Home

Control of Stand-Alone Instruments

1. error cut ee Py l ig ta JRG VISA Open Status Contig Measurement Take Datawi Close SRQ v1 SRQ vi Config vi Sample Count Return to the front panel and arrange the object there as desired Save your work 4 Data Sampler vi Front Panel E X File Edit View Project Operate Tools Window Help tt 15pt Application Font i 85 sie E S Function Parameters Data Function J DC Voltage Voltage Range 100 mV Resistance Range J100 ohm Resolution J 4 1 2 Digits Autozero Sample Count ry error out A0 4 m Run Data Sampler with various choices of front panel settings and then pat yourself on the back for a job well done Now that you know some of what goes into writing an instrument driver here s some very good news In many cases the LabVIEW instrument driver you will need for a particular instrument in your laboratory has already been written and is available for your use free of charge National Instruments provides an extensive library of down loadable instrument drivers at Attp www natinst com idnet You can also access this resource within LabVIEW by selecting Tools gt gt Instrumentation gt gt Find Instrument 485 9 9 2008 9 03 49 PM Chapter 13 Control of Stand Alone Instruments Drivers Most of these drivers are written using VISA icons and so by using the interface appropriate VISA resource name for your instrument can be used to com
2. aoe Command Response VISA resource name te GPIBO 22 INSTR error out General Settings Timeout Value byte count Return to the front panel of Simple VISA Query Long Delay With the command to perform 100 samples programmed into Command run the VI About 33 seconds later you should see something like the following front panel Note that the delimiter used by the Agilent 34401A to separate neighboring data values is a comma 457 9 9 2008 9 03 37 PM Chapter 13 Control of Stand Alone Instruments 29 Simple VISA Query Long Delay vi Front Panel File Edit View Project Operate Tools Window Help it et Appicaion Fon 2o sa Command Response CONF VOLT DC 10 0 00001 SAMP COUN 100 INIT FETC 5 13807650E 00 5 13807520E 00 5 13807650E 00 5 13807010E 00 5 13807260E 00 5 13808030E 00 5 13808160E 00 5 13807770E 00 5 13808030E 00 5 13807520E 00 5 13808160E 00 5 13807770E 00 5 13806750E 00 5 13807650E 00 5 13807260E 00 5 13807650E 00 5 13807520E 00 5 13807520E 00 5 13807260E 00 5 13807260E 00 5 13806880E 00 5 13807010E 00 5 13807260E 00 5 13807010E 00 5 13807390E 00 5 13807900E 00 5 13807390E 00 5 13807770E 00 5 13807650E 00 5 13807900E 00 5 13807900E 00 5 13807390E 00 5 13808030E 00 5 13807900E 00 5 13807520E 00 5 13807390E 00 5 13807390E 00 5 13807260E 00 5 13807390E 00 5 13807390E 00 5 13807650E 00 5 13807520E 00 5 13807140E 00 5 138
3. 13 Essick Chap13 indd 461 MEASUREMENT VI BASED ON THE SERIAL POLL METHOD Status Contig Serial Poll wi VISA resource name VISA resource name out read buffer error in no error error out Eleje Here s how the VI works assuming that the instrument referenced by VISA resource name is the Agilent 34401 A multimeter Within the chain of VISA icons VISA Clear found in Functions gt gt Instrument I O gt gt GPIB executes first The Help Window for this icon is shown below Context Help VISA Clear VISA resource name VISA resource name out error in no error error out Clears the input and output buffers of the device Else Although not an absolute necessity for inclusion in Status Config Serial Poll this VI per forms the precautionary action of clearing the Agilent 34401A VISA Clear instructs the multimeter to abort all measurements in progress disable its triggering circuitry clear its interface related output buffer and prepare to accept a new command string Next VISA Write sends the concatenated command string CLS ESE 1 SRE 0 OPC to configure the Agilent 34401A for status reporting using the Serial Poll Method Note that since the component strings are all IEEE 488 2 common commands leading colons are not required in the concatenation In this sequence of commons CZS clears the contents of the SESR and SBR As described in the beginning of this chapter ESE 1 enables the SESR s
4. 5 ESB Event Status Bit An event associated with an enabled SESR bit has occurred 4 MAV Message Available Data is available in the instrument s output buffer 3 0 May be defined for use by instrument manufacturer An instrument can be configured to assert a Service Request SRQ which is a digi tal signal carried on a dedicated wire within the 24 wire GPIB cable in response to either of two events an event detected by the Standard Event Status Register or the presence of previously requested data in the instrument s output buffer that 1s the assertion of 432 Esek ndi 432 9 9 2008 9 03 27 PM 13 Essick Chap13 indd 433 STATUS REPORTING the ESB or MAV bit respectively This configuration process 1s accomplished on IEEE 488 2 instruments by using the SRE Service Request Enable command For example if you wish an event detection by the SESR to trigger a request for service by the instru ment initialize the instrument by writing the ASCII command SRE 32 to the instrument Since 32 00100000 the setting of the SBR s fifth ESB bit will then be the criterion for the instrument asserting a SRQ If instead you wish the presence of data in the output buffer signaled by the MAV bit being set to trigger a SRQ then write SRE 16 Finally SRE 0 will disable the instrument ability to assert a SRQ The relationship between the Standard Event Status Register the instrument s out put buffer and the Status
5. less times by the GPIB device so that its status can be continuously monitored While effective the Serial Poll Method is rather inefficient because of its excessive use of the interface bus and processor time 465 9 9 2008 9 03 41 PM Chapter 13 Control of Stand Alone Instruments 13 14 MEASUREMENT VI BASED ON THE SERVICE REQUEST METHOD The Service Request Method provides status reporting with a minimum of interface bus activity To configure the 34401A for status reporting using the Service Request Method open Status Config Serial Poll then create Status Config SRO using Save As and save it in YourName Chapter 13 The front panel and terminal assignments can remain as is but the icon should be redesigned as shown here Ja status Config SRQ vi Front Panel File Edit View Project Operate Tools Window Help Contig n enci VISA resource name VISA resource name out i en Say Sey See Sn Sa Sa Sen ae aS ee ee ee nay Panay aay pe VEE SI P j kaa read buffer error in no error error out mh f N O an Y Sr Pe status code code status 2 JE Status Config SRQ vi VISA resource name VISA resource name out read butter error in no error i error ou Only two modifications of the block diagram are needed First by changing SRE 0 to SRE 32 in the command string sent to the instrument the Agilent 34401A will assert the 466 13 Besick lt Chapia sada 466 9 9 2008
6. 10 0 00001 SAMP COUN 100 INIT OPC FETC m error out empar status code JE Source Switch to the block diagram and modify it as shown next 467 9 9 2008 9 03 42 PM Chapter 13 Control of Stand Alone Instruments sacha Command Response VISA resource name timeout I GPIBO 22 INSTR 60000 event type VISA Open Status Config VISA Write WaitforRQSvi VISA Read VISA Disable Event VISA Close 2000 ROJ wi byte count Here VISA Disable Event found in Functions gt gt Instrument I O gt gt VISA gt gt VISA Advanced gt gt Event Handling must be included in order disable VISA servicing of SRQ events before the VISA session is closed Wait for RQS vi also found in Functions gt gt Instrument I O gt gt VISA gt gt VISA Advanced gt gt Event Handling Help Window shown below sits idly until the instru ment denoted by VISA resource name asserts a SRQ However there is a limit to the patience of this icon It will only wait up to a total time of timeout with a default value of 25000 ms 25 seconds Because our measurement requires over 33 seconds a constant larger than 33 000 must be wired to the timeout input of Wait for RQS vi as shown in the above diagram Context Help Wait for ROS vi timeout 25000 ms VISA resource name VISA resource name out status byte error in mo error error out Waits for a service request from a device Before you ca
7. 7 A dialog box appears where we are told that a timeout expired at VISA Read before the requested operation 1 e take 100 data samples could be completed 455 9 9 2008 9 03 36 PM Chapter 13 Control of Stand Alone Instruments l Explain Error Error Cluster Explanation Status Code Error 1073807339 occurred at VISA Read in Simple VISA Query Long Delay vi x 1073807339 Hee Code Possible reason s Ox BFFFOOLS H VISA Hex OxBFFFO0L5 Timeout expired before Source operation completed VISA Read in Simple VISA Query Long Delay After some head scratching and checking of the Agilent 34401A user manual the fol lowing explanation then emerges for the error we observed when running Simple VISA Query Long Delay Simply stated voltage sampling takes time In its default configura tion it takes the Agilent 34401A multimeter 10 power line cycles PLC for each volt age sample Additionally the Agilent 34401A has an autozero feature which is enabled by default This feature operates as follows After each voltage measurement the mul timeter internally disconnects the input signal and takes a zero reading The instrument then subtracts the zero reading from the preceding measured value to prevent offset voltages in the multimeter s internal circuitry from affecting measurement accuracy Since the zero reading also takes 10 PLC each complete voltage sample by the multim eter takes 20 PLC Assum
8. Byte Register along with the common commands that con figure and query each is illustrated in the following diagram Standard Event Event Register Enable OR T ESR ESE lt value gt ESE O NU PRAAN Status Byte Register Enable Request Service Event Status Bit Message Available OR Read STB SRE lt value gt STB SRE Output Buffer As an alternative to the use of the SRQ serial polling is a common method for determin ing the status of an instrument In a serial poll process the interface bus queries an instru ment and the instrument responds by returning the value of the bits in its Status Byte Register A serial poll is easily accomplished in LabVIEW using VISA Read STB found in Functions gt gt Instrument I O gt gt VISA This icon s Help Window is shown here 433 9 9 2008 9 03 27 PM Chapter 13 Control of Stand Alone Instruments Context Help VISA Read STB VISA resource name VISA resource name out status byte error in no error error out Reads a service request status byte byte from the message based device specified by VISA resource name Els 13 5 DEVICE SPECIFIC COMMANDS Finally each stand alone instrument is designed for a specialized purpose and has its own idiosyncratic methods for accomplishing its objectives Thus every programmable instrument comes with a set of device specific commands that allow the user to control its functions remotely and to tr
9. INSTR error out ASA Lisa pr o t abe abe s F eae eee ee And E VISA Open VISA Write VISA Read VISA Close byte count Return to the front panel arrange the objects as you wish and then save your work 4 Simple VISA Query vi Front Panel File Edit View Project Operate Tool Window Help it 35pt Application Font orior Command Respo nse Enter IDN into Command then run your VI If all goes well Response will display the instrument s identification string upon completion of the VI execution as shown 443 9 9 2008 9 03 32 PM Chapter 13 Control of Stand Alone Instruments if Simple VISA Query vi Front Panel File Edit View Project Operate Tools Window Help it 35pt Application Font gorio RA Command Response HEWLETT PACKARD 34401A4 0 11 5 2 error out i i i ah T e i istatus code ff Eo Simple VISA Query will leave the multimeter in remote mode with its triggering circuitry idled You can return to local mode which continuously triggers measurements by pressing the instrument s front panel SHIFT LOCAL key 13 9 MESSAGE TERMINATION At the conclusion of a message transfer process some method must be used to sig nal that the complete message has been passed The IEEE 488 2 standard appoints the ASCII LF line feed also called new line as its special end of string EOS character That is when receiving a mess
10. Interface Bus 13 1 THE VISA SESSION When using VISA icons to facilitate message based communication between a com puter and a particular stand alone instrument the instrument is termed a VISA resource and the communication activity is called a VISA session To query a VISA resource 1 e send it a command then receive back its response the required VISA session consists of the following four steps open the session write the command message to the resource read the response from the resource close the session In Functions gt gt Instrument I O gt gt VISA and its subpalette VISA Advanced the following four icons are available to perform the four given steps VISA Open VISA Write VISA Read and VISA Close To understand how to wire these four icons together to query an instru ment we will first briefly describe the function of each individual icon The Help Window for VISA Open is shown in the following illustration The job of this icon is to begin a VISA session between your computer and the resource defined at 426 SEs Chapi ndd 426 9 9 2008 9 03 26 PM 13 Essick Chap13 indd 427 THE VISA SESSION its VISA resource name input The VISA resource name consists of text that specifies the interface type being used e g GPIB or USB the address of the resource a number we ll discuss in a few minutes and the resource type For our work the resource type will be a stand alone instrument denoted by JNSTR To pass t
