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1. Once the operator has completed the search of the ladder logic program the Runtime System re displays the Man Machine Interface and continues monitoring the PLCs 118 Control then returns to FIG 5 120 Summary In summary a menu driven system has been de Scribed which system is used for developing Man Machine Interfaces to graphical monitor ladder logic programs executing in PLCs Man Machine Interfaces graphically depict plant processes controlled by the PLC Data from one or more PLCS representing plant process events flows state changes tank levels etc are then communicated to the Man Machine Interface for display The Development System of the preferred embodi ment provides a programmer s tool box for construct ing the Man Machine Interfaces Ladder logic pro grams are imported and accessed by the Development System for use in the development of the Man Machine Interfaces The Runtime System of the preferred embodiment provides the operational software that executes the Man Machine Interfaces The Runtime System has the ability to access ladder logic programs during monitor ing operations An operator can hot key to the ladder logic program for trouble shooting purposes The foregoing description of the preferred embodi ment of the invention has been presented for the pur poses of illustration and description It is not intended to be exhaustive or to limit the invention to the precise form disclosed2. 116 RE DISPLAY 118 MMI 120 FIG 8 U S Patent June 14 1994 Sheet 9 of 9 5 321 829 HOT KEY STRUCTURE 122 124 126 128 GOTO LADDER LOGIC FIG 9 5 321 829 1 GRAPHICAL INTERFACES FOR MONITORING LADDER LOGIC PROGRAMS BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates in general to monitoring sys tems for programmable logic controllers PLCs and in particular to a system for developing Man Machine Interfaces to graphical monitor ladder logic programs executing in PLCs 2 Description of Related Art PLCs perform many of the control functions for assembly line machines machine tools and other types of industrial equipment For example a part arriving at a workstation may contact and close a limit switch or other type of sensing device As a result an electrical circuit is completed or broken and a signal is sent to a PLC indicating the change in condition The PLC re sponds to the input as directed by a ladder logic pro gram which to a large degree simulates the response of what used to be accomplished by older systems with a set of relays and other control devices Ladder logic programs instruct the PLC how and when to react to the different signals it receives The PLC in turn in structs the devices it controls in an appropriate manner as specified in the ladder logic program In the prior art operators can communicate with PLCs from personal computers o
3. PLC addresses element addresses polling in tervals and action identifiers FIG 5 is a flowchart illustrating the steps performed by the Runtime System in executing an MMI Project FIG 6 is a flowchart illustrating the steps performed by the Runtime System when a polling interval is trig gered FIG 7 is a flowchart illustrating the steps performed by the Runtime System when a response to a poll is received from the PLC FIG 8 is a flowchart illustrating the steps performed by the Runtime System when user input is received either from a keyboard or other input device and FIG 9 describes the format of a hot key data struc ture DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description of the preferred embodi ment reference is made to the accompanying drawings which form a part hereof and in which is shown by way of illustration a specific embodiment in which the invention may be practiced It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention The preferred embodiment provides PC based menu driven software for developing Man Machine Inter faces MMI to graphically monitor ladder logic pro grams executing in PLCs The Man Machine Interfaces graphically depict plant processes controlled by one or more PLCs Data from the PLCs representing plant process events flows state changes tank lev
4. Many modifications and variations are possible in light of the above teaching It is intended that the scope of the invention be limited not by this detailed description but rather by the claims appended hereto i What is claimed is 1 A computer based method of monitoring a ladder logic program executing in a programmable logic con troller comprising the steps of a drawing a graphical object on a monitor attached to a computer to depict a plant process controlled by the programmable logic controller wherein the graphical object is selected from a group compris 5 321 829 7 ing Bargraph objects Numeric objects Trend ob jects and State Change objects b electronically associating the graphical object to an element of the ladder logic program c setting a polling interval on the computer to indi cate how often a value for the associated element is to be retrieved from the ladder logic program exe cuting in the programmable logic controller d specifying action parameters on the computer to indicate changes in the display of the graphical object that are to occur in response to changes in the value of the associated element e responding to a triggering of the polling interval by transmitting a query message from the computer to the programmable logic controller requesting a value for the associated element and by resetting the polling interval and f responding to the value for the associated element being
