Programming system for programmable logic controller
Contents
1. 1 A system for generating a display useful for creat ing a program for a programmable logic controller PLC the program comprising one or more alphanu meric ladder logic commands the PLC being adapted to execute said ladder logic commands each ladder logic command comprising an output variable and logic specifications indicating the manner in which the output variable is to be determined the system comprising 5 237 652 9 means for accepting user input data specifying one or more ladder logic commands in an alphanumeric format and means for converting one or more selected ladder logic commands in alphanumeric format into a corresponding ladder logic graph and for display ing said ladder logic graph the ladder logic graph including an output symbol corresponding to the output variable and one or more connection sym bols that graphically indicate the logic specifica tions of the selected ladder logic command 2 The system of claim 1 wherein each ladder logic command comprises one or more input variables and wherein the ladder logic graph includes an input sym bol corresponding to each input variable 3 The system of claim 2 wherein each connection symbol comprises one or more line like elements inter connecting the input and output symbols 4 The system of claim 3 wherein the logic specifica tions include an AND specifier specifying a logical AND operation an OR specifier specifying a logical OR condition2. Door SwTime 5 237 652 5 continued indicates that the oven will remain on until a predetermined temperature is reached This input is true in temperature mode false in timer mode This input has the value true when the mechanical timer has counted down to zero and otherwise has the value false This input has the value true when the temperature has reached a preselected level and otherwise has the value false The oven is on when this output is true and off when this output is false As described below this internal variable represents an internal relay that is not associated with any IO point Its significance is further described below This internal variable represents a second internal relay SwTemp TimeZero AtTemp MicroWaves Warming TimerRunning In the present system the fact that the Warming and Timer Running are internal variables is indicated by the numbers of the IO points 119 and 118 associated with these variables An internal variable can be both an input variable and an output variable For example in line 10 internal variable Warming is an output variable while in lines 12 and 13 internal variable Warming is an input variable Line 10 of the PLC program set forth above states that when input SwTemp is true AND input AtTemp is false then internal variable Warming is true When this ladder logic command is selected in block 86of FIG 5 display function 88 determines and produces t
3. line 254 The next token the variable name B results in the drawing of input symbol 256 The next token is a comma and block 228 therefore sets the logic flag equal to OR draws line segment 258 and moves the current 20 25 30 35 40 45 50 55 60 65 8 grid pointer back to point 252 The next token is the input name C and input symbol 260 is then drawn after which the program encounters the right parentheses token This causes the grid pointer to move down in block 224 drawing line segment 262 The process then continues as previously described with the production of line 264 and output symbol 266 Most PLCs include means for emulating commonly used electronic hardware elements such as counters and timers Each timer or counter has an associated variable that controls its operation Such variables will herein be referred to as timer IO points and counter IO points Tespectively A timer counts up or down from a preset value for as long as its associated timer IO point is on Counters on the other hand will count one count whenever the associated counter IO point makes a tran sition from off to on The output of a timer or counter can be in normal one shot toggle or latch mode as with a conventional output variable In a preferred embodiment the programming system of the present invention uses a two step process to cre ate a ladder logic graph that includes a counter or timer For example the following