11. P To verify that the instrument is indeed properly communicating over the GPIB right click on the Instrument 0 folder and select Communicate with Instrument Alternately you can click on the Communicate with Instrument button near the top of the window 439 9 9 2008 9 03 28 PM Chapter 13 440 13 Essick Chap13 indd 440 Control of Stand Alone Instruments Fi x Instrument 0 Measurement amp Automation Explorer File Edit View Tools Help Configuration ta Communicate with Instrument S4 Interactive Control amp NI Spy gt 152 Hide Help 39 w Kis FO gt pack ral al t a oe oe GA cpieo 22 INsTR a 5g Devices and Interfaces GPIB jE GPIBO PCI GPIB Device Type GPIB Instrument d Instrument E a A E Basics JG NI DAQr m Communicate with Instrument N PX PXI Syste 2 Interactive Control System eke Serial amp G gt Communicate all Scales in ii with my 6 Software Device Status aaa MI Drivers This device is working properly r9 y 4 amp Remote Systems Attributes 6 VISA Properties Launches the NI 488 2 Communicator utility An interactive dialog box will appear Here after typing an ASCII message in the Send String box a mouse click on the Query button will carry out a write then read action That is the message in Send String will be written over the GPIB to the selected instrument and then the instrument s ASCII r
12. functions properly For example with Function Voltage Range and Resolution equal to DC Voltage 10 V and 6 1 2 Digits respec tively Range and Resolution should equal 0 0 and 0 00001 Next open a blank VI and save it under the name Command String in YourName Chapter 13 Switch to the block diagram and write the following code which constructs the desired ASCH command string The Function Parameters control cluster and output string string indicator are made using the autocreation feature in pop up menus 476 13 Besick lt Chapiasndd 476 9 9 2008 9 03 45 PM 13 Essick Chap13 indd 477 CREATING AN INSTRUMENT DRIVER VOLT DBC VOLT AC RES Pick Line a Function Format fol 26 7 f Value H T E I cu m Commanel Range cg ae z a p Range ho Fea k Function Range and Unbundle By Name Farameters Resolution Decoder wi This diagram constructs the desired command string in a three step process First all three possible commands begin with the keyword CONF so this sequence of ASCII charac ters 1s wired to the string input of Pick Line found in Functions gt gt Programming gt gt String gt gt Additional String Functions with its Help Window given below The value of the line index input an integer given by the front panel Function Enum control then selects which of the three possible lines programmed into the String Constant wired to multi line string is to be appended to CONF C
13. i i i i i bo i i 4 i 4 i i i i i i i status code code status Ei z 0 0 S0UrCE S0UTCE 483 iS Tsik Chapi ada 483 9 9 2008 9 03 49 PM Chapter 13 Control of Stand Alone Instruments Context Help Take Data vi VISA resource name VISA resource name out Data error in no error error out timeout 60000 VISA resource name VISA Write Wait for ROS vi VISA Read ia error in mo error ggg error out byte count Note Take Data cannot be run independently without generating an error However if you first run one of the other VIs that you have written can you figure out which one then Take Data can be run successfully 13 16 USING THE INSTRUMENT DRIVER TO WRITE AN APPLICATION PROGRAM Ultimately the merit of an instrument driver is measured by the ease with which you can use it to write an application program to fulfill some specialized need in your labo ratory work Let s quickly write an application program called Data Sampler that can be configured to take a multisample voltage or resistance measurement With VISA Query SRQ open use File gt gt Save As to create Data Sampler and save it in Your Name Chapter 13 Rewrite the block diagram using your modular driver software as shown below 484 13 Bssick Chapi3andd 484 e 9 9 2008 9 03 49 PM 13 Essick Chap13 indd 485 USING THE INSTRUMENT DRIVER TO WRITE AN APPLICATION PROGRAM VISA resource name Ie GPIBO 22 INSTR
14. minus output of the CJC to the HI and LO Voltage Inputs of the Agilent 34401A Multimeter Then write a program called Thermocouple Thermometer VISA that every 250 ms until a Stop Button is clicked reads the thermocouple voltage converts this value to the corresponding temperature in Celsius and then display this temperature in a front panel indicator To convert the thermocouple voltage to its corresponding temperature use Convert Thermocouple Reading vi which is found in Proogramming gt gt Numeric gt gt Scaling with its CJC Voltage input wired to O the CJC Sensor and Type of Excitation inputs can be left unwired Program Thermocouple Type for your particular type of thermocouple e g T Run Thermocouple Thermometer VISA and use it to measure room temperature as well as the temperature of your skin As written in this chapter Serial Poll is flawed in that if the bit being monitored in the Status Byte Register is never set this VI will loop endlessly With Serial Pall open use Save As to create a new VI called Serial Poll with Timeout Then modity the block diagram so that if the bit being monitored is not set within 10 seconds the While Loop is stopped When LabVIEW is installed on your computer a driver for the Agilent 34401A Multimeter is included This driver is found in Functions gt gt Instrument I O gt gt Instrument Drivers gt gt Agilent 34401 Use the icons from this built in driver to write a program cal
15. municate over various interface buses RS 232 GPIB Ethernet and USB LabVIEW itself comes equipped with the VISA driver for the Agilent 34401A Digital Multimeter in Functions gt gt Instrument I O gt gt Instrument Drivers gt gt Agilent 34401 Take a look at some of these subVIs and see if you can decipher them Finally a useful Agilent 34401A instrument driver utility would perform the follow ing task Take the instrument s Data string data samples delimited by commas and the string terminated by a LF character and convert it to a numeric array and a spreadsheet format One manifestation of that utility called Reformat Data String is shown next On the front panel the string control and string indicator have been resized and scrollbars have been activated by selecting Visible Items gt gt Vertical Scrollbar in the pop up menus ia Reformat Data String vi Front Panel File Edit View Project Operate Jools Window Help gt e 1 15pt Application Font l5pt Application 15pt Application Font Pra ax aeons Data Spreadsheet String z i i i i i i i i i i 4 i io Numeric Array Joe A Context Help Reformat Data String vi Data a Spreadsheet String Format Numeric Array EEA 486 13 Essick Chap13 indd 486 D 9 9 2008 9 03 50 PM 13 Essick Chap13 indd 487
16. nse CONF VOLT DC 10 0 00001 INITs FETC l 5 15804200E 00 Numeric Voltage 513804 error out 13 12 SYNCHRONIZATION METHODS Although most ASCII commands are completed quickly after being received by a pro grammable instrument some commands start a process that requires a significant amount 452 13 Essick Chap13 indd 452 D 9 9 2008 9 03 35 PM 13 Essick Chap13 indd 453 SYNCHRONIZATION METHODS of time such as acquiring a large amount of data or moving an object from Point A to Point B The time required for such processes must be taken into account when writing a data acquisition program else upon execution the program may request data before it is available induce undesirable motion or cause some other chaotic outcome As an example in its default configuration the Agilent 34401 A multimeter acquires one data sample after receipt of the JN Tiate command then stores this measured value in its internal memory However through use of the SAMPle COUNt lt Space gt lt Value gt command the multimeter can be instructed to take and store multiple data samples upon receiving N Tiate The Agilent 34401A is configured to acquire 100 DC voltage samples with 6 2 digit resolution via the following concatenated string of commands CONF VOLT DC lt Space gt 10 0 00001 SAMP COUNK lt Space gt 100 INIT FETC The FETCh command will load the 100 acquired samples from the multimeter s inter nal memory whic
17. root form its subsystem So for example to configure the Agilent 34401A Digital Multimeter to measure a DC voltage whose value is expected to fall within the range of 10 Volts an action within its SENSe subsystem the appropriate command is as follows SENSe VOLTage DC RANGe 10 434 Esok Chapi nd 434 9 9 2008 9 03 27 PM 13 Essick Chap13 indd 435 DEVICE SPECIFIC COMMANDS Here SENSe is the root keyword and colons represent the descent to the lower level VOLTage then DC then lowest level RANGe keywords Finally 0 is a parameter asso ciated with RANGe While the full command mnemonic given here can be sent to the instrument it is only absolutely necessary to send the capitalized characters In 1990 aconsortium of equipment manufacturers defined the Standard Commands for Programmable Instruments SCPI in an effort to standardize the device specific command sets of computer controlled instrumentation While this standard has not been universally adopted it is not uncommon to discover that your post 1990 instru ment is SCPI compliant As a means of categorizing generally applicable command groups the SCPI standard posits the following model for a generic programmable instrument An instrument that performs measurements on an input signal is assumed to have the root functions shown in the next diagram Here for example SENSe includes any action involved in the actual conversion of an incoming signal to internal data such a
18. status registers you can tell what events have transpired The Standard Event Status Register which is schematically shown here records eight types of events that can occur within a data taking instrument Standard Event Status Register SESR 7 6 5 4 3 2 1 0 PON URQ CME EXE DDE QYE RQC OPC The eight events associated with the eight bits ofthe SESR are described in the following table In our work the OPC bit will be most useful Table 13 2 Eight SESR Events BIT ASSOCIATED EVENTS OF SESR 7 MSB PON Power On Instrument was powered off and on since the last time the event register was read or cleared 6 URQ User Request Front panel button was pressed 5 CME Command Error Instrument recetved a command with improper syntax EXE Execution Error Error occurred while instrument was executing a command DDE Device Error Instrument is malfunctioning 2 QYE Query Error Attempt was made to read the instrument s output buffer when no data was present or a new command was received before previously requested data had been read from the output buffer 1 RQC Request Control Instrument requests to be controller 0 LSB OPC Operation Complete All commands prior to and including an OPC command have been executed 431 9 9 2008 9 03 27 PM Chapter 13 Control of Stand Alone Instruments The SESR exists as an event signaling tool for you to use in your programs However this status register completely lac
19. the ASCII response string consisting of a maximum of byte count number of bytes and outputs this string at its read buffer terminal This string typically contains the results of a data taking measure ment performed by the instrument Additionally the VISA resource name is available at the VISA resource name out output terminal VISA Read VISA resource name VISA resource name out byte count read buffer error in mo error eturn count Error out Reads the specified number of bytes from the device or interface specified by VISA resource name and returns the data in read buffer Else Finally VISA Close s Help Window is given below This icon closes the VISA ses sion specified at its VISA resource name input VISA Close VISA resource name error in no error error out Closes a device session or event object specified by VISA resource name Else Note that all four of these VISA icons include error reporting via an error cluster which is appears at the error in and error out terminals The four step VISA session to query an instrument is accomplished by wiring these four VISA icons together as follows 428 iS Esik Chapi nda 428 9 9 2008 9 03 26 PM 13 Essick Chap13 indd 429 THE IEEE 488 2 STANDARD read butter VISA resource name write buffer le GPIBO 22 INSTR IDM ja abc error out fed on isd oo n g abe abow kK Ez a Se ee R fee _ _ F VISA Open VISA Write VISA
20. to create an Array Constant and label it Voltage Ranges Next program the index 0 through index 4 elements of this Array Constant as 0 1 0 10 0 100 0 and 1000 0 respec tively The Array Constant then will serve as a look up table of the multimeter s allowed voltage ranges given as double precision floating point numbers Complete the code for the DC Voltage case shown below Here the integer associated with a selected Voltage Range on the front panel Enum control provides the index of the desired look up table element This element is then output by Index Array Function Parameters Voltage Ranges Inclex Array 4 1000 0 Clone the Voltage Ranges Array Constant mouse click while holding down lt Ctrl gt and place the copy somewhere on the block diagram Then switch to the AC Voltage case and using your cloned Voltage Ranges write the code shown 473 14 Besick Chapiaindd 473 9 9 2008 9 03 44 PM Chapter 13 Control of Stand Alone Instruments Function Parameters Voltage Ranges Index Array 4 1000 0 Finally switch to the Resistance case and program it as shown Here the index 0 through index 6 elements of the Resistance Ranges Array Constant are 7 0E2 1 0E3 1 0E4 1 0E5 1 0E6 1 0E7 1 0E8 respectively Function Parameters CEETTITILILIELELELLIL LEE ttt Fesistance Range Add a second Case Structure and complete the diagram as shown next Remember to pop up on the Case Structure and sel
