data ee ee ee ee eee ee LER EBELELELELLELELELELELLELELELELELLELELELELELLELELELELELLELLEELELELE ELE ELELELELELELELL a a a a a a a a a a a a a A a a This section contains examples of icons and connectors corresponding to instrument drivers The NI Instrument Driver Guidelines covers comprehensive requirements for the development of LabVIEW Plug and Play compatible instrument drivers Most of the Plug and Play requirements are consistent with the rules presented in this chapter as well as throughout this book with a few minor exceptions that are noted where they appear Figure 5 17A shows the Initialization VI for a fictitious medical instrument the PM 1000 The icon consists of three lines of text and a glyph of a bedridden patient on a plain white background The glyph is cute but the icon is a bit too text heavy giving it a complicated appearance The connector uses the 5x3x3x5 connector pattern and 12 of the 16 terminals are assigned to controls and indicators with recommended priority If all 12 of these terminals are wired on the caller s
286. h task as a result of these shortcuts 2 3 Configure the LabVIEW Environment Developers do you e Begin most applications with a blank VI e Use LabVIEW s default settings for the size color and fonts of GUI controls and indicators e Create user interfaces that are functional but bland Alternatively do you spend excessive time e Reconstructing your favorite design patterns o Searching previous projects for reusable source code e Deselecting View As Icon or disabling auto wire routing e Making similar repetitive edits throughout most applications If you answered yes to any of these questions you need to customize the LabVIEW environment If you start with blank VIs and default control properties you might have decided that attractive panels and ease of use are not important for your particular application or you are not artistically inclined to design a better scheme More likely you have discovered and applied good combinations of colors fonts and properties that go very well together In this case you might be starting with the defaults because you do not have a good system of tracking and reusing elements of your previous applications Your favorite design patterns and user interface themes should be readily accessible as templates The palette views and front panel and block diagram options should be customized to your personal preferences This section covers how to configure the LabVIEW environment to save time a
287. hanged but the system no longer works properly perhaps the only reasonable explanation left is that we the developers have lost our sanity As you might have guessed the proof of concept positively influences LabVIEW style Because the proof of concept normally addresses the most critical requirements of the application the software will likely either incorporate or be modeled after the proof of concept Implementing a routine that has already been tested and debugged reduces the risk of significant changes Additionally the proof of concept might help define some of the application s primary constructs such as the design pattern and data structures When these elements are proven the corresponding application requires fewer changes and fewer changes equates to neater code per Theorem 2 2 Rule 2 3 Maintain good LabVIEW style throughout the proof of concepts My personal pet peeve is when developers are sloppy with their proof of concepts For example we have all met developers who prototype VIs using default control labels such as numeric and numeric 2 and VI names such as Untitled 1 They expect to show us their prototype and have us understand what it does as if we have some type of telepathy As discussed a good proof of concept should be archived and should function as a project reference This makes sense only if it is developed using good style Ideally developers should apply the same good style conventions on
288. has a simple graphic that has been copied from a Windows system prompt The type of confirmation is not identified by the icon View full size image Confirm Quit Connector Assignment Confirm Quit vi 9 Choice Figure 5 21B The icon has been modified to indicate the type of confirmation The standard 4x2x2x4 connector pattern has been applied as well as the assignment of error terminals to facilitate execution ordering View full size image Confirm Quit Confirm Quit Revised vi Connector Assignments it Print VI in Landscape Mode VI shown in Figure 5 22 is composed of three quarters graphic and one quarter text The error in no error graphic consists of a rectangular control panel that represents a GUI panel A single line of text in 10 point small font simply reads Print VI The VI uses the default 4x2x2x4 pattern with the appropriate assignments It is neat and effective and looks great in color Figure 5 22 Print VI in Landscape Mode VI is composed of three quarters demonstrative graphic and one quarter text View full size image Print YI in Landscape Mode vi Connector Assignments Path to YI to Print error in no error error out Dynamic VI Path Builder VI shown in Figure 5 23 contains an attractive icon that is two thirds graphic and one third text The graphic depicts a fork in a yellow road with both roads leading through green grass into an aqua blue horizon Again you pro
289. hat operates in the background This error handling technique is preferred for a deployed application to prevent an error dialog from confusing the user Finally the Quit LabVIEW function is replaced with a Close LabVIEW Conditional VI This routine checks whether the VI is running as an executable before ending the LabVIEW session The quit confirmation error logging routine and close LabVIEW conditional are standard VIs that are available from the organization s software reuse library The experienced peer identified the opportunity for code reuse A brief visual comparison of the Torque Hysteresis VI before the code reviews shown in Figure 10 2A and the VI after improvements from all the three code reviews shown in Figure 10 5 illustrates the substantial difference in software quality that can be achieved through code reviews Additionally code reviews increase developer skill level depth of knowledgeable resources and development productivity Moreover code reviews enforce the style conventions which improve the ease of use efficiency readability simplicity performance maintainability and robustness of your LabVIEW applications Endnotes 336 l A custom test configuration file and a style checklist document in electronic form can be downloaded from www bloomy com lvstyle Advice for VI Based Code Reviews NIWeek2006 presentation by Doug Norman of National Instruments 337 A Glossary This glossary contains
290. he Boolean menu alone controls the case selection of the top Case structure Therefore any code that executes within the Case structure cannot affect the selection of the next state This is counter to the Classic State Machine design patterns in which the desired next state is passed via shift register and wired to the case selector The first alternative presented in Figure 8 27C uses a Classic State Machine Specifically an enumerated type definition maintains the states and one Case structure contains a case for each state The Run VI and Quit Boolean controls are appended to the Boolean menu and polled in the Idle Default frame of the Case structure The state selection is passed via shift register to the next loop iteration Function labels are not visible and free labels have been added to wires exiting shift registers The state machine has states for Initialize Shutdown Blank and Run in addition to Idle Default Finally all control labels are visible 288 Figure 8 27C This diagram contains the same functionality implemented with the Classic State Machine design pattern View full size image mm mo j a ah a ok a ps ERG Po lnia CEI gt State gt F gt VI Ref gt f o In this example it is noteworthy that there is a one to one correspondence between Boolean controls and application states with three exceptions Initialize Blank
291. he Dialog window appearance and System controls to maximize consistency with operating system dialogs Although maintaining operating system consistency is desirable for desktop applications it is generally undesirable for industrial applications Instead industrial GUI VIs including dialogs should complement the industrial application and equipment they are designed for As discussed this usually means larger panels objects and text 3D style controls have a larger and more industrial appearance and help improve visibility over dialog or classic style controls Rule 3 9 Enlarge and center the objects of an industrial GUI VI in proportion to their importance The default size of LabVIEW s controls and indicators is adequate for most desktop GUI VIs but is not sufficient for many industrial GUI VIs Important objects on an industrial GUI should be readily visible from several feet away from the monitor The more important the object is the more prominent it should appear The object s size and location help distinguish its importance Larger and more centered objects are the most visible Resize important controls and indicators larger than the default and position them near the center of the screen Figure 3 3 shows separate desktop and industrial versions of a custom dialog VI used for stereo receiver measurements The VI prompts the user to configure some test parameters and adjust the volume control of the receiver
292. he Torque vs Angle subcluster Next the Torque vs Angle subcluster is bundled into the corresponding element of the single run cluster An array of single run clusters is initialized based on the specified Number of Runs forming the Multiple Run Data array The Multiple Run Data array is bundled into the corresponding element of the single UUT cluster The single UUT cluster then replaces the element of the Multiple UUT Data array corresponding to the UUT Number Initialization of the nested data structure for multiple test runs using one UUT is complete Populate Data Structure VI is inserted into the top level VI just prior to the For Loop as shown in Figure 6 l11E Figure 6 11D Populate Data Structure VI is a subVI that initializes the nested data structure s Multiple Run Data array with Torque vs Angle data containing the NAN constant View full size image PUN Bio ee a n a erie E BLP Ee eN es Tee UL Reed F on Ee Figure 6 11E Populate Data Structure VI is inserted into the top level VI just before the For Loop that performs multiple test runs View full size image 193 Number of Runs Max Rate Sa deng Figure 6 11F illustrates a reliable and efficient method of implementing Run Test VI utilizing the previously populated nested data structure During the time sensitive acquisition loop the data manipulation is reduced to replacing the elements of the torque and angle arrays with real meas
293. he code control contained in an error cluster that is represented as a 32 bit signed integer The code is used to uniquely identify the errors that functions and VIs generate LabVIEW maintains an internal database for which error codes are associated to descriptions Error log file Error reporting technique that utilizes an ASCII or datalog file for logging error information to file Event Driven State Machine Design pattern that combines a Classic State Machine Event structure and queue into a hybrid single loop design pattern that performs GUI event handling as well as buffered state transitioning The Event Driven State Machine 1s appropriate for top level and GUI VIs for applications of medium complexity Event Handling Loop Design pattern that facilitates event driven programming in LabVIEW It consists of an Event structure within a While Loop The Event structure contains an event subdiagram for each configured event and executes the subdiagram code when the event occurs The Event Handling Loop can process a wider variety of user interface activity in a more efficient manner than polling the controls within a continuous loop Event Machine Design pattern that consists of a single Event structure within a While Loop with user events defined for each state and subdiagrams configured for any combination of user and GUI events The Event Machine is recommended for applications where multiple Event structures are requir
294. he diagram opens this way by default The most important frame is either the most frequently executed logical branch or the frame that reveals the most nodes Finally note that maximizing data flow and minimizing Sequence structures goes a long way toward preventing excessive nesting Throughout this book I generally recommend avoiding local and global variables and Sequence structures because they undermine dataflow principles Now let us take a moment to consider their practical applications In some circumstances they serve important and useful purposes Rule 4 29 Use write local variables for initializing control values Rule 4 30 Use global variables for simple data sharing between parallel loops or VIs Write local variables are the best method for writing to controls from the diagram This is necessary to programmatically initialize control values such as configuration parameters that are read from file Also local and global variables represent a fast and easy method of sharing data between parallel processes such as continuous loops or VIs However read local and global variables have no inputs and write local and global variables have no outputs Therefore the execution order of a sequence containing local and global variables cannot be specified using data flow Property Nodes and Shared Variable nodes have error terminals and are viable dataflow alternatives to local and global variables A Property Node configured
295. he icon terminal labels and description for each are shown in the Context Help windows in Figure 7 8A Curiously Simple Error Handler VI is more prevalent throughout the LabVIEW shipping examples than General and appears to be the more popular choice However the Simple Error Handler VI is a trivial subVI that violates Rule 4 9 The diagram is shown in Figure 7 8B Simply stated it does not add enough value to warrant its existence Instead it reduces flexibility and adds unnecessary processing overhead Each subVI layer entails a nominal amount of processing time including the call to Simple Error Handler VI that contains General Error Handler VI Therefore the General Error Handler VI is the more flexible and efficient choice for dialog error reporting Figure 7 8A The Simple and General Error Handler VIs are selected from the Dialog amp User Interface palette The primary difference is the number of terminals exposed View full size image Context Help Simple Error Handler yvi error code no error 0 ae error source 7 Dialog amp User Interface type of dialog OK msg 1 gt Indicates whether an error occurred If an error occurred this a aa i a Context Help General Error Handler vi user defined descriptions user defined codes error code 0 error source type of dialog OK mg 1 error in no error exception action none 0 exception code exception source Indicates whether
296. he icons throughout the driver Input and output operations performed by nodes that make calls to device drivers DLLs or shared libraries the operating system or any application or resource that is external to the LabVIEW environment including 345 remote instances of LabVIEW I 0 operations include all of the nodes available on the following palettes File I O Measurements I 0 Instrument I 0 and Data Communication I O names References to a specific instance of an open resource such as an instrument or device They function similarly to a pointer to a data structure describing the resource I O names provide an intuitive display of the current value For example VISA IVI and DAQmx Name Controls provide information about the specific hardware that they reference kink A very small bend in a wire L LabVIEW 2 style globals Another name for functional globals because this was the only method of sharing data without data flow prior to the introduction of local and global variables in Version 3 0 LabVIEW Advanced Virtual Architects LAVA Organization committed to the open and unbiased exchange of ideas on intermediate to advanced topics in LabVIEW The URL is www lavausergroup org LabVIEW Tools Network An NI sponsored site within the LabVIEW Zone that lists developer resources including add on products books tutorials and more LAVA See LabVIEW Advanced Virtual Architects loop subVI C
297. he project repository and choose an existing folder Repeat this process for each folder in the repository that you expect to utilize during LabVIEW development This generally includes all LabVIEW source folders appearing under LV source as well as requirements specifications instrument manuals and other reference materials Figure 2 8 provides an example Dissimilar to the repository on disk however you are free to move file references within Project Explorer as often and as randomly as you like How you arrange files and folders on the project tree has no effect on the location of the corresponding files and folders in the repository Figure 2 8 The LabVIEW project contains an organized folder hierarchy that reflects the application s architecture similar to the project repository View full size image 50 gt Project Explorer Project Name vproj File Edit View Project Operate Tools Window Help jagal taxe ee Ry I ee S Q My Computer Documentation B Reference t Bill of Materials cS Correspondence H O Instrument Manuals SS Schematics as Specifications l Test Plans Shipping Docs Config Files L ab Application Name Config ini Help Files Log Files Application Name Documentation 1 0 0 dorc E Read Me html Read Me txt Graphics Logos O Screen Shots B 9 LV Source i Analysis Configuration DAQ Data Manipulation 5 Diagnostic VI
298. he simplistic GUI aids the user in error free operation Spaghetti VI see Figure 1 3 however suffers from unreliable and sluggish performance because of the inefficiency of the single loop architecture and other style related factors previously discussed Consider subVI performance as its execution speed Section 1 1 6 mentioned the relationship between simplicity and performance Often the fewer nodes that are used to implement a subVI the faster the subVI executes For example the three implementations of the Remove Backspace VI in Figure 1 11 have different execution speeds Figure 1 12 shows the Profile Performance and Memory window containing the timing statistics after each VI was called 1 000 times within a loop with a long string passed to the string in terminal Thirteen Nodes VI consumed only 5 6ms Twenty Two Nodes VI consumed an average of 5 9ms per run and Twenty Five Nodes VI consumed 12 8ms Although 5 6ms 5 9ms and 12 8ms might 26 seem insignificant if the three development styles are extended to every subVI throughout a large application the performance difference becomes substantial Figure 1 12 The Profile Performance and Memory window displays the timing statistics after each Remove Backspace subVI runs 1 000 times View full size image Profile Performance and Memory H Timing statisties Profis memory usage C Timing details Memory usage Time unk Sre unk miisaconds w Hlotytes
299. he standard error in and error out clusters on the panel and assign them to the bottom left and right connector terminals respectively These standard connector assignments conform to Rule 5 26 Next surround the completed subVI diagram with a Case structure and wire the error in cluster to the selector terminal By default the code will be contained by the No Error case Use the Positioning tool to move the selector terminal near the bottom of the Case structure and align the error in case selector and error out terminals vertically Propagate the error cluster from the selector terminal to all nodes within the Case structure that have error terminals Pass the error cluster through a tunnel on the right border to the error out indicator terminal Inside the Case structure s Error case pass the error cluster data from the selector terminal straight through the output tunnel on the Case structure s right border The Error case is empty aside from the error cluster In the No Error case you can also modify the error information such as applying a user defined error code and description if an error occurs The error source can be provided by reading VI Server s VI Name property or reading and indexing the call chain returned by the Call Chain function The latter function returns the entire hierarchy of callers which helps pinpoint the specific instance of a subVI call that generates an error The Case structure ensures that all
300. he terminal assignments should resemble the front panel layout Recall that panel layout rules were provided in Section 3 2 Text Consistent placement of controls and indicators on the subVI panels and consistent connector terminal assignments improve the recognition and ease of use of subVIs throughout your applications Additionally assigning common controls to consistent terminals of a standard connector pattern ensures that the terminals will line up properly and can be wired together without wire bends In the FieldPoint measurement and control example VIs from Figure 5 12 note that the refnums are appropriately assigned to the upper left and right terminals and the error clusters are assigned to the lower left and right terminals Rule 5 27 Choose left and right vertical edge connector terminals for high priority inputs and outputs Rule 5 28 Choose top and bottom horizontal edge connector terminals for lower priority inputs and outputs Be sure to prioritize your terminals appropriately Priority can be specified explicitly using the required recommended and optional priority settings from the context menu and implicitly using the locations of the terminal assignments The most commonly used inputs and outputs are normally assigned to terminals on the left and right vertical edges of the connector pane and the less commonly used inputs and outputs are on the top and bottom horizontal sides The front panel layout should
301. his consists of bold 13 point Application font with default values in plain text enclosed in parentheses View full size image gt Interval Jimer vi Front Panel File Edit ew Project Operate Tools Window BOM SS 77 Visibility is very important for industrial GUI VIs The following rules enhance the overall visibility of front panel text for multiuser industrial applications Rule 3 25 Maximize the contrast between text color and background color Rule 3 26 Use large text size for command buttons and critical data Important data and objects on the GUI VI panels of an industrial application should be readily visible from several feet away Use large objects and text and maximize the contrast between the text and background colors Specifically when using the default black text color lighten the shade of the background and increase the size of the text Increase the font size from the Font Style dialog or select the text and press the lt CIRLO lt F gt keyboard shortcut Each instance of the keyboard shortcut adds 1 point to the text size Alternatively use white light or bright colored text ona dark background Since many people have trouble distinguishing certain colors the level of contrast is more important than the actual colors Rule 3 27 Allow extra space between labels and objects for multiplatform applications As discussed in Section 3 2 1 General Rules font appearance is platform and
302. hows the diagram of Torque Hysteresis VI implemented without clusters Many wires and terminals are required to pass the data from the first three subVIs to the successive subVIs The subsequent subVI Compute Statistics VI generates six additional calculated values that are used by Generate Report VI and Save Data VI However the latter two subVIs do not have enough terminals to receive this data using wires despite the use of the connector pattern with the maximum 28 terminals as shown in Figure 6 4B Instead global variables are used to pass the excess data between subVI diagrams Additionally maintenance of this VI is a chore If an additional parameter is added to the UUT Information Dialog corresponding controls terminal assignments and wires must be added to three subVIs or new global variables are required 172 Figure 6 4A Top level diagram of Torque Hysteresis VI implemented without clusters The wire and subVI terminal density complicates maintenance View full size image Part Status i 2 Production Tear i 2 Test Reference gt ii Gadin Number TTT gt Teal Description axana Cycles i0 Board 15 1 Fa aon DI ee ae rT Fimi Angle deg gt Figure 6 4B Although Generate Report VI has the connector pattern containing the maximum 28 terminals it is not sufficient for the six additional values calculated in Compute Statistics VI Applying Rul
303. ht click on an existing wire and select Clean Up Wire to automatically reduce kinks loops and bends Rule 4 12 Maintain even spacing of parallel wires When propagating parallel wires maintain consistent even spacing between each node or bend There are two useful techniques for ensuring even spacing First use similar connector pattern and terminal assignments for all subVIs that are intended to be used together Connector pattern conventions are covered in Chapter 5 Second align the nodes horizontally before wiring Simply select the nodes and choose any of the horizontal alignment tools from the Align Objects menu on the toolbar In Figure 4 4 a data logging routine is developed using several File I O functions that comprise a dataflow sequence The file refnum and error cluster are propagated using parallel wires The File I 0 functions have similar connector patterns which helps facilitate even spacing However even slight offsets in the horizontal alignment cause wire kinks In Figure 4 4A the diagram is initiated by dropping the functions from the palettes onto the diagram In Figure 4 4B the Bottom Edges alignment tool snaps the nodes into perfect horizontal alignment You can also distribute the nodes with even horizontal gap using the horizontal distribution tool In this example an asymmetric horizontal gap is desired to provide extra spacing within the While Loop In Figure 4 4C clear wiring proceeds with even vertical spacing
304. ht to left data flow On the surface it appears to represent good right to left data flow style In a pinch one neatly labeled right to left wire never hurt anyone However it is still right to left nonetheless a rule violation Figure 4 17 Right to Left VI is a top level VI containing low level functions for forming the file path a local variable a vertical tunnel and right to left data flow View full size image rer ie et tare H i Close inspection reveals that the right to left wire is a file path that is formed by some low level functions within an inner Case structure Whenever a collection of low level functions that work together to perform a cohesive routine appears on the diagram of a high level VI it is a good opportunity for a subVI as per Rule 4 7 Additionally the Case structure s False case contains a read local variable for the File Path indicator This represents an opportunity for a shift register to replace a variable Finally a wire is routed from the Lot control terminal vertically through 126 a tunnel in the inner Case structure s bottom border This violates Rule 4 13 Hence one right to left flowing wire leads us to four style rule violations e Layout This is a sparsely populated Classic State Machine design pattern which is discussed in Chapter 8 e Modularity This application is relatively simple containing only 6 user VIs and 173 nodes for a modularity index of 3 5 As
305. iagram Put them to good use 7 1 Error Handling Basics 204 An error is a failure of a function or VI to complete its programmed task Most nodes in LabVIEW propagate error data using the error in and error out clusters The error clusters consist of a Boolean for the status an integer for the code and a string for the source and are located in the Array Matrix amp Cluster controls palette The status uniquely indicates whether an error has occurred The code identifies the type of error and is used by the error reporting VIs to look up a corresponding description The source identifies the function or VI that generates the error The error cluster s default value consists of status FALSE code 0 and source lt empty string gt indicating no error or warning If the status is FALSE but the code is nonzero and the source is not empty then a warning occurred A warning is similar to but considered less severe than an error For example the node is successful at performing its programmed task but there is something unusual about the input values or result Examples of the error out cluster containing no error a warning and an error are provided in Figure 7 1 The default no error value is shown on the left The warning shown in the middle is commonly returned from a successful call to VISA Read It indicates that the number of bytes transferred is equal to the requested input count but more data might be available The error clust
306. ight gray background to maximize the contrast and clarity Icons are effectively used to maximize ease of use Figure 3 19 Parafoil Guidance Interface is a virtual cockpit It contains a high density of data while maintaining good readability View full size image Br iard Pte al AE hi 00 02 Q ps OA Ig iat i 1 it Application Parafoil guidance interface Conditions Virtual cockpit with maximum object density and visibility Color scheme White or light gray background with black text Layout VSI and HSI graphs are prominently sized on the panel Controls within Control decoration are Disabled and Grayed Out during auto control mode Heading text 15 and 18 point bold black MS Sans Serif font on a white or light gray background Boolean text 13 point normal black Arial font on default gray background Numeric and String Control Data 13 point normal black Arial font on white background Secondary Fonts 14 point normal black Arial font on white or gray background 93 Navigation Tab controls and menu bar Endnotes l Ritter David LabVIEW GUI Essential Techniques New York NY McGraw Hill 2002 2 Instrument Driver Guidelines online tutorial available from NI Developer Zone 3 Rosenthal Odeda and Robert H Phillips Coping with Color Blindness New York NY Avery Publishing Group 1997 94 4 Block Diagram The LabVIEW block diagram excels at conveying source code A really good diagram i
307. igure 3 1 The GUI VI panel contains objects for two test fixtures grouped using spacing decorations and clusters View full size image 57 Ce e a oo 1625 oo oe oo 00 Teal 0 0 f E I e f Chamber Temp pH 7 died Show Charts Rule 3 2 Apply symmetry and spacing to front panel objects My favorite LabVIEW editing tools are Align and Distribute Objects accessible from the toolbar I use these tools on every front panel I create whether it is a GUI VI or a subVI These tools are instrumental to effective and professional looking front panel layouts When viewing a panel for the first time you can immediately see whether the developer uses these tools Panels with aligned and evenly distributed objects are neat and symmetric whereas panels without them appear more haphazard The best thing about these tools is that they are quick and easy to use There is no good reason not to use them The front panel of Figure 3 1 is symmetric with the left and right halves of the screen appearing as mirror images of each other The indicators within the temperature and power clusters are also symmetric and evenly spaced The 32 temperature indicators are arranged in 4 columns of 8 indicators tightly compressed so that each indicator is as close as possible without overlapping the next one This is achieved by selecting a set of objects such as one column of indicators and then using the Vertical
308. igure 7 23 The diagram of Read Lines from File VI includes error handling via the General Error Handler VI The error dialog is not desirable for unattended operation View full size image EEZ mark after read chars EFF EOF The Load Script case contains a sequence of subVIs that interact with a spreadsheet application The error cluster propagates among all subVIs but the error is not trapped or reported The last subVI in the error chain s error out terminal is not wired allowing the error data to disappear Likewise the error handling throughout the remaining cases is incomplete This is evident at a glance because the error cluster does not enter or exit the Case structure or While Loop Incomplete error trapping combined with no error reporting equates to nonexistent error handling 236 In Figure 7 24 Test Executive VI has been modified with proper error handling First the error cluster is read from the error in control terminal and initializes a shift register near the bottom of the While Loop The error is read from the shift register and passes through a tunnel near the bottom of the Case structure Each case of the Case structure including Initialize and Load Script propagates the error cluster among all nodes that have error terminals and returns the error cluster through the tunnel at the bottom right of the Case structure Therefore an error generated by any node in any case is trapped inside the error
309. ily Note that any terminals not contained by a repeating structure are read only once This is a problem for Boolean controls configured with latching action In this case the control is not able to reset itself If the terminal s control is associated with an event that is registered by an Event structure place the terminal within the corresponding event case This ensures that the terminal s value will be read each time the event fires For unwired terminals not associated with any events simply place them to the left of the diagram s primary structure Rule 4 20 Modularize wires of related data into clusters It is much easier to implement clear wiring techniques and maintain organized diagrams if you reduce the overall number of wires you have to work with Use clusters to group related data and reduce the quantity of wires Wherever you have several wires of related data that are needed in the same areas of your diagrams replace the individual wires with a cluster This is analogous to modularizing low level routines into cohesive subVIs The data elements in the cluster should be related and serve a common purpose For example consider the measurement routine from an optical filter test application shown in Figure 4 5 The application prompts the user to define the laser scan parameters configures a wavelength tunable laser source measures the filter s transmission characteristics graphs the data and saves the data to fi
310. improves the interchangeability of development machines and is a good CM practice The LabVIEW options are maintained in the LabVIEW ini configuration file located in the root of the LabVIEW installation directory I recommend backing up this file or perhaps even saving screen shots of the options dialog box This enables you to reconfigure a new or existing machine quickly It is also possible to distribute the LabVIEW ini file to maintain consistent options across multiple machines However beware that the contents of the LabVIEW ini file are version platform and system sensitive In addition to environment preferences the LabVIEW ini file maintains paths to the most recently used files and directories which are relevant only toa specific machine Most LabVIEW developers reorganize their palettes frequently to suit their particular editing needs at any given time This consists of arranging the top to bottom order of the categories as well as expanding minimizing showing and 40 hiding them Additionally custom palettes are configured using Tools Advanced Edit Palette Set Custom palettes are strategically important with respect to code reuse Code reuse is an essential technique in all modern programming environments The LabVIEW VI is a self contained software module that can be called from higher level VIs as a statically linked subVI or dynamically by configuring Call Setup or using the VI Server Call by Reference Node
311. in I have observed a discrepancy between hardware and software development practices For hardware the design phase is a given Schematics always precede the soldering of circuits Block diagrams and CAD drawings precede the fabrication of mechanical assemblies fixtures and equipment racks Design documentation is essential for hardware Why should software be any different A few explanations albeit excuses might exist In some applications the software design is discovered by experimenting with an implementation For example one or more prototype LabVIEW VIs are developed to help characterize the application that the software is intended to solve Specifically prototypes are often developed to determine how fast a data acquisition and online analysis routine can run on a given computer The working prototype VIs might eventually evolve into an integral portion of the software Many LabVIEW developers prefer to expand the prototype into the finished application following the Code and Fix development model This can lead to trouble as you saw in the Spaghetti VI example in Chapter 1 Discipline is required to stop with a working prototype and to go back and complete the specification and design phases before continuing with software development Another explanation is that a design phase is not necessary for very simple applications Indeed LabVIEW applications range in complexity from very simple to very complex If a
312. in the Context Help window and promotes wire crossovers in the diagrams of the calling VIs Figure 5 20 DAQ Assistant Express VI has inputs assigned to terminals on the right causing wire crossovers on the calling VI diagrams View full size image DAQ Assistant pit Connector Assignments stop T timeout sec data HIP a error in seenen rror out task out 156 Figure 5 21A shows the icon and connector of Confirm Quit VI The icon contains a familiar Windows system symbol that accompanies various confirmation dialogs The glyph appears big bold and unmistakable However it does not indicate the specific type of confirmation It contains only one output assigned to the middle terminal of the 3x3 connector pattern In Figure 5 21B the Confirm Quit dialog is revised with error clusters a 4x2x2x4 connector pattern and an icon combining text with the graphic The type of confirmation is specified on the icon by reducing the glyph size and adding the word QUIT using 9 point small fonts Error cluster terminals have been added to facilitate execution ordering via data flow The 4x2x2x4 connector pattern allows the input and output terminals to line up with VIs utilizing this standard connector pattern In the future the VI could be enhanced to handle more than one type of confirmation by adding an enumerated control to specify the confirmation type and changing the icon text as appropriate Figure 5 21A Confirm Quit VI
313. in the context of this book dialog VIs need not contain exactly the same properties as the Dialog window appearance and are not necessarily modal Collectively I refer to the set of all VIs that have user viewable panels including top level VIs and dialog VIs as GUI VIs VIs that do not open their panels during program execution are subVIs This is a simplification because many top level and dialog VIs are called as subVIs In this book however GUI VIs have panels that users can view and operate on whereas only developers can view subVI panels Therefore the scope and function of GUI VI panels substantially differ from those of subVI panels and separate guidelines apply to the front panels of each These two VI categories are distinguished whenever separate guidelines are applicable as each style topic is presented throughout the chapter GUI VIs intended for personal use in an office private laboratory or other individual computing environment are referred to as desktop GUI VIs GUI VIs designed for multiple users to interface with industrial equipment are industrial GUI VIs Desktop GUI VIs appear and behave similar to other native applications on the computer s operating system whereas industrial GUI VIs complement the industrial application and associated equipment regardless of the computing platform upon which they are installed Hence separate rules apply for desktop and industrial GUI VIs Some subVIs are application specifi
314. ing Hence the Error Case Structure maximizes efficiency in the event of an error Additionally notice that the diagram nodes have been horizontally aligned about an axis defined by the error terminals 220 Figure 7 12A An instrument driver subVI contains incomplete error trapping and inefficient recovery when an error is received View full size image VISA session lie ELS dup VISA session error in mo error EE pee wE a EE out Figure 7 12B The error cluster is shared among all functions that have error terminals and an Error Case Structure maximizes efficiency in the event of an upstream error View full size image LabVIEW functions and subVIs that do not skip their code on error include most nodes that terminate a session to a resource This includes Close Reference VISA Close Close File and several nodes that have Destroy Stop Close and Clear in their name SubVIs that call these nodes should not skip them on error Instead confirm their behavior by carefully reading the error in terminal description in the node s detailed help or inspecting its diagram If the description indicates the node runs normally if an error occurs before it the node should be placed outside the subVI s Error Case Structure As an example all LabVIEW Plug and Play instrument drivers include a Close VI that calls the VISA Close function These subVIs do not contain an Error Case Structure Rule 7 15 Use the SubVI with Error
315. ing The more layers of structures within structures there are ona diagram the more difficult it is to comprehend the data flow This equates to poor readability and maintainability Most design patterns have three or fewer layers of nesting 240 8 1 Simple Design Patterns The simple design patterns are common and very easy to understand and implement They include the Immediate SubVI Functional Global Continuous Loop and Event Handling Loop The Immediate SubVI design pattern consists of the nodes that comprise the subVI and error trapping There are no continuous loops dialog windows or GUI panels Instead an immediate subVI executes its code straight through to completion in the order in which the error cluster propagates through the nodes on the diagram Error trapping is incorporated per the subVI error handling rules presented in Chapter 7 Error Handling This includes an Error Case Structure for skipping the diagram if an error is detected standard error in and error out clusters and connector assignments and error propagation via error cluster wires As the name implies the Immediate SubVI design pattern is very common with subVIs and is the simplest design pattern You can use the SubVI with Error Handling template to develop immediate subVIs This template is available from the New dialog by selecting VI From Template Frameworks SubVI with Error Handling It consists of error clusters the standard 4x2x2x4 c
316. ing for parallel error chains and For Loops First the error cluster is propagated to the error in terminal of all nodes that have error terminals Second the error out terminal of the last node in each chain is wired to the Merge Multiple Errors VI Error propagation between For Loop iterations is accomplished using a shift register Finally the Wait n mSec VI replaces the Wait ms function and Case structure This VI s error terminals facilitate execution ordering via natural data flow and the icon s unique size and shape allow easy insertion within an error chain The delay now reliably occurs at the beginning of the bottom error chain before the four Find Screw Ends VIs Figure 7 21C is similar to Figure 7 21B except that a While Loop replaces the For Loop and a few nodes are rearranged The While Loop allows the loop to terminate if an error occurs in any loop iteration Nodes are rearranged so that the error cluster is the bottom most output tunnel Anew input tunnel is required to specify the desired number of loop iterations The input tunnel is located below the error cluster input tunnel to pass it to the expression that evaluates the stop condition without crossing other wires and objects Hence Rule 7 17 has been sacrificed in favor or Rule 4 15 Figure 7 21B Error trapping is completed by extending the error cluster to all error terminals merging parallel error chains and passing the error cluster between loop iterations using a