5. hot key command by an operator of the computer by accessing a hot key data structure in the com puter and extracting therefrom an electronically stored filename which identifies a location on the computer where a copy of the ladder logic pro gram is stored and displaying the copy of the ladder logic program on the monitor at a specific rung thereof 12 The apparatus of claim 11 wherein the computer further comprises means for preventing unauthorized access to the copy of the ladder logic program
6. interval and 6 means for responding to the value for the asso ciated element being input from the programma ble logic controller to the computer by changing the display of the graphical object in accordance with a change in the value of the associated ele ment 8 The apparatus of claim 7 wherein the means for electronically associating comprises means for accept ing an address entered by an operator at an entry field displayed on the monitor the address identifying a memory location in the programmable logic controller associated with the element 9 The apparatus of claim 7 wherein the means for electronically associating comprises i means for accepting a designated key entered by an operator into the computer and displaying a list of symbols used in the ladder logic program on the monitor in response thereto ii means for searching the list for a symbol repre senting a specific element in the ladder logic pro gram and iii means for electronically associating the graphical object to the specific element 5 321 829 9 10 The apparatus of claim 7 wherein the means for electronically associating comprises i means for accepting a designated key entered by the operator into the computer and displaying the ladder logic program on the monitor in response thereto ii means for searching through the ladder logic program for a specific element and iii means for electronically associating the graphical
7. object to the specific element 11 An apparatus for monitoring a ladder logic pro gram executing in a programmable logic controller comprising a a computer coupled to the programmable logic controller having a monitor attached thereto the computer further comprising 1 means for displaying a graphical object on the monitor to depict a plant process controlled by the programmable logic controller the graphical object being electronically associated with an element of the ladder logic program wherein the graphical object is selected from a group com prising Bargraph objects Numeric objects Trend objects and State Change objects 2 means for initializing a polling interval on the computer for the graphical object the polling 10 15 20 25 30 35 45 50 55 65 10 interval indicating a waiting period before a value for the associated element is to be retrieved from the programmable logic controller 3 means for responding to a triggering of the polling interval by transmitting a query message from the computer to the programmable logic controller requesting a value for the associated element and by resetting the polling interval 4 means for responding to the value being input from the programmable logic controller to the computer by changing the display of the graphi cal object in accordance with a change in the value for the associated element and 5 means for responding to an entry of a
8. these events are illustrated in FIG 6 7 and 8 Polling Interval FIG 6 is a flowchart illustrating the steps performed by the Runtime System when a polling interval is trig gered A query message requesting a particular element value is generated by the Runtime System for transmis sion to the PLC 86 The Runtime System then resets the polling interval 88 and control returns to FIG 5 90 Poll Response FIG 7 is a flowchart illustrating the steps performed by the Runtime System when a response to a poll is received from the PLC The input is a message gener ated by the PLC in response to a prior query by the Runtime System or an unsolicited message generated by the PLC in response to an Exception Scan The input typically comprises an element identifier and the current value therefor in the PLC The Runtime System searches the data structures described in FIG 4 to match the element to an object 92 In the next series of steps of FIG 7 the Runtime System processes the input according to the type of object associated with the element Bargraph objects 94 96 Numeric objects 98 100 Trend objects 102 104 and State Change objects 106 108 As indicated in FIG 7 each graphi cal object is redrawn if the element value causes it to be altered Note that the State Change object may be drawn for the first time After the input has been pro cessed control returns to FIG 5 110 Keyboard Event FIG 8 is a f