21. 07650E 00 5 13807010E 00 5 13807520E 00 5 13807260E 00 5 13807900E 00 5 13807260E 00 5 13807770E 00 5 13807260E 00 5 13807520E 00 5 13807770E 00 5 13807900E 00 5 13807520E 00 5 13807140E 00 5 13806880E 00 5 13808160E 00 5 13807770E 00 5 13807260E 00 5 13807650E 00 5 13807650E 00 5 13807520E 00 5 13807650E 00 error out 5 13807390E 00 5 13807260E 00 5 13806750E 00 5 13807260E 00 5 13807520E 00 5 13807650E 00 5 13808030E 00 5 13807390E 00 5 13807390E 00 5 13807390E 00 5 13807520E 00 5 13807010E 00 5 13807140E 00 5 13806620E 00 5 13807010E 00 5 13807260E 00 5 13807390E 00 5 13806880E 00 5 13806370E 00 5 13807010E 00 5 13807390E 00 5 13806750E 00 5 13807390E 00 5 13807770E 00 5 13807390E 00 5 13807390E 00 5 13807520E 00 5 13807650E 00 5 13807010E 00 5 13806880E 00 5 13807520E 00 5 13807390E 00 5 13806880E 00 5 13807390E 00 5 13807140E 00 5 13807010E 00 In the preceding example we found that with a detailed knowledge of the measure ment process being implemented it was possible to troubleshoot a malfunctioning VISA based VI Please note that lack of communication in particular the GPIB device not correctly knowing when the instrument s data will be available is the root problem that led to the malfunction Fortunately powerful tools exist that allow one to monitor the status of tasks being performed by a programmable instrument For IEEE 48
22. 13 Essick Chap13 indd 425 Chapter 13 ee Control of Stand Alone Instruments HARDWARE REQUIREMENTS FOR THIS CHAPTER To perform the exercises in this chapter you must have a National Instruments General Purpose Interface Bus GPIB device connected to your computer This device might be a PCI GPIB board plugged into a PCI expansion slot or a GPIB USB device attached to a USB connector A stand alone instrument equipped with an IEEE 488 2 compliant interface ideally the Agilent 34401A Digital Multimeter is also needed This instru ment is connected to a PCI GPIB board using a GPIB cable a GPIB USB device con nects directly to the instrument If while performing the chapter exercises the communication between the GPIB device and instrument breaks down e g caused by an accidental error in your pro gramming communication can often be restored either by turning the instrument off and then on again or by restarting your computer In previous chapters you have used LabVIEW software to transform a personal com puter connected to an appropriate National Instruments DAQ device into several handy laboratory instruments In particular you programmed this system to become a DC volt meter digital oscilloscope spectrum analyzer waveform generator and digital thermom eter Pause to consider the following tantalizing prospect Perhaps the only instrument required in a modern day laboratory is a DAQ device equipped computer controlled by
23. 8 2 compliant instruments these tools are the Standard Event Status Register SESR and Status Byte Register SBR that were discussed at the beginning of this chapter With proper use of the SESR and SBR many potential data taking glitches such as the one just experienced can be avoided The status reporting capabilities of the SESR and SBR can be employed in several ways We will explore two commonly used techniques the Serial Poll and Service Request Methods The core operation for both of these methods is the same the com pletion of an assigned task triggers the Operation Complete OPC bit in the Standard Event Status Register to be set which in turn sets the Event Status Bit ESB of the Status Byte Register In the Serial Poll Method the setting of ESB is detected by directly checking the Status Byte Register whose state is obtained by serial polling the instrument The com plete step by step process of this method is shown in the following diagram 458 13 Essick Chap13 indd 458 D 9 9 2008 9 03 37 PM 13 Essick Chap13 indd 459 MEASUREMENT VI BASED ON THE SERIAL POLL METHOD Serial Poll Method Standard Event Status Byte Event Register Enable Status Byte Register Enable 7 Ld 6 5 4 Message Available 3 4 2 Td 1 Request Control S 0 Operation Complete o ESE 1 Read STB SRE 0 Check Bit 5 In the Service Request Method the Status Byte Register is configured such that when its ESB is set the Request Servic
24. 9 03 41 PM 13 Essick Chap13 indd 467 MEASUREMENT VI BASED ON THE SERVICE REQUEST METHOD SRQ line when the SBR s fifth ESB bit is set The already present ESE 7 command configures the instrument to set the ESB in response to the SESR s OPC Operation Complete bit being set Second in order for VISA icons to detect service request SRQ events during this VISA session VISA Enable Event with Service Request wired to its event type input must be included in the diagram as shown VISA Enable Event is found in Functions gt gt Instrument I O gt gt VISA gt gt VISA Advanced gt gt Event Handling read buffer event type write buffer CLS ESE 1 SRE 32 OPC VISA resource name k F Lice Lise Per are Pier are h E e a E R M VISA Enable Event VISA Clear VISA Write VISA Read ke Do mE a error in no error error out byte count Save your work as you close this VI Open VISA Query Serial Poll then use Save As to create a new VI named VISA Query SRQ and store it in YourName Chapter 13 The front panel is fine as is T VISA Query SRQ vi Front Panel jk File Edit View Project Operate Tools Window Help tt 25Pt Application Font 85 ie E S Command Response en Cn ne ne OnE SO SOY Un OnE OY S CY SY ET VERE RENT SEES SEY VEIIEIT VERRY CORNY Cann On S S S VERY VOR OY VOR VOR USES VE SY VOR OS UE VOR VO VOY VE SY VO SO UR SO YS CONF VOLT DC
25. CONF command which obeys the conven tions of the SCPI language A comma separates the parameters from each other and a lt Space gt separates the mnemonic from the parameters Once the multimeter has been configured for the desired measurement function as described in the previous paragraph the data taking process is begun by sending the JN Tiate command from the TRIGger root subsystem Upon receipt of INIT the multimeter will acquire the requested voltage sample and then store this value in its internal memory Finally the FETCh command from the MEMory root subsystem instructs the instrument to transfer the reading in its internal memory to its interface related output buffer We d like now to place this command sequence into Simple VISA Query Since there are three commands to be sent it appears that we must modify the VI to include a sequence of three successive implementations of VISA Write While you are free to do so a much easier solution is available The SCPI language allows the programmer to concatenate several commands into one long multicommand string that can be sent in a single VISA Write statement The syntax for this concatenation process is as follows e Use a semicolon to separate two commands within the string e Begin a command with a colon if it has a different root level than the command preceding it The first command in the concatenated string and IEEE 488 2 common commands which begin with an aste
26. LabVIEW software That is by simply writing a collection of appropriate VIs it might be possible for you the contemporary scientific researcher to satisfy all of your laboratory instrumentation needs with this single LabVIEW based data acquisition and generation system This system s tremendous flexibility would then obviate the need to purchase an expensive collection of stand alone electronic equipment such as power sup plies function generators picoammeters spectrum analyzers and oscilloscopes The functioning VIs that you have written in previous chapters demonstrate that the above tantalizing prospect can at least in certain situations be realized But don t discard your stand alone instruments just yet The timing speed sensitivity and 425 9 9 2008 9 03 25 PM Chapter 13 Control of Stand Alone Instruments simultaneous data taking requirements of many contemporary research experiments are beyond the capabilities of your DAQ device For instance while the Lab VIEW based digital oscilloscope we constructed worked well for observing audio range frequen cies less than 20 kHz it would prove miserably inadequate at displaying the several nanosecond wide voltage pulses emanating from a photomultiplier tube In this latter situation a stand alone digital scope with a very fast analog to digital converter on the time scale of several gigasamples per second would do the job nicely Thus stand alone instruments play a cen
27. OPC bit to set the ESB in the Status Byte Register and SRE 0 disables the instrument from asserting a SRQ Then OPC requests the instrument to return a 7 to the instrument s output buffer after this command is completed This last command is included simply as a method of checking that the entire sequence of commands has been executed Finally VISA Read reads the contents of the instrument s output buffer If all goes well there should be a single ASCII character read into the computer 461 9 9 2008 9 03 38 PM Chapter 13 Control of Stand Alone Instruments Test drive your VI as follows Click on VISA resource name control s menu button with the 4 S es aay 49PT Appuication ront VIA resource name VISA i O p T p read bu Cc Then from the list presented select the VISA resource name for your computer controlled instrument VISA resource name yl iH COMI IRAR LPT1 thy error oul Refresh Eas iin dn os Saas coon ia ft E Then run Status Config Serial Poll Upon completion does the Buffer Reading string indi cator display an ASCII character Next construct a VI called Serial Poll which continuously reads the Status Byte Register of an instrument until a given bit is set A suggested coding of Serial Poll is shown in the following diagrams and explanations of the unfamiliar icons are in the subsequent paragraphs Save Serial Poll in YourName Chapter 13 VISA resource n
28. PIBO 22 INSTR Maximum Queue Length Resource Lock State Interface Information Resource Name Version Information Resource Class General Settings Message Based Settings hs Register Based Settings User Data GPIB Settings Serial Settings PXI PCI Settings TCP IP Settings USB Settings VXIUVME Settings FireWire Settings T F FF FF FF FF FF F F F F Interface Information Note that within the Timeout terminal a small arrow at the right points outward from the terminal s interior This outward directed arrow indicates that the Timeout terminal is configured as an indicator that is it reads gets the current Timeout value Using Create gt gt Indicator create a front panel indicator to display the value of Timeout Is GPIBO 22 INSTR Property Mode l l pe General Settings Timeout Value Switch to the front panel change the indicator label to Timeout Value ms then save your work Run the VI As shown below the default Timeout value for my system is 3000 ms 3 seconds File Edit View Project Operate Tools l Sjn Timeout Value ris 3000 44 9 9 2008 9 03 33 PM Chapter 13 Control of Stand Alone Instruments You can also use a Property Node to set the value of a VISA property To demonstrate this procedure with Get Timeout Value open use Save As to create a new program called Set Timeout Value and store it in YourName Chapter 13 When r
29. Read VISA Close byte count In this example the message DN is sent over a GPIB interface to an instrument at address 22 JDN a command recognized by most instruments instructs the instru ment to identify itself After receiving this command the instrument s response which is an ASCII string consisting of identification information is received by the computer over the GPIB and displayed in the read buffer front panel string indicator Similar to the File I O and DAQmx icons that you have studied previously the wiring scheme of VISA icons takes advantage of the principle of LabVIEW programming called data dependency Simply stated data dependency means that an icon cannot execute until data is available at all of its inputs In the previous diagram all of VISA Open s required inputs given in bold on the Help Window are wired So when this diagram is run VISA Open executes immediately Upon completion VISA Open outputs a VISA resource name at its VISA resource name out terminal which is passed through the wiring to VISA Write s VISA resource name input Because of data dependency VISA Write can not execute until it receives the VISA resource name from VISA Open In a similar way VISA Read cannot execute until VISA Write completes and so on Thus through this programming scheme we are assured that the icons will execute in the desired sequence VISA Open followed by VISA Write followed by VISA Read followed by VISA Close Als
30. Search String is Comma Replace String is Tab Constant USING THE INSTRUMENT DRIVER TO WRITE AN APPLICATION PROGRAM replace all 7 IT Eg search and Replace String Search String is Line Feed Constant Replace String is End of Line Constant Pea E o at m Search and Replace String Array Type is 1D DBL Array Constant p Spreadsheet String H Get format string 7ol2 6e Numeric Array EZA jez i HH EI Spreadsheet String To Array If interested try writing Reformat Data String It implements the Search and Replace String icon found in Functions gt gt Programming gt gt String Help Window shown below to coerce the original Data string into the spreadsheet format by replacing comma delimiters and the LF terminator with tabs and an EOL respectively Do you understand how it works Once written include this program as a subVI in Data Sampler and watch it perform its magic Context Help Search and Replace String input string search string replace string offset 0 error in ne error result string number of replacements offset past replacement error out Replaces one or all instances of a substring with another substring To include the multiline Boolean input right click the function and select Regular Expression 487 9 9 2008 9 03 50 PM Chapter 13 Control of Stand Alone Instruments DO IT YOURSELF Assume that you have a widget in your laboratory that is provid