317. ing to the specification is an excellent exercise for a developer preparing to design the application During all meetings I attend I always strive to understand the project s requirements as thoroughly as possible No matter how confident I may feel at meeting s end I have never proceeded to sit down and write a specification without new questions and ideas arising in the process Following up on these ideas translates to better understanding and agreement of project scope which translates to a better LabVIEW application and more satisfied end users Theorem 2 2 Unforeseen scope changes hinder good style Unanticipated expansion of scope can hinder good style and in the most severe instances may even diminish an application into spaghetti A developer may choose to begin with the simplest architecture that satisfies the application s initial requirements in order to expedite development For example a simple top level architecture consisting of a single While Loop may be chosen This architecture is often not amenable to an application s expanded requirements When the scope changes substantially the While Loop is expanded to accommodate more nodes and wires and may become large and unwieldy Also as the diagram s node quantity increases there are more obstacles preventing neat placement of additional nodes wires and structures resulting in a sloppy diagram Indeed this may have happened in the Spaghetti VI examp
318. ions Specifically register custom error codes by creating an XML based text file in the LabVIEW user lib errors folder This is accomplished using the Error Code File Editor that is launched by selecting Tools Advanced Edit Error Codes The user defined error codes are registered within LabVIEW similar to a predefined error This has several advantages over the diagram definition method First you need not wire additional inputs to the General Error Handler VI Second the user defined error codes may be used within any VI that runs under the instance of LabVIEW that contains the XML file Finally the Explain Error feature recognizes and explains errors defined in this manner Explain Error is a built in utility that displays information about an error code in a dialog It is invoked by selecting Explain Error from the shortcut menu from any error cluster or by selecting Help Explain Error When deploying applications be sure to create an installer or source distribution that includes the XML file Rule 7 12 Use negative codes for I 0 device errors and positive codes for warnings 217 With I O devices such as data acquisition hardware positive codes are used to represent warnings that may not directly affect the application s success and negative codes represent significant errors that require corrective action or termination of the VI As previously discussed the error cluster s status Boolean uniquel
319. is being dragged and merged onto a diagram 266 Figure 8 17A Custom palette view contains several icons representing single loop design patterns The icons are configured to merge the corresponding design pattern onto the selected diagram ae eet LOCAL Figure 8 17B Click drag and drop the design patterns from the Functions palette to merge them onto a diagram View full size image G i Parallel loops are required for applications that have multiple continuous tasks that must execute in parallel For example you might have a data acquisition application that acquires and logs data to file Concurrently the user needs to process and display the data on demand in response to user interface events However the data processing and display tasks cannot interfere with the continuous acquisition and logging task Consequently you cannot put both tasks in separate cases of an Event Handling Loop or state machine Using any of the single loop design patterns any subVI event or state temporarily suspends the loop preventing other subVIs events and states from executing until the code completes Instead the best way to architect this application is to divide the simultaneous tasks into separate 267 parallel loops Use a Continuous Loop exclusively for the continuous acquisition and logging task so that it operates without any interruptions from the user interface Likewise implement the user interface event hand
320. ise Property Nodes and local variables that are created for them may appear ambiguous Figure 5 19A The lengthy terminal labels of Form Com Params Cluster Into a String VI are truncated by the Context Help window View full size image Form Com Params Oluster into a String vi Conmector Assignnvents Lommuncation Parameters Cl eT Communication Parameters St oll In Figure 5 19B abbreviated terminal labels are applied along with the suffixes in and out for the control and indicator respectively The corresponding terminal labels are succinct intuitive and unambiguous As discussed in Chapter 3 succinct and intuitive labels are very important However I am not an advocate of abbreviations and acronyms unless they are either universally familiar or extremely intuitive as they are in this case Additionally error terminals have been added to promote proper data flow Figure 5 19B The label lengths have been reduced and error terminals have been applied to promote proper data flow View full size image Form Com Params Cluster into a String Improved yvi Connector Assignments Com Params in Com Params out error in no error error gut The example shown in Figure 5 20 is the DAQ Assistant Express VI icon and connector assignments with View as Icon selected from the shortcut menu This particular configuration generates inputs assigned to terminals on the right instead of the left This causes wire crossovers
321. isition VIs must share their task and error cluster by propagating wires between the corresponding input and output terminals of each VI Likewise instrument control VIs share a common VISA resource name and error cluster Wire bends are avoided if the related VIs have the same connector pattern with the common inputs and outputs assigned to the same terminals As we can see from Figure 5 10 the DAQmx VIs all use the 5x3x3x5 connector pattern with the task assigned to the top left and right terminals and the error cluster assigned to the bottom left and right terminals In Figure 5 11 the niDMM VIs all use the 4x2x2x4 connector pattern Figure 5 10 Cont Acq amp Graph Voltage Int Clk VI is a LabVIEW shipping example containing several DAQmx VIs The standard icon view is shown at the top These VIs utilize the 5x3x3x5 connector pattern as shown at the bottom View full size image 144 Pani Vaha HALE Maaien Vale Pei Sampie Rate Physical Channel eerie fo fel__tal f __ S m I LB Se Seas po i gt E Minimum Wahua Terk Sample Fate Samek bo fial s Pe iia ri Hadma Yeka Em fae ee ra Pirika Thane H E l lail Bill sega lll Tie Ty miii gt fl A vokas E a Figure 5 11 DMM Measurement VI is an example VI from the niDMM instrument driver These VIs all utilize the 4x2x2x4 connector pattern as shown at the bottom View full size image el Measurement Resolution Degts
322. k and help ensure reliable and robust applications Finally when creating routines that contain nodes without error terminals error propagation via Error Case Structure or subVI with error terminals is optional In the latter case propagating the error cluster to the subVI is useful for data flow maintains standard connector terminal patterns and assignments and optimizes error reporting and recovery Figure 7 14A contains the front panel block diagram and Context Help window for Wait n mSec VI This subVI simply inserts a delay within an error chain The unique icon shape and connector pattern are discussed in Chapter 5 Icon and Connector The diagram contains the Wait ms function and proper subVI error handling Note that the Wait ms function does not have error terminals and cannot generate an error It is an example of a low risk function However the subVI incorporates error propagation and an Error Case Structure for skipping the code on error per the rules in Section 7 2 Moreover the subVI s error terminals allow the calling VIs to use data dependency to specify when the delay occurs and to skip the delay if an error is already present Figure 7 14A Wait n mSec VI consists of the Wait ms function a low risk function without error terminals and proper subVI error handling including the Error Case Structure View full size image Wait m misec wi Front Pamel fie Edt Yew Prot Gpernte Took Window bel Conbest H
323. l it is inherently not textual Documentation serves to supplement and reinforce the readability of the graphical source code Theorem 9 1 Good development style reduces source code documentation effort Readability is one of the key attributes of good style as discussed throughout this book The more human readable the source code implementation is the less documentation effort is required Indeed some overlap exists between readable source code containing intuitive embedded text and documentation For example in Chapter 3 Front Panel Style the importance of succinct and intuitive control labels is emphasized In Section 9 2 the relationship between visible control labels and diagram readability is discussed Multiple constructs in LabVIEW use text to facilitate readability including the Bundle and Unbundle By Name functions and the enumerated data type As another example if your top level diagram architecture is directly implemented using one of the standard design patterns discussed in Chapter 8 Design Patterns then you need not write a lengthy document describing how the design pattern works Other references are available for this purpose Standard design patterns are recommended for good readability maintainability and performance However if you decide to create your own custom VI architecture then 296 it will require documentation The more unique or complex the implementation appears
324. l feasibility It is extremely important to get an idea of how much funding is available and the required completion date at the outset of the project It is common to identify significant obstacles at this point 34 For example you might find that the lead time or expense of some of the required elements exceeds the requirements The earlier in the project that you identify such discrepancies the more time and effort you will save I like to organize the requirements into four primary categories acquisition analysis presentation and test methodology The acquisition section describes the required measurement and control hardware interfaces I find it useful to create a spreadsheet that itemizes every physical parameter or input and output 1 0 point that must be measured or controlled including the engineering units range accuracy and rate at which the parameters must be acquired or generated This information can then be used to specify transducers signal conditioning digitizers instruments and control devices These choices are then entered right into the same spreadsheet providing further definition of the system Note that the hardware interfaces and I 0 requirements help determine the scope of the LabVIEW development required to access these interfaces such as readily available instrument drivers lower level development using VISA or DAQmx or simple interactive configuration using Express VIs The analy
325. l of an instrument driver subVI contains numeric controls for programming the measurement function and resolution The subVI call uses numeric constants for specifying these values which are not meaningful and prone to error View full size image Manual Resoktion 5 5 5 5 igts 6 5 Manual Range 1 00 ETE Figure 6 8B The instrument driver subVI panel contains text ring controls instead of numeric controls Constants created from the terminals shortcut menus provide pull down lists containing a discrete number of intuitively labeled choices View full size image Manual Resolution 5 5 5 5 gsis mm meea pemg me n pe Arme er Ler Reun ss at S a ee j E PEHA be ss oo f z i a n ae Hee m reri ee fi be Rule 6 16 Use enums liberally throughout your applications The text selections for enums and rings form a very important source of documentation on the panel and diagram that is built into these controls I recommend enums over rings for maximum readability including the case selector text However there are a few exceptions where a ring control must be used over an enum Use a ring control when the text labels map to nonsequential negative or fractional values or if localization is an issue Also use a ring control 180 when creating an instrument driver or developer toolset intended for international distribution The text embedded within an enum uniquely defines its data in
326. l terminal labels visible on the diagram 9 2 Apply free label comments in select locations 9 3 Label every subdiagram of every multidiagram structure 9 4 Label algorithms constants and Call Library Function Nodes 9 5 Use default 13 point plain black application font for all diagram text 9 6 Leave notes for the development team 9 7 Use enumerated data types with Case structures 9 8 Create the icons and descriptions as you develop your source code 9 9 Provide online documentation with deployed applications Chapter 10 10 1 Enforce your organization s style convention using code reviews 10 2 Utilize a combination of self reviews and peer reviews for best results 10 3 Perform a self review prior to every peer review 361 10 10 10 10 10 10 10 10 10 10 1 Enforce your organization s style convention using code reviews 4 Use the VI Analyzer to automate tedious inspections 5 Customize the VI Analyzer s test criteria 6 Customize the rank for each test according to the priority of each rule 7 Use a manual checklist to perform a comprehensive style review 8 Perform at least one peer review per project 9 Involve the following people in a peer review project manager lead developer experienced peer and an inexperienced peer 10 Bring the requirements specification and style rules checklist to the peer review 11 Designate a neutral party to take notes on desired
327. l text based source code Thousands of lines of plain black text with some colored keywords and comments interspersed Myriad cryptic symbols function names modifiers and hex codes Tracing data flow entails repeated searches for text based variable names Source code consists of text colored text and cryptic text while editing involves manipulating text Let us just say that the overall zest and appeal is somewhat lacking Enter LabVIEW Colorful and intuitive diagrams comprise the source code Functions and routines are represented by icons Data flow is observed by visual inspection Welcome LabVIEW to the formerly mundane world of software engineering Icons are a distinguishing feature of graphical source code They are fun to create and are much more intuitive and visually appealing than lines of text If a picture is worth a thousand words then an icon is worth a thousand lines of code in a conventional text based language It is not often that engineers programmers and scientists have the opportunity to draw cute little pictures LabVIEW is truly a unique environment The icon and connector are often the finishing touches in VI development enabling the VI to be called as a subVI The icon is a graphical representation of a subVI call The connector contains terminals that allow data to flow from the wires of the calling VI s diagram through the controls on the subVI s panel to the terminals wires and nodes of the diagram upon s
328. lave application from Figure 4 11 is modified to execute the 12 step test sequence from Figure 4 12 utilizing the Queued State Machine design pattern for the slave loop View full size image Si Figure 4 13B The functionality of the Queued State Machine design pattern is expanded to accommodate an initialization routine in addition to the test sequence The Initialize case contains the File I O functions that read the scaling data View full size image Test Results a jl n l L E M y Ej gt Test Results gt mannii a et zz z a Sensor Scaling i 121 Figure 4 13C illustrates how the loops stop each other in the event of a user interface event or error If the user presses the Quit Boolean control the Quit Value Change event case adds the Shutdown state to the front of the queue via the Enqueue Element at Opposite End function This causes the Test Sequence Loop to run the Shutdown state and terminate the loop If an error occurs in the Event Loop the Error case of the Case structure releases the Queue causing the Dequeue Element function in the Test Sequence Loop to return with an error and execute the Shutdown state since it is the Case structure s Default case Finally if an error occurs in the Test Sequence Loop it calls the Shutdown state which releases the Queue clears a DAQ task and fires the Quit Value Change event causing the Event Loop to terminate The Queued State Machi
329. le Figure 4 5A contains the panel nonvisible indicators and Context Help window for Define Scan VI the dialog used for selecting the laser scan parameters This is the first subVI called in the measurement sequence shown in Figure 4 5B Define Scan VI provides multiple parameters used by the subsequent VIs As shown in the Context Help window the connector pane is densely populated with individual terminal assignments for each parameter In Figure 4 5B there are many individual wires flowing through a relatively simple routine The subVI connector terminal for Save Scan VI the last subVI in the dataflow sequence is also very densely populated The wiring is kept reasonably neat because of even spacing and judicious terminal assignments However much toil is required to achieve this result and even more toil is required to modify the VI Specifically any change to the required measurement parameters entails changes to the wires subVIs and connectors throughout the diagram Figure 4 5A Define Scan VI is a dialog that prompts the user to specify laser scan parameters It returns 15 parameters through separate connector terminals for each 104 View full size image Caries I _____________ Define Wavelength Scan Paraeetars Tray Abp Toker iarte a Wasa iei seb m rd This WI prompts the une for paramelers of he kasr piai The parameters ane mead ard presi fo Convig He mhen ihe weer clicks Oe i E nberwciead fine Wane bey L
330. le from Chapter 1 The Significance of Style If you take a second look at Figure 1 3 you will see a single While Loop containing many functions subVIs wires terminals and Case structures This is an application that might have started 30 out neat at one time and evolved as the work scope evolved into spaghetti The architecture might have been appropriate for the original requirements but was inappropriate for the eventual requirements Realistically if this were the case the developer certainly should have changed the architecture among many other possible improvements after the work scope changed It is also likely that the application was developed from the outset without a fundamental understanding of the requirements and without a style convention In any event the process of writing and reviewing a specification significantly reduces the potential for large unexpected changes of scope reducing the extent of corresponding source code revisions thereby improving style If a specification document does not exist either the developer does not fully understand the requirements or they reside in the developer s memory The two situations are very similar In the former situation the software will require substantial modifications as the developer s understanding evolves This is also known as the Code and Fix software life cycle development model The more overall edits there are including change
331. lements of documentation There are a few considerations when using this tool First many of the selections are not relevant for a user s manual If this is your objective then you really need to select only documentation elements that apply to the graphical user interface GUI including the VI Description Front Panel Controls Control Descriptions and Control Labels You can deselect everything else Also collect documentation only for the GUI VIs including the top level VI Users may not need to see the inner workings of your subVIs unless they will also maintain the source code or if they need to 311 see the details of any mathematical algorithms Also there are some additional items that flow to the document that will not be useful for the end users Specifically Print VI Documentation prints the specified documentation for every control on the VI s front panel These may include error clusters and any controls or indicators that you use during development for diagnostic purposes that are invisible or have been scrolled off the visible area of the screen Finally the control terminals are provided in the document as they normally appear on the diagram These items are not meaningful for typical end users Consequently you may need to perform significant manual edits to remove the documentation related to the error clusters invisible controls and terminals An alternative method of automating the documentation process invol
332. ling GUI objects Additionally several software components from the organization s software reuse library were recommended including an industrial style two button confirmation dialog a close LabVIEW conditional VI and an error logging routine Diligent notes were taken and the Torque Hysteresis VI was further improved after the code review as shown in Figure 10 5 Figure 10 5A contains the revised front panel Program flow is improved by the addition of the New UUT Boolean control on the Main tab This command button initiates a new test sequence beginning with a dialog that prompts the user for the UUT Information The Prompt UUT Information dialog VI uses large individual controls with tab key navigation which simplifies ease of use compared to the corresponding cluster on the Configuration tab Similarly a Prompt Motion Params dialog VI prompts for the motion cycle parameters and updates the corresponding cluster on the Configuration tab Figure 10 5A Front panel window for Torque Hysteresis lImproved w Peer Review VI Navigation of the GUI is improved via New UUT command button and dialogs that prompt the user for the configuration data including the Prompt Information dialog View full size image 334 a a BL Pay at bod Wy pop Sree A Customer Hodel Part Status Production Froduction Year 2007 Test Reference Spec Number Spec Description Figure 10 5B contains the r
333. ling and display in an Event Handling Loop Create a messaging scheme to share data between loops Rule 8 16 Use queues shared variables or RT FIFOs for parallel loop messaging Messaging between loops can be accomplished using a variety of constructs including local or global variables functional globals notifiers user events queues shared variables and real time first in first out RT FIFO buffers Local and global variables are the simplest but the least desirable in terms of performance and style As discussed in Chapter 4 local and global variables undermine LabVIEW s dataflow principles Functional globals offer performance and flexibility improvements over local and global variables but they require some time and effort to develop and maintain Notifiers are effective but they are limited to storing and passing one message at a time User events buffer the event data but they are more difficult to configure and maintain and the buffer is less functional than queues I recommend using queues or shared variables for desktop applications and using RT FIFOs for real time applications requiring programmatic configuration Queues and shared variables are built in LabVIEW constructs that have evolved over time Queues are available from LabVIEW s Synchronization palette and shared variables are configured from the Project Explorer window You can configure queues and shared variables to buffer more than one message or data element at
334. locate simpler alternatives to string controls For example enums and rings provide a discrete set of alphanumeric choices as previously described Always use path controls for entering or programming alphanumeric data representing a file path LabVIEW formats the path using the standard syntax for the host s operating system This ensures platform portability The Browse button makes it simple for a user to interactively navigate and select a directory or file path and should be kept visible on GUI panels Configure the Browse Options including Selection Mode to restrict the user s choices and validate the data The picture data type stores a sequence of operation codes and data for drawing an image in a picture indicator Operation codes are low level instructions that tell the CPU what to draw There are about 50 operation codes representing a wide variety of geometrical shapes and bitmap configurations The data are the operands for each operation code They include coordinates colors and other operation specific data The operation codes and data stored by the picture data type are transparent to the developer Use the Picture VIs on the Picture Functions palette to create the pictures Because 182 the picture data type is stored in contiguous memory it 1s a relatively simple data type similar to string and numeric arrays 6 3 Data Constructs Data constructs are collections of one or more of the fundamental data
335. lose any open references report errors and exit LabVIEW if desired Because the Panel Close and Application 252 Instance Close events normally cause the VI or LabVIEW to shut down immediately these events must be discarded for the shutdown routine to run This is achieved by wiring a TRUE Boolean constant to the Discard filter event terminal Figure 8 6C Three separate events that trigger a shutdown include the Application Instance Close Panel Close and Quit Value Change The former two events are discarded to allow the shutdown routine to run View full size image re ey cure ean i att In Figure 8 7 the Torque Hysteresis VI has been implemented as an event handling loop based on the template The front panel is identical to Figure 8 5A and is not shown Each of the controls from the Boolean menu have a corresponding Value Change event case that executes the same code contained in the Case structures in Figure 8 5B The Event Handling Loop is more compact flexible and efficient than the Continuous Loop implementation A single Event structure with multiple event cases is more compact than the multiple Case structures Also it is easy to add more event cases or divide the existing event cases into multiple event cases without increasing the height and width of the diagram Moreover it is more efficient than the Continuous Loop because the diagram sleeps until a registered event fires freeing the pr
336. lp Error in nc error Password 2 E e F TEETE AP EA Admin ID Mumber Figure 7 13C Functions that call external device drivers such as VISA have a high level of risk of generating an error and the corresponding errors are often detrimental to the calling application Error handling is essential with I 0 functions View full size image VISA Write YISA resource name mAs VISA resource name out write buffer pa n return count error in ino error F error out Wirkes the data from write buffer bo the device or interface specified by VISA resource name His2 Trigger Count Rule 7 16 Error trapping is required for nodes that perform I O operations recommended for nodes that contain error terminals and optional for diagrams that do not contain nodes with error terminals 224 Trap all errors from all nodes that have error terminals to the maximum extent possible However when style conflicts arise prioritize error trapping according to the three classes of nodes just presented A firm minimum requirement is to positively handle any errors generated by nodes that perform I 0 As discussed I O functions and subVIs have a higher risk of generating errors and greater consequences when errors happen Additionally error trapping is highly recommended for the medium risk nodes that have error terminals but do not perform I 0 Errors returned from these nodes help the developer understand how they wor
337. lthough the VIs in the example use no more than 11 of the 16 available terminals The DAQmx VIs are polymorphic and are designed to support a very wide variety of hardware and applications Indeed some configurations of the DAQmx VIs exceed the 12 terminals available from the 4x2x2x4 pattern and future expandability is critical 145 There will always be exceptions to the 4x2x2x4 rule However you can save at least 1 minute on every VI you create and improve the neatness and ease of maintenance of all of your VIs if you start with a template containing the 4x2x2x4 pattern and a few common terminal assignments such as error clusters LabVIEW has a SubVI with Error Handling template that meets this criteria It may be selected from the New dialog by choosing File New VI From Template Frameworks Rule 5 22 Assign controls to left terminals indicators to right terminals Rule 5 23 Never cross wire stubs in the Context Help window The terminal assignments of the connector pane must promote left to right data flow without wire bends and crossovers Therefore controls must be assigned to terminals on the left and indicators to terminals on the right Otherwise input wires flowing in from the left will cross output wires flowing out to the right The middle terminals may be assigned to controls or indicators as long as they are not intermixed causing crossovers To check your terminal assignments for crossovers inspect the
338. lthough three or fewer is by far the most common Also you can create arrays containing clusters as elements which can contain arrays and clusters ad infinitum Likewise you can create clusters containing any combination of simple data types arrays and clusters which may contain arrays and clusters The only limitation is that arrays cannot directly contain other arrays unless the contained arrays are bundled by a cluster In any event there is no practical limit to the number of layers of arrays and clusters within arrays and clusters that LabVIEW allows us to create 189 Data constructs containing multiple layers of arrays and clusters are referred to as nested data structures and are considered complicated for two reasons First they are confusing for the developer to manipulate on the diagram For example changing a single element of data may require multiple calls to Index Array and Unbundle By Name to access the data element followed by multiple calls to Bundle By Name and Replace Array Subset to rebuild the data structure These cluster and array manipulation functions are frequently intermixed with nested looping structures used to index through the elements of each array layer This becomes tedious and confusing after the first two layers The second reason nested data structures are complicated relates to how LabVIEW stores them in memory Clusters are represented by a heading that describes the order and type
339. ly on different platforms Additional maintenance is often required to resize or reposition the text fields or labels The Greek letter u which I use extensively becomes the letter m on some machines In fact some of the specialized fonts do not even display consistently between the front panel and diagram windows of the same VI Specialized fonts are not recommended if you are distributing applications for use on multiple platforms An alternative is to create the specialized label in an external application save it as an image file and import the image into LabVIEW Rule 3 20 Choose only one font and vary the size boldness and color to obtain multiple styles Application System and Dialog are named to connote three different contexts Yet the world s most popular operating systems Microsoft Windows XP and Vista use one font for all three My recommendation is to choose only one font either Application font for portability or any universal sans serif font suchas Arial for consistency Vary the size boldness and color to obtain different styles for the different contexts Choosing only one font prevents the clashing appearance caused by multiple dissimilar fonts Power Monitor System VI contains eight different font styles as shown in Figure 3 9A Specifically the Application font is used with the following variations of point size and color 42 point bold for the heading Power Monitor System 24 point bold for tab lab
340. ly right click and choose Visible Items Label from the shortcut menu and enter the desired text Likewise the default icon and description of Call Library Function Nodes are not very informative It is useful to label each Call Library Function Node with the name of the DLL and function that is being called Alternatively select Names format by choosing Name Format Names from the shortcut menu This resizes and populates the icon with the DLL s configured function name Previously developers were forced to navigate to the configuration dialog to identify the Call Library Node s function call Rule 9 6 Leave notes for the development team Another good use of free label comments is to leave temporary notes for the members of a multideveloper project team Commercially available source code control tools are highly recommended for multideveloper projects These tools normally prompt for comments when source files are checked in after editing Additionally LabVIEW s VI Differencing tool can identify source code changes between revisions However using free labels you can enter comments directly on the diagrams 302 close to any source code that you want to draw attention to This technique complements or extends the conventional source control and differencing techniques One scenario in which I use this technique is when the developers are sharing one computer collaborating in series instead of parallel This happe
341. m and Chapter 5 Icon and Connector present the basics for VI layout and development Chapter 3 provides rules for layout text color and navigation It distinguishes separate rules for the front panels of GUI VIs and subVIs where appropriate Chapter 4 presents rules for layout wiring and data flow along with techniques for optimizing data flow Chapter 5 discusses good icon development practices and editing shortcuts and covers standard connector terminal patterns assignments and conventions Chapter 6 Data Structures provides rules on data type selection and array and cluster development A methodology is integrated with several useful reference tables for simplifying data type selection and configuration Rules and examples for optimizing VIs involving complex data structures also are presented in this chapter Chapter 7 Error Handling Chapter 8 Design Patterns and Chapter 9 Documentation expand upon the basics Chapter 7 presents comprehensive rules for thorough error handling along with special considerations for error handling within subVIs Chapter 8 discusses common VI architectures that promote good style beginning with simple subVI design patterns and progressing to single and multiple loop design patterns It also describes several variations of the LabVIEW state 7 machine Additionally Chapter 8 presents three complex application frameworks including a dynamic framework that us
342. many definitions In this discussion we consider application and subVI performance Application performance refers to how well the application or VI completes its intended mission Therefore application performance measures are related to the requirements For example if an application is developed to reduce test time test time is the primary performance measure If improving product quality is the primary objective a reduction in the number of defective products or customer returns might be the performance measure Most applications have multiple objectives The most common objectives include increasing the speed and efficiency by which a task is accomplished while improving ease of use Therefore application performance can be considered a combination of efficiency see Section 1 1 2 and ease of use see Section 1 1 1 Robustness see Section 1 1 5 also is required If the application is not reliable and robust performance is meaningless Style affects application performance Referring to the application examples the primary objective of Meticulous VI see Figure 1 1 is to perform reliable acquisition logging and display of physiologic data This objective is achieved using the Multiple Loop Application Framework which prioritizes the important tasks in dedicated loops Nested VI see Figure 1 2 performs configuration of a scientific instrument and then uploads data acquired from a variety of remote sample sites In this case t
343. matically during PC boot up and the top level VI panel covers the entire screen This is accomplished by using the Maximized Window Run Time Position property selected from File VI Properties Choose Window Run Time Position from the Category ring and select Maximized from the Position ring Beware that extra front panel real estate becomes visible when the display resolution increases This can reveal controls that were intentionally scrolled off the visible area during development Be sure to make such controls invisible if you do not want the user to find them Rule 3 5 Size dialog VI panels much less than full screen Rule 3 6 59 Center dialog VI panels Center the front panels of dialog VIs and size them much smaller than full screen This helps distinguish them as dialogs Specifically selecting the Centered Window Run Time Position property ensures that the panel appears in the center of the screen when opened regardless of the host computer s display resolution Without centering your panels would open at the exact location where they were last saved and might require manual positioning by the user Additionally your panel position and appearance is affected by variations in display resolution If you expect the display resolution to change select Maintain proportions for different monitor resolutions and Scale all objects on the front panel as the panel resizes You can select these properties in the Window Siz
344. mbers and letters both displayable and nondisplayable Path Stores the location of a file or directory using the platform s native syntax Picture Contains a set of instructions and data for displaying pictures created with LabVIEW s graphics VIs Digital data Contains digital signal data JB He E Digital waveform Data structure consisting of an initial time t0 represented as a time stamp the time interval between samples dt represented as a double precision floating point numeric and a digital waveform Y represented as a digital data type 163 Table 6 2 LabVIEW Data Types and Descriptions Listed in Top Down Order of Memory Efficiency Boolean Stores a TRUE or FALSE value as a byte integer Waveform Data structure consisting of an initial time t0 represented as a time stamp the time interval between samples dt represented as a double precision floating point numeric a waveform Y represented as a 1D array of any numeric data type and attributes Dynamic Provides signal data and attributes represented as an array of analog waveforms used only with Express VIs Variant Encodes a name data type data and attributes into a generic data type Used with Activex DataSocket and subVIs requiring a fixed interface to handle a variety of data Similar to the list of control types in Table 6 1 the order of data types in Table 6 2 is approximate and overlapping For e
345. med for all testing debugging documentation revisions and upgrades that occur throughout the entire life cycle of the application When taken in this context it behooves us to use good style Theorem 1 2 Good style reduces development time and effort Good development style might require more up front time and effort but the quality of the software is higher from the start and the software is more useful throughout the life cycle of the application I cannot overemphasize this concept In numerous instances Bloomy Controls has been contracted to make minor functional modifications and bug fixes to customers preexisting applications that required disproportional effort In some instances we were forced to rework the application as if starting over from scratch because bad style prevented modifications Using proper style from the outset reduces development time throughout the application s life cycle and makes modifications easy Furthermore this book provides only rules that are practical to implement in real world situations including projects with tight deadlines I have eliminated rules that are not practical One such rule that we had many years ago was a requirement for developers to edit the VI History every time any VI was saved regardless of the project development model or stage of development As a rule all the Bloomy engineers had LabVIEW configured to automatically prompt for revision history comment It imme
346. mes are used as section names and key value pairs conespond bo the named cluster elements ard their values within the tection formatted custes Oniy the ert within tha aan ee are ee eee File Edit Format View Help UUT Information Date 3 12 2007 Customer Toyota Mode l Camry Status Production Year 2 007 Ref Torgue To Rotate Number 19 8 04 Description 0 14 Nm Product Validation Motion Parameters Velocity Amplitude trpmj 30 000000 ee Amplitude deg 360 000000 ocity Ramp Angle Cdeqj 30 000000 Torque Units fin oz In oz Torque Limit torque unitsJ 37 500000 Figure 6 15A contains a data structure that was thrown together rather haphazardly It consists of an array of clusters containing a subcluster and three subarrays of clusters It appears unkempt because it violates several of the front panel rules presented in Chapter 3 The alternative presented in Figure 6 15B contains several improvements including transparent name labels aligned and compressed controls and boldface labels with plain text units in parentheses Additionally these data structures contain more controls with fewer layers of nesting Multiple two dimensional arrays of clusters in Figure 6 15B are generally more memory efficient than one nested data structure of Figure 6 15A However the alternative increases the number of wires and subVI terminals required by a factor of four The best alternative might be to maintain the single data structure
347. mmary This appendix lists all the style rules presented throughout The LabVIEW Style Book Use this as a manual checklist a when performing code reviews as discussed in section 10 1 2 Manual Checklist of Chapter 10 Code Reviews You can also use it as a quick look up reference when discussing style with peers An electronic copy is available from www blooomy com lvstyle Chapter 2 2 1 Maintain a LabVIEW project journal Dai Write a requirements specification document as Maintain good LabVIEW style throughout the proof of concepts 2 4 Document your LabVIEW options and back up the LabVIEW ini file eco Develop reusable SubVIs 206 Make reusable libraries accessible from the LabVIEW palettes 2al Place reusable templates in the LabVIEW templates folder 2 Maintain an organized repository on disk 2 9 Create an LabVIEW source folder hierarchy that reflects your application s architecture 2 10 Create the folder hierarchy before you begin coding 2 11 Organize LabVIEW source files into cohesive project libraries where appropriate 2 12 Create unique and intuitive source filenames 2 13 Do not abbreviate filenames 2 14 Never use LabVIEW s default filenames 2 15 Identify the top level VIs 2 16 Follow your organization s CM Rules 2 17 Avoid moving source files on disk Chapter 3 eal Group related controls using decorations spacing tabs and clusters dea Apply symmetry and spacing to front panel object
348. mmon requirements for different certifications such as ISO 9000 Rule 2 16 Follow your organization s CM Rules If your organization has a CM process in place follow all Rules that have been established including source control There could be a specific procedure that you must follow that includes specific tools and configuration settings Placing a project under source control usually involves creating or moving your repository to a designated location adding the files to source control and configuring the access rights for the files within the source control software You can then proceed with development from the LabVIEW environment adding new files to source control as they are created checking out existing files for editing checking them back in and so on For more information refer to the LabVIEW Help Rule 2 17 Avoid moving source files on disk One point of emphasis is that after development has started project files can be organized freely within Project Explorer but should not be moved on disk Otherwise broken links will result within the LabVIEW project and any source control tools that reference the dislocated files Now we are ready to begin coding Endnotes 1 Howard M Kanare Writing the Laboratory Notebook Washington D C American Chemical Society 1985 2 Free downloadable materials are available from WWW bloomy com resources 3 Software design references Larman Craig Applying UML and