9. 989 Yamanaka 4 843 538 6 1989 Lane etal 4 991 076 2 1991 Zifferer et al 364 147 5 127 099 6 1992 Zifferer et al 395 725 OTHER PUBLICATIONS Allen Bradley Company Inc Programming and Op erations Manual Bulletin 1772 Mini PLC 2 05 Pro cessor Icom Inc PLC 2 Ladder Logistics User s Manual Aug 1987 Icom Inc PLC 5 Ladder Logistics User s Manual Sep 1987 Taylor Industrial Software Product Bulletin 24 Apr 1987 Icom Inc PLC 3 Ladder Logistics User s Manual Sep 1987 Taylor Industrial Software Product Summary Feb 1988 Taylor Industrial Software Product Bulletin 21 Apr 1987 PLC 5 Ladder Logistics User s Manual Sep 1987 Icom Inc MMI Logistics User s Manual May 1990 Icom Inc Primary Examiner Paul V Kulik Assistant Examiner Jennifer M Orzech Attorney Agent or Firm Merchant Gould Smith Edell Welter amp Schmidt 57 ABSTRACT A menu driven system for developing Man Machine Interfaces MMI for use in the graphical monitoring of ladder logic programs executing in programmable logic controllers PLCs The Man Machine Interfaces graphi cally depict plant processes controlled by the PLC Data from the PLC representing plant process events flows state changes tank levels etc are communi cated to the Man Machine Interfaces A Development System provides a programmer s tool bo
10. United States Patent 9 Zifferer JIHAN NAHH ELE LLL DU US005321829A 11 Patent Number 4453 Date of Patent 5 321 829 Jun 14 1994 54 GRAPHICAL INTERFACES FOR MONITORING LADDER LOGIC PROGRAMS 75 Inventor Scott C Zifferer Mequon Wis 73 Assignee ICOM Inc West Allis Wis 21 Appl No 556 958 22 Filed Jul 20 1990 511 Tnt CE za msc De GO6F 9 00 52 U S G 395 575 395 159 371 29 1 364 147 364 DIG 1 364 275 1 58 Field of Search 395 575 600 650 191 192 395 147 159 364 147 188 371 29 1 56 References Cited U S PATENT DOCUMENTS 3 686 639 8 1972 Fletcher et al 3 813 649 5 1974 Struger etal 3 882 305 5 1975 Johnstone 3 964 026 6 1976 Wanauchi et al 4 038 533 7 1977 Dummermuth et al 4 200 914 4 1980 Kintner 4 4 227 247 10 1980 Kintner 4 244 034 1 1981 Cherba 4 247 901 1 1981 Martin et al 4 316 260 2 1982 Hideshima 4 396 974 8 1983 Imazeki et al 4 415 965 11 1983 Imazeki 4 445 169 4 1984 Wakiti et al 4 449 180 5 1984 Ohshima et al 4 486 830 12 1984 Taylor Jr et al 4 488 258 12 1984 Struger et al 4 513 379 4 1985 Wilson et al 4 533 997 8 1985 Furgerson 4 635 183 1 1987 Isobe et al 4 644 478 2 1987 Stephens et al 4 661 899 4 1987 Usuda 4 663 704 5 1987 Jones et al 4 703 414 10 1987 Inoue et al 4 718 025 1 1988 Minor et al 4 815 014 3 1989 Lipner et al 4 833 592 5 1
11. aphical object is drawn 36 The graphical object is tied to an element of a ladder logic program 38 A polling interval is specified which indicates how often the element value is retrieved from the PLC 40 Also specified are specific action pa rameters for the graphical object that indicate how the object changes in response to changes in the element value 42 Finally the MMI Project may be saved for later retrieval 44 FIG 3 describes three methods used by the Develop ment System to tie elements of ladder logic programs to graphical objects 38 The first method is the typing in of the PLC address associated with the element at an entry field displayed on the computer screen 40 The second method is to press an input key desig 10 15 20 25 30 35 40 45 60 nated as the SYMBOL key thereby displaying a list of 65 all symbols in the ladder logic program 42 The list may be searched for the desired element 44 and the desired element selected by positioning the cursor over 4 it and pressing the ENTER key 46 This sequence of steps eliminates most common typographical errors This operation is further described in the co pending and commonly assigned patent application Ser No 07 374 487 filed Jun 30 1989 now pending by Scott C Zifferer et al entitled METHOD AND APPA RATUS FOR SYMBOLIC LADDER LOGIC PRO GRAMMING WITH AUTOMATIC ATTACH MENT OF ADDRESSES which application i
12. ct 24 Otherwise the graphical objects are corrected if they are in error 26 Usually these steps are sufficient to resolve any errors Once the errors are resolved the user can begin monitoring the PLCs 28 Thus there are only a few steps in the development and debugging cycles of the preferred embodiment Naturally time savings turn into money savings for the user In addition to the description herein more specific information on the Development System can be found in the MMI Logistics TM User s Manual ICOM Inc which manual is incorporated herein by reference MMI Project An important concept of the preferred embodiment is an MMI Project An MMI Project is the name given to an overall application Although the MMI Project consists of many inherently unique entities e g graphi cal objects ladder logic programs etc all of these become integral parts of an application The MMI Project s rves as a superintendent for the application allowing the user to define all the parts thereof and tie them together in a meaningful way for particular needs Development System FIG 2 is flowchart illustrating the steps performed by the Development System in constructing a Man Machine Interface The MMI Project is defined 30 along with its associated PLCs i e station addresses on a network ladder logic programs and associated data bases 32 The animation type is defined 34 and the corresponding gr