31. TT PACKARD 34401A 0 11 5 2 n error out i i i status code fl E SOQUrCE To deactivate backslash coding pop up on Response and select Normal Display 13 10 GETTING AND SETTING COMMUNICATION PROPERTIES USING A PROPERTY NODE In addition to message termination there are other low level functions connected with message based communication Many of these low level functions have an associated parameter setting which is termed a VISA property VISA assigns default values for these properties and as long as the VISA based programs that you write fall within the scope of these default settings the VISA icons will automatically take care of these low level functions without any programming effort needed by you as demonstrated by the message termination example shown above At times however you will most likely write programs that fall outside the scope of the default VISA property settings and so you will need to assign nondefault values to these quantities Reading getting and writing setting VISA property values can be done within your programs using a Property Node and also can often be done in MAX As a concrete example of a VISA property consider VISA Read s timeout which is a fail safe feature of VISA Read that prevents a program from running endlessly if an error occurs If for example an instrument which is being queried doesn t seem to be responsive perhaps a nameless experimenter forgot to fl
32. Value string format string walue 0 output string Converts a number inte a regular string according to the format specified in format string and appends this to string Else Switch to the front panel and change the label of the String Indicator from output string to Command Run the VI with a given choice of the controls within Function Parameters and verify that the correct command string appears in the Command indicator Save your work 4 Command string vi Front Panel File Edit View Project Operate Tools Window Help it 15pt Application Font 8 iie Function Parameters Command B Function ic 0 N FVO LT DE 1 00E 11 00 E 5 7 ie DC Voltage Voltage Range Sith fe Resistance Range 14100 ohm Resolution 36 1 2 Digits 478 13 Essick Chap13 indd 478 D 9 9 2008 9 03 47 PM 13 Essick Chap13 indd 479 CREATING AN INSTRUMENT DRIVER Add a Push Button found in Controls gt gt Modern gt gt Boolean and a Numeric Control to the front panel and label them Autozero and Sample Count respectively Change the rep resentation of Sample Count to U16 Switch to the block diagram then include code to control the multimeter s autoze roing feature and to program the desired number of data samples to be taken The 5d format in the SAMPle COUNt command specifies a five place decimal integer because the maximum allowed value for SAMPLe COUNt according to the Agilent 34401A user
33. age based communication If instead you have some other interface equipped instrument available try reading the following pages to understand the generic issues being investigated Then by consulting the user manual it may be fairly easy for instance to use the interface appropriate VISA resource name and substitute an ASCII command string here and there in order to adapt the exercises to your particular instrument and interface bus If neither of the above describes your situ ation simply read through the following pages I believe you will learn some valuable features of instrument control that will serve you well in future work 13 7 MEASUREMENT amp AUTOMATION EXPLORER MAX To carry out the exercises in this chapter you must have a National Instruments GPIB device connected properly to your computer which in turn is connected e g by a GPIB cable to a GPIB equipped stand alone instrument Also the GPIB device s driver software called NI 488 2 must be correctly installed To verify that these conditions are met we will use the handy utility Measurement amp Automation Explorer which 1s nicknamed MAX To open MAX either select Tools gt gt Measurement amp Automation Explorer if you have an open VI or Getting Started window or else double click on MAX s desktop icon if available After MAX opens double click on Devices and Interfaces in the Configuration box This action will command MAX to determine all of the Nati
34. age string the LF character is always interpreted by the receiver as the last byte of a message Thus appending LF to a command string is one method of signaling message termination in IEEE 488 2 communication Alternately the IEEE 488 2 standard allows the assertion of an end or identify EOI while the last character in the string is being passed as another acceptable termination method The EOI is a digital signal on a dedicated wire within the GPIB cable When using VISA icons to control an IEEE 488 2 compliant instrument message termination is taken care of automatically allowing you to remain blissfully ignorant of this lower level activ ity If you d like to view an example of this usually invisible message termination activity pop up on the Response indicator on the front panel of Simple VISA Query then select V Code Display The n character you see at the end of the identification string is the backslash code for LF The multimeter appended this termination character to its identification string to the alert the receiver in this case the GPIB device that the message has ended 44A i Esok Chapi ndi 444 9 9 2008 9 03 32 PM 13 Essick Chap13 indd 445 GETTING AND SETTING COMMUNICATION PROPERTIES USING A PROPERTY NODE 4 Simple VISA Query vi Front Panel File Edit View Project Operate Tool Window Help Lit 35pt Application Font gor fiore 2 Command Response PION i i SOCOCOCCC HEWLE
35. ame E error in no error VISA resource name out VISA Read STB spe Number To Which Bit Boolean Array m ee 11 23 a O o Index Array 462 jS Esik Chapi ndi 462 9 9 2008 9 03 39 PM 13 Essick Chap13 indd 463 MEASUREMENT VI BASED ON THE SERIAL POLL METHOD Ja Serial Pollwi Front Panel File Edit View Project Operate Tools Window Help it pt Applicaton Fore Sor oe A VISA resource name VISA resource name out g I I p Ej i Which Bit errar in no error error out status code e ao S0UTCE Context Help Serial Poll wi VISA resource name VISA resource name out Which Bit o error out error in no error VISA Read STB found in Functions gt gt Instrument I O gt gt VISA is the workhorse of this VI With each iteration of the While Loop its status byte output returns the current values of the SBR s eight bits in the form of an integer For example if the SBR s fifth bit ESB is set then status byte outputs the integer 32 since 00100000 32 The Help Window for VISA Read STB is shown next 463 9 9 2008 9 03 40 PM Chapter 13 Control of Stand Alone Instruments Context Help VISA Read STB VISA resource name VISA resource name out status byte error in no error i error ou Reads a service request status byte byte from the message based device specified by VISA resource name S The individual bits of status byte can be checked through the use of Num
36. and Open the front panel of Command String then run Measurement Config again Check that the concatenated command in the Command indicator on Gommand String s front panel has a form as given above make sure all of the colons semicolons and spaces are included If there is an error correct it on the block diagram of Command String After running Measurement Config the multimeter will be left in remote mode You can switch to local mode by pressing the instrument s front panel SHIFT LOCAL key The Agilent 34401A can then be triggered equivalent to sending the INIT command over the GPIB with the SINGLE TRIG button A star annunciator will blink on the instrument s front panel display as it acquires each voltage sample Does this annucia tor blink Sample Count times after the SINGLE TRIG button is depressed Save Measurement Config as you close it Write a final modular VI for your Agilent 324401A instrument driver called Take Data as shown below and save it in YourName Chapter 13 The leading CLS command assures that all bits in the SESR and SBR register are set to zero prior to each data taking process Ta Take Datawi Front Panel File Edit View Project Operate Tools Window Help Take 11 15ptAppicationFont_ E VISA resource name VISA resource name out i i i i i i i i i i i i immm i i i i i in L e 3 j Data i i i i i error in no error error out i i i i
37. ansfer the information it produces into a computer s memory The array of device specific commands for an instrument is listed in its user manual This set of commands is defined by the instrument s maker and therein lies a problem When surveying the user manuals for programmable instruments of varying models and manufacturers you will find a great diversity in the style of the various command sets Some especially those associated with older model instruments are an alphabetized collection of cryptic one or two character strings the designers thinking was obviously shorter commands yield quicker and therefore better computer instrument communi cation At the other extreme are the user friendly sets with similar commands logically grouped each represented by an easy to read and remember mnemonic As programmable instruments have come into wider use it has become apparent that development costs and unscheduled delays can be diminished markedly by simpli fying the instrument programmers task whenever possible Thus user friendly device specific command sets are the rule rather than the exception for instruments being currently manufactured Commonly these command sets are organized in a hierarchical tree structure similar to the file system used in computers Each of an instrument s major functions such as TRIGger SENSe alternately MEASure CALCulate and DISPlay define a root and all commands associated with that
38. ber To Boolean Array found in Functions gt gt Programming gt gt Boolean with its Help Window shown next This VI creates an array of TRUE and FALSE values that mirror the sequence of zeros and ones starting from the least significant bit in the binary representation of the integer input number For example if number equals the decimal integer 48 then the Boolean array output will be F F F F T T F F since 4810 00110000 Index Array can then be used to ascertain the value of a particular element in this array Serial Poll s While Loop will continue to iterate until Which Bit becomes TRUE Context Help Number To Boolean Array number vw Boolean array Converts an integer or fixed point number to a Boolean array If you wire an integer to number Boolean array returns an array of 8 16 52 or 64 elements depending on the number of bits in the integer If you wire a fixed point number to number the size of the array that Boolean array returns equals the word length of the fixed point number The Oth element of the array corresponds to the least significant bit of the two s complement of the binary representation of the integer Elele Run Serial Poll under Highlight Execution and through your observations gain a bet ter understanding of its operation Remember to input values for VISA resource name and Which Bit on the front panel When run in this isolated manner the VI will most likely never be able to exit the Wh
39. cation fy M Drivers Ba Instrument 0 22 None HEWLETT PACKARD BS y 4 amp Remote Systems s w s Primary gt Properties Address Because up to 15 instruments can be connected to a single GPIB device each instru ment has an identification number called its GPIB address A GPIB address can be any integer from 0 to 30 and is typically defined via a hardware DIP switch setting within the instrument or a sequence of button pressing and or knob turning on its front panel The instrument s user manual will describe the method for setting its address The GPIB address of the instrument found in the Scan for Instruments operation is found by double clicking on the Instrument 0 folder After the double click in the box associated with the Attributes tab we find that the instrument s GPIB address is 438 4 Resick lt Chapi3andd 438 9 9 2008 9 03 28 PM 13 Essick Chap13 indd 439 MEASUREMENT amp AUTOMATION EXPLORER MAX 22 the address of your instrument may be different A text description identifying the instrument also appears Q Instrument 0 Measurement amp Automation Explorer l File Edit View Tools Help Configuration fa Communicate with Instrument S3 Interactive Control Se NI Spy gt 39 Hide Help 9 My Syst 9 eam Name Value Gea l A g Data Neighborhood ay Devices and Interfaces Primary Address 22 GPIB gt GPIBO PCI GPIB Secondary Address None I
40. d measurement function CONFigure VOLTage DC lt Space gt lt Range gt lt Resolution gt CONFigure VOLTage AC lt Space gt lt Range gt lt Resolution gt CONFigure RESistance lt Space gt lt Range gt lt Resolution gt Here the possible values of lt Range gt for both the DC and AC voltage measure ments are 0 1 1 10 100 and 1000 Volts For the resistance measurement the allowed lt Range gt values are 100 1k 10k 100k 1M 10M and 100M ohms In all cases the measurement precision may be 4 5 2 or 6 2 digits which corresponds to lt Resolution gt being 107 10 or 10 times the lt Range gt value respectively We will write two programs called Range and Resolution Decoder and Command String which will allow a user to construct the desired command string using front panel controls On Range and Resolution Decoder given range and resolution choices from a user friendly front panel listing of the multimeter s available offerings the program will convert these choices to the double precision floating point numeric format needed in Command String Command String will construct the appropriate ASCII command string to be sent to the Agilent 34401A based on selections made on its front panel controls Create anew VI named Range and Resolution Decoder and save it in YourName Chapter 13 Place four Emun controls found in Controls gt gt Modern gt gt Ring amp Emun on the front panel and label them Function Vol