349. modes of atypical application suchas configure acquire analyze and present Controls that are required during the configuration mode might not be relevant during the latter three modes Consider using separate GUI VI panels or a tab control to group controls that the user needs access to on separate panels or tabs for each mode The application programmatically selects the desired panel or tab and prevents the user from changing it or limits which panels can be navigated during a given mode Specifically use the Visible and Disabled control properties to programmatically show hide enable and disable tabs or controls based on the current context or state of the application Hence once the application begins acquiring the user is not allowed to access the Configure panel Section 3 5 2 Dialog Utility VI presents an analogous GUI with tab control Additionally a GUI VI might have some parameters that are applicable for specific configurations For example consider an application that performs current voltage capacitance or resistance measurements using a digital multimeter The panel has a measurement configuration dialog VI that contains a Resistance Type control with selections for 2 wire and 4 wire measurements Because this control applies only to resistance measurements it is inactive when the measured parameter is not resistance At the same time avoid excessive showing hiding or positioning of the controls Your GUI
350. monstrative as an example The front panel in Figure 8 26A contains controls for the resource name bytes to read timeout response and total bytes in addition to the standard error clusters The diagram shown in Figure 8 26B monitors and reads the number of bytes available at serial port and builds a response string and byte count that are stored in shift registers A 25ms delay is applied using the Wait ms function The elapsed time is monitored and compared to the timeout condition using Tick Count ms and several arithmetic functions The loop repeats until the byte count reaches the bytes to read or an error or timeout condition occurs Figure 8 26A The front panel of Poll Instrument Response VI a subVI that polls a serial instrument for a response View full size image 285 Poll Instrument Response vi Front Panel ES Re ee ies Spree T tee Figure 8 26B The diagram is a hybrid between the Continuous Loop and Immediate SubVI design patterns View full size image Erbas ho read 104 fae Notice that this VI is a hybrid between the Continuous Loop and the Immediate SubVI design patterns Specifically it contains a While Loop with multiple termination conditions including the timeout Also it has subVI error trapping including error clusters and an Error Case Structure Figure 8 26C presents an alternative diagram with some minor enhancements Specifically it uses Elapsed Time VI the functional global in
351. most of us have developed subVIs for manipulating strings and arrays reading and writing files performing computations and more Any such subVIs are reusable if they satisfy certain criteria First they should perform a specific useful task ina manner that is flexible and general purpose In some cases this involves using controls assigned to connector terminals instead of constants on the diagram Second the subVI must be developed employing good style per the recommendations in this book For example suppose we are developing an application in which we need a routine to parse multiple data packets returned from an instrument that are delimited by carriage return and line feed character combinations We can write a subVI that calls Match Pattern within a While Loop If my regular expression is a string formed by concatenating the carriage return and line feed constants on the diagram the subVI will not be very general purpose and reusable Instead I should create a string control on the front panel label it Search String and assign it to a prominent connector terminal Now the subVI performs a useful and specific task It parses data packets with variable delimiter and is general purpose and reusable 42 Rule 2 6 Make reusable libraries accessible from the LabVIEW palettes e Place reusable libraries in LabVIEW user lib e Customize the LabVIEW Functions and Controls palettes The final step in making the subVIs reusable is to mak
352. most shift registers near the top of the loop Rule 4 37 Label wires exiting the left shift register terminal Shift registers are terminals on looping structure borders that shift data between loop iterations They are functionally and conceptually similar to terminals that extend wires from the end of one loop iteration to the beginning of the next Shift registers are viable alternatives to local and global variables when the required scope of data sharing is limited to a single While Loop Timed Loop or For Loop Unlike variables shift registers do not create copies of their data when read and are maximally efficient To avoid wire clutter and maintain organization space most shift registers tightly and group them near the top of the loop This creates a data highway that is limited to an area near the top of the loop and minimizes wire crossovers Leave just enough space between the shift registers to apply free labels on each wire near the left terminals Exceptions to shift register grouping include error clusters and case selectors Error clusters normally enter and exit near the bottom of loops Also case selectors are frequently positioned near the middle Therefore error clusters and case selectors are usually kept separate from the data highway at the top Rule 4 38 Use looped Case structures over Sequence structures A looped Case structure is a Case structure embedded within a loop It functions similarly to a
353. mpact that display type has on readability Hence the Overview tab provides summary information for both stations and the user can flip to the individual station tabs to view the details This reduces the amount of navigation that is required when running both stations and reduces the overall density of each tab Figure 3 6 An overview tab provides a summary of both stations using a limited number of controls and indicators including bar graphs View full size image 65 E Poet Manitar Syiti p g a a F i wee ae ee TE D iis W ay me i Jz FR F P F _ 1S i hes iL ite if te 5 yaz pi i i p PR 0j H Ji r F 7 al f i Q ra D Pa a r Pe L H J p gi 1 p A 7 i i e P n a zri l l F I i wil x I 1 1 i i T L d l z Paro r a i s i p L ait e L L i at n F Fa mal i 7 E Th Ejj m T i m aman am mr a r TG Ohe C Chane Rule 3 11 Avoid overlapping visible objects LabVIEW performs less efficiently when front panel controls and indicators overlap In particular never overlap a graph indicator with another indicator because LabVIEW must redraw the entire graph each time either indicator s value changes Graph updates are one of the most processor intensive operations that LabVIEW performs so avoid making them happen unnecessarily Also graph indicators contain numero
354. mponent View full size image 272 Dynamic ramea k lor Generic Application vi Front Panel LJ as The diagram contains the Event Driven State Machine design pattern without the queue and three primary states shown Initialize Idle and Call Plugin The Initialize state reads the contents of a plug in directory using the List Folder function and writes the VI filenames to the listbox control s Item Names property The Idle state simply waits for an event on the graphical user interface Specifically when the user selects a component from the listbox a Value Change event is fired the path to the selected plug in is created and the Call Plugin state is selected as the next state to run The Call Plugin state loads the component into memory using VI Server s Open VI Reference function and opens the front panel as achild of the subpanel using the Insert VI method Data is passed to the component s connector terminals and the component executes via a Call By Reference Node similar to a subVI call When the component stops running it is unloaded from memory using Close Reference and the child window closes from the subpanel using the Remove VI method Rule 8 21 Use the Call By Reference Node over the Run method Rule 8 22 Choose standard connector terminal assignments Rule 8 23 Maintain all components within a dedicated directory 273 An alternative to VI Server s Call By Reference Node is multiple calls to VI Serv
355. mpty Project GE Project ron Wind E Real Time Project Figure 2 6B An organization s templates are accessible from the New dialog box when placed in the Ups Habs template bo buld a standard state machine design patbem Each shaba enecubet code and detennines which state bo braresition ba Contrast this design pattem with the per interface Event Handler pattem in which code emecubes in neeporee bo weer actions Conr thie design patter ako with the Quas Message hander pattern in which each message handing code can queue Zeno oF more new messages Een project zi LabVIEW templates folder View full size image Create Mew a Bl im Blank VI E Polymorphic VI BES From Template BE Tutorial Getting Started E Tuad BEG AT EE irebrumert I0 GPi Bo Frameworks oa n B ip omy Ti Subt with Eror Handing BE Sate Machine Templates il Gloomy Template Check Quaus Status wi far Bloceny Simpie Stabe Machine Temnplahe wit i Gloomy Ewent ptanding Loop vt ile Elarre Sirge Loop wi fe iy Liner SE Project by Empty Project BE Project fren Winer E Real Time Project 2 4 Project Organization File Naming and Control MMe TE a ternplate for a quewe beced state machine with 12 previred shift registers kti sole baase the only 1 loop 1 sub VI and 1 Eypedef control Coro co a Lx _coe_ tee 46 LabVIEW 8 0 introduced Project Explo
356. multivalued data items use clusters for grouping multiple distinct items When a multivalued data type is required a cluster is often more appropriate than an array Arrays are normally used when all of the elements of a data structure have the same data type whereas clusters are used for defining data structures composed of mixed data types However you cannot document the individual elements of an array On the diagram all of the array manipulation functions access the array elements by index number only Consider that each element of a cluster has an independent owned label and description Also the cluster wire is bundled or unbundled by name using the corresponding diagram functions If an index number sufficiently identifies the data you are accessing then an array is appropriate However if unique identifiers and descriptions for each item are desired then use a cluster Array and cluster considerations are discussed in greater detail in Chapter 6 Rule 6 26 Always use Bundle and Unbundle By Name On the diagram use the Bundle By Name and Unbundle By Name functions to access and replace the elements of a cluster These functions provide more flexibility and readability versus Bundle and Unbundle Specifically you can access or update any element of the cluster in any order using Bundle By Name and Unbundle By Name Also these operations do not break the VI if your cluster changes unless the specific elements that are being bundle
357. my icons Type the VI s name or a succinct description using 8 or 10 point small fonts I find that this technique minimizes the required imagination time and effort If you can name the VI then you can create an icon containing its name or a meaningful subset of characters abbreviating the name Later when your application is nearing completion you can return to them and enhance the icons if desired If you run out of time and cannot return to it as we often do then you can rest assured that you have satisfied the requirements that unique and meaningful icons were created via text and related VIs are unified via the background color Figure 5 5 The top illustration is a new VI containing a default icon Select the desired background color and double click the Filled Rectangle tool to initiate an icon with a colored background and black border View full size image Icon Editar 1 Select background color W 2 Double click the filled va rectangle tool Jeon Editor Initial icon with colored background and black border Rule 5 14 Create an icon template for related VIs Rule 5 15 Reuse one glyph color scheme and font for related VIs 139 Commercial developer tools such as instrument drivers and toolkits demand a professional grade icon convention Create a project library or VI template containing a standard icon for use throughout the toolkit For a project library simply
358. n no error 7 error in 3 no emor 3 emor atray in amm Merges error I O dusters from different Functions error out Upctorwerber error in no error Merge Errors VI violates connector Rule 5 25 Assign error clusters to bottom left and right terminals by replacing the error in cluster that is normally assigned to the bottom left terminal with the error in array This causes a misalignment of the icon and terminals when Merge Errors VI is used with scalar error chains as shown in Figure 7 3 Likewise misalignments occur with loop indexed arrays of errors in parallel with reference numbers as shown in Figure 7 bA Moreover the error in array terminal has limited value Arrays of error clusters are formed by indexing errors on loop tunnels or building an array that combines multiple error clusters As discussed in Section 7 1 1 Trapping Errors indexing errors is not recommended Instead most loops check the error status and terminate on error or propagate a scalar error cluster using a shift register Also the Merge Errors VI s merging operation defeats the purpose of forming the array if one existed Figure 7 15B presents Merge Multiple Errors VI an alternative to Merge Errors VI containing five additional error in terminals and no error in array terminal It uses the standard 4x2x2x4 connector pattern with all left and middle terminals assigned to error in clusters Hence it merges up to eight
359. n a buffer View full size image ss LT GF O REEERE Indiase i gt eri ur acer 7 i Sd ed Le G wa a ee gt Laaj gt 5 gt Lipad 2 gt Lined gt gt Lingid I 0i DIDDIG Ma Error FP The queue is implemented as follows The Obtain Queue and Enqueue Element functions initiate the queue on the left of the While Loop The enumerated type definition control terminal is wired to the Obtain Queue function s element data type terminal This specifies the data type of the queue Enumerated constants are created from the enumerated type definition and wired to the element terminals of the Enqueue Element functions Initialize and Idle are the first two items added to the queue These are the first two states executed by the state machine The Dequeue Element function is placed within the No Error case of an Error Case Structure outside of the main Case structure If no error is present in the error cluster the next state is removed from the Queue and passed to the main Case structure s selector terminal If an error is present the error is reported using the General Error Handler VI and the Shutdown state executes Within each case of the Case structure additional states are added to the queue using the Enqueue Element function A variation of the Queued State Machine uses a queue element data type consisting of a cluster that contains the enumerated type definition bundled together with a variant The enumer
360. n additional operation that requires additional memory to store each representation Eliminate coercions when possible particularly with larger data structures such as arrays and clusters Techniques for maintaining similar data types to prevent coercions are discussed in Chapter 6 Keep your eyes open for coercion dots and try to eliminate them Rule 4 25 Create controls and constants from a terminal s context menu One simple way to avoid coercions and save development time as well is to create controls indicators and constants from a node s terminals on the diagram This is accomplished by right clicking on the desired node s connector terminal and selecting Create lt Control Indicator Constant gt from the context menu The corresponding control indicator or constant is automatically created having the data type that matches the terminal it is wired to Additionally the wiring assignment is completed and the item inherits the label of the node s terminal Hence several editing steps are completed from just one menu selection Rule 4 26 109 Disable dots at wire junctions Dots at wire junctions have no functional purpose except for drawing attention to wire junctions They are larger and usually more prevalent than coercion dots During the development of a dense diagram they help distinguish junctions from overlapping wires However they can mildly interfere with identifying coercion dots Also if you generally avoi
361. n be replaced by a wire and the Curve local variable is unnecessary View full size image PEPE T Reeds Bee eb eed ee eed eed ee ee eee ee eed eel F i ms aa Au Tak ee Se Flow Zero ELF CELE EEE ence Ch een cece c none nCEC econ ncn Cnc no nnn P ncn cane Gn Cnn nn none nono mn nnn nn ace ener Rule 4 33 Avoid polling variables within continuous loops Rule 4 34 Avoid variables if wires are feasible Polling is the condition in which a loop continuously monitors a resource until it reaches a specific value or state Never poll a variable within a loop In Figure 4 10A the DAQ Loop executes at top speed inefficiently utilizing the processor to detect a change in value of the Acquire global variable Instead use a synchronization construct such as an occurrence notifier or queue These functions allow the slave loop to sleep until the synchronization construct fires an event Another observation from Figure 4 l10A is that the Sensor Scaling write and read local variables can be replaced by a wire The write local variable might still be useful if the Sensor Scaling data read from file needs to be displayed in a control However the read local variable inside the DAQ Loop is unnecessary Finally the Curve local variable is not necessary because it initializes a graph indicator to its default value which is already the control s state when the VI loads into memory In Figure 4 10B the Acquire global variable
362. n is preferred for a GUI VI Figure 6 6C The TRUE and FALSE states of the vertical toggle switch are appropriately labeled Closed and Open The default value of each control is identified within the labels using parentheses Rule 6 12 Avoid using buttons or switches as indicators and LEDs as controls LabVIEW s front panel objects have a useful property whereby any control can be converted to an indicator and vice versa In some instances this does not make sense For example buttons and switches should not be used as indicators 176 and LEDs should not be used as controls These reversed associations are counterintuitive and might confuse people thereby violating Rule 6 1 Boolean controls that are part of a cluster maintain the same appearance when the cluster is used as both control and indicator In this case it is difficult to avoid violating Rule 6 12 because consistent data structures are desired per Rule 6 3 There are two alternatives First a command button can be customized to neutralize its appearance similar to the status control of the error cluster This is the preferred approach Alternatively separate clusters can be created for controls containing the buttons or switches and indicators containing the LEDs In this case coercions will result if the clusters are saved as type definitions and then wired together Because clusters are often saved as type definitions this approach violates Rul
363. n patterns and templates promote standards that ensure consistent quality software throughout the organization As discussed in Chapter 1 The Significance of Style good style means good readability maintainability reliability efficiency robustness simplicity and performance In this chapter multiple design patterns are discussed with emphasis on the ones favored by the engineers at Bloomy Controls Because the topic is fairly lengthy Table 8 1 provides a quick reference For illustration purposes the design patterns are applied to the Torque Hysteresis VI discussed in previous chapters Before we begin a few block diagram style rules presented in Chapter 4 Block Diagram are worth briefly reviewing here because they form the foundation upon which good design patterns are formed Table 8 1 Summary of the Design Patterns and Application Frameworks Presented in This Chapter SubV GU Top Lev Simple Medium Advanced Deskto Real Ti Event Driv Inefficient Poll Sectio Page I I jel VI Complexi Complexi Complexi p OS me OS en ing n Numbe VI ty ty ty a Immediate Y 1 Y Y 8 1 1 241 SubVI Functional 1 Y i 1 8 1 2 244 Global Continuous 1 x Y Y Y 8 1 3 246 Loop Event Handli 1 y y y 8 1 4 250 ng Loop Classic Y Y 1 Y Y 8 2 1 257 State Machine Queued State Y y y x y 8 2 2 260 Machine Event Driven i 1 ri x x 8 2 3 262 State 239 Table 8 1 Summary of the Design Patterns and Application Frameworks
364. n stops the loop immediately if an error occurs the latter continues the loop but skips all successive File I O operations In both scenarios the error is trapped and eventually passes through the While Loop to the error out indicator terminal upon loop completion Figure 7 4A Error data from the While Loop s input tunnel resets the error data and error data from the file writing operation overwrites the output tunnel data within each successive iteration of the loop View full size image Write Interval mS te error cut Arna it res mince a aoe tt da sco TEE 13 Figure 7 4B The error status is checked within each iteration of the While Loop and the loop is terminated if an error occurs Any error is successfully trapped within the error chain and passes through the output tunnel to the error out indicator terminal View full size image 207 Figure 7 4C A shift register is used to propagate the error data between successive iterations of the loop If an error occurs the loop continues but the file writing function skips its operation View full size image Write Interval mS a iy Coa error in ina error Pal OP eee etree eee i i ETEF rer ot hy Rule 7 4 Disable indexing of errors with continuous loops Note that For Loops execute a predetermined number of iterations cannot be terminated prematurely and have indexing enabled on their output tunnels by defa
365. n the front panel and diagram are intuitively labeled and evenly spaced The diagram contains free labels in every frame of every structure However the VI control and indicator descriptions are absent throughout the application Descriptions are an important source of documentation as discussed in Chapter 9 Documentation Figure 1 7 shows the Context Help window appearance for a VI and control without descriptions Figure 1 7 The Context Help window reveals missing VI and control descriptions on the panel of Nested VI a a a Context Help Context Help Nested vi Sample Interval No description available The front panel of Spaghetti VI see Figure 1 3 contains simple controls indicators clusters and decorations Intuitive labels exist for most controls but not all of them For example each station cluster contains two vertical fill slides that have the same abbreviated name Comp These controls are ambiguous Each control should be uniquely named and the abbreviation should be replaced with a more intuitive and meaningful term Chapter 3 presents rules regarding front panel text The diagram looks like well spaghetti This is because the wiring scheme is haphazard with data flowing in all directions and the architecture is not adequate for the application s complexity It is extremely difficult to visually trace most wires from source terminal to destination terminal Chapter 4 presents rules
366. n with some general rules that apply to all varieties of GUI VI and subVI panels Have you ever read an entire software license agreement that is displayed in a dialog when installing new software Or do you instinctively press the Accept button When you begin using new software do you learn it by reading the documentation online help and readme files or do you immediately operate the GUI by navigating the controls and menus Have you ever been annoyed by lengthy text on a GUI Rule 3 17 Minimize front panel text 70 Intuitive front panels are graphical not textual Users and developers alike do not like to read a lot of text on the front panels Instead we prefer buttons controls objects icons and images that are universal and self explanatory As a result you should avoid sentences long phrases and paragraphs of text within labels and controls It either slows people down or is ignored entirely The more text your front panels have the more complicated and user unfriendly they appear There are more appropriate places for lengthy text and documentation Sometimes as a developer is preparing to bring a new application online there is a tendency to embed instructions on the front panels of the GUI VIs to aid the user in operating the software However text that is applied primarily for this purpose is meaningful only for a new user running the software for the first time at best Thereafter the text is probably ignored a
367. nager and VI Analyzer are tools that can be used to test the platform portability of your filenames Rule 2 14 Never use LabVIEW s default filenames Never rely on a default name and number to uniquely identify a source file This contradicts LabVIEW s built in default VI naming convention in which LabVIEW automatically assigns the name Untitled lt number gt and Control lt number gt to each new VI and custom control or type definition respectively Avoid default names and numbers at all costs In my opinion this is a severe violation of good style Many developers append numbers to the filenames to indicate the version number Note that this is not necessary if your source files are maintained within project libraries or if the project repository is under source control Additionally VI revision history Tools Source Control Show History is a built in LabVIEW feature that maintains the revision number of your VIs For example it can be configured to automatically increment and prompt for comment each time you save or close a VI with changes This is the only method in which the revision number is maintained directly within the source file Legacy LabVIEW Plug and Play instrument driver files developed prior to the LabVIEW project follow a specific file naming convention Each filename contains an instrument prefix which is an abbreviation of the instrument vendor name and the instrument model For example the Keithley 2000 digital
368. nal real estate while maintaining compatibility with mainstream PC display technology Avoid resolutions much higher than 1280x1024 because higher resolutions are less universally supported and the larger work area promotes larger diagrams and 95 potentially less modularity Also depending on the monitor size very high resolutions may strain your eyes Although I have 20 20 vision several years ago I went through a phase where I wore tinted prescription glasses during LabVIEW development An adjustment to the resolution setting along with general improvements in display technology eliminated this problem for me Today many computers support multiple monitors It is particularly useful to utilize two monitors for LabVIEW development This allows you to dedicate one monitor to the front panel and the other monitor to the block diagram and have both windows simultaneously visible without having to navigate between them Rule 4 2 Leave the background color white Rule 4 3 Use a high object density Rule 4 4 Limit the diagram size to one visible screen or limit scrolling to one direction Do not color the diagrams Leave the background of the diagram and every subdiagram of every structure default white Data flow must be easy to visualize A high density of objects is desired without crowding objects too close and causing wires and objects to overlap In general try to limit the diagram size to one display screen In some situations
369. nce It is generally desirable to minimize complexity by choosing the simplest implementations that require the fewest nodes 24 The VI Metrics window provides a useful interface for evaluating simplicity Choose a VI in the Select a VI control such as the top level VI for an application The totals displayed in the top row of the metrics table indicate the simplicity Specifically the total number of nodes indicates the total number of functions subVIs structures front panel terminals constants global and local variables and property nodes within the VI and all subVIs combined The smaller the nodes total the simpler the application Note that VIs that are called by reference are not visible to the VI Metrics tool and are not included in the nodes total Simplicity is primarily dictated by the requirements of the application The more functionality that is required the more source code is required and the more complex the application is The three example applications from Figures 1 1 1 2 and 1 3 all have statically linked subVIs with the exception of the component subVIs used by the subpanel of Meticulous VI You can compare the complexity of each application using the total number of nodes reported by the VI Metrics window With 4 947 nodes Meticulous VI is by far the most complex followed by Spaghetti VI with 1 944 On the contrary Nested VI is a simple application with only 471 total nodes However simplicity also rel
370. nce materials Instead notepads function more like a personal hard copy of one individual s daily activities related to a project 4 If the raw notes are not formalized and expounded upon the composer does not gain new insights and perspectives Moreover it is not possible to receive feedback build consensus and confirm the requirements among multiple project team members if the project team does not share and review the requirements Rule 2 2 Write a requirements specification document Compose a requirements specification document for each application using the raw notes from the project journal as a reference A requirements specification is a statement of the project s primary objectives and a prioritized listing of the required features It contains sufficient detail to clearly describe all important requirements that contribute to the objectives It should contain very few design and implementation details Hence the requirements specification should not place constraints on how the project is developed except where these constraints affect the project s outcome The Institute of Electrical and Electronics Engineers IEEE offers a Recommended Practice for Software Requirements Specifications standard 830 1998 and a Guide for Developing System Requirements Specifications standard 1233 1998 These are excellent references for writing software and system specifications that apply to any programming languag
371. ncing style Search online for products references examples drivers toolkits colleagues and consultants Start with WWW ni com NI Developer Zone in particular is a forum for developers in need of LabVIEW resources such as articles example code and support The LabVIEW Tools Network is an NIsponsored site within the NI Developer Zone that lists add on products books tutorials and more Additionally visit OpenG org and the LAVA user group OpenG is an organized community committed to the development and use of open source LabVIEW tools the URL is WWW Openg Org The LAVA website is dedicated to the open and unbiased exchange of ideas on intermediate to advanced topics in LabVIEW the URL is www lavausergroup org Do not overlook the many offline resources and media available as well The LabVIEW environment contains hundreds of example VIs that are searchable using NI Example Finder Help Find Examples The LabVIEW Help contains application notes and white papers You can purchase many LabVIEW books from your favorite bookstore or online reseller Additionally NI offers hands on instructor led training through NI Certified Training Centers and customized solutions through the NI Alliance Program The NI Alliance program is a worldwide network of more than 600 consultants systems integrators developers channel partners and industry experts For example Bloomy Controls is a Select Alliance Partner and hosts two NI
372. nd a waste of space A good question to consider is how often will a new user operate the software versus a more experienced user Unless the application is explicitly designed for new or infrequent users such as a wizard or installation utility the text should reside elsewhere I recommend the following alternatives to lengthy front panel text e Create intuitive GUI VIs that are robust and self explanatory and do not require textual explanations e Provide a Boolean control labeled Help or and a menu labeled Help that launches help information on demand such as an online document the LabVIEW Context Help window or a dialog VI with string indicator displaying the text e Create a user manual or help topic in an external document or compiled help file and launch it from LabVIEW on demand Rule 3 18 Delete template instructions immediately after edits are performed Another situation in which developers use lengthy text is when leaving notes or instructions for other developers on the panels of reusable templates For example the LabVIEW instrument driver template VIs contain text labels that tell the developer exactly what changes are required to each VI Always delete the template instructions immediately after the prescribed edits have been made Otherwise the application appears incomplete or raises uncertainty It is very disappointing to open a VI that contains editing instructions within a complet
373. nd effort throughout an application s life cycle Additionally neat development practices need not be overly time consuming If you know the style rules and how to implement them you eliminate the toil This chapter presents style rules that ensure neat and organized diagrams that are practical to implement in real applications with tight deadlines Combined with the rules in other chapters they ensure readable and maintainable LabVIEW source code Moreover mastery of these style rules may lead to awe inspiring LabVIEW diagrams 4 1 Layout This section covers rules for block diagram layout including layout basics and subVI modularization The following rules pertain to the general layout of the block diagram Rule 4 1 Use 1280 x 1024 display resolution The display resolution affects the visible area the developer has to work with and how the diagram appears when opened on a given target computer It is beneficial to standardize on one display resolution so that the diagram window maintains a consistent appearance when opened on PCs with similar display capabilities The higher the resolution setting the smaller the diagram objects shrink relative to the screen size and the more code fits on one screen A fairly high resolution is recommended to maximize the viewable diagram area without straining your eyes The LabVIEW development environment is designed for a minimum 1024x768 resolution A resolution of 1280x1024 provides additio
374. nd improve the specification before proceeding A word processor with comment insertion and change tracking features is simple but nonetheless useful for incorporating comments and changes from multiple reviewers More specialized requirements management tools such as Telelogic DOORs and IBM s Rational RequisitePro provide clear visibility change management and traceability Additionally NI Requirements Gateway 1s an add on package that provides a traceability link between requirements specifications formed with any of the previously mentioned tools and your LabVIEW source code This enables you to analyze and report the compliance of your software to the project requirements 2 2 Design Software is designed using traditional techniques such as flow charts and pseudo code or using Unified Modeling Language UML including class sequence collaboration state and component diagrams Many excellent references describe these standard design techniques I do not describe them here Instead I would like to note that Theorem 2 1 can be extended to include design documentation The more overall planning there is including design documents as well as specifications the better the LabVIEW style will be Many developers are tempted to skip straight from the requirements specification to the software development If fewer than 25 of LabVIEW developers write specifications far fewer are apt to create design documents Aga
375. nd improve your programming style The LabVIEW Options dialog box accessed from the menu bar by selecting Tools Options is used to set preferences related to the work environment Many of these options are related to style For example some block diagram options I avoid include Show dots at wire junctions Show subVI names when dropped and Place front panel terminals as icons Diagrams tend to look neater without the dots labels and icons Dots at wire junctions help developers distinguish between the junction of multiple wire segments and overlapping wires However many LabVIEW developers search for dots that represent coercions which have negative connotations Triple clicking any wire can reveal its branches and dots at junctions can be avoided Terminals as icons and subVI labels are simply a waste of precious real estate I recommend disabling these options Figure 2 2 shows my own personal preferences for the block diagram options Figure 2 2 LabVIEW options dialog box with block diagram preferences shown View full size image 39 e Options Category Bock Diagram New and Changed for LabviEW fx Paths C Show dots at wire junctions H Shows tip strigs cer barmin Use control caption for subil tio strips C Show subWl names when dropped Denion pare kainak from daya Enable subo wiring Mirus dist arcs Mazin Ge anes 4 3z Lise ire a Lasts Pisce Front panel benminals as icons Erat e subomatic anor hand nsg in i
376. nd memory consumption Good LabVIEW developers strive to optimize the performance and memory use in their applications Consistent data types reduce the level of programming effort required while ensuring efficient and reliable performance Rules 6 1 6 2 and 6 3 must be considered together when developing an application s data structures Controls that are simple to operate data types that are memory efficient and overall consistent data structures are desired How do we ensure that each consideration is satisfied Carefully examine the data requirements of each component or subVI and choose data types that are intuitive efficient and consistent When compromises between memory efficiency 6 2 and data structure consistency 6 3 are required always choose consistency 6 3 Per Theorem 6 1 memory access time is the universal latency The more memory operations are performed by our applications such as buffer allocations the greater the negative impact is on performance All data type conversions entail buffer allocations that are susceptible to this issue However larger data types such as DBL instead of 116 do not necessarily affect the number of memory buffer allocations Indeed allocating a 4 096 element array of DBL may require the same memory access time as allocating a similar array of 16 Rather allocating an array of 16 and later converting to an array of DBL entails multiple buffer allocations Consider the e
377. nd tab control is embedded within the Configure tab and colored transparent It organizes DAQ channel configuration parameters by transducer type and is programmatically navigated based on the value of the Sensor Selection control Chapter 6 examines the embedded tab control and corresponding data Aside from the various logos this application uses only the Application font with three different styles Figure 3 17 Centrifuge DAQ research application for seismic event simulation View full size image 90 P Leriri ugr DA Siem Em GR Yes Eoimt gpa Tode indon bei Configure blonitor Acquire Analyre Semn Sekcion girai Gage Moh 1 Settings Aco onesies LWT CEEE Scia stot number 6 Ci Aa 6 IT Lie z Location iem sens Sraling Eng bmt limia fog UA pertation Mirr rake E3 F Serial apd Pheer ai i HA Wisg Sething Cater ag a iter MEE cc fioe Bols ee E M fo fo ee roo rel E E E CE A i 5a os T Tarara meam BE a ba 5 a cc Co pt zi 7a M ET oe ee glo C a Co oe E Application Centrifuge DAQ top level VI panel Conditions 1024x768 resolution high object density Color scheme Maroon gray white Layout Nested tab control with outermost tab control organizing controls into tabs for each operational mode Additionally hundreds of configurable parameters are organized using an invisible tab control within the Configure tab Heading text Tab control headings are 18 point bold black Application font
378. ndex array method 208 Figure 7 5B A shift register extends the error chain between iterations of the loop This approach is more efficient than array indexing View full size image umber of Sangles D S Wrba miava mS Wiebe te deci BIBIE BIG IS BIBT Ie ee ee B zy BIS 1S Bl B08 ee ELIET TILILLE 5 ELS l msm h ey jl EEN ini na arar oo Many VIs of medium or greater complexity benefit from parallel error chains similar to what is illustrated in Figure 7 3 Parallel error chains are particularly useful when the parallel chains of nodes share common data such as an I 0 name or reference number For example in Figure 7 3 a task is propagated among the DAQmx VIs and a file reference number is propagated among the File I O functions Positioning these groups of nodes in parallel helps facilitate the task and reference number propagation among the nodes in addition to the error cluster However in some applications the parallel nodes are functionally interdependent and should be combined into a common error chain In this situation the nodes can be arranged in parallel to facilitate I O name and reference number propagation but a single error chain is shared by both groups of nodes For example in Figure 7 6A the DAQmx VIs acquire waveform data that is displayed to a waveform graph while the File I O functions log random data to file These parallel nodes are functionally independent and separate e
379. ndividual controls that are not part of a cluster are configured for simple and immediate tab key navigation by default The tab order of controls on the front panel can be edited by selecting Edit Set Tabbing Order The elements within a cluster can also be tabbed according to their cluster order after the Key Focus property is applied to any element within the cluster Subsequently tabbing remains confined to the cluster until Key Focus is applied to a control outside the cluster Therefore tab key navigation requires additional programming for a cluster that 1s not necessary using the controls on the panel Figure 6 10 illustrates how a cluster can be used in conjunction with a dialog that prompts the user for the UUT information The visible area of the panel simply contains string controls that have been customized for GUI interaction as shown in Figure 6 10A The UUT Information strict type defined cluster is actually scrolled off to the side of the panel that is not visible as shown in Figure 6 10B along with the error clusters and Cancelled indicator Tab key navigation has been disabled for the noninteractive controls This is accomplished by selecting Advanced Key Navigation Tab Behavior Skip this control when tabbing Notice that the type defined cluster is much more compact than the corresponding controls on the panel Hence in this example the controls on the panel are customized for best GUI behavior and the type defined cl
380. ne design pattern is described in greater detail in Chapter 8 Figure 4 13C The loops stop each other if the Quit Value Change event occurs in the Event Loop or an error occurs in either loop View full size image 4 4 Examples In this section a variety of block diagram examples are presented both good and bad Let us begin with the bad and gradually transition to the good The bad examples are particularly effective at illustrating the reason for many of the block diagram style rules presented thus far As observed in Chapter 3 Front Panel Style the SubVI from Selection utility is the world s most flagrant style violator This is the process of selecting an area of the diagram and choosing Edit Create SubVI The worst possible programming practice is to create a subVI from selection and not clean up the aftermath The terminal locations and labels wiring connector assignments icon and description all require corrective action Sometimes the resulting subVI diagrams appear as if a bomb went off inside them SubVIs created using this tool never conform to good style and rework is mandatory Figure 4 14A corresponds to the diagram of the SubVI from Selection VI front panel example presented in Chapter 3 It contains several telltale signs of a subVI from selection The wiring is kinked the objects are tightly spaced and the control labels are nonsensical For example the task ID and error control terminals are impro