13. der logic programs executing in PLCs The Man Machine Interfaces graph ically depict plant processes controlled by the PLC Data from the PLC representing plant process events flows state changes tank levels etc are communi cated to the Man Machine Interfaces for display A Development System in the preferred embodiment provides a programmer s tool box for constructing the Man Machine Interfaces Ladder logic programs and databases associated therewith are imported and 5 20 40 45 50 55 65 2 accessed by the Development System for use in the development of the Man Machine Interfaces A Runtime System in the preferred embodiment pro vides an execution environment for the Man Machine Interfaces The Runtime System has the ability to access ladder logic programs during monitoring operations A user can hot key to the ladder logic program for trouble shooting purposes BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings in which like refer ence numbers represent like elements throughout the several views FIG 1 is a flowchart illustrating a typical develop ment and test cycle in the preferred embodiment FIG 2 is flowchart illustrating the steps performed by the Development System in constructing a Man Machine Interface FIG 3 describes three methods used by the Develop ment System to tie elements to graphical objects FIG 4 describes a data structure for storing graphical objects
14. els etc are communicated to the Man Machine Interfaces for display The preferred embodiment operates in a hardware environment of the type described in FIGS 13 16 and the description thereof found in the co pending and commonly assigned patent application Ser No 07 373 826 filed Jun 30 1989 now U S Pat No 4 991 076 by Scott C Zifferer et al entitled METHOD AND APPARATUS FOR CREATING CUSTOM DISPLAYS FOR MONITORING LAD DER LOGIC PROGRAMS which application is incorporated herein by reference 5 321 829 3 FIG 1 is a flowchart illustrating a typical develop ment and test cycle in the preferred embodiment The user develops a ladder logic program on a personal computer 10 and stores it in a disk file 12 Typically computer based development systems are used to develop the ladder logic program One example of a computer based ladder logic development system is described in the PLC 5 LADDER LOGISTICS TM User s Manual ICOM Inc which manual is incorpo rated herein by reference The user draws graphical objects representing the plant processes to be monitored 14 and ties or asso ciates the graphical objects to elements in the ladder logic program 12 The user tests the monitoring sys tem 16 and if errors are found 18 the graphical objects are reviewed for correctness 20 If no errors are found in the graphical objects 22 then the user checks that the ladder logic program is corre
15. input from the programmable logic control ler to the computer by changing the display of the graphical object in accordance with a change in the value of the associated element 2 The method of claim 1 wherein the electronically associating step comprises the step of accepting an ad dress entered by an operator at an entry field displayed on the monitor the address identifying a memory loca tion in the programmable logic controller associated with the element 3 The method of claim 1 wherein the electronically associating step comprises the step of 1 accepting a designated key entered by an operator into the computer and displaying a list of symbols used in the ladder logic program on the monitor in response thereto 2 searching the list for a symbol representing a spe cific element in the ladder logic program and 3 electronically associating the graphical object to the specific element 4 The inethod of claim 1 wherein the electronically associating step comprises the steps of 1 accepting a designated key entered by the opera tor into the computer and displaying the ladder logic program on the monitor in response thereto 2 searching through the ladder logic program fora specific element and 3 electronically associating the graphical object to the specific element 5 A method of monitoring a ladder logic program executing in a programmable logic controller compris ing the steps of a displaying
16. lowchart illustrating the steps performed by the Runtime System when operator input is re ceived either from a keyboard or other input device Typically such an input could indicate many things including an exit from the Runtime System The Runtime System also has the ability to access ladder logic programs The operator may hot key to ladder logic programs for trouble shooting etc Multi ple hot keys can be assigned to different MMI Projects different ladder logic programs within the MMI Project and different rungs within the ladder logic program FIG 9 describes the format of a hot key data struc ture This structure is usually created at the same time the MMI Project is constructed in the Development System The first field 122 defines the keystrokes of the hot key The second field 124 contains an identifier which indicates that upon accepting the input key the Runtime System should display the ladder logic pro gram Other actions such as zooming the display of graphical objects downloading element values to the PLC or exiting the Runtime System could be specified as well The third field 126 provides a filename where a copy of the associated ladder logic program is stored and a station address of the PLC executing the ladder logic program The fourth field 128 identifies a specific rung if any within the ladder logic program Thus the operator can display a specific portion of a specific ladder logic p