41. e bit is induced to be set also This action then causes the instrument to assert a SRQ which alerts the GPIB device that the assigned operation is complete This method is pictured here Service Request Method Standard Event Status Byte Event Register Enable Status Byte Register Enable Message Available OR O NU PRAAN ESE 1 We ll write VIs that implement both of these approaches to status reporting 13 13 MEASUREMENT VI BASED ON THE SERIAL POLL METHOD Let s try the Serial Poll Method first To configure the Agilent 34401A for status report ing using the Serial Poll Method write the following VI called Status Config Serial Poll and save it in YourName Chapter 13 First code the VI s block diagram as shown next Use the autocreation feature in pop up menus to create all of the constants controls and indicators 459 9 9 2008 9 03 38 PM Chapter 13 Control of Stand Alone Instruments read buffer Sabo ape VISA resource name out write butter CLS ESE 1 5RE GOPC VISA resource name rs Part abou pT ES e vis abo ws error in no error error out byte count Switch to the front panel and arrange the object logically Design an icon and assign the connector terminals consistent with the Help Window shown Status Config Serial Poll vi Front Panel File Edit View Project Operate Tools Window Help 460 13 Bssick lt Chapi3sndd 460 9 9 2008 9 03 38 PM
42. ect Add Case for Every Value The Range amp Resolution indicator cluster is created by popping up on Bundle and using Create gt gt Indicator 474 13 Rssick Chapi3 ndd 474 D 9 9 2008 9 03 45 PM 13 Essick Chap13 indd 475 CREATING AN INSTRUMENT DRIVER Function Parameters Range amp Resolution a Resistance Unbundle By Name Sc Function Resistance Ranges Index Array pa i 7 6 1 0E 8 eee i Resistance Range c O Fesolution Return to the front panel Within the Range amp Resolution indicator cluster label the top and bottom Numeric Indicator as Range and Resolution respectively Design an icon and assign the connector terminals as shown Save your work 415 9 9 2008 9 03 45 PM Chapter 13 Control of Stand Alone Instruments iia Range and Resolution Decoder wi Front Panel File Edit View Project Operate Tools Window Help At 15pt Application Font 8a te Function Parameters i Function rj D C Volta ge Range amp Resolution a A i i i Range _ Voltage Range fo EES a Resolution Resistance Range bo 100 ohm Resolution 4 1 2 Digits Context Help Range and Resolution Decoder vi Function Parameters Eleje Run Range and Resolution Decoder and verify that it
43. es the value you input to Set Timeout Value as a suggestion rather than an order and sets Timeout to the nearest allowed value 13 11 PERFORMING A MEASUREMENT OVER THE GPIB Now that Simple VISA Query has given us a template for the VISA query process let s try controlling a real measurement Hook up some known DC voltage difference say 5 or 6 Volts between the HI and LO Voltage Inputs of the Agilent 34401 A Multimeter This instrument s user manual instructs us that delivering the following sequence of ASCII commands will result in one DC voltage sample being acquired and then loaded into the instrument s output buffer which is part of its interface circuitry CONF VOLT DC lt Space gt 10 0 000001 INIT FETC Here is the meaning of this secret code First the Agilent 34401A can be programmed to perform 11 different types of measurement functions including DC voltage AC volt age DC current AC current resistance and frequency Given these options the first command instructs the instrument that we desire to take a DC voltage measurement The full command is CONFigure VOLTage DC lt Space gt lt Range gt lt Resolution gt this command actually executes a collection of commands drawn from the Agilent 34401A s INPut SENSe TRIGger and CALCulate root subsystems The command mnemonic CONFigure VOLTage DC is constructed in the hierarchical tree structure typical of SCPI compliant instruments CONFigure is the root level key
44. esponse will be read back over the GPIB to the computer and displayed in the String Received box When the dialog window opens its Send String box is preloaded with JDN the IEEE 488 2 common com mand for an instrument to identify itself NWI 488 2 Communicator GPIBO Primary Address 22 Globals ibsta Ox 100 Write Read iberr None ibeni 4 Instrument 0 Send Strina IDN Query Configured String Received Show Sample Configure EQS 9 9 2008 9 03 29 PM 13 Essick Chap13 indd 441 SIMPLE VISA BASED QUERY OPERATION Click on the Query button If your GPIB communication is configured properly the identification string received from the instrument will appear in the String Received box For the Agilent 34401A Digital Multimeter used here this identification string identifies the instrument s manufacturer and model number followed by some integers that denote the version numbers of installed firmware that controls the multimeter s three internal microprocessors NI 486 2 Communicator GPIBO 3 InstrumentO Primary Address 22 Globals Send String IDM sen mg ibsta 0x2100 Query Write Read iberr None END Configured india String Received HEWLETT PACKARD 4407 4 0 11 5 2 Configure EQS Show Sample For future reference this interactive dialog window is a handy tool for use in determin ing correct command syntax when developing message based communication
45. h by the way can hold up to a maximum number of 512 measured values into the instrument s interface related output buffer Let s write a VI that uses the given command string to gather a sequence of 100 voltage samples Open Simple VISA Query then use Save As to create a new VI called Simple VISA Query Long Delay Delete Numeric Voltage from the front panel and enlarge Response so that it can display a very long string which is the concatenation of 100 voltage values Type the command given above into the Command control Once entered into Command you can keep this command permanently loaded there by selecting Edit gt gt Make Current Values Default E mea VISA Query Long Delay vi Front Panel File Edit View Project Tools Window Help STE i eeren Font EE Command Response SEEE OOY ny OS S O S S S O SE WORSEN WOE V S S S S S O O O E S S O S E S SO VOI S S S S S S S SOO O S S S S S S NOSIS E S S S S S S S O S S CONF VOLT DC 10 0 00001 SAMP COUN 100 INIT FETC m error out status code if 0 source 453 9 9 2008 9 03 35 PM Chapter 13 Control of Stand Alone Instruments Switch to the block diagram Delete Fract Exp String To Number After the given command string is written to the multimeter by VISA Write VISA Read will receive a string containing the 100 voltage samples Since each voltage sample is reported as a 15 byte string and a single delimiting ASCII characte
46. he VISA resource name to other VISA icons this quantity is available at the VISA resource name out output terminal VISA resource name VISA resource name out duplicate session F i error out error in no error Opens a session to the device specified by VISA resource name and returns a session identifier that can be used to call any other operations of that device Else VISA Write whose Help Window follows below performs the actual ASCII mes sage transfer from your computer to the stand alone instrument Once presented with the open session s VISA resource name this icon writes the ASCII string at its write buffer input to the instrument This string is one of the commands recognized by the instrument and when received by the instrument configures it properly for a desired data taking measurement Additionally the VISA resource name is available at the VISA resource name out output terminal VISA Write VISA resource name VISA resource name out write buffer return count error in no error es error out Writes the data from write buffer to the device or interface specified by VISA resource name Else 421 9 9 2008 9 03 26 PM Chapter 13 Control of Stand Alone Instruments Next the Help Window for VISA Read is shown This icon transfers the response message from the stand alone instrument into your computer s memory When given the open session s VISA resource name this icon receives
47. ile Loop so you ll have to stop it using the Abort Execution button in the toolbar We re finally ready to write VISA Query Serial Poll This top level program imple ments serial polling to synchronize the GPIB activities necessary in acquiring 100 volt age samples using an Agilent 34401A multimeter 464 jS Esik Chapis ndd 464 9 9 2008 9 03 40 PM 13 Essick Chap13 indd 465 MEASUREMENT VI BASED ON THE SERIAL POLL METHOD Open Simple VISA Query Long Delay then use Save As to create VISA Query Serial Poll The front panel can remain unchanged If it s not already there by default type the following command into the Command control CONF VOLT DC lt Space gt 10 0 00001 SAMP COUNK lt Space gt 100 INIT OPC FETC Switch to the block diagram and modify it as shown below with Status Config Serial Poll and Serial Poll used as subVIs TENA Command Response VISA resource name Ie GPIBO 22 INSTR error out es ee oe el VISA Open Status Config VISA Write Serial Poll vi VISA Read VISA Close Serial Pollj wi Which Bit byte count Bit 5 ESB Here is how this diagram works The concatenated command string is sent to the instru ment by VISA Write After configuring the multimeter for the desired DC Voltage measure ment function the acquisition process is begun by the ZNIT command The succession of 100 samples is acquired and temporarily stored in the multimeter s internal me
48. ing this instrument is plugged into a 60 Hz power source that is 60 PLC per second 100 voltage samples will take about 100 x ee X 33 3 sec 60 PLC sec There s the problem A few moments ago we found that the default timeout value for VISA Read is 3 seconds but the measurement we have initiated takes over 30 seconds 456 13 Essick Chap13 indd 456 9 9 2008 9 03 37 PM 13 Essick Chap13 indd 457 SYNCHRONIZATION METHODS Thus long before the requested data is available VISA Read terminates the execution of Simple VISA Query Long Delay There are a couple of crude solutions to this dilemma First on the block diagram of Simple VISA Query Long Delay you can insert a single frame Sequence Structure into the VISA execution chain that simply contains a Wait ms icon wired to produce a delay of about 33 seconds between the issuance of the data taking command and the order to read the gathered data samples The resulting diagram would appear as follows PENA Command VISA resource name error out Visa ay be x abc mA SSS SS aE q n A VISA Read VISA Close byte count Second for a slightly more elegant fix you can use a Property Node to change the Timeout value for VISA Read from its default value on my system of 3000 ms to something larger than 33 seconds Use this approach to modify the block diagram of Simple VISA Query Long Delay as shown below Here the Timeout value is chosen to be 60 seconds
49. ing you with some interest ing information about X where X might be the position of an object or the intensity of a light source Additionally say the widget provides this information about X in the form of a voltage code that is it produces an output voltage V that is some known function of X Then with an Agilent 34401A multimeter and an appropriate application VI called it Time Evolution of X you can monitor X via measurement of V as a function of time Using your Agilent 34401A instrument driver programs as subVls write Time Evolution of X When run this top level VI continuously obtains a single DC voltage sample every Wait Time seconds where Wait Time is given by the value on the similarly named front panel control until the front panel Stop Button is clicked While running the VI provides real time graphing of the Voltage versus Time data on a Waveform Chart with the Chart s Time axis properly calibrated The front panel also provides the option of storing the all of the accumulated data in a spreadsheet file with Time and Voltage in the spreadsheet s first and second column respectively Define Time O at the moment that the first voltage sample is acquired The front panel of Time Evolution of X should appear as shown below All needed parameters without a front panel control should be input on the block diagram After building this VI run it to observe a time dependent voltage input e g from a function generator to
50. instruments in use in research and industry worldwide the GPIB most likely retain its popularity for many years to come For the instrument used in this chapter s exercises I chose the Agilent 34401A Digital Multimeter for the following reasons First of all this instrument mea sures voltage current and resistance vanilla flavored quantities that require no spe cialized knowledge to understand unlike for instance the control of grating angle and slit size in a spectrometer Second for such a high quality and useful GPIB equipped instrument its price tag of approximately 1000 makes it extremely affordable Every lab should have one and many do Third this instrument is both IEEE 488 2 and SCPI compliant Thus the following exercises rather than being narrowly tied to one specific device can be much more universally applicable by demonstrating the generic features of these widely used standards such as command syntax and status reporting In the best circumstance an Agilent 34401A earlier purchased units of this same instrument are named the Hewlett Packard 34401A is already available for your use or with a modest investment you can purchase this worthwhile instrument Then with out need for modification you can straightforwardly work your way through the given 436 14 Besick Chapiasndd 436 9 9 2008 9 03 28 PM 13 Essick Chap13 indd 437 MEASUREMENT amp AUTOMATION EXPLORER MAX exercises to learn the basics of mess