381. ned approximately near the middle of the pack of parallel error chains In Figure 7 15B the Merge Multiple Errors VI is used to combine all error chains Because the lower left terminal conforms to Rule 5 26 this subVI is vertically aligned with the error terminals and subVI of the bottom error chain Also the Build Array function is eliminated and the diagram contains fewer wire bends Finally a middle connector terminal is utilized to merge five error chains in this example Error handling identifies two classes of misbehavior errors and warnings Errors are uniquely distinguished by a TRUE value of the error cluster s status Boolean Warnings correspond to any nonzero codes accompanying a FALSE error status As discussed when an error is trapped within an error chain most nodes upstream from the error skip their code The exceptions include most functions that close a reference such as File Close VISA Close Close Reference and Release Queue In some situations it is desirable to either downgrade or ignore specific errors so that upstream nodes execute normally As an example consider a communications server application in which client connect and disconnect operations are routine tasks The communications functions return an error when the client disconnects The server program needs this information to clear its connection with the client and begin looking for new client connections However if the disconnect error 1s
382. nes Wis Enable suhomatic error Marcil disiogs C Erh automatic wire routing Auto incert Feedback Mode in cycles Ci Pime mbhTs a aparabis _ Place shruackures with Auba Grow enabled C Show red Xs on broken wires Labels snap to preset positions on terminals C Labets locked try defauk Defaut Lsbed piiiti r Deak w _ Show conshant fobding of wires C Show constant folding of structures On the front panel I prefer most of the default options Seasoned LabVIEW developers really appreciate transparent labels and modern style controls In previous versions many of us repetitively colored our labels transparent on each and every control of every panel we created Transparent labels look much nicer than the raised labels of the past Modern style controls also known as 3D controls were one of the great innovations of LabVIEW 6 enabling our VI front panels to resemble or transcend their physical counterparts the panels of traditional boxed instruments Also related to front panel style is the Alignment Grid option Show front panel grid and other associated properties The grid appears while in edit mode and aids the developer in visually aligning and spacing front panel objects Rule 2 4 Document your LabVIEW options and back up the LabVIEW ini file In an ideal world all developers within an organization would standardize on the same LabVIEW environment settings In addition to promoting consistent style this
383. nes and the block diagram in general View full size image 287 L MEDS PS BE 28 2 ee ee es ee EELEE ee i ee i i ss a a B a E a a E a Chee ppkn or YI Reberence E FI H E a e Wires enter structure through top horizontal border see Rule 4 13 e Right to left data flow see Rule 4 22 e No labels on wires exiting shift registers see Rule 4 37 e Function labels visible see Rule 4 45 e Errors are not trapped see Rule 7 3 e Simple Error Handler VI instead of General Error Handler VI see Rule 7 7 e Error cluster is not the bottom tunnel see Rule 7 17 e GUI terminal polling instead of Event structure see Rule 8 4 e No enumeration for case selectors see Rule 8 12 e Two parallel Case structures see Rule 4 13 e Control labels not visible see Rule 9 1 The Boolean menu is used to select the case of the top Case structure The bottom Case structure is controlled by the Run VI Boolean This is not readily apparent because the control label is not visible Because both Case structures are controlled by Boolean controls the two Case structures can be merged into a single Case structure This entails appending the Run VI and Quit Booleans to the array that is searched for TRUE and simply adding cases to the Case structure for the corresponding states Note that the shift register maintains the previous values of the Boolean menu and does not pass any state information T
384. ng errors Expanding the term velocity eliminates two occurrences View full size image 326 P Wi A nalyror Results Window Dobie a faise oortree bo bhigi the fahre object inthe V OF thee 8 dagal nce you performed the aars the wrong object may be haghbghted Reks Lit E Tome Hysteresis Bed Siew Backes Area a B S bh ba He kk Nili el t 7 1 a r iar LELE A W S i ex d W Tha gomina Weel Asa ppn haa iha moinen niaaa Wer Habe rpn lates Lusi Figure 10 2J The Wire Bends test identifies a wire segment with fewer than 3 pixels known as a wire kink View full size image E V1 Analyzer Kesulis Window Fist Renis Double dick Madura occurrence bo haghight the fadure object in the WI of the wi changed since you performed the anatycis the wrong cbgect may be highighted i W 4 i E S Torque Hysteresis Bad yev Biei Wires Error Guster Wired i Uaia Codes j PE RI EI Dumari tion Wires Linder Objects Coercion Dots Lorient Less Gishalt and Locals Speed Check Wire Berk Concur eran Coourence 2 i d umaa nour ance 5 Paine Fabre Reported E i Combo Terminal Wiring 3 Ei String Constant Strie 1 E Show Fabres Oniy 2 Sort by V1 Hgh Rank Test Deacrigkion Cy Sort by Test i Lra Rank Test ies Foe iro ovement ut eer haniou podnapis nh a ii i
385. ng in Publication Data Blume Peter A The labVIEW style book Peter A Blume p cm Includes bibliographical references and index ISBN 0 13 145835 3 hardback alk paper 1 Scientific apparatus and instruments Computer simulation 2 LabVI EW I Title Q185 B568 2007 005 38 dc22 2006035871 Copyright 2007 Pearson Education Inc All rights reserved Printed in the United States of America This publication is protected by copyright and permission must be obtained from the publisher prior to any prohibited reproduction storage in a retrieval system or transmission in any form or by any means electronic mechanical photocopying recording or likewise For information regarding permissions write to Pearson Education Inc Rights and Contracts Department One Lake Street Upper Saddle River NJ 07458 Fax 201 236 3290 Text printed in the United States at RR Donnelly in Crawfordsville Indiana First printing February 2007 Dedication To the employees of Bloomy Controls who contributed to the evolution of this topic within our organization To my wife Phia and daughter Eva for their patience love and support Contents OP VA sissy seta E E E E au uian E E 1 EOC ATO a AE E E T A EE E N E E A T E N A 2 ON OW UG MOPE E E E 6 PACS ae va EA EEA E E aseinua nara hteasitenaponaevosmvaeee 7 NATO CG Rg ON scccccraticcstrand csr erases E E E E nei adteeeanhanciuntt 7 Organi atiOM ay cercoctaser
386. ns when the single computer has exclusive access to some critical instrumentation or equipment and is secured behind a firewall Indeed I have worked on projects that were supported by several developers across several shifts per day and weekends Each developer writes notes describing any significant changes that were made in an area adjacent to the modifications We might color code the labels varying the background shade to denote the shift The author includes the date and appends her initials to each comment Some references recommend applying free labels liberally I have deliberately used the term selectively instead of liberally Free labels should never be so abundant that they dominate the diagram In some situations too much text can actually hinder readability For example if you implement a standard design pattern such as the Classic State Machine and choose to include paragraphs of text explaining how it works using multiple free labels you may not recognize the design pattern among the comments Instead I recommend applying relatively succinct comments in select locations as described Longer explanations belong within the VI description help files or external documentation Alternatively if long comments on the diagram are necessary either place the long comments in a location adjacent to the diagram or use string constants with scrollbars String constants can be used in a similar manner as free labels except that string c
387. nsistency as well as symmetry The Edge Point is an important output but is assigned to a bottom middle terminal to allow consistent assignment of common terminals among related VIs Figure 5 13 Context Help window for Find Screw Ends VI indicates required priorities for Original Image in and Grayscale Image in and recommended priority for all other terminals Se a ee ee ee a SE EE TE Context Help Find Screw Ends vi Original Image in m BREW Penne Image Dst Out Grayscale Image in pe A fey Grayscale Image out Line Endpoints error in mo error Inputs are an array cluster of line endpoints The VI creates a straight line from the first to the second point and finds the first edge point moving in the direction from 1st to 2nd The point is drawn overlayed onto the image Note that many subVIs have an input for the index of looping structures These subVIs may initialize or reset a resource or variable upon the first iteration of the loop iteration 0 Because the index terminal is normally at the bottom left corner of a looping structure assigning the control to a bottom middle terminal helps minimize wire bends and overlaps 5 3 Examples This section contains examples of icons and connectors including some that highlight reinforce and extend the syle rules from the prior sections as well as a few exceptions to the rules Some are quite clever while a few are downright obnoxious We begin with the
388. nstitutes a critical phase of the application development cycle Absence of a specification leads to significant time and effort expended developing software that may not satisfy the intended objective Alternatively the requirements that are understood today may not be the same as what is required tomorrow a phenomenon known as scope creep Moreover how do we know when our application is complete without documented specifications In practice I would estimate that fewer than 25 of LabVIEW developers write formal requirements specifications Most projects are initiated in meetings in which hardware and overall system requirements dominate the agenda Hardware seems to take priority over software because the products features part numbers prices and lead times seem more tangible 29 and inflexible Since LabVIEW provides unlimited flexibility with respect to software functionality the features prices and lead times of a LabVIEW application appear virtual or even self imposed In fact every software feature has a lead time and price in terms of development hours This simple fact is often overlooked or underestimated As a result misunderstood requirements and scope creep abound throughout the LabVIEW community Scope creep is not purely evil and forbidden In R amp D environments in particular measurement and automation requirements often change naturally as the research emphasis or product or process design evolves LabVIEW s
389. nt In this manner each component will have read and write access to all data structures whether they are used within the component or not On the diagram propagate the data structures from control terminals to any destination nodes to the indicator terminals Any unused data structures are wired directly from control to indicator terminals Assign the controls and indicators to the 4x2x2x4 connector pattern according to the connector rules presented in Chapter 5 This includes the error in and error out clusters assigned to the bottom left and right terminals Rule 8 25 Assign one input and one output terminal as type variant The controls and duplicate indicators of the prescribed four data structures combined with the standard error clusters occupy ten connector terminals This leaves two spare terminals using the standard 4x2x2x4 connector pattern However spare terminals are useless with plug in components When the Open VI Reference type specifier is configured within the framework based on the connector pattern of a representative plug in the connector assignments of all components must exactly match the type specifier Hence the terminal assignments cannot be added or removed after the framework is deployed So why limit ourselves to a maximum of four data structures To accommodate unknown data requirements in the future assign one additional pair of input and duplicate output terminals as type variant and propagate the variant betw
390. ntinuous operation while preventing the application states and user interface activity from interfering with the acquisition and display of the torque and angle data Data is transferred between loops using a shared variable The DAQ Loop incorporates a 10ms delay and receives a higher priority than the Main Loop which contains a 100ms delay Figure 8 18B Data acquisition and state transitioning are performed in separate parallel asynchronous loops The DAQ Loop performs continuous acquisition of torque and angle data independent of the Main Loop View full size image 270 pee LODE mig a ead Chota De pe EEH ah o ee een L L iid i an Fi Jes a ry Ti A ERPF Taea m ar pees Erer F E S T E bat e Sasehepeorpreag raion 1 Ol P Can Were ie niu a An application that uses parallel loops is quickly constructed by merging two or more of the single loop design patterns onto a diagram and incorporating data sharing using queues shared variables or RT FIFOs Alternatively you can construct a multiloop template by saving a VI containing multiple single loop design patterns and messaging as type VIT and maintain it in the LabVIEW Templates folder VI templates are loaded from the New dialog and selected by choosing New from the File menu or selecting New from a node s shortcut menu in the ProjectExplorer Window 8 4 Complex Application Frameworks Many complex applications share several
391. o guide us through this process The sections that follow discuss rules that apply to specific types of LabVIEW data structures 160 Rule 6 1 Choose controls that simplify the operation of the panel The term simple has several connotations with respect to LabVIEW controls and data structures Simple data structures store data in contiguous memory addresses They include all scalar data types such as Boolean numeric and string arrays of Boolean and numeric and clusters containing only the aforementioned simple data types Simple controls are controls that are intuitive and easy to operate and that represent simple data structures Simple controls possess properties that can be configured to help validate user and programmatic input When properly configured simple controls are reliable and memory efficient To some degree control types can be classified in order of operational and data simplicity For example numeric controls are very intuitive can be configured to restrict values to a specified range and are stored efficiently in memory according to the representation String controls can contain any alphanumeric data without restriction and are stored as blocks of contiguous bytes 1 byte per character Boolean controls are maximally restrictive allowing only a value of TRUE or FALSE and are stored as byte integers Therefore of these three control types Boolean is the simplest followed by numeric and then string Table 6 1 cont
392. o use the preferred 4x2x2x4 connector pattern The Serial Port terminal is an I O name which is rightfully assigned to the top left terminal per Rule 5 26 152 In Figure 5 17B the icon has been improved by replacing the top two lines of text with a banner containing PHPMI1k as the instrument prefix Additionally the glyph is centered for better visibility The icon appears clear and simple as a result of these changes Also five serial port settings have been modularized into a cluster and the 4x2x2x4 connector pattern is applied The ID Query and Reset controls have been assigned to the middle terminals of the icon which implies a slightly lower priority than the Serial Port Settings and other input terminals on the left border Figure 5 17B The VI has been improved with a banner containing the instrument prefix a centered glyph cluster input and fewer terminals View full size image PHPM1k Initialize vi Connector Assignments Reset F Dont Reset Serial Port COMI Pri ea VISA session Serial Port Settings mae Returned String HE error in no error Te i error out ID Query F Dont Check Figure 5 17C contains the icon and connector assignments for the Initialize VI from the instrument driver template This VI is generated automatically by the Create New Instrument Driver Project utility that is selected from Tools Instrumentation Create Instrument Driver Project The Reset and ID Query controls are a
393. object inthe Yi If the I changed ince ou performed the snaks the wrong object may be highighted Rents List B Torque Hysteress Bad Style vi El Birre Wie fe l Error Chaba Wiaj E Unused Cece Ocourerce 2 Wi Gocurertation Wires Linder Objerts Conca Dots Comment Lage Chak and Locals Spell Check Wire Bends i Control Termins Wiring j String Constant Sy i he Pb a n gn E E E E E E E Sort byi f Highs ari Tost Sort by Test i L Rink Text This Brum LE content b not being used Consider nemeving i from your dayam jA eot J toot sere tore _ vee _ Figure 10 2F Several controls do not have descriptions including the Save button This results in multiple VI Documentation failures View full size image 324 e i Analyzer Poets Ar irda Double check faire occurrence bo highlight the Faire object inthe wl OF che Vi changed highlighted cnoe you performed the aaye e mong object may be z Tongue Hytherestis Bad Style eo Badrah Wires l Error Cluster Wired J l Urueed Code 1 Vi Documentation Occurrence L GH Be Ee ag m ig a A Figure 10 2G A segment of the cluster wire runs underneath the Sequence error View full size image P yj Amalvo Kesulis Windi Test Resuts Testing Errors summary Double cick a future sccurence to Highight the fare object inthe yr D the Vp changed finns you performed the analy the
394. ocessor to work on other tasks Figure 8 7 The Torque Hysteresis VI is implemented as an event handling loop This is a more compact flexible and efficient alternative to the Continuous Loop implementation in Figure 8 5 View full size image 253 pikes ero in 3 8 2 State Machines State machines are the most highly touted LabVIEW design patterns of all time There are many variations most of which consist of a Case structure within a While Loop with a shift register or messaging construct wired to the case selector terminal Each case of the Case structure contains a subdiagram corresponding to a state of the application The case selector is an integer string or enumerated data type identifying the states The shift register or messaging construct passes the next state selection from a previous case to the selector terminal in the next loop iteration This is illustrated in Figure 8 8 using the classic form of the state machine In a typical application the state selection is determined by an event on the user interface by a step in a sequential test or measurement routine or from the result of a previous state Figure 8 8 State machine design patterns consist of a Case structure within a While Loop and a shift register or messaging construct wired to the Case structure s selector terminal Each case of the Case structure corresponds to a state of the application View full size image Pe oe gt
395. odern computing devices Frequent operations involving nested data structures can reduce application performance Rule 6 28 Organize complex data using nested data structures Rule 6 29 Avoid manipulating nested data during critical tasks Despite these grim sounding side effects there are very practical uses for nested data structures that have limits to the number of layers and data size Nested data structures help organize data in a hierarchical manner If your target is amodern PC you probably have plenty of memory resources at your disposal and you can minimize the impact on performance if the nested data structures are applied properly Specifically try to manipulate the nested data at select locations in your application where performance is the least critical For example a nested data structure can be designed to organize a mixture of configuration data raw acquired data and post processed data The configuration data may be queried from various locations and bundled or unbundled prior to initiating any resource demanding operations Accessing the nested data structure should be avoided during the execution of critical tasks such as high speed data acquisition deterministic control and online data processing Specifically never bundle or unbundle continuously acquired measurement data into a nested data structure on the fly 190 as it is being acquired As the size of the data increases LabVIEW s memory man
396. of the subVI s code will be skipped if an error is present This maximizes the efficiency and responsiveness of your subVIs and the applications that call then Figure 7 11 illustrates subVI error handling incorporated by DAQmx Read VI In Figure 7 11A the panel contains the standard error clusters assigned to the lower left and right connector terminals Figure 7 11B shows the diagram including No Error and Error cases of an Error Case Structure As shown the No Error case primarily consists of a call to a Call Library node that performs the read operation as well as a call to a utility VI named DAQmx Fill In Error Info VI The Error case is empty except for the error cluster passthrough and a constant Figure 7 11C shows the contents of 218 DAQmx Fill In Error Info VI This utility VI checks if a new error has occurred and creates the error source information using a combination of data returned from Call Chain and any message passed to extended error info Because the error occurred in the subVI s caller the call chain is indexed by one element Figure 7 11A The front panel of DAQmx Read VI contains the standard error clusters assigned to the lower left and right connector terminals View full size image gt DOs Read Analog FEL Chan 1Sampi wi Front Panel De Ede ew njeti ipres ook Wincker bsp Fl Ili Figure 7 11B The block diagram of DAQmx Read VI uses an Error Case Structure to skip its code if an err
397. og amp User Interface utility VI SubVI that performs a low level task that complements or extends the capabilities of the built in LabVIEW functions UUT See unit under test V variant Self describing data type that encodes the data name data type data and attributes or information about the data into a generic format Conceptually variant can be thought of as a wrapper that converts any data into a new format that is described in a universal manner vi lib Folder containing all of LabVIEW s standard shipping VIs provided on the palettes VI Analyzer Toolkit An add on product from NI that automatically inspects VIs for style and performance The VI Analyzer contains more than sixty tests that can be configured and run interactively or programmatically The VI Analyzer Toolkit is very useful for performing self reviews W warning An error cluster value consisting of a nonzero code and FALSE status Warnings are considered to be similar to but less severe than an error waveform data type WDT A special type of cluster consisting of three elements and attributes The elements include a start time stamp to a time interval between data points dt and a one dimensional array of numbers representing the samples of an analog or digital waveform Y The attributes can contain any number of developer specified name and value pairs white space Empty space on a GUI window 352 353 B Style Rules Su
398. oint numeric prior to the mathematical operations This is indicated by the coercion dot on the subtract function The coercion creates a new buffer of data expanded to 8 bytes 167 per array element in addition to the previous buffer Hence the coercion increases the memory use by a factor of 4 in addition to allocating a new memory buffer Figure 6 1C Fetch Waveform VI scales the raw array to a scaled array of double precision floating point numbers Arithmetic operations between dissimilar data types cause a coercion which allocates a new memory buffer 116 dup YISA session visa Di AE R E ser Data Data error out IEA Actual Points a DBL E py gt beL Waveform Array DEL DEL OBL Increment TES A gt Initial x Next the scaled array passes through Read Waveform VI without modification as shown in Figure 6 1D Finally the top level VI shown in Figure 6 1E bundles the scaled array together with the Initial X and X Increment and updates the Waveform Graph The bundle function causes the scaled array to be copied into the cluster that it builds thus forming an additional memory buffer Figure 6 1D Read Waveform VI passes the scaled array without modification Waveform Start 1 0324 Actual Points Source Channel 1 Lait PDBL Initial x Waveform End 10000 O324 POBL Increment VISA session THTDSSR a nnne IFO dup VISA session error in no error k rr pjesen
399. olor and text conform to the rules for a subVI discussed in Chapter 3 The block diagram contains the code within an Error Case Structure Note that none of the functions that comprise the VI has error terminals However subVI error handling maximizes the efficiency in the presence of an error and helps facilitate data flow on the calling VI Figure 8 2A The layout color and text of this Immediate SubVI panel conform to the front panel rules for a subVI discussed in Chapter 3 View full size image Calculate Target Positions vi Front Panel Ele Edt view Project Operate Tools Window Ee Figure 8 2B SubVI error handling maximizes efficiency in the presence of an error and facilitates data flow on the calling VI View full size image If you are curious why the design pattern is called Immediate SubVI although the LabVIEW template is called SubVI with Error Handling allow me to explain In the literal sense both the terms subVI and error handling are misused The complexity of today s LabVIEW applications is such that subVIs can have any level of complexity from the principal 243 components down to the low level instrument drivers The Immediate SubVI design pattern is only intended for very simple subVIs Also in Chapter 7 error handling is defined as the practice of trapping and reporting errors The SubVI with Error Handling template performs error trapping but not error reporting Therefore an immediate subVI execu
400. onal Nested VI makes good use of subVIs which increase memory efficiency but uses Sequence structures which can decrease processing efficiency SubVIs reduce the quantity of nodes on the diagram which reduces block diagram memory In addition LabVIEW can reclaim memory buffers used by subVIs when the subVIs are not executing thereby reducing data memory use However the nested structures particularly the Sequence structures are a potential source of inefficiency When a 18 Sequence structure is called all frames must execute through completion in consecutive order In many situations it is preferable to reorder or abort the sequence to improve its efficiency The style rules presented throughout this book promote maximum efficiency Chapter 4 Block Diagram and Chapter 8 Design Patterns discuss rules that influence processing efficiency Chapter 6 Data Structures presents rules for efficient memory use Readability refers to how easily the developer can comprehend the source code This includes both the front panel and the block diagram The objects on the front panel should be clearly labeled and easily identified The diagram should be neat orderly and easy to follow The application should be well documented throughout The front panel of Meticulous VI appears both simple and perhaps a little mysterious Figure 1 6 shows the front panel in edit mode The menu controls along the bottom an
401. onnector pattern error terminal assignments and a Case structure with error in terminal wired to the selector terminal A variation is shown in Figure 8 1 It may save only 1 or 2 minutes of editing but if you create an application containing 100 or more immediate subVIs the minutes can add up to hours Also the template helps promote the subVI rules for front panels connectors and error handling discussed in Chapter 3 Front Panel Style Chapter 5 Icon and Connector and Chapter 7 Error Handling You can incorporate additional features to the template such as standard documentation icon type definition controls and more to save more time Figure 8 1A The front panel of the Immediate SubVI template contains error clusters the standard 4x2x2x4 connector pattern and terminal assignments View full size image 241 immediate SubV i vit Front Panel Template Figure 8 1B The diagram contains a Case structure with the error in terminal wired to the case selector View full size image immediate Sub wit Block Diseram Lemp bate 242 Figure 8 2 contains an example of the Immediate SubVI design pattern applied to a subVI called by the Torque Hysteresis VI Specifically Calculate Target Positions VI calculates several positions based on the user specified motion parameters The front panel layout c
402. onstants can have vertical scrollbars that hide a portion of the text From the shortcut menu select Visible Items Vertical Scrollbar Rule 6 16 Use enums liberally throughout your applications Rule 9 7 Use enumerated data types with Case structures Enumerated controls or enums map text selections to numeric values similar to a text or menu ring control However unlike a ring control when an enum is wired to a case selector the Case structure replaces its selector labels with textual labels that match the items within the enumeration Therefore enums describe the function of each case diagram of the Case structure using intuitive text Separate free labels for documenting every case of the Case structure per Rule 9 3 are often not necessary when enums are utilized In the example illustrated in Figure 9 3 Test to Run is a numeric control that is used to select one of several tests to run It is wired to the Case structure s selector terminal Each case of the Case structure contains one or more subVIs that perform the selected test In Figure 9 3A Test to Run is a text ring control and the corresponding selector labels are integers Free labels are applied in each case to associate the case number with the actual test name This requires effort to create and maintain the association of the free labels test names and case numbers As the Case structure is modified several items must be edited In Figure 9 3B
403. onstruct comprised of a loop that performs one of the application s principle tasks modularized into a subVI looped Case structure Construct comprised of a Case structure embedded within a For Loop or While Loop It functions similarly to a flexible alternative to a Sequence structure or Flexible Sequencer when the sequentially ordered code is placed in cases of the Case structure instead of the frames of the Sequence structure 346 maintainable The ease with which the source code is understood by other LabVIEW developers besides the author The code can be edited to change or add new functionality matrix Type definition consisting of two dimensional array of either double precision floating point numbers in a real matrix or complex numbers in a complex matrix The matrix data types are used widely throughout the Linear Algebra VIs in the Mathematics library Microsoft Compressed HTML Help CHM Standard help file format under Windows XP CHM files are heavily indexed and include a hyperlinked table of contents that normally resides outside the main body text modal GUI VI behavior in which the front panel window is locked on top of all open windows in the application until the interaction is completed and the window closes Modal behavior is a default property setting with LabVIEW s Dialog window appearance as selected from File VI Properties Window Appearance N NaN Abbreviation for Not a Numb
404. oop is configured using the input node s priority terminal Many real time applications have precise or complex timing requirements and the advanced features of a Timed Loop are beneficial For desktop applications prioritize loops using delays for maximum simplicity For real time applications use the Timed Loop s Priority property for precise performance Rule 8 18 268 Size parallel loops to the same width and align vertically Rule 8 19 Minimize space between loops Rule 8 20 Label each loop in the top left corner Here are a few rules that promote neat organized diagrams containing parallel loops Size each loop to the same width align the loops vertically and minimize the empty space between loops In Chapter 4 we discussed limiting the size of our diagrams to one screen with 1280x1024 resolution If a diagram contains multiple parallel loops the one screen rule is not feasible unless the loops are modularized into subVIs If you use parallel loops on the same diagram be sure to minimize the space between loops in an effort to maximize the diagram area that is visible on one screen Finally apply a free label to the top left corner of each loop identifying the primary task performed by the loop Consider the Torque Hysteresis application example The primary objective is to acquire analyze and present torque versus angle data In the single loop implementations presented in the previous sections the motion
405. operty The tabbing order specifies the order in which controls are traversed using the Tab key Set the tabbing order by selecting Edit Set Tabbing Order Key Focus is acontrol property that highlights a specific control and makes it capable of receiving input from the keyboard without the user having to select it first For example a common technique for dialog VIs is to use Key Focus to programmatically select and highlight an initial control then the user navigates to subsequent controls using the Tab key in the order specified by the tabbing order If you have any controls that are not intended to be operated by the user such as an error cluster or some controls that have been scrolled off the visible area of the panel remember to remove them from the panel s tab order This is accomplished from the control s properties dialog which is opened by selecting Properties from the control s shortcut menu Select the Key Navigation tab and choose Skip this control when tabbing Finally specify key assignments for various actions of any controls that users must operate without a mouse Control actions vary by control type Key assignments are specified from the Key Navigation tab Rule 3 38 Customize the runtime menus for top level VIs Rule 3 39 82 Always include a Help menu or button LabVIEW s default menu bar is marginally useful at best for applications deployed to users The majority of the selections are intended to help
406. operty Node is associated with the Quit Boolean control and is configured to set the Quit Value Signaling property which fires a Quit Value Change event within the Event Loop The Quit Value Change event case runs and stops the Event Loop Consequently variable polling is no longer necessary and all instances of the Stop global variable are eliminated This implementation optimizes the master slave synchronization efficiency as shown in Figure 4 11B Figure 4 11A A notifier replaces the occurrence and the Stop global variables are eliminated This implementation optimizes the synchronization efficienctly 116 View full size image Figure 4 11B The Quit Value Change event case stops the loop The Release Notifier function is called outside the loop causing the Wait on Notification function in the DAQ Loop to wake up and return an error The error is cleared in the False case and the DAQ Loop terminates View full size image QEvert Loop Master T G eiae Signaling As discussed mastering LabVIEW s dataflow principles is synonymous with eliminating local and global variables and Sequence structures when feasible The previous sections identify the practical and impractical uses of these programming constructs This section presents alternatives that optimize data flow including shift registers and looped Case structures Rule 4 35 Use shift registers over local and global variables Rule 4 36 117 Group
407. or Handler VI is used instead of the Simple Error Handler VI Figure 7 18B Minor improvements to the shipping example include vertically aligned error tunnels at the bottom of the loop loop condition at the bottom right and General Error Handler VI View full size image File pant inks serpy a Sree La Eel hie Figure 7 19 contains the diagram of an instrument driver VI that controls a semiconductor wafer probe system The diagram incorporates subVI error handling that includes error cluster terminals and an Error Case Structure The No Error case of the Error Case Structure executes the Call Library Node and an error routine The Call Library Function Node calls a function that resides within a dynamic link library DLL The function returns a parameter with integer data type named Error Number This parameter returns a value of 0 if the function succeeds or an integer in the range of 500 to 600 identifying any errors that occur within the DLL The error routine evaluates the Error Number and generates an error cluster using Error Cluster from Error Code VI The resulting LabVIEW error consists of a code that is derived from the original Error Number returned from the DLL but shifted into the range of LabVIEW user defined errors 8999 to 8000 5000 to 9999 Specifically an Error Number from 500 to 600 is shifted to an error code from 8500 to 8600 231 Per Rule 7 12 negative error codes are preferred for I O device error
408. or floating point numbers 6 14 Use automatic formatting unless a specific format is required 6 15 Show radix for hex octal or binary data 6 16 Use enums liberally throughout your applications 6 17 Save enums as type definitions 6 18 Avoid string controls on GUI VI panels unless required 6 19 Use enum ring and path controls in place of string controls where possible 6 20 Keep the Browse button visible for path controls on GUI VI panels 6 21 Use arrays for multivalued data items use clusters for grouping multiple distinct items 6 22 Use arrays to store large or dynamic length data sets 6 23 Enter descriptions for array and cluster shells and control elements 6 24 Use alignment tools to keep clusters neat and compact 6 25 Save all clusters as type definitions 6 26 Always use Bundle and Unbundle By Name 6 27 Avoid clusters for interactive controls with Dialog VIs 6 28 Organize complex data using nested data structures 6 29 Avoid manipulating nested data during critical tasks 6 30 Limit the size of arrays by initializing to maximum length Chapter 7 7 1 All VIs must trap and report the errors returned from error terminals 7 2 Trap errors by propagating the error cluster among the error terminals 7 3 Trap errors from all iterations of loops 7 4 Disable indexing of errors with continuous loops 7 5 Trap all errors from all nodes that have error terminals 7 6 Report errors using a dialog and or log file 7 7 Use General Error Handler VI o
409. or is present View full size image Figure 7 11C DAQmx Fill In Error Info VI creates the error source information from a combination of data returned from the Call Chain function and any message passed to extended error info View full size image 219 a E i E DALE Pall im Error bite Rule 7 14 Use unwired defaults over constants for output tunnels of Error case As a side note the maintainability of DAQmx Read VI can be improved by using unwired defaults for output tunnels of the Error case instead of constants Specifically the numeric constant wired to the output tunnel leading to the data indicator terminal should be eliminated and replaced with the tunnel s unwired default value This is accomplished by selecting Use Default If Unwired from the tunnel s shortcut menu This provides several advantages over wired constants First indicators can be modified or deleted without requiring the developer to manually edit the constants and wires Second it is easier to confirm that the defaults are being used by simple inspection Finally the diagram appears cleaner without the constants Wire constants to output tunnels only if the value is not the data type s default Prior to LabVIEW version 8 2 the Call Library Node did not have error terminals Consequently the Case structure was the only method of preventing a dynamic link library function call from executing Error Case Structures are also used to incre
410. oreover any application containing a graphical user interface benefits from the efficiency of an event handling loop The only caveat is that the Event structure is not available in the LabVIEW Base package Rule 8 5 Use the Value Change event for most GUI controls The Value Change event detects any control data changes by the user and is common to all types of controls If a control s value changes whether it is a Boolean command button a numeric a string or another control type the Value Change event fires and the corresponding subdiagram runs For basic GUI applications use the Value Change event to detect control value changes This promotes consistency in the VI s performance readability and maintenance Use other events such as Key Down Mouse Down and events that are specific to a particular control for special effects Rule 8 6 Place control terminals within their Value Change event case Note that the Event structure allows you to read the new and old control values from the Event Data Node Boolean controls configured with any type of latching action however will not reset their value to the default state until their data is read directly from the control terminal Placing the control terminals within the Event structure s corresponding Value Change event case ensures that the terminals are read and latching Booleans are reset each time a value changes It is a good practice to place all control terminals insid
411. orm graph No modifications to the data are required Waveform Start Waveform End 110000 a Waveform Graph In this example consistent data types modestly reduce memory consumption while substantially reducing the number of buffer allocations Hence overall performance is optimized Additionally the waveform data type is a convenient data structure that requires fewer terminals and wires for the multiple VIs of the call chain When the waveform data type is formed the developer can use the high level VIs on the Waveform palette to perform common waveform operations Hence Rules 6 1 6 2 and 6 3 have been combined for best overall performance It should be noted that in some applications the raw waveform is preferred in the smallest format instead of either waveform data type or double precision floating point numbers For example if the objective is to acquire and stream the raw data to file the smallest file size and fastest disk streaming rate are desired In this instance streaming the raw binary data to file in the format that it is read directly from the instrument provides the optimum result Be sure to log the waveform scaling information to convert the data to the desired engineering unit offline 170 After the controls have been selected the next step is to configure their properties For Boolean command buttons it is common to configure the Mechanical Action and Boolean Text properties
412. ost top level VIs are configured with the Run when opened Execution property selected Therefore the LabVIEW toolbar is usually not needed Rule 3 13 Include your company logo for a professional appearance A nice finishing touch is to apply your company logo to the panel of the top level GUI VI LabVIEW can import most common graphic files Select Import Picture to Clipboard from the Edit menu and then paste the image onto the panel I normally apply the logo in a margin near the top of the panel 67 This section presents rules for the layout of subVI panels which remain closed during the execution of the application Rule 3 14 Use default appearance for subVI panels objects and most text Because only developers view the panels of subVIs it is usually not necessary to expend a lot of effort to make them attractive Instead it is a convention that subVI panels appear bland This helps identify them as subVIs This way when a developer opens a VI and sees default colors control appearances and fonts she generally recognizes it as a subVI I recommend expending just enough effort to make subVI panels organized and professional Use the default appearance for the panels objects and most text This means gray panels sized less than full screen containing controls and indicators that resemble their appearance on the palettes with 13 point Application font for most text Rule 3 15 Arrange controls to resemble the conn