17. o ciating them with graphical objects 48 FIG 4 describes a data structure for associating an element with a graphical object The data structure stores a copy of the graphical object 56 a station address for the PLC being monitored 58 an address for the element in the PLC 60 a polling interval 62 and identifiers that describe what animation actions occur when the element value changes 64 Each MMI Project may have multiple screens for display on the computer Each screen may have multiple objects thereon for display on the computer each accessing different elements in the same or different PLCs Runtime System FIG 5 is a flowchart illustrating the steps performed by the Runtime System in executing an MMI Project The Runtime System provides an execution environ ment for the MMI Project The Runtime System itself is executed by a computer which communicates with one or more PLCs The MMI Project is loaded 66 and the initial screen of graphical objects is displayed 68 The Runtime System initializes polling intervals for each object con tained in the MMI Project 70 A loop is entered whereby the Runtime System waits for an event to occur 72 The Runtime System may handle a number of different events the triggering of a polling interval 74 76 a response from the PLC 78 80 or the entry of commands by the operator from the computer 5 321 829 5 82 84 As indicated in FIG 5 details regarding each of
18. ogram on the monitor at a specific rung thereof 6 The method of claim 5 further comprising the step of preventing unauthorized access to the copy of the ladder logic program 7 A computer based apparatus for monitoring a lad der logic program executing in a programmable logic controller comprising a a computer coupled to the programmable logic controller having a monitor attached thereto the computer further comprising 1 means for drawing a graphical object on the monitor to depict a plant process controlled by the programmable logic controller wherein the graphical object is selected from a group com prising Bargraph objects Numeric objects Trend objects and State Change objects 2 means for electronically associating the graphi cal object to an element of the ladder logic pro gram 3 means for setting a polling interval on the com puter to indicate how often a value for the asso ciated element is to be retrieved from the ladder logic program executing in the programmable logic controller 4 means for specifying action parameters on the computer to indicate changes in the display of the graphical object that are to occur in response to changes in the value of the associated element 5 means for responding to a triggering of the polling interval by transmitting a query message from the computer to the programmable logic controller requesting a value for the associated element and by resetting the polling
19. r other control devices to monitor the status of ladder logic programs trouble shoot ladder logic programs force the status of I O status bits ON or OFF to simulate events and perform a number of other functions However prior art sys tems offer little else in the way of monitoring aids For example in the prior art an engineer must manu ally build a monitoring system separately from the de velopment of the ladder logic program Thus the engi neer constructs a database for the monitoring system including PLC stations element addresses possibly including textual descriptions and other information such as scaling factors and mathematical functions to be performed Further keeping these separate entities i e the monitoring systems the databases and the ladder logic programs synchronized can be a logistical night mare Furthermore in the prior art an engineer does not have access during monitoring to the ladder logic pro gram Thus the engineer typically must exit the moni toring system and separately access the ladder logic program or use a separate computer for accessing the ladder logic program SUMMARY OF THE INVENTION To overcome the limitations in the prior art described above and to overcome other limitations that will be come apparent upon reading and understanding this specification the present invention discloses a menu driven system for developing Man Machine Interfaces MMI to graphically monitor lad