51. ip on the instrument s power switch VISA Read will only wait for the instrument s response for a certain number of milliseconds given by the value of the VISA property named Timeout before aborting the read operation and issuing an error message 445 9 9 2008 9 03 32 PM Chapter 13 Control of Stand Alone Instruments The default timeout value for VISA Read on your system can be determined using a Property Node The Help Window for a Property Node which is found in Functions gt gt Instrument I O gt gt VISA gt gt VISA Advanced is shown next Context Help Property Node Gets reads and or sets writes properties of a reference Else Write the following VI which reads the current Timeout value on your system Open a new VI and save it under the name Get Timeout Value in YourName Chapter 13 On a new block diagram place a Property Node and then using Create gt gt Constant wire the VISA resource name for your instrument to the Property Node s reference input as shown Property Node le GPIBO 22 NSTR Next using the thy click onthe Property terminal and select General Settings gt gt Timeout Value You might explore what other Properties appear in this menu many of which are specific to a particular interface bus 446 BESC Chapi ndd 446 D 9 9 2008 9 03 33 PM 13 Essick Chap13 indd 447 GETTING AND SETTING COMMUNICATION PROPERTIES USING A PROPERTY NODE Property Node le G
52. ks initiative and will not perform any work unless you request it to do so Thus when initiating communications with an instrument one of the messages that you may wish to send is an instruction that activates the subset of event reporting SESR bits that are of interest to you For instruments that conform to the IEEE 488 2 standard this activation process is accomplished via the ESE Event Status Enable command For example supposed you wished the QYE bit to be activated and thus record any execution errors in the SESR s bit 2 Since 000001002 410 the QYE bit can be activated by performing a VISA Write of the ASCII command ESE 4 to the instrument In our work to come we will activate the OPC bit with the command ESE 1 The Status Byte Register which is schematically shown next records whether data is available in the instrument s output buffer whether the instrument requests service and whether the SESR has recorded any events Status Byte Register SBR 7 6 5 4 3 2 l 0 RQS ESB MAV The functions of the eight bits of the SBR are described in the following table The SBR bits are studious performing their status reporting duties without need of a request from you Table 13 3 Functions of the Eight SBR Bits Bit Function of SBR Bit 7 MSB May be defined for use by instrument manufacturer 6 RQS Request Service The instrument has asserted the SRQ line because it requires service from the GPIB controller
53. led it Time Evolution of X Built In Driver which carries out the task described in this chapter s Do It Yourself project The icon VI Tree gives a helpful overview of the built in driver Regardless of the chosen resolution the Agilent 34401A Multimeter always reports data sample values with eight digits to the right of the decimal point Thus some of these decimal place values are not significant With Take Data open use File gt gt Save As to create a new VI called Take Data Accurate Resolution Add a Function Parameters front panel control to this new VI so that the selected resolution setting can be input and then modity the block diagram appropriately so that the data output reports values with the actual resolution selected eg 41 2 digits if that is the selected resolution Use the Instrument I O Assistant Express VI to query the Agilent 34401A Multimeter Place an Instrument I O Assistant found in Functions gt gt Express gt gt Input on the block diagram of a VI called Simple VISA Query Express When this Express VI s dialog window opens select the desired instrument and then click on Add Step In the Add Step dialog window that appears double click on Query and Parse In the Enter a command box type 489 9 9 2008 9 03 51 PM Chapter 13 Control of Stand Alone Instruments CONF VOLT DC 10 0 00007 INIT FETC and then click Run this step The command will be sent to the Agilent 34401A Multimeter and its stri
54. ll this VI first enable the event by calling the VISA Enable Event function and specifying Service Request as the event type HAAR t Save your work then run VISA Query SRQ Does it successfully acquire the requested 100 DC voltage samples Do you understand the operation of this program and how the Service Request Method manages to work with a minimum of interface bus activity To simplify the block diagram of VISA Query SRQ you might consider packaging VISA Disable Event and VISA Close together in a subVI called Close SRQ since both of these icons are involved in closing down the service request based VISA session To a this feat easily simply create a highlighting box around the two icons using the 468 14 Resick lt Chapi3andd 468 9 9 2008 9 03 42 PM 13 Essick Chap13 indd 469 MEASUREMENT VI BASED ON THE SERVICE REQUEST METHOD Command VISA resource name timeout Ie GPIBO 22 INSTR error out Status Config VISA Write Wait for RQS vi SRO vi VISA byte count Then select Edit gt gt Create subVI A new subVI icon will appear wired on your diagram Command VISA resource name timeout I GPIBO 22 INSTR error out Status Config VISA Write Wait for ROSwvi VISA Read SRQ vi VISA byte count Double click on this new icon to open it Then relabel the front panel objects appropri ately design an icon and assign the connector consis
55. manual is 50000 The format string entry for this command should be SAMP COUNK Space gt 5d CONF VOLT DC VOLT AC RES Pick Line a Function Format 3 f Value A El a Format Value Range Hen a i Range pon Ea Function Range and Unbundle By Name a Parameters Resolution False String Decoder wi ERO AUTO ON fERO AUTO OFF rs T Format Command To E cr a bi m L j ao H fa E a tal s Sample Count Autozero can be either turned on and off with the following commands ZERO AUTO lt Space gt ON ZERO AUTO lt Space gt OFF Append True False String found in Functions gt gt Programming gt gt String gt gt Ad ditional String Functions whose Help Window follows provides an easy way to select which of these two choices is concatenated to the command string Remember to include the leading colon and final semicolon in the false string and true string entries to assure proper command concatenation 479 9 9 2008 9 03 47 PM Chapter 13 Control of Stand Alone Instruments Context Help Append True False String runny string true string false string selector output string Selects either a FALSE or TRUE string according to a Boolean selector and appends that string to string CAK To guarantee that the instrument fully processes the sent command string before exit ing the configuration VI which you will write in a moment conclude the string with OPC Return
56. mory After the hundredth sample is obtained OPC instructs the instrument to set its SESR s OPC bit which in turn sets the SBR s ESB then FETC loads the contents of the inter nal memory into the instrument s output buffer At that point Serial Poll detects the setting of ESB which then triggers the instrument s output buffer to be read by VISA Read One might be tempted to write the concatenated command with OPC after FETC rather than sandwiched between JNIT and FETC as above It is best however to avoid send ing OPC after a query a query is a command like FETC that ends in a question mark as such commands cause a message to be loaded into an instrument s output buffer If the message exceeds the finite size of the output buffer as happens in our present situation the query must be immediately followed by VISA Read as the program executes in order to read the long message string over the bus successfully Return to the front panel save your work and then run VISA Query Serial Poll Does the VI obtain the requested 100 DC voltage samples successfully If so try running it again with Highlight Execution activated for both VISA Query Serial Poll and its subVI Serial Poll This exercise will illustrate the weakness of the Serial Poll Method namely the large volume of interface bus traffic required by this technique During the 30 odd seconds while the 100 data samples are being gathered the instrument is polled count
57. ng response will be displayed Click the Auto parse button to convert the response string to numeric format and then close the dialog box by clicking the OK button When returned to the block diagram simply create an indicator for the icon s token output terminal Run Simple VISA Query Express and demonstrate that it successfully obtains a DC Voltage sample from the Agilent 34401A Multimeter 490 ESk Chapi mdd 490 D 9 9 2008 9 03 51 PM
58. nstrument l ra Identification HEWLETT PACKARD 344014 0 11 5 2 Basics 7 ga NI DAQmx ices W34 GPIB Interface ID 0 What do you PX PXI System Unidentified Sy Serial amp Parallel gt Communicate 4 Scales with my 6 Software a MI Drivers ral y 4 9 Remote Systems want to do r m p 4 m p GPIB Attributes 8 VISA Properties Instrument m 4 Clicking on the VISA Properties tab we find that the correct VISA resource name for this instrument is GPIBO 22 INSTR and also are told under Device Status that the instrument is working 1 e communicating properly Instrument 0 Measurement amp Automation Explorer gt e File Edit View Tools Help Configuration a Communicate with Instrument 2 Interactive Control E NI Spy a K Hide Help 9 My System CS Back fes Al Data Neighborhood 1 gl Data Neighborhoo Av GPIBO 22 INSTR 5ga Devices and Interfaces GPIB GPIBO PCI GPIB Device Type GPIB Instrument J Instrument la E Basics ga NI DAQmx Devices Dici a es PX PXI System Unidentified VISA Alias on My System want to do Serial amp Parallel a 4 Scales with my inctriuimant 6 Software Device Status MI Drivers This device is working properly X Remote Systems Context Help This window Se ee ee Attributes 387 VISA
59. nterfaces o GPIB Interface i NI DAQmx De gi Scan for Instruments b i a P 1 E2 Interactive Control a E E PXI PXI System U fa lists checkboxes and i Serial amp Parall i text boxes to change vi NI 488 2 Troubleshooting Util the GPIB board 44 Scales Es sien aa settings Select Save nd l 4 WNI 488 2 Hel en 6 Software i i E y IVI Drivers XM Delete 4 Remote Systems TT senrcerarcneonw rite ZA a gt GPIB Board Properties Cattinac h Perform scan to detect connected instruments In a few moments MAX will complete the scan To view its results double click on the GPIBO folder For the case shown below one stand alone instrument was found and information about it is stored in the folder labeled Instrument 0 B C GPIBO PCI GPIB Measurement amp Automation Explorer n File Edit View Tools Help Configuration H H A Scan for Instruments gt 59 Hide Help Q My System GPIB Interface properties can only be changed by an Administrator CA Back fes Al a Data Neighborhood g gy Devices and Interfaces ame ji gt m p GPIB i Primary Address Interface gw Instrument 0 Secondary Address one Basics HE NI DAQmx Devices System Controller Use the drop PXI PXI System Unidentified dovm lists Azs f I O Timeout 13 10 sec a and i E diaa Erue Connected Instruments l l ra rr to ii 6 Software Instrument PAD SAD Identifi
60. o the correct manner of chaining together VISA icons for error reporting is shown above If an error does occur at one point in the chain subsequent icons will not execute and the error message will be passed to the error out indicator cluster Because VISA based programming is so robust you can write highly dependable data taking programs with just the information already presented However with a bit more grounding in the message based communication scheme you ll be able to create programs in which you can have near total confidence The following paragraphs will take you to the next level of sophistication in stand alone instrument control 13 2 THE IEEE 488 2 STANDARD When remote control of laboratory instruments first became possible there was a cha otic period during which more or less each instrument manufacturer defined its own communications protocol through a unique blend of parallel and serial modes positive 429 9 9 2008 9 03 26 PM Chapter 13 Control of Stand Alone Instruments and negative polarities and assorted handshaking signals In 1965 Hewlett Packard now named Agilent ended this cacophony by designing a universal instrument inter face called the Hewlett Packard Interface Bus HP IB and offered it as the only option on all of its new computer programmable instruments Because of its high transfer rates HP IB quickly gained popularity with other instrument manufacturers and in 1975 was accepted as an industr