413. ot perform its intended function it will not provide a positive user experience Chapter 3 Front Panel Style contains style rules for front panel design that promote consistency and ease of use for your GUIs Chapter 6 Data Structures and Chapter 8 Design Patterns provide additional style rules that help optimize the responsiveness of the GUI Efficiency pertains to the use of processor memory and input output 1 0 resources An efficient LabVIEW application executes quickly without performing unnecessary operations particularly ones that are performed repeatedly within looping structures An efficient application also conserves memory by limiting the size of the four LabVIEW memory components the front panel block diagram data space and code Front panel and block diagram memory store the graphical objects and images that comprise the front panel and block diagram respectively Data space memory contains all the data that flows through the diagram as well as the diagram constants default values for front panel controls and the data that is copied when written to variables and front panel indicators Code is the portion of memory that contains the compiled source code Finally efficient applications minimize I 0 operations such as GUI updates instrument and network communications and data acquisition DAQ calls Execution speed and memory use are related Memory and disk operations are a princip
414. ot satisfactory for industrial GUI VIs In many industrial settings where the users are multitasking while an application runs a LabVIEW dialog might open unnoticed Yet dialogs are often used for critical purposes such as prompting for manual actions or displaying an unexpected error or warning Fortunately it is very simple to create more industrial alternatives that follow the style rules for industrial GUI VIs Figure 3 2 compares desktop and industrial style dialog windows Figure 3 2 LabVIEW s Two Button Dialog function is well suited for desktop applications as shown on the left A larger Two Button Dialog VI is preferred for industrial applications as shown on the right This is a desktop two button dialog 60 This is an industrial two button dialog Rule 3 8 Use system controls for desktop dialogs use 3D controls for industrial dialogs Custom dialog VIs can be developed for desktop as well as industrial applications For desktop applications simply set the Window Appearance property to Dialog and use only system style controls from the System palette You set the Dialog window appearance from File VI Properties Window Appearance The panel background color modal behavior and other properties are selected to resemble an operating system dialog Likewise the controls indicators and decorations available from the System palette resemble the style of objects used by the operating system Use only t
415. ot viable ad pibon of ihe e eneeetod nme be mg Bij ated eee abe aed fe ap Fite cee fae ay The ia ete ie ie Ee Peay Poe pfi ape aes Cap ee Pig Sera leet a h a a m na u in dines pirer ard re n D e Tie Mat Mid a ure DEE Bai o E Pe bout Biere Kl a umerii eraa vie Chee nra ae i beL Baa ope by at th in praj WE n The WT el pona ty whukdossn shen cunrang m r soen aie Baloe depe edi WP p Akraj appear arog 2 H e Uncheck Shige T ibar Mien Running ame uncheck Shoe Sere Baar j i Figure 8 22B The framework s top level diagram is a compound design pattern that contains an Event Handling Loop a Main State Machine and an Error Handling Loop The framework is rich with features that have evolved over time View full size image nan Getic ies ET er Le ET m Ws 0 ore E oy 280 This application framework and all subVIs have evolved over time and represent a reliable starting point for complex application development Additionally the framework and subVIs are all recognized standards within our organization that facilitate commonality performance maintainability and good programming style The framework is integrated into a LabVIEW project for file organization and management similar to what is presented in Chapter 2 In Section 8 3 Compound Design Patterns the performance of the Torque Hysteresis application was improved using parallel loops In Figure 8 23 the application
416. p GUI event handling instrument communications data parsing data logging data review data display and error handling A total of eight parallel loops are used Each loop is finely tuned for optimum performance However the top level diagram is extremely large resulting in very large data and diagram memory components The four components of memory use are provided in the VI Properties window shown in Figure 1 5 You access the VI Properties window by selecting File VI Properties and then selecting Memory Usage for the category Figure 1 5 The VI Properties window indicates the four categories of memory use for Meticulous VI View full size image E VI Properties Memory Usage Front Panal Objects 174 8K Block Diagram Objects 2721 6K Code 518 0K Data 2935 08 Total matk Total I Size On Disk wt FS11K Help Event structures are the most efficient method of capturing user interface activity The GUI event handling loop sleeps using no processor time until it receives a registered event from the operating system This is both more efficient and much more responsive than polling the control terminals in a loop Hence Meticulous VI is maximally responsive to GUI events Finally Meticulous VI makes extensive use of shift registers for passing data between loop iterations and queues for passing data between parallel loops Shift registers are much more efficient alternatives to variables and queues are much more functi
417. p subVI contains a Timed Loop as shown in Figure 8 31 Messaging between VIs is accomplished using RT FIFO buffers Figure 8 30 The top level diagram for a distributed control system uses the Modular Multiple Loop Application Framework View full size image 293 ib Block Name Figure 8 31 The diagram of one of the loop subVIs consists of a Timed Loop Messaging is accomplished using RT FIFO buffers View full size image Endnotes 294 Templates for several of the design patterns discussed in this chapter can be downloaded from www bloomy com lvstyle Using LabVIEW to Create Multithreaded Applications for Maximum Performance and Reliability and LabVIEW and Hyperthreading are available for free download from NI Developer Zone WWW zone ni com 295 9 Documentation Documentation is any description of the structure components or operation of a system application or source code Documentation includes specifications design documents source code documentation user manuals and online help Specifications and design documents are discussed in Chapter 2 Prepare for Good Style Source code documentation consists of comments descriptions and text that are useful for the developers to understand the source code A user manual is a document describing how to operate the software from the end user s perspective Online help is documentation that is electronically integrat
418. panel control back in version 3 0 of LabVIEW previously packaged as an Attribute Node that did not have error terminals you might be able to justify skipping the error trapping on this one However beware that there have been some subtle changes in the behavior of various functions and properties from one version of LabVIEW to the next The Synchronization and Communication functions for example seem to incorporate new enhancements with each new release Error handling helps us identify and learn about the differences between versions and port our applications more reliably LabVIEW s functions and VIs can be categorized in terms of risk to help understand the types of errors they generate and evaluate handling strategies with respect to a given application The probability and severity of an error depends on the application and the system However different types of operations have different levels of risk of generating errors and different types of associated misbehavior regardless of the application or platform Specifically I find it useful to categorize the nodes on the palettes in three levels of risk high medium and low Then prioritize the error trapping based on the perceived risk LabVIEW s error out terminal is the primary method by which LabVIEW errors are returned from nodes that generate errors An exception includes nodes that return an error code as a scalar integer such as the Mathematics VIs and many Call
419. particular HTML is the easiest format to cross reference multiple documents using hyperlinks Also if your documentation requires frequent updates you can maintain the content as a website instead of shipping the documentation with the application You can launch an HTML document in the host computer s default web browser using Open URL in Default Browser VI located in the Help sub palette under Programming Dialog amp User Interface Help Portable Document Format PDF was created by Adobe Systems for representing two and three dimensional documents in a fixed layout cross platform compatible file format PDF files encode the exact look of a document in a device independent manner They are easily viewed or printed using any type of computer Use PDF for documents with fixed content and format such as a user manual PDF documents can be viewed using Adobe Acrobat Reader as well as most web browsers Use the System Exec VI located on the Connectivity Libraries amp Executables palette to launch the document in the reader you specify You must build the proper command line for the desired browser application and document path Alternatively use Open Acrobat Document VI located in lt vl lib gt Platform browser llb This VI automatically forms the command line for you using the computer s default reader Unfortunately this VI is not well documented or style conforming and is not made accessible from the palettes For commercial ap
420. perations performed on the diagrams Control types determine how users understand and operate the front panels This includes both GUI VIs that users interact with and subVIs that only the developers see As presented in Chapter 3 Front Panel Style judicious selections for controls make the panels intuitive user friendly and reliable Additionally each control type supports one or a limited set of data types Consequently the developer s choice of controls helps determine the application s data structures Data constructs are used to organize data and reduce wire clutter on the diagram Consistent use of data constructs throughout an application help make it easy to understand and maintain as well as memory efficient Nested data structures have special considerations with respect to performance and memory use as discussed later in this chapter Theorem 6 1 Because memory and data storage access rates are the principal performance barriers of modern computing devices software execution speed is inversely proportional to memory use It is a fact common to all modern software that execution speed is inversely proportional to memory use Specifically memory and data storage access rates are the principal performance barriers of modern computing devices A LabVIEW application s memory use is defined by its data structures and the operations performed on the data LabVIEW does not impose restrictions on the data st
421. perly labeled task ID out and error out Additionally one of the indicators has the generic label Numeric What is curious is that the subVI has a custom icon and description and the control terminals are within reasonable proximity to the structure 122 Perhaps it was partially repaired The wiring and terminal labels have been cleaned up in Figure 4 14B Also Figure 4 14C contains equivalent code using the waveform data type and the newer sound output VIs released with LabVIEW 8 This is the preferred implementation Figure 4 14A This diagram is the product of the SubVI from Selection utility without the mandatory repairs View full size image bask ID gut butput chunk ET Figure 4 14B The VI is revised to incorporate appropriate control labels object spacing and clear wiring View full size image Stream recorded data to sound card Figure 4 14C The VI is rewritten to utilize the waveform data type and LabVIEW 8 sound VIs View full size image A tai ID out Be error out Layout The diagram 4 14A is small and the objects are poorly spaced Modularity Not applicable because there are only a few nodes However the VI itself is the outcome of the developer s attempt to modularize the calling application Wiring Scheme The wires are excessively kinked a common byproduct of the subVI from selection utility Additionally one wire is not lined up with the tunnel in which it
422. pers who have mastered the fundamentals and are ready to take their skills to the next level will learn the most from this material No doubt you have experienced the power and flexibility of LabVIEW and are ready to concentrate on style Advanced developers will strongly identify with the contents reinforce their knowledge and experience and have a useful reference to share with colleagues You might use The LabVIEW Style Book to help reduce the training and support burden you might have within your organization Managers and Organizations that employ multiple developers and users can gain maximum benefit by standardizing on these style rules across the organization This approach ensures quality and consistency throughout an organization and helps satisfy industry quality standards Organization The chapters of The LabVIEW Style Book present style rules and examples organized by topic Chapter 1 The Significance of Style discusses the relationship between style and ease of use efficiency readability maintainability robustness simplicity and performance Chapter 2 Prepare for Good Style presents considerations that influence style before you begin programming including specifications configuration of the LabVIEW environment and project and file organization Additionally it presents a specialized standard for LabVIEW project specifications Chapter 3 Front Panel Style Chapter 4 Block Diagra
423. pin he Prag Da vas erry orei dd th rdi ar hal The Classic State Machine is not appropriate for complex VIs top level VIs and GUI VIs Alternative state machine implementations that utilize queues and Event structures are more functional and efficient for these applications The Classic State Machine design pattern s shift register method of state transitioning is limited to one new state specified per loop iteration or state of the application In VIs of medium or greater complexity it is often desirable to buffer multiple states that comprise a sequence and execute them one at a time The Queued State Machine design pattern uses a queue to buffer multiple states Any state of the application can add any number of new states to the end of the queue using calls to the Enqueue Element function States are removed from the queue one element at a time and passed to the case selector using the Dequeue Element function This is similar to a first in first out buffer Additionally 259 use the Enqueue Element At Opposite End function to add a state to the front of the buffer for immediate execution This allows the application to respond immediately to high priority actions or events such as executing the Shutdown state when the user quits the application A template for the Queued State Machine design pattern is provided in Figure 8 12 Figure 8 12 A template for the Queued State Machine design pattern which maintains multiple states i
424. plication s primary purpose is to run a test involving a lengthy data acquisition process upon user command via the Run Test Boolean control Because the data acquisition process is lengthy the application is divided into separate parallel master and slave loops including an Event Loop for processing user interface events and a DAQ Loop for performing the data acquisition task The Event Loop is the master that triggers the data acquisition task within the slave DAQ Loop The parallel loops enable the user interface to remain responsive while the data acquisition task runs in the background LabVIEW spawns separate execution threads for each parallel loop A local variable is used to initialize the Sensor Scaling cluster with values read from file Global variables are used to share Boolean data between parallel loops The Acquire global variable triggers the data acquisition task from the Event Loop The Stop global variable stops the DAQ Loop when the Event Loop stops and vice versa The application has a Boolean control that writes to the Stop global from within the Event structure s Quit Value Change event case not shown Specifically when either loop stops it passes a Boolean TRUE value through a While Loop tunnel to a Stop write global variable Upon the next iteration the other loop reads this value from its corresponding Stop read global variable and stops Global variables require less configuration and effort than alternative constru
425. plications consider a compiled help file Microsoft Compressed HTML Help CHM is the standard help file format under Windows XP CHM files are heavily indexed and include a hyperlinked table of contents that normally resides outside the main body text First you create an HTML document using your favorite word processor or HTML editor and then you compile it into a CHM file using a third party help file compiler such as Microsoft HTML Help Workshop or Adobe s Macromedia RoboHelp From LabVIEW you can programmatically launch the help file using the Control Online Help function Wire the String to search for input to open the help file to a specific topic The required content of a compiled help file varies widely based on application and is beyond the scope of this chapter Windows Vista utilizes Microsoft Assistance Markup Language AML a new generation of Help in which documents are defined by their context As of this writing the AML format is not available as an authoring platform Therefore CHM remains the preferred help file format for commercial applications Several varieties of online documentation are incorporated into the Torque Hysteresis VI shown in Figure 9 5 They include Context Help and a user manual in PDF HTML and CHM format These resources are selected from the Help menu as shown in Figure 9 5A Figure 9 5B shows the diagram for handling menu selection events Each menu selection corresponds to 309 a case in
426. ports Microsoft Visual SourceSafe Perforce MKS Source Integrity IBM Rational ClearCase Serena Version Manager PVCS Seapine Surround SCM Borland StarTeam Telelogic Synergy PushOK CVS and 53 SVN plug ins and ionForge Evolution Under Windows LabVIEW uses the Microsoft Source Code Control Interface On non Windows platforms LabVIEW supports only Perforce via the command line interface Source control is essential in a multideveloper environment It allows a project team to collaborate on the same project at the same time by controlling access to the project folders and source files This prevents multiple developers from editing the same source files at the same time Also most source control packages enable you to configure permissions for each user For example you could configure a project documentation repository to give write access only to managers while you could configure your source code repository to give write access only to software developers Alternatively some source control tools give multiple developers unlimited access to the source files and the source control tools resolve conflicts when changes are merged back into the repository Source control tools track changes and revisions to all types of project files and maintain a comprehensive backup These features are as beneficial for a single developer project as they are for a multideveloper project Additionally CM and source control are co
427. possible opportunities Imagine going through final checkout of an application and a report or display screen contains the company name address and logo of a different company Very embarrassing to say the least Additionally code that has been hacked tends to appear much messier than code that was designed for its intended purpose from the outset Hence maintenance of the source code is generally more difficult and time consuming in this scenario For best style I recommend maintaining a reusable application framework that uses several of the 271 functional elements common among the applications you encounter Make the application framework generic reusable and architected in a highly flexible manner This section presents three application frameworks that are useful for complex applications It is important to note that many variations to these frameworks are possible The specific implementations including examples and illustrations are kept relatively simple to facilitate discussion The Dynamic Framework uses VI server functions to dynamically load and run its high level VIs from source files on disk The high level VIs that comprise the application are known as components VIs that are dynamically loaded by the framework whether high or low level are plug ins Prior to the VI Call Configuration feature introduced in LabVIEW version 8 0 the Dynamic Framework was widely used to help improve the load time and memory efficiency of ve
428. properties of a control specify the Type Definition Status and save it to a CTL file You access the control editor from the control s shortcut menu by selecting Advanced Customize A custom control or type definition enables you to reuse the customized control in multiple places throughout the GUI and maintain a consistent data type and appearance Simply add the type definition to your LabVIEW project and drag and drop it from the project tree to your VI front panels The control loads from the file that contains the customized properties specified in the file 83 The Type Definition Status is specified using the ring control at the top of the control editor The choices are Control Type Def and Strict Type Def A control is simply a custom control with appearance and properties specified in the control file The custom control can be edited on the panel after loading from file without affecting the appearance properties and data type of the source file Each instance of a type definition can be customized as well with the exception of the data type A strict type definition maintains the exact appearance properties and data type that are defined in the source file for each instance of the strict type definition I normally create multiple instances of the controls that I customize and often make changes to their appearance Within a project I normally need the changes to be made to all instances Therefore I prefer to use the strict
429. put to a limited number of discrete selections and are represented as integers Finally note that enumeration and variant appear in each table as both controls and data types This is because they are both An enumeration is a special data type consisting of a numeric with associated text strings A variant is a self describing data type that encodes any LabVIEW data into a generic format Similarly arrays and clusters can be considered data types as well as controls However the data type of arrays and clusters is undefined until they are populated with specific data structures Unlike enumerations and variants wiring assignments cannot be made from the terminals on the diagram until the data type definition is completed by depositing controls into the array and cluster shells Otherwise any wires are broken Therefore arrays and clusters do not become valid data types until populated Rule 6 3 Choose controls and data types that facilitate consistent data structures throughout an application Referencing Theorem 6 1 the most important consideration in choosing controls and data types is consistency Dissimilar data types require conversions either explicitly via the formatting and conversion functions or automatically via coercions that appear as dots on terminals LabVIEW allocates separate memory buffers to store each representation of data Therefore dissimilar data types lead to extra programming processing buffer allocations a
430. r If a cluster element is added removed or relabeled the Bundle By Name and Unbundle By Name elements become invalid if they are affected Therefore Bundle By Name is much more readable and maintainable than Bundle Figure 6 10C The string controls are bundled into a cluster on the diagram The terminals of the Bundle function are ambiguous EHAA E 4 0 OK Value Change Customer Model i i Source ba cee UUT Information Part Status Production Year Test Reference Spec Number Spec Description G Figure 6 10D Bundle By Name provides terminal labels that improve readability and maintenance View full size image Dae omeni Model Labim Model UUT Information Part Statusi abe Production Year EE ES pe a Beam Spec Number Spec Spec Description 188 A few special data constructs include matrix error waveform dynamic and variant The former four are variations of arrays and clusters that are predefined by LabVIEW and serve a specific purpose The real and complex matrices are simply two dimensional arrays of double precision floating point numbers and complex numbers respectively These arrays have been customized and saved as type definitions They are used widely throughout the Linear Algebra VIs in the Math library Likewise the error in and error out clusters used extensively for error propagation are just cluste
431. r convenience Rule 4 11 Minimize wire bends eliminate kinks and loops Rule 4 12 Maintain even spacing of parallel wires Rule 4 22 Always flow data left to right Rule 4 23 Propagate the error cluster We begin with some connector rules that help prevent wire clutter Rule 5 19 Select a pattern with extra terminals It is common to add and remove inputs and outputs from subVIs throughout an application s development and maintenance cycles which necessitate changes to their terminal assignments A change in the connector pattern is required when the number of inputs and outputs exceeds the number of available terminals Changes to the connector pattern cause the connected wires in the callers diagrams to move become misaligned with their terminals or break altogether Additionally every instance of the subVI call must be manually relinked by right clicking and selecting Relink to SubVI from the shortcut menu on each instance These undesirable side effects can ripple throughout an application requiring close inspection and fine adjustments to the subVI calls However connector pattern changes can be avoided by always selecting a pattern that contains more terminals than you will likely need starting from the outset Rule 5 20 Choose a unified pattern for related VIs Related VIs that are commonly used together typically share some common inputs and outputs that are normally wired together For example data acqu
432. r in ro anor error mut Thiet Vi prompts the er for parameter of he leer config Mie her the uner chin OE D bn d for wee by veers legge on eth erage privieses E peime Wareleneth Scan Parameters Figure 4 5D The cluster of laser scan parameters propagates throughout the diagram Wire clutter is reduced Parameters can be added and removed from the cluster without affecting wiring and subVI terminals assignments View full size image In some situations it makes sense to define a cluster for just two data elements A small cluster is useful to associate data that is very closely related and is frequently used together As an example the high and low limits of a measured parameter may be read from a database edited by the user in a dialog VI passed to another routine that compares the measured data to the limits and passed to additional routines where a report is generated and the data and limits are logged to file A two element cluster containing the high and low limits eliminates one wire and logically binds the data together In this case the cluster is more beneficial for associating related data than for eliminating wire clutter The optical filter measurement routine propagates the wavelength and power data arrays using two separate wires As shown on the right half of Figure 4 6A the two wires are used together in most places with the exception of Convert Power Units VI which requires only the power data In Figure 4 6
433. r reviews for best results Code reviews require one or more knowledgeable resources to provide objective feedback Peer reviews involve coordinating a meeting with colleagues who are experienced with LabVIEW and the organization s style conventions This is the traditional method of conducting code reviews However objective feedback can be obtained in more than one way Self reviews are performed utilizing alternate resources such as the summary of style rules in Appendix B Style Rules Summary and the LabVIEW VI Analyzer Toolkit Self reviews and peer reviews are complementary techniques that should both be used for best results 10 1 Self Reviews As the name indicates self reviews are techniques for reviewing your source code by yourself Automated and manual methods exist for conducting self reviews The automated method involves the LabVIEW VI Analyzer Toolkit which is an add on product from NI Additionally the style rules summarized in Appendix B can be utilized as a manual checklist Simply go through the rules and check off whether your VIs conform to each rule Rule 10 3 Perform a self review prior to every peer review 316 At a minimum perform at least one self review prior to each peer review This helps you prepare for the peer review by considering your design decisions and identifying and correcting any style issues you discover before the peer review This increases the productivity of
434. ransparent is inconsistent 305 Figure 9 4A Configure Test Instrument VI front panel and block diagram Documentation is nonexistent on the diagram View full size image Se Figure 9 4B This implementation contains documentation consisting of visible terminal labels labels on wires exiting shift registers and labels in each case However the diagram is wide due to the unnecessarily lengthy label View full size image Figure 9 4C contains the optimal documentation for this VI The text ring control for Please Select Instrument to Configure has been replaced with an enumerated type definition labeled Instrument The enum maintains the same instrument names for text items as the former text ring control However the instrument names now appear in the Case structure s selector area This documents the Case structure s cases and eliminates the need for the free label comments in each case All of the labels are succinct which reduces the space they consume on the diagram Bundle By Name replaces the Bundle 306 function The terminal names simplify and reinforce the wiring assignments and reduce maintenance if the cluster order changes Finally all of the labels throughout the diagram have transparent backgrounds Figure 9 4C Several improvements have been performed including succinct labels enumerated Case structure and Bundle By Name function View full size image
435. rce code Apply shortcuts to help expedite icon development but never sacrifice unique meaningful icons and cohesive VI descriptions 308 The relationship between cohesive subVIs and VI descriptions is discussed in Chapter 4 Block Diagram An extensive Ww discussion of icon style including shortcuts and examples is provided in Chapter 5 Icon and Connector 9 4 Online Documentation Integrate stand alone documents such as help files and user manuals into your LabVIEW applications as online documents Online documents include HTML portable document format PDF and compiled help CHM files These documents can be programmatically launched from the Help menu of a top level GUI VI or from a Boolean control labeled Help or from a dialog VI Use an Event structure to programmatically launch the document when the menu is selected or when the Help button s value changes This is very similar to launching the Context Help window as discussed in the previous sections HyperText Markup Language HTML is the universal language for the creation of web pages Today most word processors and many applications can create HTML documents For example LabVIEW s built in printing features as well as the VIs from the Report Generation toolkit readily generate HTML Create your online documents using the HTML format if you want to view the document in a web browser or if you want to provide network access to the documents via the Web In
436. rder Because sequence local terminals can be written to only once 11 terminals are required to propagate the error cluster among 12 frames This clutters the appearance and causes wiring and dataflow rules violations Most frames contain one right to left data flow and a wire crossover caused by reading from a sequence local terminal on the right border or writing to a sequence local terminal on the left border Additionally the Stacked Sequence structure implementation contains 48 local variables These include two local variables per frame for updating the Test Results table indicator plus two local variables per frame for reading and writing the test limits 118 Figure 4 12A A test sequence is implemented using a 12 frame Stacked Sequence structure It contains 48 local variables plus 11 sequence local terminals and right to left data flow a 11 0 11 Pecan Pressure Leak Test Test Results fpr vq Test Results est Pr ssirg Limits LEAK Limits T Eon Figure 4 12B utilizes a looped Case structure using a For Loop as the looping structure The code within the cases of the Case structure is functionally equivalent to the code within the frames of the Stacked Sequence structure from the prior implementation However only five shift registers are required to replace 48 local variables and 11 sequence local terminals used by the Sequence structure implementation The wiring is clear with no right to l
437. re not artistically inclined Also an artist s creativity can be compromised by deadlines and time constraints Realistically it is impractical to create an imaginative new icon for every VI within a large application Shortcuts are essential The following rules help to minimize icon development time while ensuring meaningful and effective icons Rule 5 11 Use textual foreground colored background and a black border for fastest results Rule 5 12 Contrast the text and background colors Rule 5 13 Choose a color convention for the type of VI From my experience the fastest way to create a unique and meaningful icon for every VI is to create icons that consist of a text based foreground a colored background and a black border In other words eliminate the glyph Choose text and background colors that provide sufficient contrast such as black or dark text on a white or light background or 138 vice versa Choose the background color to designate the type of VI via color code convention In the Icon Editor start with a new VI containing a default icon select the desired background color and double click the Filled Rectangle tool as shown in Figure 5 5 Note that black should already be selected as the default foreground color unless you have changed it This operation effectively replaces the default icon contents with an empty icon filled with the background color and bounded with a black border This is how I begin most of
438. receives error information from the parallel loops via queue and evaluates reports and logs the errors View full size image 22 The error handling scheme in Nested VI is less thorough and sophisticated than Meticulous VI yet it is simple and functional Referring to Figure 1 10 the first subVI called in each case of the inner Case structure has an unwired error in terminal whereas the error out terminal is propagated between the nodes of its case through the Case structure s output tunnel to a Sequence Local The Sequence Local passes the error information to the error handling subVI in frame I of the outermost Sequence structure You can improve error handling style by propagating the error cluster between loop iterations and wiring it to the error in terminals of all nodes Efficiency also can be improved if the error handling code is not called unless an error occurs Figure 1 10 The error handling scheme of Nested VI is functional but not optimal The error cluster propagates between subVIs through the Case structure to the sequence local The error is processed using the Error Handler VI in frame l of the Sequence structure View full size image 23 Pele Deeb eee eee ee eee ee E E E G Djia F SEY a A E E a E a a a tS a E E i t Ta GET STATUS Eee Loe Coo GOOO eat P a cd Cpa eth edo Ot el Oe et te gol Colt tt GOOO Coot feb tt Ga et ol Gat feb tel Gal GCC T m eRe he Paha he Pee he ae he a i a e
439. rer an interface for organizing project files within a project tree The properties that specify the project are stored within an XML file called the LabVIEW project lvproj The files referenced by the project can reside almost anywhere can be named almost anything and can target on a growing variety of computing devices This presents new opportunities as well as new challenges for developers and organizations Conventions for organizing naming and controlling project files are desirable Many of us take it for granted that source files are organized named and controlled appropriately From experience I have learned that this assumption is entirely not valid For example it seems that some developers strive to minimize the length of their filenames I have seen naming conventions that consist of very short acronyms followed by an underscore and number as if there was an 8 3 character limitation reminiscent of DOS Surprisingly this is not uncommon I have seen large applications that consist of hundreds of such VIs within a single folder or LLB none of them marked as top level In these situations when the developer is long gone and documentation is scarce I wish I could buy a vowel Furthermore I have seen test labs that contain multiple PCs each maintaining dozens of variations of the same application The users edit constants on the block diagram that represent transducer scaling factors as well as default valu
440. rminated and the error is reported The General Error Handler VI s placement outside of the loop ensures that the dialog will appear only once even if the loop s termination logic changes This example represents an extremely common error handling methodology applied to a simple application s architecture Figure 7 10 The Torque Hysteresis VI traps errors via propagation of the error cluster terminates the loop on error and reports the error using General Error Handler VI Notice that the General Error Handler VI is called only once View full size image Rule 7 10 Avoid subVIs with built in error reporting LabVIEW s palettes contain several VIs that have error reporting built in These VIs can be recognized on the palettes as high level VIs appearing in the top row of their subpalettes that do not have error terminals They include all 214 VIs formerly known as the Easy I 0 VIs which are available from the traditional Data Acquisition subpalettes and several of the high level File I 0 VIs All such VIs are inflexible providing only the dialog method of reporting errors Additionally the absence of the error terminals makes them more difficult to use in a dataflow sequence Finally these VIs are less efficient than the lower level VIs Hence I recommend avoiding then The error cluster contains a code control represented as a 32 bit signed integer Codes are used to uniquely identify the errors tha
441. rocessing required for the conversion to double precision floating point numbers In Figure 6 2B the raw array is scaled using arithmetic operations applied to consistent data types eliminating the coercion and extra memory buffer versus Figure 6 1B Also a waveform data type is assembled In Figure 6 2C the waveform passes through Read Waveform VI without modification Finally in Figure 6 2D the waveform passes up to the top level VI and updates the waveform graph without modification Figure 6 2A Get Raw DBL Waveform VI returns an array of double precision floating point numbers which compromises memory efficiency within this VI for data structure consistency throughout the application Also an intermediate conversion is eliminated View full size image Comeet cate coring Wiehan Aira pie iF nie or maA rta See per pot L Figure 6 2B Fetch Waveform VI performs arithmetic operations using similar data types avoiding the coercion and constructs a waveform data type View full size image 169 poret i xl iss lerror out 32 Actual Points Figure 6 2C Read Waveform VI passes the waveform without modification Waveform Start 1 0324 Source Channel 1 Labet Waveform End 10000 0324 Actual Points dup VISA session error out VISA session JLo error in tno error k MITI LIEEESEEEEEE Waveform Figure 6 2D The top level VI simply updates the wavef
442. rols On the diagram lay out the nodes into data or sequence dependent groups positioned from left to right in the order they will execute Use the vertical alignment tools in the Align Objects pull down menu to align the icons and simplify wiring Use the horizontal distribution tools from the Distribute Objects pull down menu to space the nodes for adequate terminal access Each group of sequential nodes comprises an error chain Initiate a wire from the error in control terminal to the error in connector terminal of the first node in each error chain Wire the node s error out terminal to the next node s error in terminal and repeat until all of the error terminals are wired with the exception of the last node of each error chain s error out terminal For a VI with a single error chain wire the last node s error out terminal to the error out indicator terminal or to an appropriate error reporting subVI If a VI has multiple error chains merge the error out terminals from the last node in each chain using Merge Errors VI Figure 7 2 illustrates a simple error chain made up of File I 0 nodes Figure 7 3 contains two parallel error chains including DAQmx VIs and File I 0 nodes Figure 7 2 The error cluster propagates among several File I O nodes to form an error chain View full size image F i error in no aides O fal Fa error out m dl l 4 a IE ee Te ee EERE EEE i LDR RRR ese BPs Figure 7 3
443. ronment The layout is logical and intuitive with heading data at the top a graph prominently positioned in the center statistical data grouped in clusters below the graph and large Boolean controls for navigation at the bottom There is excellent contrast between the foreground text and background color such as the white text on the charcoal and blue backgrounds However it contains more fonts than necessary For example the four large Boolean controls contain four different font styles STOP is a single succinct and intuitive word that uses 29 point bold Dialog font and all capital letters Change Work Order uses 16 point bold font Print uses 29 point bold Dialog font and Exit System uses 20 point bold font Hence there are four different fonts and four different succinct wordy caps text conventions on four similar command style Boolean controls The following rules are violated by the Booleans alone 3 17 3 19 3 21 and 3 40 Additionally the panel contains the standard LabVIEW menu bar which 88 is not appropriate for a deployed application because most of the menu selections apply only to developers Finally a numeric indicator and Boolean LED overlap a graph indicator violating Rule 3 11 Figure 3 16A The main GUI VI panel of a Capacitor Test amp Sort application has a logical and intuitive layout but it violates several rules regarding text menu bars and overlapping controls View full size image gt Lapacitor lest