4. to a grid element at the left center of the drawing area Block 200 then proceeds to draw the entry point for the ladder logic graph the entry point being shown by reference number 136 in FIG 6c Each time that a given graph element is drawn the grid pointer is updated in a corresponding manner Thus in the case of entry point 136 the grid pointer would be incremented to specify that the next portion of the ladder logic graph will be in the middie row second column of the grid array The program diagrammed in FIG 7 then proceeds in block 202 to obtain the next token from the alphanu meric ladder logic command A token is a unit of the ladder logic command Thus for program line 12 the tokens are the variable names TimerRunning Warming Door Start and Microwaves and the punctuation sym bols lt comma period backslash and gt Block 204 examines the next token to determine whether it is a variable name or a punctuation symbol In program line 12 the first token is the punctuation symbol lt so that program control flows to block 206 that is further dia grammed in FIG 8 For the token lt block 232 will cause all flags to be reset to their default values since 5 237 652 7 this token corresponds to the beginning of a program line Control will then return to block 212 in FIG 7 and from there back to block 202 to retrieve the next token The next token is the variable name TimerRunning so that block 20
5. 88 returns the user to 20 25 30 35 40 45 50 55 65 4 input edit module 80 from which point the user may save the PLC program select other ladder logic com mands or download the PLC program to PLC 70 via download function 90 In order to provide a clear understanding of the oper ation of the present invention a preferred embodiment of display function 88 will now be described in connec tion with a specific PLC program The PLC program used for an example will be a program for controlling the operation of a conventional microwave oven The statements comprising the program are as follows 1 Start 1 2 Door 2 3 SwTime 3 4 SwTemp 4 5 TimeZero 5 6 AtTemp 0 7 MicroWaves 7 8 Warming 119 9 TimerRunning 118 10 lt SwTemp AtTemp gt Warming 11 lt SwTime TimeZero gt Timer Running 12 lt TimerRunning Warming Door Start gt MicroWaves L 13 lt Warming TimerRunning Door gt MicroWaves R The numbers in the left hand column above are line reference numbers inserted for the purpose of the pres ent description and do not form part of the PLC pro gram itself In a typical PLC each IO point is identified by a unique number and such numbers could be used in the program created by computer 60 However to make the PLC programs more readily comprehensible a preferred embodiment of the present invention provides a method for associating an alphanumeric label with each IO point number
6. similar manner FIGS 6b 6c and 6d illustrate the ladder logic graphs created by display function 88 for PLC program lines 11 13 respectively FIG 6b illus trates the setting of the internal variable TimerRunning FIG 6c shows how the microwave oven is turned on while FIG 6d shows how the microwave oven is turned off Examining FIG 6c in detail this ladder logic graph includes input symbols 122 124 126 and 128 and output symbol 130 Input symbols 122 and 124 corre 10 15 20 25 30 35 40 45 50 55 60 65 6 spond to the internal variables TimerRunning and Warming respectively These input symbols are con nected in parallel by line segments 132 and 134 respec tively indicating a logical OR operation Line segment 132 includes segment 136 that indicates the beginning of a ladder logic graph The symbol L in output symbol 130 reflects the latch indicator L appearing at the end of program line 12 The latch indicator specifies that the Microwaves output will be latched into the true state by the PLC whenever the conditions shown in FIG 6c are satisfied Thus an operator of the microwave oven is only required to momentarily depress the Start switch in order to turn the oven on In a preferred embodiment of the invention each output variable can have one of four modes These modes are normal N one shot O toggle T and latch L The N mode is the default and causes the output var
7. two statements added to a PLC program gt tm0 10 502 TA lt A B gt tm0 would produce the ladder logic graph shown in FIG 10 The first statement set forth above beginning with a right hand angle bracket is a definition of a particular timer here labeled tm0 The four parameters following the timer name provide the preset value the value set into the timer or counter when the timer is reset the name of the output variable C controlled by the timer and the output mode N O T or L The fourth parame ter A is an optional parameter When included it will cause the timer or counter to be automatically reset when the count hits zero FIG 10 illustrates the ladder logic graph correspond ing to the second program statement set forth above As shown the timer symbol tm0 produces the display of a timer symbol 280 that includes a rectangle with the timer label tm0 above the rectangle the preset and auto parameters within the rectangle and the output variable C to the right of the timer symbol connected to it by line segment 282 A similar display would be created for a counter While the preferred embodiments of the invention have been illustrated and described variations will be apparent to those skilled in the art Accordingly the scope of the invention is to be determined by reference to the following claims The embodiments of the invention in which an exclu sive property or privilege is claimed are defined as follows