61. onal Instrument devices present within your computing system C My System Measurement amp Automation Explorer File Edit View Tools Help Configuration amp My System lg Data Neighborhood Ja Devices Eana National Instruments 1 T E sottware Measurement amp Automation j M Drivers Explorer 1 Remote Systems WI What is Measurement amp Automation Explorer Measurement amp Automation Explorer MAX provides access to your National Instruments products Category Help MAX will list the findings of its device survey in hierarchical tree fashion as shown next If a GPIB device is connected correctly to your computer a folder labeled GPIBO will appear in the resulting list your system may have a different number than 0 in the fold er s label To find all of the stand alone instruments properly connected to this device right click on the GPIBO folder and select the Scan for Instruments option Alternately you can click on the Scan for Instruments button near the top of the window 437 9 9 2008 9 03 28 PM Chapter 13 Control of Stand Alone Instruments x GPIBO PCI GPIB Measurement amp Automation Explorer File Edit View Tools Help Configuration H H E A Scan for Instruments gt 52 Hide Help 9 My System GPIB Interface properties can only be changed by an Administrator G Back fez Al gl Data Neighborhood i ii z F ai Devices and I
62. ormat Such conversion operations can be easily accom plished in LabVIEW using the array of conversion icons found in Functions gt gt String In the present case use Fract Exp String To Number in Functions gt gt Programming gt gt String gt gt String Number Conversion The Help Window for this icon is given next Context Help Fract Exp String To Number number Interprets the characters 0 through 9 plus minus e E and the decimal point usually period in string starting at offset as a floating point number in engineering notation exponential or fractional format and returns it in number Els 451 9 9 2008 9 03 34 PM Chapter 13 Control of Stand Alone Instruments Place a Numeric Indicator on the front panel of Simple VISA Query and label it Numeric Voltage Use Display Format in this indicator s pop up menu to make its Digits of precision equal to 5 and disable Hide trailing zeros Then modify the block diagram as follows Fract Exp String To VISA resource name Response Number Numeric Voltage Ie GPIBO 22 INSTR f co ey error out lisa py isa py So p abep abor kK a ha Lo ae E VISA Open VISA Write VISA Read VISA Close byte count Run the VI to verify that the string to number conversion icon performs as expected TA Simple VISA Query vi Front Panel File Edit View Project Operate Tools Window Help it 35pt Application Font ar te 8 5 Command Respo
63. presented with the list of VISA resources that MAX found when it performed the Scan for Instruments operation You can then simply choose the desired instrument from this list Alternately you can manu ally enter the appropriate VISA resource name for your instrument as found using MAX into the k The syntax for a VISA resource name is nterface Bus Name Resource Address Resource Type VISA resource name Command Response I GPIBO 22 INSTR e co k FE VISA Open Complete the block diagram as shown next using the VISA icons found in Functions gt gt Instrument I O gt gt VISA and its subpalette VISA Advanced Wire the Command con trol terminal to VISA Write s write buffer input and the Response indicator terminal to VISA Read s read buffer terminal When executed VISA Read will read up to NV bytes from the selected resource where N is equal to the integer wired to its byte count 44 13 Essick Chap13 indd 442 9 9 2008 9 03 30 PM 13 Essick Chap13 indd 443 SIMPLE VISA BASED QUERY OPERATION input In a moment we will read the Agilent 34401A identification string According to this instrument s user manual its identification string can be up to 35 characters long Thus wire the byte count input to an integer U32 greater than or equal to 35 I used 50 as shown Create the error out indicator cluster using the pop up menu option Create gt gt Indicator pna Command Response VISA resource name Is GPIBO 22
64. programs for a new instrument Exit this dialog window and then close MAX 13 8 SIMPLE VISA BASED QUERY OPERATION Let s begin by writing a VISA based program that carries out the query 1 e write then read action that you just completed using MAX On the front panel of a blank VI place a String Control and a String Indicator then label them Command and Response respectively As shown in the next diagram you ll want to resize these objects so that they can display strings much larger than their default sizes allow Using File gt gt Save first create a new folder named Chapter 13 within the YourName folder then save this VI under the name Simple VISA Query in YourName Chapter 13 44 9 9 2008 9 03 30 PM Chapter 13 Control of Stand Alone Instruments TA Simple VISA Query vi Front Panel File Edit View Project Operate Tool Window Help at 15et Application Font vf forli E ZL Command Respo nse bP Switch to the block diagram and place a VISA Open icon found in Functions gt gt Instrument I O gt gt VISA gt gt VISA Advanced there Pop up on its VISA resource name input and create a VISA Resource Name Constant using Create gt gt Constant VISA resource name Command Response Ee abe You must now load the VISA Resource Name Constant with the VISA resource name of the instrument with which you wish to communicate By clicking on the VISA Open Constant s menu button M with the thy you will be
65. r will be needed to separate each sample this 100 sample string is expected to be about 100 X 15 100 x 1600 bytes long Input an integer larger than 1600 to byte count as shown TN Command Response VISA resource name le GPIBO 22 INSTR Error out es W Eeen ee R fe i VISA Open VISA Write VISA Read VISA Close byte count Run Simple VISA Query Long Delay Count down the seconds 3 2 1 Disappointed You will find that your VI does not display even one voltage sample let alone the expected 100 values but rather produces an error A Simple VISA Query Long Delay vi Front Panel File Edit View Project Operate Tools Window Help at Command Response g CONF VOLT DC 10 0 00001 SAMP COUN 100 INIT FETC m error out J 1073807339 VISA Read in Simple VISA Query Long Nela wi 4 ml 454 13 Besick lt Chapiasndd 454 D 9 9 2008 9 03 36 PM 13 Essick Chap13 indd 455 SYNCHRONIZATION METHODS To find out what produced this error pop up on the code indicator within the error out cluster and select Explain Error error out stan code x VISA Read in Sin VISA Query Lon Melari wi p 107335 Visible Items Change to Control Description and Tip Create Replace Data Operations Advanced Fit Control to Pane Scale Object with Pane Adapt To Source Representation Display Format Properties FT F F F
66. reate the three lines in this String Constant by the following sequence of keystrokes VOLT DC lt Space gt lt Enter gt VOLT AC lt Space gt lt Enter gt RES lt Space gt Be sure to include the lt Space gt character at the end of each command string You can make the invisible space and line feed characters visible by popping up on the String Constant and selecting Codes Display The correct entry will then appear as VOLT DC s nVOLT AC s nRES s Context Help Pick Line string multi line string line index output string Chooses a line from multi line string and appends that line to string CAK 471 9 9 2008 9 03 46 PM Chapter 13 Control of Stand Alone Instruments Format Value from Functions gt gt Programming gt gt String Number Conversion Help Window shown next then is used to attach two more string fragments each with embedded ASCI coded numbers that program the lt Range gt and lt Resolution gt set tings of the multimeter This icon takes the number at the value input and converts it to an ASCII string representation with the format defined at the format string input This ASCII string is appended to string and presented at output string In the above dia gram the scientific notation format 7 2e see section 5 5 is used for both lt Range gt and lt Resolution gt parameters Note a comma and semicolon follow lt Range gt and lt Resolution gt respectively Context Help Format
67. risk do not require a leading colon Since each of our three commands has a different root level applying the above rules results in the following concatenated string CONF VOLT DC lt Space gt 10 0 00001 INIT FETC Type this command into the Command control on the front panel of Simple VISA Query as shown next Run the VI Your computer will instruct the multimeter to acquire a 6 2 digit voltage reading retrieve this value and then display it on the front panel in the Response indicator Cool eh 450 Esik Chipi ndd 450 D 9 9 2008 9 03 34 PM 13 Essick Chap13 indd 451 PERFORMING A MEASUREMENT OVER THE GPIB ita Simple VISA Query vi Front Panel File Edit View Project Operate Tool Window Help it 35pt Application Font gorio 2 Command Response 5 13802910E 00 CONF VOLT DC 10 0 00001 INITs FETC Note that while extra digits are displayed the value within Response is only accurate to the fifth decimal place As shown above the Agilent 34401A reports its data samples in the form of an ASCII character string using the exponential format SD DDDDDDDDESDD where S is a positive or negative sign D is a numeric digit and E is an exponent For future reference note that the string that represents a data sample is 15 bytes long If you want to use this reading as input to a mathematical calculation a common situation you will need to convert the string representation into a numerical f
68. ront panel arrange the objects there as you wish Then design an icon and assign the connector terminals consistent with the Help Window shown below Save your work 481 BESokCp ada 481 D 9 9 2008 9 03 48 PM Chapter 13 Control of Stand Alone Instruments a Measurement Config vi Front Panel File Edit Wiew Project Operate Tools Window Help Lit 35pt Application Font te 22 5 VISA resource name Function Parameters VISA resource name out i GPIBO 22 INSTR Function P GPIBO 22 INSTR Joc Voltage Voltage Range 10 y read buffer Resistance Range 1 a 100 chm Resolution 6 1 2 Digits error in no error error out Autozera Context Help Measurement Contig vi Autozera VISA resource name VISA resource name out Function Parameters read buffer Sample Count error out error in no errar Input the VISA resource name for your instrument into the VISA resource name control and then run Measurement Config with front panel control settings shown above so that the following command is sent to the instrument CONF VOLT DC lt Space gt 1 00E 1 1 00E 5 ZERO AUTOK lt Space gt ON SAMP COUNKSpace gt 5 OPC 482 13 Essick Chap13 indd 482 D 9 9 2008 9 03 48 PM CREATING AN INSTRUMENT DRIVER If the command is successfully sent over the GPIB an ASCII 1 will appear in output buffer If the Agilent 34401A beeps there is most likely an error in the sent com m
69. s setting the range resolution and integration time while INPut consists of actions that condition the signal prior to its conversion such as filtering biasing and attenuation SIGNAL MEASUREMENT INSTRUMENT CALCulate Signal FORMat Routing Bus TRIGger MEMory DISPlay Alternately an instrument that generates signals is modeled by the following diagram SIGNAL GENERATION INSTRUMENT Signal Routing FOR Mat Bus SOURce i CALCulate TRIGger MEMory The SCPI command set is organized in a hierarchical tree structure using the syntax illustrated above by the Agilent 34401 A Multimeter command Yov ll learn more about 435 9 9 2008 9 03 27 PM Chapter 13 Control of Stand Alone Instruments the SCPI command syntax as you work your way through this chapter But maybe now is a good time to dive in and actually control a stand alone instrument 13 6 SPECIFIC HARDWARE USED IN THIS CHAPTER In designing this chapter I faced the following problem There are thousands of computer controllable stand alone instruments available for purchase from the myriad of worldwide scientific instrument makers Each of these instruments communicates using one or many times a few of the handful of available interface buses I have a small subset of these instruments in my laboratory and I can use them there to practice the art of message based communication You also hopefully have a small subset of such instruments available
70. tage Range Resistance Range and Resolution respec tively Pop up each Enum select Edit Items and then program each with the items given in the following list Function DC Voltage AC Voltage Resistance Voltage Range 700 mV 1 V 10 V 100 V 1000 V Resistance Range 00 ohm 1 kohm 10 kohm 100 kohm 1 Mohm 10 Mohm 100 Mohm Resolution 4 7 2 Digits 5 1 2 Digits 6 1 2 Digits 471 9 9 2008 9 03 44 PM Chapter 13 Control of Stand Alone Instruments Then place these four Enum controls in a Cluster shell found in Controls gt gt Modern gt gt Array Matrix amp Cluster labeled Function Parameters as shown next if Range and Resolution Decoder vi Front Panel File Edit View Ga er a Tools Window Function Serer Function B DC Voltage Voltage Range 100 mV Resistance Range i 100 ohm a i Resolution 4 1 2 Digits Switch to the block diagram place a Case Structure there and complete the code shown Pop up on the Case Structure and select Add Case for Every Value and then verify that it has three cases labeled DC Voltage AC Voltage and Resistance Function Parameters k Unbundle By Name 472 13 Essick Chap13 indd 472 D 9 9 2008 9 03 44 PM CREATING AN INSTRUMENT DRIVER Select the DC Voltage case and then place an Index Array icon within it Pop up on Index Array s n dimension array input and select Create gt gt Constant