444. rresponding LabVIEW code is generated automatically However several of the state machine implementations described in this chapter cannot be generated by the State Diagram Toolkit s code generator Instead you should use templates for initiating each design pattern Rule 8 11 Divide the primary states into additional states Sometimes an application s functional states do not translate perfectly into LabVIEW subdiagrams that fit appropriately within the state machine s case boundaries As you develop the source code implementation details are exposed that may lend themselves to a different delineation of states For example a given functional state may have a task in common with several other functional states which warrants breaking out the task into its own separate case so that it can be called from multiple states Also when it becomes apparent that a state s code is too large for one case create more cases as needed Consequently one functional state may ultimately be implemented using multiple subdiagrams in multiple cases Remember to modularize your code into subVIs and define the states for maximum reusability A combination of planning experience and intuition is normally applied for best results Rule 8 12 Use an enumerated type definition for the case selector An enumeration saved as a type definition provides documentation and improves maintenance When an enumeration is wired to a Case structure
445. rror chains are utilized In Figure 7 6B the DAQmx VIs acquire data that is logged to file using the File I O functions In this case the parallel nodes are functionally interdependent Specifically the Write to Binary File function depends on the DAQmx Read VI for its data If DAQmx Read VI returns an error the data it returns is not valid and should not be written to file Likewise the data acquisition task need not execute if an error occurs in selecting or creating the file In this example placing the nodes in parallel facilitates propagation of the task and reference number and sharing a common error cluster creates error dependency while maximizing efficiency Figure 7 6A DAQmx VIs and File I O functions are independent and parallel error chains are utilized View full size image peee me me m ee aasma mee m mmn on ee ee eee me ee noe ee ne ee oe merer error in mo error LL mor Figure 7 6B DAQmx VIs and File I O functions are interdependent Sharing a common error chain facilitates error dependency and maximizes efficiency View full size image 209 error in he erren Rule 7 5 Trap all errors from all nodes that have error terminals For best results trap all errors returned from all nodes that have error terminals This maximizes the application s reliability Some challenges and possible exceptions to this rule are explored in Section 7 3 Prioritizing Errors
446. rs The waveform data type WDT can be considered a special type of cluster consisting of three elements plus attributes The elements include a start timestamp to a time interval between data points dt and a one dimensional array of numbers representing the samples of an analog or a digital waveform Y The attributes may contain any number of developer specified name and value pairs An example is a name or description for the waveform In addition to the existence of attributes WDT has conceptually different semantics with respect to math operations than standard clusters Consequently the WDT has a palette of dedicated functions for accessing and manipulating its data Conversely the standard cluster functions do not support the WDT Dynamic is a universal data type used with Express VIs It was designed for novice developers as a very flexible data type that can store many types of data without learning about data types data storage and type conversion You can wire the dynamic data type to any input terminal or indicator that accepts Boolean numeric or waveform data LabVIEW automatically inserts the appropriate Convert to from Dynamic Data function when required In memory the dynamic data type is represented as an array of analog double precision floating point waveforms A variant is a self describing data type that encodes the data name data type data and attributes or information about the data into a generic format
447. rs like to create cute glyphs describing a word or an action indirectly referencing the desired word or action For example an axe wielding lumberjack or a pile of timber may be used to describe data logging a gun may represent arming a measurement trigger or a toilet may be used to describe the flushing of a buffer or file Additionally Figure 5 9 contains a fisherman that is used to represent the word cast which indirectly references the VI s file casting operation All of these examples are cute and fun but not universal Feel free to develop such icons for your own applications but avoid them for developer tools intended for international distribution Figure 5 9 A graphic of a fisherman is used to describe a file casting operation The association between a fisherman and the term cast exists only in the English language Cast Data File vi Path Operation Text string in error in not an error File Text String out error out 5 2 Connector Pane The connector pane is the subVI s wiring terminal interface Conventions for connector panes are presented in this section Rule 5 18 Choose connectors and terminal assignments that promote clear wiring and proper data flow 143 Judicious terminal assignments promote clear wiring and proper data flow on the diagrams of the subVI s callers Style rules related to wiring and data flow are presented in Chapter 4 A few of them are repeated here fo
448. ructures that an application may use Therefore an application s performance is directly related to the developer s choice of data structures and corresponding operations This chapter presents style rules for choosing data structures that provide intuitive reliable and efficient operation 6 1 Data Structure Design Methodology Evaluate the data structures during the design phase of application development Describe the data required for each major subsystem including the GUI data acquisition and instrument I 0 analysis report generation file I 0 database queries and network and interapplication communications Examine all of the application s primary data sources and destinations Prototyping front panel development is a very useful technique for specifying the GUI as discussed in Chapter 2 Prepare for Good Style Additionally use prototype front panels to specify the data required for each component of the application When coding begins the data structures are implicitly declared by the developer s choice of controls and indicators on the front panels as well as the inputs and outputs required by the nodes on the diagrams Implementing the data structures for a VI is generally a three step process First select the controls and data types Second configure the properties of those controls Third group the controls into data constructs if appropriate In this section we start with some general rules t
449. rument driver that controls the laser contains generic icons and there are no free labels You cannot really understand by inspection what the VI is doing In Figure 9 7B the VI is improved using the Unbundle By Name function meaningful icons and several free labels The laser driver s icon convention consists of a light beam glyph and text describing the function of each VI Also a free label is placed under each subVI describing its operation Unbundle By Name contains terminals revealing the intuitive labels of the controls that comprise the cluster The number of terminals is reduced to only those that are wired Additionally there are free labels on the long cluster wire and above the arithmetic functions that calculate the number of wavelength steps The VI s overall readability is substantially improved Figure 9 7A The optical filter measurement routine contains clear wiring and data flow but inadequate documentation via the Unbundle function generic icons and no free labels View full size image SUE EEE Figure 9 7B Readability is improved utilizing the Unbundle by Name function a meaningful icon convention and free labels View full size image pia Pashia ET gt gt gt pm mah a ae aaa EE Figure 9 8 contains a cluster of controls that are used to configure serial communications with one or more pressure transducers Each control is very well documented with labels and descriptions conforming
450. rument drivers and utility VIs Likewise modularize the diagrams of your high level component VIs into lower level subVIs Using the top down design and development approach modularize any low level routines into cohesive subVIs Anywhere you have a collection of related functions that work together to perform a specific routine replace them with a subVI A subVI is cohesive if you can clearly describe its purpose in two or three sentences such as when entering the subVI s description Cohesion Test A subVI is cohesive if its purpose is clearly described using two or three sentences Also using the bottom up approach develop or reuse instrument drivers and utility VIs Instrument drivers encapsulate the low level device communications including command string assembly VISA functions and response string parsing Utility VIs complement or extend the capabilities of the built in LabVIEW functions and VIs available on the Functions palette Thousands of reusable instrument drivers and utility VIs are available for free download on the Web Code reuse is discussed in Chapter 2 Prepare for Good Style In Figure 4 2 we revisit the calibration interval diagram from Figure 4 1 This is an example of a high level subVI Inspecting Figure 4 2A we observe three low level routines reading and appending the time stamps to file searching and counting clock wraps and prompting the number of days until the next calibration In
451. ry large applications This is also referred to as a dynamic loader However the VI Call Configuration dialog window facilitates dynamic loading with the stipulation that the desired components are statically linked to the application as subVIs The VI Call Configuration dialog window is opened by selecting Call Setup from the shortcut menu of a subVI call Currently the Dynamic Framework is required if the components are not linked to the application and are not identified until runtime Specifically the framework reads the available components from a dedicated directory on disk and loads them into memory on demand This capability allows components to be developed and distributed after the application has been deployed facilitating upgrades and maintenance A template for the Dynamic Framework is shown in Figure 8 19 The front panel simply consists of a tab control with pages for Main and Error Info The Main page contains a listbox control subpanel and Quit Boolean The listbox displays a list of the components read from disk and the subpanel displays the front panel of the selected component The component panel is considered a child panel when loaded into the subpanel The child panel is fully interactive for as long as the component is running Figure 8 19A The front panel of the Dynamic Framework VI Template contains a listbox control displaying a list of components and a subpanel that displays the front panel of the selected co
452. s C Error Handling File 10 Menus C Prototype YIs H 2 Remote Targets Type Definitions User Interface C Utilities gt Variables YI Server mi Application Name Top Level vi Dependencies me Build Specifications Rule 2 11 Organize LabVIEW source files into cohesive project libraries where appropriate 51 The LabVIEW project library is a file with the lvlib extension that maintains properties shared by a collection of LabVIEW source files The properties include a name prefix version access rights password palette icons and menu names Project libraries are intended to maintain cohesive collections of source files that work together to perform a specific set of functions such as an instrument driver Project libraries prevent namespace conflicts that arise when two files containing the same name are opened Specifically all the source files that comprise the project library inherit the library name as the prefix for the source filename This allows project libraries to contain identically named source files as long as the library names are unique For example multiple instrument drivers can contain an Init VI and a Close VI which can be loaded in memory at the same time The file prefix is provided by the lvlib filename which is unique to each instrument driver Additionally you can use the Library to define VIs as either public and accessible to all users or private and usable
453. s ean Size similar objects the same 3 4 Maximize the top level VI panels for industrial applications Geo Size dialog VI panels much less than full screen 3 6 Center dialog VI panels 354 lt 10 TA Te Th 14 mts mele eae sdo sea pen al on PA 24 om lt 20 rel Te jee lt 90 sad MEy 00 34 sod Group related controls using decorations spacing tabs and clusters Use LabVIEW s dialogs for desktop applications avoid them for industrial applications Use system controls for desktop dialogs use 3D controls for industrial dialogs Enlarge and center the objects of an industrial GUI VI in proportion to their importance Limit the quantity of information displayed on a GUI VI panel Avoid overlapping visible objects Hide the toolbar Include your company logo for a professional appearance Use default appearance for subVI panels objects and most text Arrange controls to resemble the connector assignments Resize the panel for a snug fit Minimize front panel text Delete template instructions immediately after edits are performed Apply consistent fonts and capitalization Choose only one font and vary the size boldness and color to obtain multiple styles Use succinct intuitive control labels and embedded text Apply 13 point black Application font for most subVI panels Provide default values and units in parentheses at the end of owned labels
454. s Also the error cluster s source is populated with an intuitive message consisting of the call chain and the response returned from the DLL Figure 7 19 Instrument driver subVI uses a Call Library Function Node to call a function in a DLL If an error is returned by the DLL the Error Number is shifted into the range of user defined errors View full size image error in me error a mE TIaLICliticticiely an ee eee a ee ee a a re Be al ar Leer A BES PS IEE Pe 2 ee 2 ee 2 EdE Se ESE E E E Ps PS Pe PS Pees Figure 7 20 illustrates an initialization routine that reads configuration parameters from file using Config VIs sets several control properties using Property Nodes and initializes all control values to default using VI Server These nodes form three parallel error chains that are merged prior to exiting the Case structure through an output tunnel Merge Errors VI has a nonstandard error in array assigned to the lower left terminal This causes a kink in the error cluster wire passed from the VI Server Close Reference function as shown in Figure 7 20A In Figure 7 20B Merge Multiple Errors VI contains only scalar error terminals including the bottom left terminal that is normally reserved for the error in terminal This subVI eliminates the wire kink and promotes standard error terminal assignments Figure 7 20A Merge Errors VI has a nonstandard error in array terminal that causes a wire kink when used to merge parall
455. s The diagram consists of a compound design pattern based on the Multiple Loop Application Framework It contains an Event Handling Loop for processing GUI activity a Queued State Machine for state transitioning and a Continuous Loop for error handling allowing multiple tasks to run in parallel For example several transducers can acquire and log data while another transducer is configured without any interruption to the acquisition and logging Figure 8 29A The main front panel of a transducer control utility consists of a simple Boolean menu View full size image 291 Welcone to the OXO Setup Utility DARD Setup Utility Main wij Front Panel Setup Utility Figure 8 29B The diagram uses the Multiple Loop Application Framework View full size image 292 bs wens mi n Bayani viam oe i Mei ae n a mr 4 L b I DA h Tl ee et i i o 1E EPE 3 H i os E PRESAK P Se ee He hapi EE Seer ee d i Lag Sart Tiron Ja ma F Lk iE PT F l EF rh rj a e DE ahem poe Ty F H B kss tin N Lit Papel Te m r E eh f i mk Bi me fies lerh Tas prn iE EF PALE ia c i F Figure 8 30 contains the top level diagram for a distributed control system implemented using LabVIEW RT It is a Modular Multiple Loop Application Framework The diagram of a loo
456. s AP A i P i i RE Figure 10 2K The Torque Hysteresis VI is revised based on the recommendations provided by the VI Analyzer View full size image 327 b je pe Pe bp sei d o Riel peel F Se p fe b e a ee e ill i D As demonstrated in the previous section the VI Analyzer is an effective tool for fast personal code reviews However the VI Analyzer does not test many style considerations For example you might have noticed that many additional style improvements can be made to the front panel and block diagram of Figure 10 2 above and beyond those recommended by the VI Analyzer Indeed the VI Analyzer performs approximately 60 tests including 40 style related tests but The LabVIEW Style Book contains more than 200 style rules Rule 10 7 Use a manual checklist to perform a comprehensive style review A manual checklist is the most comprehensive method of going through every rule reviewing the rule s significance and then checking your code s adherence You can use the rules summary in Appendix B as the style checklist or as a reference to derive your own Simply not enough time exists during a peer review to evaluate all 200 rules The manual checklist approach is tedious but thorough and has the added benefit of reviewing each of the style rules If your organization s style rules differ from the ones listed in the appendix simply create your own checklist Let us now perform a self review of the Torque
457. s and connectors make software development fun This chapter provides style rules for the icons and connectors 5 1 Icon The icon is a 32x32 pixel symbol containing any combination of graphics and text Icons are created from scratch using the tools available within LabVIEW s icon editor or copied from an existing image file or application Consequently any drawing application may also be used to create or manipulate icons Most of the rules described in this section can be implemented using LabVIEW s icon editor We begin with some basic style rules for icon development Rule 5 1 Have fun creating icons It is readily apparent that LabVIEW s inventors intended for us to enjoy developing software As much as text based programmers may try to steal our joy by teasing us for drawing pretty pictures we know that deep down inside they are jealous because they are not having as much fun as we are nor are they nearly as productive Creating icons is an enjoyable experience for most developers The more tips you learn the more efficient and rewarding it becomes Rule 5 2 Create a unique and meaningful icon for every VI Rule 5 3 Never use LabVIEW s default icons Rule 5 4 Save VIs with subVI icons visible instead of terminals Icons graphically identify the subVI promote readability and contribute to documenting the diagrams of the caller VIs Consequently it is important to create icons that meaningfully depict the VI s p
458. s and nobody is present to acknowledge it the application may suspend itself indefinitely Additionally beware that LabVIEW s web server cannot publish dialogs for remote clients accessing the application via web browser In this case the dialog will open on the server computer but is transparent to any remote clients Avoid dialog error reporting in any of these situations It is important to carefully consider the placement of the error reporting routine All error chains in an application must eventually terminate with a call to the error reporting VI However avoid calling the error reporting VI within continuous loops or within subVIs that might be called within continuous loops unless corrective action is taken to gracefully recover from the error Otherwise the error reporting VI could cause the error dialog to repeatedly appear or the log file to grow very large It is best to restrict the error reporting VIs to the top level which helps consolidate error reporting and reduces the chances of reporting within a continuous loop Additionally it is helpful to merge all errors into one error reporting VI The Merge Errors VI is a handy method of combining the errors from multiple error chains for reporting by a single VI The Torque Hysteresis VI s top level diagram is shown in Figure 7 10 Observe that errors are trapped via propagation of the error cluster and are reported by the General Error Handler VI If an error occurs the loop is te
459. s associated with a text label and numeric value similar to an enumeration Numeric Simple method of entering a scalar numeric value Can be configured for a variety of whole and fractional numeric representations Listbox Provides a user selectable list of strings associated with numeric scalar single selection mode or array multiple selections mode values Combo box Provides a user selectable list of strings associated with the same or an alternate list of strings 7 String Used to enter or display alphanumeric data Multiple display modes include normal codes and hexadecimal labe Tree User selectable list of strings organized in a hierarchy with expandable and collapsible capability 161 Table 6 1 LabVIEW Controls and Descriptions Listed in Top Down Order of Operational and Data Simplicity ape switches and lights that are used to enter and display Boolean TRUE FALSE values Array Groups multiple data elements of the same type Can be one or multidimensional H k Matrix Two dimensional array of real or complex numbers used for matrix operations such as linear algebra E E SEES gt 4H ei l Table Two dimensional array of strings arranged into a table with rows and columns of cells containing the text EERE EERE elements a Cluster Groups multiple data elements of the same or mixed data types oe Do Variant Used to pass or display variant data consisting
460. s because of misunderstood requirements the messier the source code can become per Theorem 2 2 This is generally true with all programming languages In the latter situation in which the requirements are understood but not documented several problems can arise Skipping the documentation reduces the amount of planning and consideration which can result in an inferior initial release that requires many revisions Hence the Code and Fix life cycle model still applies Additionally the memory resident specification might become the developer s exclusive intellectual property Inevitably the developer will forget some of the details and her ability to maintain the application decays over time More concerning is that the developer might not be available to support the application throughout its life cycle Most people change responsibilities jobs companies and careers and LabVIEW developers are no exception Speaking from experience Bloomy Controls routinely receives inquiries from companies that need to maintain fix or refactor their legacy LabVIEW applications When the specifications and documentation are lacking the source code is often sloppy These applications are generally rewritten from scratch beginning with a specification Conversely when good specifications exist the source code is predictably high quality not always but most of the time Good specifications originate as manually recorded no
461. s consistent with the native operating system dialog boxes and is consistent throughout your application The Standard State Machine is a common design pattern that consists of a Case structure within a While Loop a shift register for passing the case selector value and an enumerated type definition that contains the state names State Ww Machines are a popular type of design pattern that are described in much more detail in Chapter 8 Design Patterns Rule 2 7 Place reusable templates in the LabVIEW templates folder Figure 2 6A shows the LabVIEW New dialog box with the aforementioned templates that ship with LabVIEW You can expand upon the standard templates and also incorporate your own by placing them in the LabVIEW templates folder In addition to increasing productivity templates will help improve your LabVIEW programming style as long as your templates follow good style Figure 2 6B shows some templates that are part of the Bloomy Controls software reuse library Figure 2 6A The New dialog box provides access to templates that ship with LabVIEW including the Standard State Machine View full size image 45 Creahe Mew mi Bark Vi fe Polymorphic VI BES From Template BE Tutorial Getting Started Cp Tuad ep RT E E gt Iratrument 1 0 GPIB p Frameworks HI Top Liwa Repbcstion Using Events a Produce forsumer Gesign Pabam Daa ma Macher Shaws Doriy Pibe Fa i Bomy EE User SHS Project T E
462. s enlightening even awe inspiring like a work of art A careless diagram however can appear as jumbled as a bowl of spaghetti Indeed these two extremes are depicted by Meticulous VI and Spaghetti VI in Chapter 1 The Significance of Style Somewhere in the middle between artwork and spaghetti is where most applications reside Some developers have neat wiring practices but large flat diagrams Others have overly modular diagrams that disguise the architecture Still others prefer variables over data flow Many many developers skimp on documentation to save time Moreover most diagrams are characterized by tradeoffs between good style and shortcuts deemed necessary to get the job done The overall outcome is a compromise among attractive appearance personal preferences and functional performance Theorem 4 1 Great LabVIEW diagrams can be expeditiously developed Many developers wrongfully assume that attractive diagrams require a level of toil that is impractical for real world applications that have tight deadlines It seems faster and more productive to avoid getting caught up in diagram aesthetics Indeed it is possible to expend excessive time optimizing the appearance of a complex diagram and most of us must plead guilty for doing this on occasion However it is always much more time consuming in the long run to debug and modify sloppy code Per Theorem 1 1 applying good style significantly reduces time a
463. s is accomplished by choosing Advanced Customize from the control s shortcut menu select Type Def or Strict Type Def in the Type Def Status ring of the Control Editor window and save the control to a CTL file Per Rule 6 6 choose the Strict Type Def if the control has custom properties in addition to the text labels and choose the Type Def otherwise Constants as well as controls that are formed from a type definition maintain an association so that when the type definition s items are edited all instances are automatically updated as well Note that a ring control must be saved as a strict type definition in order to provide similar behavior String path and picture are variable length data types stored in contiguous memory String controls provide tremendous flexibility for entering or passing alphanumeric data However they lack filters or built in methods to restrict the format number of characters case or spelling of the data Therefore they provide the greatest opportunity for error 181 Rule 6 18 Avoid string controls on GUI VI panels unless required Avoid string controls on GUI VI panels except where free form alphanumeric data is required As developers we should never trust the user to enter data in a specific manner and the more invalid options we provide the user the less robust our software is As per Theorem 3 1 reliability is the developer s responsibility Two useful applications o
464. s section presents rules for text applied to the panels of subVlIs Rule 3 22 Apply 13 point black Application font for most subVI panels 76 As noted previously it is not necessary to get fancy with subVI panel text Therefore LabVIEW s 13 point black Application font is used in conjunction with default gray panels and default control appearances for most subVIs Rule 3 23 Provide default values and units in parentheses at the end of owned labels Rule 3 24 Combine bold text labels with plain text parentheses for control labels of commercial subVIs Append the default value and units in parentheses to the end of the labels if applicable These helpful label extensions improve the readability of the subVI front panels and cause the data to also appear in the terminal labels displayed in the Context Help window This helps expedite subVI learning and utilization Commercial subVIs such as instrument drivers and developer toolkits require some additional effort All control and indicator labels are bold and default values and units are appended to the labels in parentheses using plain text This convention is specified inNI s Instrument 2 Driver Guidelines It makes the subVI panels appear more uniform In Figure 3 11 the Interval Timer subVI from Figure 3 8 has been modified with bold text labels and plain text defaults Figure 3 11 The label text style of Interval Timer VI has been modified for a commercial subVI T
465. s such as local and global variables that can hinder applications when used improperly Good LabVIEW developers always modularize their diagrams using subVIs This involves identifying a portion of code that performs a specific task and creating a subVI to perform that task Testing and debugging subVIs is relatively easy as long as they are each dedicated to a specific task comprised of a limited number of nodes Modular applications that use previously tested and debugged subVIs are generally higher quality from the outset Any bugs that are discovered are easy to identify and isolate because of the modularity LabVIEW contains a tool called VI Metrics that reports how many user subVIs and write variables an application uses as well as the overall number of nodes on the diagram The VI Metrics utility is accessed by selecting Tools Profile VI Metrics Figure 1 8 shows the VI Metrics window with Nested VI loaded Figure 1 8 The number of nodes and the number of user VIs reported by the VI Metrics window help approximate the application s modularity View full size image 21 Vi Metrics Select a VI Show statistics For idtepram User irterFace ofuser Vis 15 v Globalsflocals ofvidib Vis 25 C CINs shared lib calls F Exchede vil files from statistics subv1 interface WI of nodes global reads global writes local reads local writes A botal 471 12 5 Neshed vi 163 i Forth Command i il No error 6 Status
466. s when long phrases are required or the desired GUI label differs from the control label Captions are label alternatives that are visible on the front panel only when selected from the shortcut menu via Visible Items Caption Rule 6 5 Enter control descriptions Rule 6 23 Enter descriptions for array and cluster shells and control elements Enter a one or two line description that further describes each control s purpose data type default value and range unless it is completely self explanatory based on the label Arrays and clusters often form the data infrastructure for an application It is useful to enter descriptions for the shells and elements before they proliferate throughout the application Additionally apply tip strips containing descriptive phrases to the controls of GUI VIs Tip strips are abbreviated descriptions that appear when the mouse cursor is placed over the control Tip strips captions and control descriptions help maintain succinct control labels and embedded text Long phrases and explanations belong in the former locations instead of the labels Rule 3 39 Always include a Help menu or button Always include a Help menu or button for GUI VIs with functional selections for providing the user help This may include Show Context Help as well as menu items for online documents or compiled help files The Show Context Help menu item opens the Context Help window which displays any descriptions a
467. scalar error in clusters into one scalar error out cluster This increases the number of error chains that can be merged while promoting good error handling wiring and connector style Figure 7 15B Merge Multiple Errors VI combines up to eight scalar error in clusters using the standard 4x2x2x4 connector pattern The subVI is vertically aligned with the error terminals and other subVIs View full size image 227 Merge Multiple Errors vi error in Fi na error memes error in no error L error in 1 no error error In 2 reo error paxa error in 3 no error m p ET error in 4 no emor i error In eo error ee error In no error s seesssenned error owt Merges error I O clusters from multiple paralel error chains by checking each input terminal For error status TRUE and error code nonzero The terminals are searched in order of increasing terminal number The first error found is returned and the remaining terminals are ignored If no errors are Found then the First warming is retumed if present or no error Passthrough a Liian gidan did ani dias i jaag ii gme i i gpi EH sabedes Pte E j error out Jeees PEE error in reo error mi 5 of prreneceseeseenecemeenaneees In Figure 7 15A five parallel error chains are combined using the Merge Errors VI The Build Array function is required to combine two of the error clusters into an array and the subVI is positio
468. ses Property Nodes to continuously update the indicators on the top level VI View full size image Wie tee may cause met Le Frew te ra ares Lo 8 5 Examples 284 This section presents several more application examples of the design patterns and application frameworks Elapsed Time VI shown in Figure 8 25 is a Functional Global that stores an initial time stamp in shift register memory and calculates the elapsed time with respect to the initial time stamp When the Mode Update terminal is set to Init the Init case of the primary Case structure calls the Tick Count ms function to acquire the initial time stamp and stores it in the shift register Subsequent calls to the Elapsed Time VI with the Mode terminal set to Update select the Update case which acquires new time stamps and calculates the elapsed time in the selected units This VI is useful for monitoring or resetting the elapsed time from multiple locations in an application or simply reducing clutter on the diagram of the calling VI Figure 8 25 Elapsed Time VI is a Functional Global that maintains an initial time stamp in shift register memory and calculates the elapsed time for each subsequent call View full size image b Llp ee ve i reni Pay Poll Instrument Response VI is a subVI that reads a message from an instrument using a polling technique This VI is as useful for serial instrument control applications as it is de
469. signate a neutral party not the lead developer or project manager to take notes during the peer review This way the developers are free to concentrate on presenting their code and receiving feedback and the meeting can progress quickly through the agenda Also aneutral party can maintain unbiased feedback and action items For convenience the developer might want to enter some free label comments within the source code reminding her of recommended changes similar to a bookmark Include a search term within each comment such as review lt date gt to quickly locate the comments after the review using the Find dialog However always avoid fixing or modifying your functional source code during the peer review This slows down the meeting while changing the level of perspective A peer review is performed on the Torque Hysteresis VI subsequent to the automated and manual self reviews Referring to the front panel and block diagram in Figures 10 3B and 10 4B the following additional rules violations were noticed by the peers that had gone undiscovered by the developer Rule 33T Set the Tabbing Order and use the Key Focus property to aid navigation Rule 6 2 Choose memory efficient data types Rule 6 27 Avoid clusters for interactive controls with dialog VIs Rule 7 8 Implement an error log file for application deployment Rule 7 9 Suppress dialog error reporting for unattended or remote operation Rule 8 4 Avoid pol
470. sis section describes any mathematical processing that is required either online as data is acquired or offline using data files and a separate routine Some types of analysis include data reduction or decimation corrections digital filters peak searches limit evaluations statistics and control algorithms For example statistical process control SPC is very common in automotive manufacturing applications SPC entails computing such parameters as mean range control limits standard deviation process capability and more and graphically displaying the results as Xbar amp R and other charts The analysis section of the requirements specification describes such calculations Presentation includes all human readable interfaces to the data that the application generates This includes the graphical user interface ASCII data files and reports One technique that is extremely useful and very easy in LabVIEW is creating a prototype user interface and including one or more screenshots in your specification Do not spend a lot of time building an actual working prototype or experimenting with fonts and colors at this point just apply enough controls and indicators on a front panel to illustrate how the user interface might appear This is a powerful technique for revealing requirements that might have been overlooked or misunderstood As the saying goes a picture is worth a thousand words Likewise it is beneficial to prototype a sample report or
471. some overlapping is required Data flow All data flows left to right An additional shift register maintains the file path value between loop iterations The diagram of Centrifuge DAQ VI shown in Figure 4 19 is neat intuitive and very well documented The primary data elements are read from shift registers and wired through tunnels into the primary Case structure facilitating good 127 data flow Each wire is labeled outside the Case structure and then bent upward to create additional space within the Case structure Near the bottom of the diagram an array of control references is passed to a subVI for manipulating properties The Queued State Machine design pattern is readily expandable via adding cases to the primary Case structure and items to the enumerated data type that is wired to its selector More features of this application are discussed in Chapters 6 and 8 Figure 4 19 Centrifuge DAQ VI contains many advanced features including control references and the Queued State Machine design pattern View full size image AFLI Dif ee b GPLUS E oo Terp Logie Pih F gt Control Ris Ae gt 0 Layout This diagram actually has two parallel While Loops only one of which is visible on the monitor and included in this illustration It utilizes the Queued State Machine design pattern Modularity There are 113 user VIs out of 2 887 nodes for a modularity index of 3 9 SubVIs are used for manipulating con
472. ssigned to terminals on the left border of the connector and the Serial Port Settings cluster is assigned to the top middle terminal These assignments are equivalent but more consistent with instrument driver standards than Figure 5 17B The icon contains a standard glyph that the wizard applies to all Initialize VIs The standard glyph helps to improve the recognition of common instrument driver VIs using graphics in addition to text Also the standard glyph reduces icon editing effort because it is created automatically by the utility However the standard glyph reduces the association of the VI with the specific instrument as well as the association of the VIs that comprise the instrument driver Instead the banner is the only means by which the instrument is identified Hence a tradeoff exists between recognizing common function types using the function specific glyphs and recognizing the instrument using a common instrument specific glyph The NI guidelines recommend the former whereas I generally prefer the latter Figure 5 17C The icon and connector of the Initialize VI generated automatically using the Create New Instrument Driver Project utility The utility uses a template to create a standard glyph connector assignments and terminal labels View full size image PHPM 1k Initialize from NI Template vi Connector Assignments Serial Port Settings VISA resource name Perera VISA resource name out ID Query T Yes aes Re
473. stant contains a pick list of the control s text selections similar to the control on the panel You can choose any of the text selections and show or hide the digital display Additionally when an enum is wired to the selector terminal of a Case structure the text selections appear in the 179 Case structure s selector area This is why the enum is used for the State Machine design patterns discussed in Chapter 8 Design Patterns Figure 6 8 shows an example of an instrument driver subVI that configures a digital multimeter In Figure 6 8A numeric controls are used to configure the Function and Manual Resolution parameters on the subVI s front panel These parameters are programmed through terminals on the subVI s connector pane The subVI call shown to the right uses numeric constants to configure these parameters The value of 4 for Function is meaningless and the value of 6 5 for Manual Resolution is risky Specifically the range of valid selections and the meaning of each selection is not very clear In Figure 6 8B text ring controls are used in place of the numeric controls The subVI call contains text ring constants that were created from the terminals context menus These constants contain a discrete number of text labels that clearly describe each selection and limit the user to valid choices Hence the text rings are more functional intuitive and reliable than the numeric controls Figure 6 8A The front pane
474. stom menu bar has been applied providing selections for File Work Orders Print and Help These selections further assist the application s users in navigating the software Additionally the numeric indicator and LED have been moved above the graph eliminating the overlap This improves efficiency by reducing the required drawing operations Figure 3 16B The panel is improved with succinct Boolean commands and consistent fonts a custom menu bar for user navigation and nonoverlapping indicators View full size image t Capacitor Test amp Sort improved vi Fle Work Orders Print Hep Ae rovox Capacitor Test amp Sort wow pes io wrptvreyt eee Fe 7 E a A3 k 7 Oe DS OE 099 100 1 0 1 1 03 Le Porformance Stats Amaut Capability funahsis Measured mean TEE c gt w e e m 3e Abave cok E standard Deviation usi ist 3 SIGMA o ios ea asic Figure 3 17 shows the GUI VI panel for a centrifuge data acquisition DAQ and control system This application is used to perform experiments on soil used for earthquake research The operators are engineers and scientists It has hundreds of configurable parameters The corresponding controls are organized using a nested tab control The outermost tab control is visible and directly operated by the user It organizes controls into the categories Configure Monitor Acquire and Analyze A seco
475. system dependent The display resolution video adapter driver and monitor settings in addition to the operating system and user preferences all affect the appearance of most fonts If your application is intended for use on multiple platforms allow extra space between your labels and other objects to prevent them from overlapping 3 3 Color Most of us have experienced a GUI with a combination of colors that turned our stomachs Even more common are GUIs with no color at all Selecting colors is an art Some great LabVIEW developers are very creative ina technical or scientific sense but not in an artistic one Furthermore approximately 10 of the world s population is color vision confused meaning they cannot differentiate two or more of the primary colors in the spectrum When you consider the color vision confused along with normal eyesight users operating software with poor color choices color confusion might be an epidemic In any event color is one of the more subjective properties of a panel Color combinations that are attractive to one person might be unattractive to another However some rules can really help Rule 3 28 Apply color judiciously e Select three complementary colors including one highlight one muted and one grayscale 78 The GUI VIs that have turned my stomach all had one thing in common too many bright colors Try to judiciously select three or at most four colors that go well together