20. rogram for troubleshooting purposes if the Man Machine Interface indicates a problem FIG 8 illustrates the steps performed by the Runtime System when the operator inputs a hot key request ing access to the ladder logic program The first step is to check security to prevent unauthorized access to the 5 15 20 25 35 40 45 55 65 6 ladder logic system 112 This security system is an extension of the one described in the co pending and commonly assigned patent application Ser No 07 375 270 filed Jun 30 1989 now U S Pat No 5 127 099 by Scott C Zifferer et al entitled METHOD AND APPARATUS FOR SECURING ACCESS TO A LADDER LOGIC PROGRAM MING AND MONITORING SYSTEM which ap plication is incorporated herein by reference Once the security check is passed the Runtime Sys tem accesses the hot key data structure of FIG 9 and extracts the filename and rung number therefrom to access and display the ladder logic program on the computer screen in place of the Man Machine Interface 114 During the display of the ladder logic program the operator has access to a number of search functions 116 including those described in the co pending and commonly assigned patent application Ser No 07 375 059 filed Jun 30 1989 now abandoned by Scott C Zifferer et al entitled METHOD AND APPARA TUS FOR CROSS REFERENCE SEARCHING IN LADDER LOGIC PROGRAMS which application is incorporated herein by reference
21. s in corporated herein by reference The third method is to press an input key designated as the LADDER LOGIC key to display the ladder logic program on the computer screen 50 The user can search through the ladder logic program for the desired element using any of the functions described in the co pending and commonly assigned patent applica tion Ser No 07 375 059 filed Jun 30 1989 now aban doned by Scott C Zifferer et al entitled METHOD AND APPARATUS FOR CROSS REFERENCE SEARCHING IN LADDER LOGIC PROGRAMS which application is incorporated herein by reference 52 The user also can search through the ladder logic program for the desired element using mouse input device as described in the MMI Logistics TM User s Manual ICOM Inc which manual is incorporated herein by reference 52 Once the desired element is found the user ties the object to the element via a Point and Click operation using a mouse input device 54 In the Point and Click operation the Development System recognizes which instruction and element of the ladder logic program are currently displayed under the cursor of the mouse input device The element address is thus read from the ladder logic program Because the Point and Click operation requires no keyboard input it eliminates most common typographical errors After the desired element has been selected by one of the three methods it is stored in a data structure ass
22. the graphical object on a monitor at tached to the computer to depict a plant process controlled by the programmable logic controller the graphical object being electronically associated with an element of the ladder logic program wherein the graphical object is one selected from a group comprising Bargraph objects Numeric ob jects Trend objects and State Change objects b initializing a polling interval on the computer for the graphical object the polling interval indicating a waiting period before a value for the associated element is to be retrieved from the programmable logic controller c responding to a triggering of the polling interval by transmitting a query message from the computer to the programmable logic controller requesting a value for the associated element and by resetting the polling interval I WN 20 25 30 35 45 50 55 60 65 8 d responding to the value being input from the pro grammable logic controller to the computer by changing the display of the graphical object in accordance with a change in the value of the asso ciated element and e responding to an entry of a hot key command by an operator of the computer by accessing a hot key data structure in the computer and extracting therefrom an electronically stored filename which identifies a location on the computer where a copy of the ladder logic program is stored and display ing the copy of the ladder logic pr
23. x for con structing the Man Machine Interfaces Ladder logic programs and databases associated therewith are im ported and accessed by the Development System for use in the development of the Man Machine Interfaces A Runtime System provides an execution environment for the Man Machine Interfaces The Runtime System has the ability to access ladder logic programs during monitoring operations A user can hot key to the ladder logic program for trouble shooting purposes 12 Claims 9 Drawing Sheets U S Patent June 14 1994 Sheet 1 of 9 5 321 829 LADDER LOGIC PROGRAMS DO NOT MATCH FIX GRAPHICS FIG 1 U S Patent June 14 1994 Sheet 2 of 9 5 321 829 DEFINE PLCs AND LADDER LOGIC PROGRAMS PROJECT FIG 2 U S Patent June 14 1994 Sheet 3 of 9 5 321 829 388 TIE ELEMENT TO OBJECT DISPLAY LIST FIG U S Patent June 14 1994 Sheet 4 0f9 5 321 829 58 60 62 64 pic ELEMENT POLLING ADDRESS INTERVAL 56 GRAPHICAL OBJECT FIG 4 U S Patent June 14 1994 Sheet 5 of 9 5 321 829 RESPONSE FIG 7 FIG 5 U S Patent June 14 1994 Sheet 6 of 9 5 321 829 86 SEND QUERY TO PLC RESET POLLING INTERVAL 86 FIG 6 U S Patent June 14 1994 Sheet 7 of 9 5 321 829 92 FIND OBJECT ASSOCIATED 94 98 IF ALTERED 102 106 DRAW REDRAW STATE CHANGE 110 FIG 7 U S Patent June 14 1994 Sheet 8 of 9 5 321 829 112 114
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