71. tent with the Help Window shown using the 4 X 2 X 2 X 4 pattern Save this VI under the name Close SRQ in YourName Chapter 13 TA Close SRQ vi Front Panel File Edit View Project Operate Tools Window Help un 15pt Application Font if Somfai VISA resource name a errar in error out status code UE g ZOLUTCE 469 9 9 2008 9 03 42 PM Chapter 13 Control of Stand Alone Instruments Context Help Close SRQ vi VISA resource name error in error out Switch to the block diagram of Close SRQ It should appear as follows event type error out VISA Close VISA Disable Event error in Close Close SRQ and return to the block diagram of VISA Query SRQ You may have to delete the originally created subVI and load a new copy of Close SRQ there using Functions gt gt Select a VI After that the finished block of VISA Query SRQ will appear as shown next Try running this VI to verify that it functions correctly Command Response VISA resource name timeout I GPIBOs22 INSTR Md A 60000 jae abc error out ne Y G i VISA Open status Config VISA Write Wait for ROS wi VI5A Read Close GRO wi SRO wi 2000 byte count 13 15 CREATING AN INSTRUMENT DRIVER An instrument driver is a collection of modular software routines that perform the oper ations required in the computer control of a programmable instrument These operations include configuring triggering sta
72. the multim eter and save the resulting Voltage versus Time data in a spreadsheet Ja Time Evolution of Xvi Front Panel File Edit View Project Operate Tools Window Help it 35pt Application Font 2 ie e2 5 Waveform Chart Plot 0 PSA 10 Wait Time second error aut je P ra 0 a status code Save Data 1 f 0 A Yes source J h oO A helpful tip The Time axis of the Waveform Chart can be calibrated using a Property Node Pop up on the Chart s icon terminal and select Create gt gt Property Node gt gt X Scale gt gt Offset and Multiplier gt gt Multiplier Then set the Multiplier property appropriately 488 13 Essick Chap13 indd 488 D 9 9 2008 9 03 51 PM 13 Essick Chap13 indd 489 USING THE INSTRUMENT DRIVER TO WRITE AN APPLICATION PROGRAM Problems if Thermocouples are widely used as temperature sensors A thermocouple is constructed by joining the ends of two dissimilar metals for example a copper and a constantan wire tor a Type T thermocouple This junction produces a millivolt level voltage which has a well documented temperature dependence where the temperature is measured relative to a cold junction reference temperature Conveniently this cold junction can be provided by a compact electronic device called a Cold Junction Compensator CJC which effectively makes the reference temperature equal to O C Connect a thermocouple to a CJC and then connect the plus and
73. tification string RST Resets instrument to known state EST Performs self test and reports results OPC Sets operation complete OPC bit in SESR upon completion of command OPCe Returns 1 to the output buffer upon completion of command WAT Waits until all pending operations complete execution CCLS Clears status registers ESE Enables event recording bits in SESR ESE Reports enabled event recording bits in SERS ESRe Reports value of SESR SRE Enables a SBR bit to assert the SRQ line SRE Reports SBR bits that are enabled to assert the SRQ line S TB Reports the contents of the SBR 9 9 2008 9 03 27 PM 13 Essick Chap13 indd 431 STATUS REPORTING 13 4 STATUS REPORTING Another IEEE 488 2 innovation is a standardized scheme for status reporting This status reporting system is available to inform you of significant events that occur within each instrument connected to an interface bus In this scheme each instrument is equipped with two status registers called the Standard Event Status Register SESR and the Status Byte Register SBR Each bit in these registers records a particular type of event that may occur while the instrument is in use such as an execution error or the completion of an operation When the event of a given type occurs the instrument sets the associated status register bit to a value of one 1f that bit has previously been enabled see the follow ing Thus by reading the
74. to practice with in your own laboratory What s the problem Well because of the high cost and specialized nature of such equipment the probability that my subset and your subset have some common instrument is most likely very small The unfortunate thing about this situation is that each stand alone instrument is designed to take specialized measurements and understands its own unique set of ASCII commands which are defined by its maker and are listed in its user manual Thus before attempting to control a particular instru ment using an interface bus the programmer must have a detailed understanding of the measurement that that instrument is designed to take the procedure that it implements in doing its work and the command list that it recognizes All of these considerations greatly constrain the writing of a set of generic laboratory exercises that everyone can perform That said I still was faced with the fact that I had to choose a particular instru ment and interface bus to work with in this chapter s exercises For the interface bus I chose the GPIB because it is the interface you will almost certainly encounter in your computer based laboratory work Although USB and Ethernet are gaining in popularity with scientific instrument makers due to the fact that most PCs come equipped with these interfaces GPIB is currently by far the most widely used interface bus for labo ratory equipment With an estimated 10 million GPIB equipped
75. to the front panel Run the VI with a given choice of the front panel controls to verify that the correct command string appears in the Command indicator Then design an icon and assign the connectors consistent with the following Help Window Save your work as you close the VI 4 Command string vi Front Panel File Edit View Project Operate Tools Window Help it 35pt Application Font 15 l ar 6 Function Pararneters Command o Function CONF VOLT DC 1 00E 1 1 00E 5 7ERO AUTO ON SAMP COUN 100 OPC Joc votege Voltage Range 10 V Resistance Range r a p 100 ohm Resolution 4a 6 1 2 Digits Autozero Sample Count i100 480 13 Rssick Chap13 indd 480 D 9 9 2008 9 03 47 PM CREATING AN INSTRUMENT DRIVER Context Help Command String vi Function Parameters Command Autozero Sample Count Finally create a VI named Measurement Config and save it in YourName Chapter 13 Switch to the block diagram and code it as shown This VI will write the command string to the instrument When this diagram runs the read buffer indicator will display an ASCH 1 if the command string was successfully read by the instrument VISA resource name read buffer VISA resource name out Function Parameters in a amea Autozero Command String wi Sample Count 1231 error in mo error error out Switch to the f
76. tral role in state of the art research and so it might not surprise you to find that they too fall under the scope of LabVIEW Over the past few decades a message based communications standard has evolved by which stand alone instruments can be software controlled using a personal com puter In this communications scheme a particular instrument obeys an array of man ufacturer defined ASCII character commands that represent all the possible ways of manually pressing buttons turning dials and viewing output data on its front panel While the hardware conduit called an interface bus through which these ASCII mes sages are passed between the PC and laboratory instrument can take on various guises including RS 232 GPIB Ethernet and USB there is a single set of LabVIEW icons available to control this communication process This icon set is named VISA short for Virtual Instrument Software Architecture and is found in Functions gt gt Instrument I O gt gt VISA In this chapter you will learn how to use VISA icons to control the message based communication between a stand alone instrument and your computer You will explore generic features of this communication process such as the Standard Commands for Programmable Instruments SCPI language and various synchronization methods while writing code that controls a particular stand alone instrument the Agilent 34401A Digital Multimeter using a particular interface bus the General Purpose
77. tus checking and sending data to and receiving data 470 jS Besick Chapi3 sada 470 9 9 2008 9 03 43 PM 13 Essick Chap13 indd 471 CREATING AN INSTRUMENT DRIVER from the instrument Above Status Config Serial Poll and Status Config SRQ are examples of configuration VIs that would be useful to include as part of the Agilent 34401A soft ware driver You will now write another configuration VI this time one that prepares the multimeter for taking a desired measurement The Agilent 34401A is capable of implementing 11 types of measurement func tions DC and AC voltage DC voltage ratio ratio of voltage at two different inputs DC and AC current 2 and 4 wire resistance 2 wire is the normal method for measuring resistance the more involved 4 wire technique is necessary only when measuring very small resistance samples frequency and period of an AC signal continuity and diode check To gain experience with some of the LabVIEW tools available for developing instrument drivers let s write a driver that offers the choice of configuring the Agilent 34401A for either a DC voltage AC voltage or 2 wire resistance measurement You of course can be more ambitious and write your VI to control up to all 11 possible mea surement functions Referring to the Agilent 34401A user manual we find that our driver must allow a user to select one of the following three possible commands in order to configure the instrument for the desire
78. un set Timeout Value will change VISA Read s Timeout to a value input from its front panel On the block diagram of Set Timeout Value pop up on the Timeout terminal and select Change To Write Property Node D D Timeout M22 Change All To Write Visible Iterns p Help For Property Node Timeout Value rns le GPIBO 22 INSTR Description and Tip Set Breakpoint Properties b Add Element k VISA Palette b Numeric Palette p Create 5 Replace b Mame Format b Ignore Errors inside Node Select Property Note that within the Timeout terminal the small arrow is now at the left pointing inward toward the terminal s interior This inward directed arrow indicates that the Timeout terminal is configured as a control that is it writes sets the Timeout value Delete the Timeout Value ms indicator terminal then using Create gt gt Control create a front panel control labeled Timeout Value ms 448 iS Esik Chapi nd 448 9 9 2008 9 03 34 PM 13 Essick Chap13 indd 449 PERFORMING A MEASUREMENT OVER THE GPIB Property Node GB GB le GPIBO 22 NSTR Timeout Value ris Return to the front panel and save your work Set Timeout Value ms to be 0000 and then run Set Timeout Value Next run Get Timeout Value Is Timeout now equal to 10000 ms 10 seconds Try setting Timeout equal to 8000 You will find that only certain values for Timeout are allowed LabVIEW tak
79. word and colons rep resent the descent to the lower level VOLTage then lowest level DC keywords While the full command mnemonic can be sent to the instrument it is only absolutely nec essary to send the capitalized characters Separated from the command mnemonic CONF VOLT DC by a lt Space gt the numerical values for two measurement parameters lt Range gt and lt Resolution gt are specified lt Range gt selects among the instrument s five available voltage measurement scales Each scale offers a different sensitivity with lt Range gt giving the maximum measurable value on a particular scale The five 449 9 9 2008 9 03 34 PM Chapter 13 Control of Stand Alone Instruments available ranges are 100 mV 1V 10 V 100 V and 1000 V In our situation of measuring a signal of approximately 5 V the 10 V scale is appropriate lt Resolution gt specifies the precision of the measurement with the options of three levels of accuracy 4 5 and 6 digits the 2 digit means that the most significant decimal place can only take on a value of 1 or 0 Thus on the 10 V scale voltages can either be resolved at the level of 0 001 0 0001 or 0 00001 Volts The trade off in requesting higher accuracy is that the measurement takes a longer time In the command sequence above the high est resolution of 6 2 digits is selected by setting lt Resolution gt equal to 0 00001 when lt Range gt equals 70 Note the syntax of the
80. y wide standard known as IEEE 488 or more commonly the General Purpose Interface Bus GPIB In 1987 an improved version of this standard called IEEE 488 2 was adopted which enhanced and strengthened message based com munication by specifically defining an instrument s minimally required communica tion capabilities a protocol for message exchange a generic set of commonly needed commands and a status reporting system Today most computer controlled laboratory instruments are IEEE 488 2 compliant even those that communicate over interface buses other than the GPIB such as Ethernet and USB 13 3 COMMON COMMANDS One important innovation of the IEEE 488 2 standard was the introduction of a stan dardized set of common commands for the many generic operations that all instru ments must perform The mnemonics for these common commands begin with asterisks to delineate them from the other device specific commands recognized by a particular instrument All IEEE 488 2 compliant instruments at the very least are required to recognize the subset of 13 common commands given in the following table Many of these commands are related to the reporting of events using two status registers called the SBR and SESR which will described in detail starting in the next paragraph Table 13 1 Common Commands for IEEE 488 2 Compliant Instruments 430 13 Essick Chap13 indd 430 MANDATORY COMMON COMMANDS FUNCTION IDNe Reports instrument iden

Control of Stand-Alone Instruments

Contents

Download Pdf Manuals

Related Search

Related Contents