476. t Avery short wire segment known as a wire kink is identified and corrected Also another wire contains four bends exceeding the default maximum of three bends which flags an additional occurrence of the Wire Bends test These bends allow the wire to roughly follow the border of the Case structure as the developer intended Because wire bends are often deliberate the Wire Bends test has a medium rank However in this particular example the bends serve no useful purpose and the wire is straightened in Figure 10 2K Figure 10 2H The diagram contains no comments including the subdiagrams of the Case structure resulting in multiple Comment Usage errors A brief comment is added to case l View full size image E Vi Analyzer Results Window Doubbe cick a Fara occurrence to haha the ara object in the Vi Fiha vl changed tines you performed the aa the wrong objet may be highighted Rents List argia Hysteresis Bac Style i Bihar Wires Error Chaer Arid Uraa Code YI Documentsicn Wires Under Objects Coercion Dats Comment Usage Crourrence Crourrence 2 Dee ee ee Doourrence 4 Ooourrence amp Micdum Folre Reported Gobis and Locals Socal Check E 4 Contra Terminal Wiring i Shrine Constant Sye l Show Fahrer Oriy E Sot by WI Description C Sort by Test This Case Structure subdiagram Ooan no comments ae Figure 10 21 Abbreviated control labels flag spelli
477. t OC ee EE Ce CT ee Dy ep CT ee Ee ee Ce ty Es CT ee Ce Cd Cee to Ee Ce Et ee Ce ET eo Ee De Eee DE ta CT ee i Eea thee newt stare EE Ri i D p Haat Saba 2 Bed pi gt WJA Agaue gt o Bae hahaha pha pepahegefapegaha pepe gaa pehahe hahaha gahe fagahe saga hagas pehahepaha hahahaha egah ahaha hee apa pajapaja papaa a 256 Figure 8 9B Space efficiency is optimized by adding states implemented as cases in the Case structure for Initialize Idle Shutdown and Blank A two element shift register ensures that the Idle state alternates with all other states View full size image F ER ene te Wride Default Y Default Hf a a Hal na ta a Gaa i Hoar aa a Ha l Poll the Stop button and check the error status shop be status Pe gt Resource gt ie error in y tl pe jem gt Error gt Let us now discuss several popular state machine implementations The diagrams illustrated in Figures 8 8 and 8 9B are examples of the Classic State Machine design pattern which uses a shift register for state transitioning GUI activity such as the state of one or more Boolean command buttons is captured by polling the terminals within the Idle state The Classic State Machine is appropriate for subVIs and routines of low to medium complexity For example it is a more functional and flexible alternative to the Sequence structure for executing a series of operations The operations are implemented as states of
478. t and measurement application it is common to apply a unit under test UUT with known characteristics and compare the application s measurements with the known values However in many real world projects a characterized UUT or some other instrumentation or equipment is not readily available In these cases the test methodology might entail developing software to simulate the UUT or 35 other parts of the system In any event the test methodology is outlined in the requirements specification and details can be added as the project evolves At Bloomy Controls we prioritize the requirements as critical high medium and low Critical items are directly related to the primary objective of the project They are showstoppers If any critical priority requirements cannot be completed we cease to proceed with the project as defined Items of high priority are very important features but not necessarily showstoppers Medium is the most common priority Items of low priority are features that we incorporate if time and budget permit after all higher priority items have been completed Priorities are referenced during the design and implementation phases to determine the order in which the requirements are addressed and the time that we budget for each The requirements specification should be reviewed by multiple people involved in developing using and maintaining the system if applicable It is common to discuss revise a
479. t even have plenty of room for further optimization The Profile Performance and Memory window is best used for measuring the change in efficiency of a single application when incremental modifications have been made An alternate method of evaluating efficiency is simply to inspect the application for sources of inefficiency Look for unnecessary operations within loops and for operations that create new data buffers The diagram of Spaghetti VI see Figure 1 3 for example contains many nodes within the main While Loop This includes read local variables which make copies of their data when read from and Property Nodes that are written within every iteration regardless of whether their value has changed 17 Inspecting the diagram of Nested VI see Figure 1 2 reveals a tight loop in which several control terminals are polled within the innermost While Loop as fast as the loop can run High speed GUI polling is not an efficient use of the processor Most humans cannot distinguish between a 1 millisecond ms and a 100ms GUI response More efficient alternatives include adding a delay using the Wait ms function or using an Event structure to service all user interface activity These alternatives free the processor to work on parallel tasks and applications Meticulous VI see Figure 1 1 uses multiple parallel loops on one diagram many shift registers and an Event structure The following tasks execute in their own separate parallel loo
480. t file This is shown in Figure 7 9A The Get Date Time String function available from the Timing palette is used to generate the date and time as ASCII character strings The General Error Handler VI returns a message describing the error The message output terminal is combined with the code source time and date and is written to a tab delimited text file The corresponding error log file can be read using any word processor or spreadsheet application It is important to wire the type of dialog input terminal of the General Error Handler VI and to set the corresponding enumeration to no dialog if you want to suppress the dialog Figure 7 9A The date and time are combined with the error message source and code and are logged to a tab delimited text file Readability is maximized using this approach View full size image 212 piken bo a hab che eet Pie Pesek error T OE ioe keg le perth ak bene of dialog 0t mesg 1 LEER errant reo error rne error Gut A second error log file method uses a datalog file format to optimize efficiency while sacrificing file readability Specifically bundle the error cluster with a time stamp and log the corresponding cluster to file as shown in Figure 7 9B The Get Date Time in Seconds function available from the Timing palette returns the current time formatted using the time stamp data type The datalog functions including Open Create Replace Datalog Write Datalog
481. t functions and VIs generate LabVIEW maintains an internal database for which error codes are associated to descriptions The categories and ranges of error codes as of version 8 2 of LabVIEW are shown in Table 7 1 This table relates the ranges of predefined error codes to the types of operations they are reserved for Table 7 1 LabVIEW Error Codes Range Error Code Table 2147467263 through 1967390460 Networking 1950679040 through 1950678977 Shared Variable 1967362045 1967362022 through 1967361997 and Security 1967345663 through 1967345609 1074003967 through 1074003950 1074000001 and Instrument Driver 1074000000 1073807360 through 1073807192 VISA 90165 through 90149 and 90111 through 90001 MathScript 41005 through 41000 and 41000 Report Generation 23096 through 23081 Formula Parsing 23096 through 23000 20141 20140 ana 20005 Mathematics 20337 through 20301 Signal Processing 20207 through 20201 Point By Point 20119 through 20001 Signal Processing 4644 through 4600 Regular Expression 2983 through 2970 2964 through 2960 2955 through Source Control 2950 and 2929 through 2901 2586 2585 2583 through 2581 2578 2575 Storage 2574 2572 through 2550 2526 through 2501 and 2552 1821 1820 1817 through 1800 Waveform 1719 through 1700 Apple Event 1300 through 1210 Instrument Driver 823 throu
482. ta dependency Figure 4 9A Two single frame Sequence structures are used for forcing the execution order of the Initialization routine two While Loops and Shutdown routine View full size image 112 7 ihre ques The Shutdown routine consists of a Sequence structure on the right containing the Quit LabVIEW function It receives data from the two parallel loops ensuring that the Shutdown routine executes last Also note that the General Error Handler VI propagates the error cluster as an input from the DAQ Loop and an output to the Shutdown Sequence structure This forces the General Error Handler VI to execute after the DAQ Loop stops but before the Shutdown Sequence structure can begin The diagrams in Figure 4 9B and Figure 4 9C are functionally equivalent to the diagram from Figure 4 9A except that they utilize three frame Stacked and Flat Sequence structures respectively Stacked Sequence structures are less desirable than Flat Sequence structures because only one frame is visible at a time they require sequence local terminal for data flow between frames and the multiple frames hide the data flow Flat Sequence structures facilitate data flow between frames using tunnels and all frames are simultaneously visible Also each frame of a Flat Sequence structure has a uniform appearance and does not require artificial data dependency Therefore the Flat Sequence implementation in Figure 4 9C is preferred in this example
483. tablished style convention and code review methodology and we were pretty set in our ways Not until I hired and trained a fresh young graduate did I become aware of the VI Analyzer s capabilities Ordinarily code reviews are very lengthy with new engineers because there are often many style issues to discuss However the quality of the code developed by one of my newest staff members significantly increased Amazingly all of her wiring was clear and nonoverlapping with left to right data flow Her code resembled that of much more experienced developers The productivity of her code reviews was substantially increased and the level of required training was reduced Her secret was the VI Analyzer Toolkit Several of my engineers now run the VI Analyzer on their source code prior to a code review Rule 10 4 Use the VI Analyzer to automate tedious inspections 317 The primary benefits of the VI Analyzer include the following 1 It helps inspect and enforce good style 2 It can be utilized by an individual 3 It is automated and reduces tedium It is important to recognize that the VI Analyzer provides value that extends beyond testing a VI s style Many of the tests inspect the likelihood of bugs and errors similar to a secondary compilation process Specifically the VI Analyzer contains a Warnings subcategory of tests including Bundling Duplicate Names Typedef Cluster Constants Hidden Tunnels Reen
484. te case for each state of the application and an enumeration wired to the case selector The State Machine design pattern is readable because each frame has an intuitive label in the selector 20 area that corresponds to the labels of the enumerated type definition and describes the function of each state It is also scalable because states can be added and removed simply by adding and removing cases to the Case structure and items to the enumerated type definition Chapter 8 discusses the State Machine design pattern and Multiple Loop Application Framework The front panel of Nested VI see Figure 1 2 uses a simple tab control that can be readily expanded by adding new tabs Each tab contains a different display page and can be selected by the user when needed The diagram however contains Sequence and Case structures nested together which obstructs the developer s view of the source code by forcing her to navigate each frame of each structure to understand how it works Moreover it is not a standard design pattern Rather it is confusing compared to the State Machine design pattern which is more recognizable and less nested Spaghetti VI see Figure 1 3 is not maintainable because the architecture is not scalable the diagram is not modular and the wiring is sloppy The architecture consists of a single While Loop with all nodes that execute more than once inside the loop and wires structures and nodes intermixe
485. ted statistics are stored in shift registers The right Case structure prints a report and logs the test data to file when the user clicks a Boolean labeled Log Data The loop terminates if the user clicks a Boolean labeled Quit or an error occurs Note that the shift register data allows each task to execute on demand instead of sequentially by providing the most recently acquired data to the subVI terminals In particular it is common for an operator to configure the UUT and motion parameters once and to execute several test runs prior to logging data Figure 8 5A The front panel of the Torque Hysteresis VI based on the Continuous Loop template contains a Boolean menu XY graph and tab control The error clusters are scrolled off the visible area of the panel View full size image Torgue Hysteresis Example Continuous Loop vi Front Panel m z Sg ig Cap A a feeasaaee SEER ECE ip FEEFEE eet fail HERH a SEE 7 f i _ Salt Ti fe Ps PRA ed EEE kodi l eor oor pi i mR et ann uy Torque hin TTI 7G Bd 8S Fy 2 oa te i TEE PTI l ee f f i il i I rr areas EETA boot ed a ae ee ae s baa ae a a a Figure 8 5B The block diagram has been divided into three primary tasks that are executed on demand via the Boolean controls Data is stored in shift registers View full size image PF Re Se i hE i
486. tem Ease of use is related to readability and responsiveness of the GUI In Figure 1 1 the front panel for Meticulous VI is intuitively laid out The principal physiologic data is displayed in stacked waveform charts and large numeric indicators in the center of the panel Ancillary data is contained on separate display screens Boolean controls facilitate navigation ina conventional manner The overall appearance resembles and behaves as a virtual instrument The application is fast and responds quickly to operations that the user performs The front panel of Nested VI in Figure 1 2 uses a tab control to logically organize the front panel objects The controls are clearly labeled and evenly spaced However the purpose of each tab and the order by which the user navigates the GUI are not immediately clear The diagram contains Sequence structures that execute each frame to completion regardless of any operations the user performs on the front panel Its capability to respond to the user clicking Quit depends upon which frame of the Sequence structure the application is executing when the button s value changes The front panel of Spaghetti VI shown in Figure 1 3 organizes controls and indicators using clusters It is logical and organized but it contains too many colors and font types and not enough empty areas or white space More important the performance is not reliable No matter how attractive a GUI appears if it does n
487. tes Many of the meetings in which projects are specified resemble fast paced brainstorming sessions and high speed diligent note taking is essential Bound notepads are a standard method for professional engineers and scientists to track their daily activities They are also an ideal method of initially capturing raw specifications Rule 2 1 Maintain a LabVIEW project journal The bound notepad serves as a LabVIEW project journal Record the date of each activity and maintain your project related notes in chronological order As notes are appended throughout the project s life cycle they need not be limited to requirements notes are commonly extended to include design notes and maintenance logs The notes can also include bug descriptions wish lists data manual calculations notes from support inquiries and artistic doodles The function of a project journal for a LabVIEW developer is similar to a laboratory journal for a scientist or inventor Laboratory journals are used to preserve the experimental data and observations that are part of any scientific investigation They are the basis for further analysis discussion evaluation and interpretation Similarly project journals contain any mixture of specifications and data that form the basis of the software design In many situations scientists and inventors use LabVIEW to conduct their experiments Indeed many scientists and inventors are also LabVIEW developers
488. tes or test the most unlikely combination of operator inputs Error handling can tell us when something goes awry and pinpoint the operation that reports a problem The application can be finetuned such as setting the Data Range property of numeric controls to help prevent invalid user inputs that may cause misbehavior When an application contains complete error handling is thoroughly tested and does not generate errors we may rest assured that it is a reliable application After deployment the error handling must remain intact to report any new problems that may arise due to unforeseen circumstances Many factors are beyond LabVIEW s control Changes to the computing device s hardware software or network configuration are common sources of new misbehavior to an otherwise bug free application The presence of thorough error handling helps users and developers quickly identify and resolve new problems Hence thorough error handling serves the purpose of preventative maintenance in our applications Not only is error handling important for identifying problems but error handling also is directly related to good LabVIEW programming style As discussed in Chapter 4 Block Diagram data flow is the fundamental principle of LabVIEW and propagating the error cluster is the primary means of establishing data flow The error clusters are LabVIEW s most universally recognized data structure on a VI s front panel connector pane and d
489. tes its code immediately without any user interaction continuous looping or error reporting that might cause the calling application to wait The Functional Global design pattern consists of a subVI that contains a While Loop a Case structure an enumeration and controls for reading and writing data The While Loop contains one or more uninitialized shift registers anda Boolean constant is wired to its conditional terminal forcing it to stop after only one iteration The loop s sole purpose is to store global data within the shift registers The Case structure contains separate cases for writing and reading the shift register data Additionally you can add cases for initializing or resizing data structures such as arrays and strings or performing calculations or data manipulation The enumeration provides programmatic selection of the read write and other supported operations from the diagram of the calling VI The subVI is applied to each location of an application that requires access to the data similar to a global variable Functional globals are also known as LabVIEW 2 style globals because they were the only method of sharing data without data flow prior to the introduction of local and global variables in LabVIEW 3 0 However functional globals have important uses today They are more memory efficient and reliable than local and global variables Each instance of a read local or global variable creates a copy of its data when read
490. the Case structure that launches the document using the appropriate browser Specifically User Manual HTML uses Open URL in Default Browser VI to launch the HTML document in the computer s default browser User Manual PDF uses the System Exec VI to launch the PDF document using Adobe Acrobat Reader User Manual CHM uses the Control Online Help function to launch the CHM file in the default browser Finally the Context Help menu selection opens the Context Help window for viewing the control and VI descriptions This operation is built in and no code is required on the diagram Figure 9 5A The front panel of Torque Hysteresis VI contains Help menu selections for user manuals in PDF HTML and CHM format in addition to Context Help View full size image gt Torque Hysteresis vi User Manual CHM About Torque Hysteresis VI my lal j i T n Figure 9 5B The block diagram of Torque Hysteresis VI captures Menu Selection events using an Event structure and launches the corresponding user manual in the appropriate browser Open URL in Default Browser VI is used to launch an HTML file System Exec VI is used to launch PDF files and the Control Online Help function is used to launch a CHM file View full size image 310 oe Th j St LIL Edt gt Shated L LH arar in LE ara fz eget Rule 9 9 Provide online documentation with deployed applications Here is a hypothetical situation You have just
491. the caller VI s diagram whereas the enum is not Thermometer vi Temp Scale Thermometer with Enum vi Temp Scale The OpenG tools include an extensive set of VIs that use the variant data type Many of these VIs are intended to operate on multiple LabVIEW data types similar to polymorphic VIs The LabVIEW data type is converted to a variant which contains the data type control name and data This information is used to increase the flexibility of the data that can be passed to a subVI through the connector terminals and the operations that can be performed on the data within the subVI The OpenG tools use this concept to extend the capabilities of the LabVIEW libraries When a LabVIEW data type is wired directly to a variant input terminal a coercion dot appears on the input terminal and the conversion to variant is performed automatically However dissimilar data types are undesirable per Rule 6 3 Ideally coercion dots should be avoided for best style However the conversion from the LabVIEW data type to variant is unavoidable As an alternative the data can be converted to variant using the To Variant function prior to wiring 197 to the input terminal of the OpenG VI This method replaces the coercion dot with an explicit function call While the two methods are functionally equivalent the explicit conversion conveys that the conversion is deliberate and not an oversight For example in Figure 6 13 the Open
492. the diagram of the main GUI VI normally the top level VI Likewise maintain any loops that perform high speed display updates at the top level to optimize the update rates Alternatively consider distributing the top level VI s user interface among the loop subVI front panels and load them into subpanel controls Figure 8 24A shows the top level diagram of the Torque Hysteresis application implemented using the Modular Multiple Loop Application Framework Three queues and a type defined cluster of control references are initialized on the left of the diagram The queues are passed to the Event structure and loop subVIs for messaging The control references are passed to the loop subVIs for reading from and writing to the controls of the top level VI using Property Nodes When the application ends errors are merged and LabVIEW closes using the subVIs on the right Figure 8 24A The top level diagram of the Torque Hysteresis application is modularized into three loop subVIs plus an Event Handling Loop View full size image te ee _ ict Soria Seta Graal TENORE E TTi Ti E a A bh E jdim Lah k kai ee ie f ge ae ual eel BOTT ae Cada Cire Leer Fa aed rep soars Tn Dan Cepia P Less arep Ga i i aire nem Eat Seen Peewee i epee rira Beyan Tp Ee vargil Lua Lat Corte EP iph riri Fop as Tega ret leat anid Fiil Tangs Libs ETETA 1 iu ae Lees oe kee F F E iF ina at Jat tee den ej we a a Verte
493. the index terminals of array and string manipulation functions DAQmx and most instrument driver VIs return DBL or waveforms containing arrays of DBL Likewise the analysis VIs process arrays of DBL Therefore using 132 for integer and DBL for floating point numbers helps maintain consistent data types throughout an application and avoids unnecessary coercions and conversions For completeness two exceptions to this standard include digital pattern I O and traditional data acquisition VIs Reading or writing digital pattern data from a data acquisition device generally requires l or 4 byte unsigned integers U8 U32 Also the analog traditional DAQ VIs use 4 byte single precision SGL numeric if configured to read or write scaled data instead of waveforms However the DAQmx VIs and driver are both functionally and stylistically superior to the traditional DAQ VIs and driver I highly recommend upgrading any legacy applications that use traditional DAQ 177 If you are like me you may have wondered why LabVIEW uses 32 for unsigned quantities such as loop iteration and count terminals versus U32 The LabVIEW inventors intended 132 and DBL to be the prominent data types for integers and fractional numbers respectively Therefore LabVIEW uses 32 and DBL to help promote their use Rule 6 14 Use automatic formatting unless a specific format is required By default LabVIEW formats floating point numbers within numeric controls
494. the native language of the LabVIEW version in which it is created The enum functions correctly when ported to localized versions but the language of the enum text is not translated For this reason the NI Instrument Driver Development Guidelines recommends text ring controls and Booleans over enums The text within the text ring gracefully ports to the localized language because the text is a symbolic mapping to the integer Finally the Strings property is read and write programmable for a ring but is readable and not writeable for an enum A comparison between the enum and ring is summarized in Table 6 4 Table 6 4 A Functional Comparison Between Ring and Enum Data Types Data Type Behavior Unsigned integer representations i a Signed integer representations i Floating point numeric representations i Nonsequential values d Text menu controls indicators and constants x Text menu case selector a Strings property is editable i Strings are localizable a Rule 6 17 Save enums as type definitions It is common to add and remove items from the enum controls many times during application development and maintenance However any clones you copy or constants you create do not maintain the same items as the original control after cloning unless the original control is saved as a type definition This is easy to overlook and a common source of misbehavior Therefore always save your enum controls and as type definitions Thi
495. the state machine instead of frames of the Sequence structure Unlike a Sequence structure the Classic State Machine can change its execution order on the fly or stop the sequence prior to completion Also the enumerated type definition helps ensure that each case is intuitively labeled Finally shift registers on the While Loop provide a graceful method of sharing data between states An example of a classic state machine applied as a Flexible Sequencer was presented in Chapter 4 257 Additionally the Classic State Machine is amore flexible and organized alternative to the Continuous Loop design pattern For example Figure 8 10 contains two implementations of the Torque Hysteresis application s data acquisition routine This routine acquires a finite number of torque and angle measurements using DAQmx VIs and publishes each sample to a shared variable Figure 8 10A implements the DAQ routine as a Continuous Loop with initialization VIs on the left of the loop and shutdown VIs on the right Figure 8 10B contains a functionally similar routine implemented as a Classic State Machine This implementation requires less horizontal space contains better documentation via state labels and expands more gracefully than the Continuous Loop by adding new states as cases within the Case structure instead of adding more objects horizontally Figure 8 10A The DAQ routine of the Torque Hysteresis application implemented as a Continuous Loop
496. their separate identities and is directly compatible with the XY graph indicator Hence the cluster of 1D arrays maintains consistent data types throughout the application satisfying Rule 6 3 Finally the torque and angle data is processed and statistics are calculated and placed in a separate cluster named Statistics The elements in this cluster all have the same type double precision floating point number but a cluster is used to maintain their unique identities via independent labels Rule 6 23 Enter descriptions for array and cluster shells and control elements Rule 6 24 Use alignment tools to keep clusters neat and compact 184 Arrays and clusters often form the data infrastructure for an application When defined they can proliferate throughout the application As such it is important that they are consistent and well documented As discussed in Chapter 3 proper documentation includes succinct intuitive labels that include the units in parentheses for any physical quantities Additionally include a description for each control element as well as the array or cluster shell Use the autosizing or distribute objects tools to keep the controls within a cluster neat and compact I find that some clusters can really expand in number of elements as an application s requirements expand so I prefer to make them compact to minimize the front panel space This includes use of the default properties for the controls that comprise th
497. tion font on white background Numeric and String Control Data 13 point normal black Application font on white background Secondary Fonts Not applicable Navigation Tab key navigation Figure 3 15B contains a functionally equivalent dialog VI that conforms to dialog style conventions The Dialog option for Window Appearance properties has been chosen and all controls and decorations are selected from the System palette A phrase describing the dialog s function replaces the VI name in the title bar Note that the VISA Resource Name control has been replaced with a System Combo Box to maximize the native dialog appearance Also the Quit and OK buttons have been replaced with OK and Cancel in the appropriate locations Figure 3 15B This revision uses Dialog window title and appearance Dialog fonts and System controls conforming to the conventions for good dialog VI style View full size image gt Configure RS 232 Communication Settings Settings C Data R5232 Logs Bloomy 26012005 C Force parse fromlog Ordinarily will not parse iF has already taken place VISA resource name stop bits 10 1 bit St i baud rate 9600 Flow control 0 none 9600 None ore 0xA n LF m A parity O none ety N Enable Termination one Char T Figure 3 16A shows the main GUI VI panel of a Capacitor Test amp Sort application This is an industrial test application operated by semiskilled users in a production envi
498. tions It behooves us to enter the description even if it seems unnecessary or we might be disappointed when a formal document is required Rule 6 6 Save custom controls as strict type definitions A strict type definition is a control that is customized and saved as Strict Type Def in the Control Editor window The Control Editor forms a CTL file that maintains the data type and properties of the strict type definition This allows the multiple instances to maintain the same customized properties including appearance and behavior The properties are then modified via the Control Editor and changes are automatically applied to all instances Strict type definitions are discussed in detail in Section 3 4 2 Consistency In general if the control has one or more properties that have been configured and will have more than one instance requiring the same properties save it as a strict type definition After the controls and properties are specified from the previous two steps the final step is to implement the data constructs This is primarily accomplished by grouping related controls into arrays and clusters and saving as type definitions Arrays are multivalued data sets of the same type Clusters combine multiple controls of any type into a new data structure Arrays and clusters are represented by a single terminal and wire on the diagram They are very useful for organizing data and reducing subVI terminals and wire clutter
499. to read or write a control s Value property is similar to a local variable but it causes the diagram to switch to the user interface thread to read or write the value directly from the front panel in a synchronous manner Local variables read or write from the control s terminal on the block diagram which does not trigger an immediate thread change and user interface update A single process shared variable functions similarly to a global variable that performs error checking Local and global variables are generally simpler and more efficient than Property Nodes and Shared Variable nodes If you must use local and global variables and you must specify the order in which they are written or read then you must use a Sequence structure Rule 4 31 111 Use a Sequence structure to order operations if no data dependency exists Rule 4 32 Use only Flat Sequence structures when required Use a Sequence structure to specify execution order when a specific order is required and no data dependency exists Specifically Sequence structures can order initialization routines including write variable operations to ensure that they are performed at the very beginning of the application Likewise Sequence structures can force shutdown routines such as Quit LabVIEW to occur at the very end of the application Figure 4 9 illustrates an example of a diagram that utilizes an initialization routine two parallel loops and a shutdown routine The ap
500. top level VI developed using a legacy version of LabVIEW Prior to version 8 0 the single terminal was the default pattern and most top level VIs utilize the default Rule 10 6 Customize the rank for each test according to the priority of each rule The test rank setting determines the order in which any violations appear in the Results window and the symbol that appears next to the test name Set the tests corresponding to high priority style rules to a rank of High and set the normal priority rules to Medium Because the VI Analyzer is utilized to inspect performance issues as well as style the default test rankings are significantly different than the priorities of the corresponding style rules provided 319 in this book For example the Error Cluster Wired test has a default rank of Low This test checks whether the error out terminals of nodes are wired enforcing Rule 7 5 Trap all errors from all nodes that have error terminals As discussed in Chapter 7 Error Handling error trapping is an essential ingredient of error handling and error handling facilitates robust applications Customize this rule s test rank setting to High to reflect the priority of Rule 7 5 You can download a customized test configuration containing recommended analyzer test settings and rankings from www bloomy com lvstyle Figure 10 2A provides a primitive version of the Torque Hysteresis VI discussed in previous chapters This version was de
501. tore data in contiguous memory addresses They include all scalar data types such as Boolean numeric and string an array of Boolean and numeric and clusters containing only the aforementioned simple data types simplicity Inversely relates to the quantity of nodes and terminals that comprise an application It is the opposite of complexity The fewer front panel objects and block diagram nodes the simpler the application standard subVI User developed subVI that is not part of a commercial product or toolkit state machine The most popular LabVIEW design pattern of all time consisting of a Case structure within a While Loop with a shift register or messaging construct wired to the Case structure s selector terminal strict type definition Custom control defined using the Control Editor window that maintains the exact appearance properties and data type for each instance of the strict type definition See also type definition subVI A VI that is called from the diagram of a higher level VI In Chapters 3 and 8 subVIs are also assumed not to open their panels during program execution subVI performance 350 The execution speed of a subVl T target Any computing device upon which a LabVIEW VI can execute Targets include personal computers embedded controllers PDAs and FPGAs task A set of operations normally encapsulated by a subVI or While Loop TBD To be determined Pertains to undefined r
502. trant VI Issues and more These are elusive sources of misbehavior that are difficult to identify through manual inspection Rule 10 5 Customize the VI Analyzer s test criteria o Set the Coercion Dots maximum number on a single wire to 0 e Set the Enabled Debugging test to Fail Test if Debugging is Disabled during development e Set Globals and Locals maximum number to 0 each o Set Comment Usage to Ensure subdiagrams contain at least one comment each e Set Control Alignment for a Pixel Tolerance of 1 e Disable the Dialog Controls test for subVIs and industrial GUI VIs e Set Connector Pane Pattern to 32 and 0 Utilize the VI Analyzer s Select Tests dialog to customize the test criteria according to your needs and save the test configuration to file for repeated use Specifically set the maximum number of coercion dots on a wire to 0 as shown in Figure 10 1 This test configured under Block Diagram Performance Coercion Dots enforces Rule 4 24 Avoid array and cluster coercions and Rule 6 3 Choose controls and data types that facilitate consistent data structures throughout an application The default value is 2 so it is necessary to customize it to identify every violation Also unless you are preparing to compile an executable set the Enabled Debugging test to fail if debugging is Disabled Most code reviews are performed during development and debugging is desirable During at least one pass through the
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504. trings then an array would be possible However the index would not be sufficient to uniquely identify the elements Instead separate controls with independent labels are needed Hence a cluster is required 183 Rule 6 22 Use arrays to store large or dynamic length data sets Another distinction between arrays and clusters is that the number of elements in an array can be determined programmatically and resized as often as necessary whereas the number of elements in a cluster is fixed Also it may be time and space prohibitive in the literal sense to create cluster controls containing a very large number of elements Therefore arrays are used instead of clusters if the quantity of data elements is large or may change during program execution and the elements share a common data type Let us review the Torque Hysteresis VI that was introduced in Section 6 1 3 Create the Data Constructs The clusters are implemented as shown in Figure 6 5 In the first step of the measurement sequence UUT Information Dialog prompts the operator for some information regarding the unit under test including the customer model production year and more This information is stored in a cluster named UUT Information Here all the data is related and is of the same string data type A cluster is chosen over an array because the data elements have unique identities requiring independent labels and descriptions Next the application prompts the op
505. troduced in the previous section to replace the timing and arithmetic functions that were present at the bottom of the diagram in Figure 8 26B Additionally the 286 error cluster wire enters and exits the Case structure and While Loop through tunnels on the bottom left and right vertical borders Hence the modified VI is stylistically superior than the original Figure 8 26C The diagram of Poll Instrument Response VI is enhanced using the Elapsed Time VI functional global introduced in the previous section Two calls to Elapsed Time VI replace several timing and arithmetic functions View full size image a p ap bies ip reed 1024 L EI r brasat 3 amc E DERECE Mi Pra wy error idee erree on The VI illustrated in Figure 8 27 contains several features in common with a Classic State Machine design pattern but the overall implementation is uncommon and includes multiple style rule violations The common features include the While Loop containing a Case structure a shift register and a menu of Boolean controls The diagram style violations are as follows Figure 8 27A The front panel window of Set Window Options VI consists of a Boolean menu and some LEDs Dialogue Box AYES Window has Title Bar Resizable Show Scroll Bars Show Menu Bars idm Figure 8 27B The diagram contains several components of a State Machine design pattern but violates multiple style rules pertaining to State Machi
506. trol properties reducing real estate occupied by Property Nodes Wiring Scheme Clusters are utilized to avoid wire clutter Wire bends create additional space within the main Case structure Data flow Data flows strictly from left to right Queues are used to pass data between parallel loops without variables Shift registers are used over variables wherever feasible The application contains 50 variables primarily used to initialize control values The Screw Inspection VI diagram shown in Figure 4 20A is clear with a staircase dataflow appeal It is modular with a particularly attractive icon convention that is featured in Chapter 5 At first glance the layout wiring and data flow look pretty good Shift registers are utilized over variables maximizing data flow Wires and objects are evenly spaced However close inspection reveals multiple rules violations as shown in Figure 4 20B These include unnecessary 128 wire bends and kinks 4 11 overlapping objects 4 15 unlabeled long wires 4 18 including wires exiting shift registers 4 37 right to left data flow 4 22 discontinuous error clusters 4 23 and coercion dots 4 24 There is unnecessary clutter including constants and labels to the left of the shift registers The constants are unnecessary because they are the same as the default values for each shift register s data type when first loaded into memory Error handling is functionally nonexistent Figure 4 20C
507. tt Sort vi fle Edt Gperste Tool rome Window Hep wow parca Z DETOVOK Sendai Test amp Sort Performance Stats mean E mm 36 Te O Standard Deviation ust Ls 3 SIGMA pe o __ J ange E nos I Change je a Work Order et System Application Capacitor Test amp Sort top level VI panel Conditions 1024x768 resolution medium object density Color scheme Blue charcoal white Layout Graph is prominently sized and centered on the panel Numeric indicator and Boolean LED overlap a graph indicator in violation of Rule 3 11 Boolean text Four different fonts and four different succinct wordy caps conventions White text on charcoal background provides good contrast Heading text 16 point bold white Application font on charcoal background Numeric Control Data 16 point normal black Application font on white background Secondary fonts Control labels and heading data are 13 point black Application font Labels are bold data is normal 89 Navigation Five large Boolean controls at the bottom of the panel and the standard menu bar facilitate navigation The standard menu bar is not appropriate for the application s users Figure 3 16B contains the same panel with improvements made to the Boolean text menu bar and overlapping indicators All Boolean controls contain a single succinct command word with first letter capitalization using 29 point bold white Dialog font A cu
508. tten to indicators is returned through subVI connector terminals wired to the indicator terminals on the top level VI There are three issues with this approach 1 If there are more than a couple front panel terminals within the loop the subVI will have excessive terminals and wiring clutter inside and outside the subVI 2 The indicator updates will not occur until the subVI completes changing the behavior of continuous updates 3 Maintenance is very tedious 282 Specifically adding and removing control references for each new control or indicator needed within the subVI entails terminal connector and wiring changes Instead always pass the control references using acluster of control references saved as a type definition and avoid the SubVI from Selection utility Rule 8 31 Keep the Event Handling Loop at the top level Rule 8 32 Keep high speed display updates at the top level The Event Handling Loop is naturally very tightly coupled to the graphical user interface Indeed many of the GUI VT s front panel terminals are maintained within this structure Maintenance is reduced if this loop is maintained on the top level diagram instead of modularizing into a loopsubVI Additionally Property Node read and write operations are several times slower than reading and writing directly to the control and indicator terminals Therefore performance is maximized and maintenance is minimized if the Event Handling Loop is maintained on
509. ttention to detail View full size image 12 PM 1000 TELA Pee Tiai ee C i RAN TEATE View full size image EN HSL E b Dii Dipy ee y hs Gerri Cee Se ee y pa layr D j emr bai Pre DE gt gt bai owas Cau F b Flr Ce 2 ees Se Bor Boarding uu J J i b Peet T afa afa ufa ual U 5 Ls i z E he EEE coe Falis prt Winrefireg Avalahia Pies INSTA COMM LOOP a ret Comm Read Read 5 790 Seva Pot CT S lt s S3S 73S 73S 7 73 7 3 PTC eles oe J Oa F wee Coe cs Te eT Loree Cae h mj A a gt hiph Gi 2 et ee ee a iy gt Erem Hanin usu e Mar a e i T o ph Ei 2 leis Ri Fr h 5 23a E Sere F p Bopper pts co f iS Pep Sat Carper fire ofl Z Commie Stee E nera Figure 1 2 Nested VI the front panel and block diagram for a simple application The diagram has a highly nested architecture 13 View full size image Hested wi Front Panel A 1O 1010 Figure 1 3 Spaghetti VI the front panel and block diagram which resembles spaghetti for a medium complexity application View full size image 14 Be fe Ber ae f po Ea gee F 1 Trah fal ere fF ae ph Lap frre y m ii fe ai F o a i ee ee a p F g p e T Pee Pl ni Pap EE a a k aaa i 4
510. turned String T Yes pr iune e error out error in mo error Before embracing the PHPM1k Initialize VI revisions in Figures 5 17B and 5 17C there are a few considerations Depending on the application s data structures the callers may now have to perform a bundle operation to program the serial parameters A tradeoff exists between reducing wire clutter and adding cluster operations to the caller s diagram According to the NI Instrument Driver Development Guidelines passing cluster information between VIs increases the complexity of the calling application However my recommendation is to use consistent style for the instrument driver VIs as for their calling applications Specifically I recommend using clusters if the data elements are closely related 153 and are likely to be used together in more than one place Also judicious use of clusters in an instrument driver promotes good data structures throughout the calling applications For example the user of the revised VI may now be inclined to use the same Serial Port Settings cluster to pass this data among other VIs in the call chain Data types and structures are discussed in greater detail in Chapter 6 Data Structures Figure 5 18 contains three versions of the Functions palette for the Suss Interface Toolkit for LabVIEW as well as the icon for DoProberCommand VI This instrument driver previously introduced in Section 5 1 2 Icon Shortcuts
511. ty 8 10 Derive the application s primary states from the specification or design document 8 11 Divide the primary states into additional states 8 12 Use an enumerated type definition for the case selector 8 13 Minimize code external to the Case structure 8 14 Include states for Initialize Idle Shutdown and Blank 8 15 Avoid timeout with the Enqueue and Dequeue Element 8 16 Use queues shared variables or RT FIFOs for parallel loop messaging 8 17 Prioritize loops using delays or thread priorities 8 18 Size parallel loops to the same width and align vertically 360 8 1 Use multiple criteria for the loop condition 8 19 Minimize space between loops 8 20 Label each loop in the top left corner 8 21 Use the Call By Reference Node over the Run method 8 22 Choose standard connector terminal assignments 8 23 Maintain all components within a dedicated directory 8 24 Pass data between components with shift registers D oO Assign one input and one output terminal as type variant 8 26 Display plug in panels within subpanels 8 27 Create a parallel loop for each cohesive parallel task 8 28 Include loops for Event Handling Main State Machine Hardware I 0 and Error Handling 8 29 Pass control references into subVIs via a type defined cluster 8 30 Avoid subVI from selection with continuous loops Be ol Keep the Event Handling Loop at the top level 8 32 Keep high speed display updates at the top level Chapter 9 9 1 Keep front pane
512. types used to form a new data type They include developer defined constructs such as array cluster variant variable and queue as well as built in data types such as matrix error waveform and dynamic Arrays and clusters provide tremendous flexibility and utility and are the primary means for organizing data in an application This section presents style rules for data constructs with emphasis on arrays and clusters Array and cluster controls consist of shells or containers within which any data type or construct may be placed The complexity of arrays and clusters depends entirely on their contents This could be as simple as a single scalar Boolean or as complex as nested arrays and clusters This section discusses simple arrays and clusters Arrays store data sets that have multiple values of the same data type In memory arrays consist of a heading containing the length of each dimension stored as 4 byte signed integers followed by the data that comprises the array Arrays containing one of the simple data types described in Section 6 2 Simple Data Types store the heading and data together in contiguous memory locations Rule 6 21 Use arrays for multivalued data items use clusters for grouping multiple distinct items Most references consider arrays as collections of related data very similar to clusters except that the data elements have the same type Indeed this is correct However there are very important
513. u might have developed in a hurry or modified several times consider whether it is clear is well documented and still makes logical sense update the documentation as needed and consider all the aspects related to style Additionally it is excellent preparation for the peer review 10 2 Peer Reviews Peer reviews are code reviews performed with the assistance of one or more peers In practice many coding issues are resolved by the developer as she explains her source code to a colleague Also some development techniques that are employed by individuals are not widely known Indeed LabVIEW is extremely rich with features and most developers never stop learning new techniques Both the developer and the participants can glean much knowledge by discussing source code with peers Theorem 10 1 Peer interaction is a powerful motivator Consequently peer reviews are an effective means of enforcing and evolving a style convention 332 Many developers are eager to learn and improve their coding style either on their own or with the help of a mentor or peer group It is common to reach a plateau at which a developer becomes comfortable with a particular design pattern or toolset after that point the developer is less likely to explore alternate styles Peer interaction is a great motivational tool and peer reviews are an excellent method of learning and improving development skills including style Moreover peer re
514. ubVI completion data returns from the diagram through the indicator and connector terminals and back to the wires on the caller s diagram This fundamental process is illustrated in Figures 5 1A 5 1D If the subVI s front panel window is not open and the diagram does not utilize Property Nodes or local variables the front panel is not actually loaded in memory Instead the data passes directly from the subVI s connector terminals to its diagram Figure 5 1A A diagram subset contains a subVI call to Find Screw Ends VI The subVI s front panel is open to monitor data flow The controls and indicators are initially empty View full size image b Pind Sonne Fad i Freni Parl Figure 5 1B The subVI call with terminals visible Data flows from the input connector terminals through the controls on the front panel to the corresponding terminals on the diagram View full size image 133 Figure 5 1C Data flows through the terminals wires and nodes of the subVI s diagram View full size image Original Image in Lo bres Edige Point Line Endpoints Lre FE Tltmage Dst Cut error In no error Lies FEiz error out Figure 5 1D The subVI passes data through the indicators to the output connector terminals and wires on the calling VI View full size image 134 Hence the icon and connector facilitate subVI calls SubVIs promote modularity organization and code reuse Moreover icon
515. ult This means that an error cluster passed through an output tunnel will accumulate into an array of size equal to the count If you have a good reason to accumulate all the errors and the loop count is limited index the errors Outside the For Loop s output tunnel the Merge Errors VI can process the array of errors as shown in Figure 7 5A Merge Errors VI searches the array and returns the first element with status TRUE or no error Indexing should never be enabled within continuous loops however because the array size is unbounded Figure 7 5A A For Loop indexes an array of errors by default Merge Errors VI returns the first element of the array with status TRUE View full size image barrier of Sarpi TEF Write interval rma URE Similar to the While Loop the shift register method of error trapping is the best method of propagating the error data between successive For Loop iterations This is shown in Figure 7 5B The shift register method maximizes efficiency in the event of an error because the functions and VIs that receive an error at their error in terminal skip their code Also the shift register method maximizes memory efficiency because it maintains the data from only one error cluster in memory Moreover the shift register extends a continuous error chain throughout the loop whereas indexing resets the error chain in each new iteration Consequently the shift register method is generally preferred over the i
516. up for each station delineated by decorations 1 4 groups of thermocouple indicators 2 5 and power indicators 3 6 for each station delineated as clusters and the group of controls and indicators common to both stations located across the bottom 7 The clusters of power indicators contain six unique indicators within each The clusters of thermocouples contain 32 indicators that are sequentially arranged and labeled The user does not have to examine all 32 indicator labels to understand them After she reads three or four of them she recognizes the ordering layout and pattern In terms of identification the group of 32 indicators is analogous to a group of only a few different objects Nonetheless there are 32 unique data items in the group Hence this VI approximately satisfies the seven groups of seven unique objects criteria Figure 3 4 Power Monitor System VI contains seven groups of objects The cluster containing 32 TCs is sequentially arranged and labeled for ease of identification 63 View full size image i 4 ca eee T TEI Power Monitor System VI is quite dense It contains 91 total controls and indicators on a panel that is sized for 1024x768 monitor resolution The overall density is easily reduced using a tab control and dividing logical groups of objects onto separate tabs Figure 3 5 contains a simple arrangement with separate tabs for each station The tab control is sized to fill the entire panel
517. urements using the Replace Array Subset function Because these arrays contain simple data types with constant size no extra buffers are created and no unpredictable latencies will occur during acquisition Likewise Replace Array Subset is used to insert the single run data into the Multiple Run Data array and the single UUT cluster into the Multiple UUT Data array The Replace Array Subset function always performs its operation in place without either moving or making copies of the input data array This is possible because the data array size is not changed by this function After the tests are completed a post processing routine can simply search for the NaN constant and resize the arrays to their actual length The post processing routine should be performed off line after any time sensitive operations have completed and we are less concerned with performance Figure 6 11F Efficiency and reliability are improved within Run Test VI by replacing the data elements of each layer of the previously populated nested data structure using Replace Array Subset The data structure remains a fixed size and extra buffer allocations are avoided View full size image PBa le Tyee LS Astute readers may notice coercion dots on the input terminals of the motion control driver VIs in Figures 6 11C and 6 11F These coercions are caused by dissimilar data types utilized within the driver VIs Specifically input and output terminals that are intended to
518. uring acquisition Unbundle and Bundle By Name are used to combine the data with the Torque vs Angle subcluster within each iteration of the While Loop which is unnecessary additional overhead within the looping structure Because the While Loop is where the torque and angle measurements are acquired this is acritical portion of the application and data manipulation should be minimized Figure 6 11C One implementation of Run Test VI uses the Unbundle by Name Bundle by Name and Build Array functions to combine the newly acquired data with the Torque vs Angle subcluster This implementation is inefficient because the Build Array function continuously resizes the arrays allowing latencies due to memory allocations during the time sensitive acquisition task View full size image Se Dee a a ae agg eee o O oa a i i a S a eT H at LN iir iie P TD s 5 gee Sa bpa tre i Figure 6 11D contains the diagram of Populate Data Structure VI a routine that initializes the Multiple Run Data portion of the overall data structure It generates a Torque vs Angle subcluster prepopulated with arrays of the NaN constant Specifically the maximum number of torque and angle samples that may be acquired per test run is calculated as the product of the Angle Amplitude deg and Max Rate Sa deg parameters An array of NaN is initialized to this maximum length and assigned to both the Torque In Oz and Angle deg elements of t
519. urpose for each and every VI The default icons generated by LabVIEW are not unique and meaningful Always replace them with something more appropriate Save your VIs with their subVI icons visible on the diagram instead of using terminals This ensures the icons remain visible each time the diagram window opens Rule 5 5 Use a black border 135 A black border drawn around the perimeter of the icon helps to clearly delineate the icon from the calling VI s diagram Always use a black border unless you have a compelling reason not to such as an imported graphic that requires every pixel Rule 5 6 Combine a glyph with color and text for best style Rule 5 7 Choose universally recognized glyphs Rule 5 8 Use 8 or 10 point small fonts for most text Because icons are only 32x32 pixels their contents must be small and punchy Create graphics and fonts that are legible and meaningful within these space constraints A glyph is a recognizable graphical symbol such as a public sign or traffic signal Signs are excellent examples of graphics and text that are combined to provide distinct meaning in a small area Libraries of glyphs graphics and art are available with most drawing applications and online as well For example NI Developer Zone hosts an Icon Art Glossary at www ni com devzone idnet library icon art glossary htm Applying the default 13 point application font we are limited to about five characters per lin
520. us properties that are programmed to create a wide variety of effects so overlapping other objects is usually not necessary In some circumstances overlapping objects do not affect performance and are permissible In one example only one of the objects is visible at a time LabVIEW does not update invisible objects so overlapping them does not cause extra processing Another example involves importing a large static image and overlapping some controls or indicators on top of the image For example consider a large image that represents an industrial process or unit being tested and overlaying digital indicators that display live data at positions that depict the locations of the measurements As long as the indicator backgrounds are not transparent indicator updates do not cause the underlying image to be redrawn Only the visible portion of the live indicator is redrawn with each update Figure 3 7 provides an example Figure 3 7 A static image of an industrial process with overlapping digital numeric indicators operates efficiently because only the digital indicators are redrawn with each update View full size image 66 ILS Temperatures gt Mix Eroi Natural Steam From Process Fuel Water From Gas ACC400 From Reformer PMP400 Exit Several Window Appearance properties need to be configured for a GUI VI Most of them fall into the categories of Top level application window and Dialog which are preconfigured and conveni
521. use it is event driven Additionally the Queued State Machine is a versatile method of implementing a state machine for routines of medium or greater complexity The Event Driven State Machine design pattern combines a Queued State Machine and Event structure into a hybrid single loop design pattern Specifically an Event structure is inserted into the state machine s Idle case allowing it to process user interface events in an efficient manner Hence one loop performs event handling as well as buffered state handling This is a powerful construct in a compact 261 form The Event Driven State Machine is appropriate for top level and GUI VIs for applications of medium complexity It is my personal favorite single loop design pattern The Torque Hysteresis VI is rewritten as an Event Driven State Machine in Figure 8 14 The front panel shown in Figure 8 14A contains a menu of Boolean controls and a graph indicator The diagram consists of an Event Driven State Machine as shown in Figure 8 14B The Event structure is configured with Event cases for each of the Boolean controls Value Change events Each Event case adds one or more states to the queue The state delineation is shown in Figure 8 14cC Each of the primary tasks of the application is configured as a state in addition to Initialize Idle Shutdown and Blank Overall the Event Driven State Machine implementation is flexible functional efficient organized and compact Figure 8
522. ussed in Chapter 8 This practice reduces maintenance when additional wires are needed Be sure to clearly label unused wires as Unused or Spare Rule 4 15 Never obstruct the view of wires and nodes e Avoid overlapping diagram objects Avoid obstructing the view of wires and nodes by overlapping them on the diagram An occasional crossover of a wire routed horizontally with a wire routed vertically is unavoidable For example sometimes it is necessary to wire the iteration terminal of a looping structure normally located at the bottom left of the structure to a location above and to the right Many of these same loops have wires routed horizontally across the entire structure via either tunnels or shift registers The error cluster is a prime example If the iteration terminal is to remain on the bottom left there may be no choice but to cross over the horizontal wires The obstruction is minimized if the vertical wire is routed through a location of minimum wire density overlapping as few wires as possible Additionally never overlap a wire with an object or a node A wire running underneath a function or subVI resembles an input and output to the node Rule 4 16 Limit wire lengths such that source and destination are visible on one screen Rule 4 17 Never use local and global variables for wiring convenience Rule 4 18 Label long wires and wires from hidden source terminals Ideally the source and destination of every wire should
523. uster is optimized for subVI propagation Figure 6 10A The front panel of UUT Information Dialog contains string controls that are customized for GUI interaction 186 View full size image Production Year Test Reference Spec Number Figure 6 10B The UUT Information type defined cluster and other controls are scrolled off the visible area of the screen gt UUT Information Dialog Customer Model Part Status Production Year Test Reference Spec Number Spec Description _Cance 187 The diagram of UUT Information Dialog bundles the data from the interactive GUI controls into the cluster when the OK button is pressed In Figure 6 10C we see that the terminals of the Bundle function are ambiguous The developer has no way of knowing which terminal corresponds to which cluster control without examining the cluster order Moreover any changes to the cluster will create maintenance challenges for the developer using this approach For example the cluster order might change after adding and deleting or renaming string controls without changing the number and data type of the elements This would result in the same configuration as in Figure 6 10C but an incorrect outcome Figure 6 10D illustrates proper use of Bundle By Name for bundling the data into the type definition As we can see the Bundle By Name function s terminals are clearly labeled Also Bundle By Name is insensitive to the cluster orde
524. ustom filter for parsing invalid characters If the number of permissible string values is discrete consider a text ring list box enum or combo box control Enter each valid string as a separate item forcing the user to select from a discrete list of valid strings The former three controls map the strings to integers the combo box maps a string to the same or another string Text ring controls are preferred for instrument drivers per NI s Instrument Driver Guidelines Personally I am partial to enums because they have a special property when wired to a Case structure s selector terminal the enum strings become the Case structure s selector labels This construct is called an enumerated Case structure Chapter 4 Block Diagram and Chapter 9 Documentation discuss this in more detail When using a text ring list box or enum over a string control the string of the selected item can be accessed on the diagram by indexing the control s Stringsl property This is shown in Figure 3 13 Figure 3 13 The string selected from a text ring is indexed from the Strings property using the value read from the terminal Function fO DC ELE Strings poocooocoocona Function 0 Y DC Rule 3 37 Set the tabbing order and use the Key Focus property to aid navigation If you would like to guide the user through the GUI use a combination of tabbing order and the Key Focus pr
525. ut terminal In Figure 10 2E an unwired enumerated constant is identified by the Unused Code test It is left over from the previous version of the File Dialog function that had an additional input terminal for selecting the operation type In Figure 10 2F the Save button does not contain a control description flagging a VI Documentation failure The cluster wire in Figure 10 2G originates within the Sequence structure on the left passes through a tunnel and briefly runs underneath the Sequence structure before connecting to the While Loop s shift register This flags a Wires Under Objects failure Figure 10 2D The error out terminal of the File Dialog function is not wired flagging the Error Cluster Wired failure View full size image 323 PV Analyzer Hesiults Winder raga Nir pl A rer tig gb F the YI changed the snakes Che wrong object may be highlighted PEH B eo i d Pi i b ai i h i Fadunest y ie Sort by WI j Hh Rank Test Description Ci Sort by Tet i Lew ark Test The enor pipat of Hh Pile Disig Fumction E not ried Dn ordear bo faisa ehecha error handing you should uaa wire ha error GAA on block day a Figure 10 2E Case 1 of the Case structure contains an enumerated constant that is not wired to anything This flags an Unused Code failure View full size image E yi Analyzer Kesulis imdi Testing Errors Summary Doublesdick a faire ocourrenoe bo highbght the Failure
526. vailable from the controls indicators and VI as the user scrolls the mouse over them Additionally LabVIEW can open online documents of a variety of formats using a browser word processor or reader application Online documents are discussed in Section 9 4 Custom runtime menus are configured by selecting Edit Run Time Menu Menus are not appropriate with dialog VIs Instead provide Boolean command buttons for any required menu selections This includes a Boolean for Help or which can launch the desired online documentation Also create shortcut menus for any actions related to a specific control Shortcut menus are created by selecting Advanced Run Time Shortcut Menu Edit from a control s shortcut menu Figure 9 l1A contains the front panel for niDMM Configure Measurement Digits VI a subVI that configures a modular instrument The Context Help window is open with the VI description icon and terminals visible The control labels are succinct and intuitive Only the error in cluster contains its default value in parentheses The default value and range for Resolution in Digits and Range depend on the value of Function and therefore cannot be specified within the control label However the default value of Function is known and should be included in its control label Default values and units in parentheses are particularly important with subVIs such as instrument drivers because they help the developer quickly decide wheth
527. veloped using a prior version of LabVIEW and a relaxed style convention We refactor this VI in incremental steps throughout this chapter using code reviews The VI Analyzer is configured to run a test utilizing custom settings recommended by Rules 10 5 and 10 6 In Figure 10 2B the VI Analyzer s Results window displays the test results The results are listed in top down order of priority with denoting the high ranking test failures no symbol denoting the normal ranking test failures and i denoting the low ranking test failures Figures 10 2C through 10 2J contain the detailed results of specific tests including all the failures identified within case 1 of the Case structure along with modifications that eliminate the failures Figure 10 2A Front panel and block diagram for a primitive version of Torque Hysteresis VI prior to performing a code review View full size image 320 t Tongue Hyiteresis Bad Style vi Front Panel 300 Eni ey a Fat aed Nal al ad fuel at E at eet Sat Peat Rade et Ba aN al ad ad al Paec atisies Figure 10 2B The VI Analyzer Results Window displays the test results in top down order of priority with denoting the high rank test failures no symbol for medium rank test failures and i denoting the low rank test failures 321 gt Vi Analyzer Results Window Test Results Testing Errors Summary Double click a Failure occurrence to highlight the Failure object in the VI IF the
528. ven State Machine but maximizes the use of the Event structure The primary advantage is that multiple Event structures can be configured to respond to the same user event whereas each queue has a unique reference for enqueuing and dequeuing This capability allows an application to have multiple 265 Event Machines that all respond to a global command similar to notifiers However there are several limitations of the Event Machine design pattern First it requires more maintenance than the Event Driven State Machine To add or remove an application state both the user event data and user event refnum clusters must be modified to add or remove the corresponding elements Also corresponding calls to Create User Event must be added or removed which also results in a new event registration refnum This is in addition to the changes to the Event structure configuration which is similar to maintaining an enumerated Case structure Second the Event Machine s event buffer is less functional than the Event Driven State Machine s queue Using a queue you can preview dequeue or even flush queue elements without firing the corresponding state cases Additionally you can add new elements to the front as well as the back of the queue I use this feature to enqueue the Shutdown state at the front of the queue whenever an immediate shutdown of the application is required The Event Machine buffers events ina strict first in
529. vention within an organization In practice not everyone will agree with all the style rules and even those who agree will not necessarily apply them consistently or effectively without a system of checks and balances in place Code reviews are systematic methods of reviewing source code quality utilizing one or more independent perspectives or reference points Code reviews have three primary benefits identifying problems exchanging knowledge and ideas that improve developer skill level and promoting and evolving standards across an organization Most importantly code reviews help ensure high quality software that is readable maintainable and robust A secondary benefit of code reviews is that they introduce additional developers to your application This provides some depth in the resources that are familiar with the application and its implementation As a professional systems integrator I find that resource options are often the difference between good customer service and excellent customer service When changes are required in the future it is very comforting to know that the code is high quality and that you have multiple resource options for further development The chances of satisfying a new change request without interrupting other project schedules are dramatically increased Rule 10 1 Enforce your organization s style convention using code reviews Rule 10 2 Utilize a combination of self reviews and pee
530. ver Simple Error Handler VI 7 8 Implement an error log file for application deployment 7 9 Suppress dialog error reporting for unattended or remote operation 359 7 1 All VIs must trap and report the errors returned from error terminals 7 10 Avoid subVIs with built in error reporting 7 11 Maintain user defined error codes within an XML file 7 12 Use negative codes for I 0 device errors and positive codes for warnings 7 13 Skip most subVI diagrams on error using an Error Case Structure 7 14 Use unwired defaults over constants for output tunnels of Error case 7 15 Use the SubVI with Error Handling template 7 16 Error trapping is required for nodes that perform I O operations recommended for nodes that contain error terminals and optional for diagrams that do not contain nodes with error terminals 7 17 Tunnel the error cluster near the bottom of structures 7 18 Leave automatic error handling disabled Chapter 8 8 1 Use multiple criteria for the loop condition 8 2 Use a Timed Loop for highly precise or complex timing and use a While Loop otherwise Bea Include a delay within continuous While Loops 8 4 Avoid polling GUI objects 8 5 Use the Value Change event for most GUI controls 8 6 Place control terminals within their Value Change event case 8 7 Resize the Event Data Node to hide unused terminals 8 8 Avoid continuous timeout events 8 9 Use a state machine design pattern in most VIs of medium or greater complexi
531. ves utilizing Control References to obtain the images of the controls as they appear on the front panel instead of the terminals and developing a custom routine using VI Server to create the documentation This approach is described in an article that appeared in LabVIEW Technical Resource Volume 10 Number 3 The important point is to be sure to thoroughly document all of your VIs and these automated techniques for generating documentation are available I recommend always providing a user manual or other form of online documentation accessible from the Help menu of the top level VI for all deployed applications You will be one step ahead of your customers 9 5 Examples As discussed in the chapter s introduction readability maintainability and ease of use are common goals of the style rules and examples provided throughout the book Due to the close relationship of good style readable source code and documentation there are related examples in every chapter A few more examples provided here include SubVI from Selection VI Filter Test VI Meticulous Control Descriptions and Temperature Profile Illustration Figure 9 6 contains the Context Help window for SubVI from Selection VI previously discussed in Chapters 3 4 and 5 As discussed in the previous chapters these VIs never conform to good style without significant manual cleanup In addition to inappropriate terminal labels and icons there is no VI description These ar
532. vi Array of Humerics pe me HHHH HHHH ETE Empty Top To Variant Empty Array vl LF PEPE EEEE SA ETTE ETE Empty Fiddle Empty Array ne ee a ER To ETE Empty Bottom The OpenG Variant Configuration File VIs really showcase the power and flexibility of the variant data type These VIs read and write data to text files in the standard configuration setting INI file format They use clusters converted to variants to read or write multiple keys in one subVI call Figure 6 14 illustrates the use of Write Section Cluster VI The Motion Parameters cluster is converted to variant and wired to the Section Formatted Cluster input terminal Write Section Cluster VI uses the data name data type and data information that the variant data type provides to 198 create a section of an INI file The elements of the cluster correspond to the section s keys and the name of the cluster is the default section name when the section terminal is unwired Figure 6 14A shows the source code in which five keys are written using one call to Write Section Cluster VI and Figure 6 14B shows the resulting section of the INI file Additionally the OpenG Write INI Cluster VI populates an entire INI file containing multiple sections As shown in Figure 6 14C a nested cluster containing subclusters for each section is wired to the INI Formatted Cluster input terminal The resulting INI file is shown in Figure 6 14D In these examples the variant d
533. views are an essential technique for enforcing a style convention within an organization Rule 10 8 Perform at least one peer review per project Rule 10 9 Involve the following people in a peer review project manager lead developer experienced peer and an inexperienced peer Rule 10 10 Bring the requirements specification and style rules checklist to the peer review The optimum number and frequency of peer reviews vary widely Some factors to consider include the experience level of the developer or team application size and complexity and primary objectives of the peer review If you are part of a multideveloper organization always perform at least one peer review for each project Involve the project manager the lead developer and experienced as well as inexperienced developers Each brings different perspectives and each contributes and benefits differently The project manager understands the requirements from a functional perspective and can monitor how those requirements are being implemented This can help identify any functional limitations of a specific software design The lead developer is the software s primary architect and the direct beneficiary of the review An experienced peer who is familiar with the organization s style convention provides constructive feedback An inexperienced peer might contribute some new ideas and question existing methods all while learning good
534. vit Front Panel fenplate Seer sayrnen zee Seer To AE AORT t tt ee BA j j f es oes A MIS EF F r a z4 f P E am rT ao t i 2 TO CEL Lt i Ct EE f ory J ae a eee SS n mun n a ae S S eee ae ES ee eee S Pepe mor CeCe tee eee 1 Ti h hall ia E tate a i z ii E aa a 3J isa a E M a paa aera uaa se aea e ae aaa n a a m oee e me lm m m T od ae Re mi a 1 i i H i i j ea m h a Se ee a ae BHE 2 2 amp 0 ee I Ei i j Haag a F ia torte BERS R SSR SRS ERR REE ESR RRS Ree BES SE See RSaaes 4 H _ i Lg y i rr E s Ja z mas LETE Figure 8 6B The diagram consists of an Event structure within a While Loop containing shift registers and a shutdown routine outside the loop The control terminals are in their respective Value Change event cases View full size image Note that three separate events trigger an application shutdown In Figure 8 6C the Quit Value Change Panel Close and Application Instance Close event cases are shown All three events display a dialog prompting the user to confirm and then stop the loop if confirmed After the loop terminates a shutdown routine outside of the While Loop runs This prevents three separate instances of the same routine to appear in three different event cases Configure the shutdown routine to c
535. w dimension comprises the bottom two thirds of the icon and two words appear across the top third Black and gray are used for the screw and text and a bright yellow 136 background is used for contrast Not the preferred color scheme for the panel of a GUI VI but great for an icon The combination of text and graphics is important because they reinforce each other A glyph can be incomplete or subject to interpretation and succinct text is an excellent enhancement Figure 5 3 The icon of Find Screw Ends VI combines a glyph of a screw with two words created using 8 point small fonts Icon Editor File Edit Help Copy From Black amp White 16 Colors 16 Colors SCREW EWDS am H Show Terminals 256 Colors 56 Colors Rule 5 9 Choose a unified style for related VIs If you have a collection of VIs that work together to perform a specific task such as data logging or instrument control choose an icon convention that associates those VIs One simple approach is to choose a common background color glyph and font and vary the text to signify the unique meaning of each VI For example Figure 5 4 contains several sets of VIs each with its own icon convention The pressure and flow instrument icons contain a glyph of the manufacturer s logo for each instrument and a unique word across the bottom describing each VI The configuration file icons contain
536. xample an enumeration consists of two components an unsigned integer and a set of string labels The efficiency of the unsigned integer component depends on its representation either 8 bit 16 bit or 32 bit The efficiency of the string component depends on the quantity and lengths of the labels These properties are configured by the developer Likewise the more complicated data types listed near the bottom of the table such as Dynamic and Variant depend on the data and attributes they contain Choose the control types and data types for intuitive operation and memory efficiency Table 6 3 presents a matrix containing the control types and supported data types The control types are organized along the top horizontal heading in left to right order of operational and data simplicity similar to Table 6 1 turned sideways The data types are listed along the left vertical heading in top down order of memory efficiency similar to Table 6 2 The cells of the table indicate the compatibility of each control type with each data type Use this table to optimize the control and data type selections in order of operational simplicity and memory efficiency Consider the type of data that is required and choose the control type that is best suited to represent that data Begin on the top left and evaluate each control type from left to right If your data element has two selections that are opposites choose a Boolean control If you need to provide a r
537. xample in Figure 6 1 An instrument driver for an oscilloscope acquires and processes a waveform through a hierarchy of subVIs as shown in Figure 6 l1A The lowest level VI is Get Raw 16 Waveform VI shown in Figure 6 1B This VI reads the raw data as a binary string converts the data to an array of unsigned byte integers combines every two successive integers into a single 16 bit signed integer and returns the raw data as an array of 16 bit signed integers 166 Because there are normally 2 bytes per sample 16 bit signed integer is the simplest and most memory efficient numeric data type for representing this data per Table 6 3 Figure 6 1A A section of the VI Hierarchy window for a top level VI that calls an instrument driver to acquire a waveform from an oscilloscope The instrument driver uses a call chain of three subVIs including read fetch and get Top Level VI Read Waveform VI Fetch Waveform VI Get Raw 116 Waveform VI Figure 6 1B Get Raw I16 Waveform VI returns an array of 16 bit signed integers This is the most memory efficient data type for storing the waveform in this VI View full size image abc U8 116 In Figure 6 1C Fetch Waveform VI converts the raw data returned from Get Raw Waveform VI into a scaled array via several arithmetic functions Because the offsets and scaling factors are double precision floating point numeric the array of raw data must be coerced to double precision floating p
538. y distinguishes an error All nonzero codes are warnings if status FALSE However it is good practice to follow the well established sign convention for I O devices 7 2 SubVI Error Handling Most LabVIEW functions and VIs with error in terminals evaluate the status Boolean and skip their code if the value is TRUE This aids the application s capability to recover from and report errors in a fast and efficient manner For example an I O function such as VISA Read may wait 10 seconds for an instrument to respond to a query before returning a timeout error to the calling application An error of this magnitude will likely degrade the application s performance If similar instrument queries execute with similar results the application appears to be hung up or unstable Trapping the error and propagating it to all subsequent nodes in the error chain prevents this from happening Instead all subsequent instrument queries that receive the error are skipped thereby accelerating the propagation of the error to the reporting routine and aiding the application in a graceful recovery Rule 7 13 Skip most subVI diagrams on error using an Error Case Structure It is desirable for subVIs to mimic the error handling behavior of the LabVIEW functions and VIs Specifically most subVIs should skip their diagrams when an error is received at the error in terminal This is implemented using an Error Case Structure as follows First place t
539. yourself reliability and ease of use are extremely important Moreover it is important to recognize that reliability is the developer s responsibility Never rely on the user to operate the GUI in the manner that you intend Never present the user with options that could cause the software or hardware to malfunction For each GUI VI consider what combination of operator actions could cause a problem For example if the GUI VI contains a button for resetting a set of instruments what would happen if the user clicked this button during a test or measurement sequence Unless the control is disabled your application or one or more of the instruments might hang up I would like to make a few points one philosophical and the others practical On the philosophical side it should never be the user s responsibility to avoid certain control values or operations It is the developer s responsibility to ensure that the user cannot cause a malfunction inadvertently LabVIEW provides the developer with 80 all the tools needed to develop a reliable GUI For the previous example the button for resetting the instruments should not be active during the measurement sequence Rule 3 33 Limit the number of controls that are visible and enabled at any one time In general limit the controls that are available to the user to controls that are relevant and will not trigger any undesirable actions Consider the different operating
540. zed shift registers and the loop s conditional terminal is wired to a Boolean constant that forces it to stop after only one iteration The loop s sole purpose is to store global data within the shift registers The subVI is applied to each location of an application requiring access to the data similar to a global variable A recognizable graphical symbol such as a public sign or traffic signal Any VI with a user viewable panel including top level and dialog VIs Immediate SubVI Design pattern that consists of the nodes that comprise the subVI and error trapping There are no continuous loops dialog windows or GUI panels Rather an immediate subVI executes its code straight through completion in the order in which the error cluster propagates through the nodes on the diagram industrial GUI VI VI with a front panel designed for multiple users to interface with industrial equipment Industrial GUI VIs complement the industrial application and associated equipment regardless of the computing platform upon which they are installed instrument prefix I 0 Very succinct text used to represent an instrument s manufacturer and model number in an instrument driver library The first two or three characters of the prefix identify the manufacturer and the following four or five characters identify the instrument model or family The resulting six to eight character acronym is used in the VI or library name and appears on t

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