8. 4 passes control to block 208 which draws the corresponding input symbol 122 shown in FIG 6c Block 210 then resets all flags and control again returns to block 202 to retrieve the next token Since the next token is a the result will be that block 288 FIG 8 will set the logic flag equal to OR The succeeding token the variable name Warming will cause control to be passed to block 208 Since the OR flag is set block 208 moves the grid pointer down one row and then draws input symbol 124 The next token resets the logic flag to AND in block 226 so that the next token Door causes block 208 to be entered with the logic flag equal to AND As a result input symbol 126 and line segment 134 are drawn to the right of the prior symbol 124 without changing rows A similar process occurs for input symbol 128 The token gt then results in the drawing of output symbol 130 and the line interconnecting this output symbol with input sym bol 128 at which point the display function is complete FIG 8 also illustrates the way in which the display function uses parentheses which may be used in the ladder logic commands to group operations As indi cated by blocks 220 222 and 224 the token causes the current grid pointer to move up by one row assum ing that the OR logic flag has not been set The token causes the grid pointer to move back down one row FIG 8 also illustrates that in block 230 the bac
9. D S 88 LADDER LOGIC GRAPH S DISPLAY CORRESPONDING US005237652A Patent Number Date of Patent 5 237 652 Aug 17 1993 11 45 Bulletin No 1745 Allen Bradley Company Industrial Control Group vol 2 5 Jan 1987 19 pages User s Manual for 6200 Series Software PLC 2 Pro gramming Software Allen Bradley Co Inc 1988 Bryan E A and Bryan L A Programmable Con trollers Theory and Implementation promotion bro chure for the text Industrial Text Co Programmable Controller Bulletin No 943 Eagle Signal Controls Co Jan 1987 12 pages The GE Series One Programmable Controller Gen eral Electric Co product brochure Apr 1986 4 pages The GE Series Three Programmable Controller General Electric Co product brochure Apr 1986 4 pages Modicon 984 Family Control Products System prod uct description Modicon Inc Jan 1990 11 pages plus Fact Sheet 5 pages Primary Examiner Robert L Richardson Attorney Agent or Firm Christensen O Connor Johnson amp Kindness 57 ABSTRACT A system for creating a program for a programmable logic controller PLC The user inputs a ladder logic command in an alphanumeric format and the system creates a corresponding ladder logic graph on a display means The user may then verify the relationship de picted in the graph before forwarding the ladder logic command to the PLC 12 Claims 6 Draw
10. In the example set forth above lines 1 9 are examples of labeling commands that pro vide such labels all labeling commands beginning with the symbol Thus for example in line 1 above the alphanumeric label Start is associated with IO point 1 The symbol lt indicates a ladder logic command Thus in the program listed above lines 10 13 are the ladder logic commands Within each ladder logic com mand a period represents a logical AND operation a comma represents a logical OR operation a backslash represents a logical NOT operation and the gt sym bol indicates that the following symbol is the output of that command Thus for example line 10 states that when input variable SwTemp is true AND input vari able AtTemp is not true then output variable Warming is true It will be assumed that the input and output variables shown in the above listed program have the following meanings This input has the value true when an operator presses the start switch of the microwave oven and has the value false whenever the start switch is not being pressed This input has the value true when the door of the microwave oven is closed and has the value false when the door is open This input has the value true when the microwave oven is in timer mode and the value false when the oven is in temperature mode Timer mode implies that the oven will be on for a specified length of time while temperature mode Start
11. R EXIT US Patent Aug 17 1993 Sheet 6 of 6 5 237 652 256 254 BY A c 258 T 252 X ogo 7262 264 D ms m 10 50 GT AL amp 282 TR 284 FIG 10 5 237 652 1 PROGRAMMING SYSTEM FOR PROGRAMMABLE LOGIC CONTROLLER FIELD OF THE INVENTION The present invention relates to programmable logic controllers also known as programmable controllers and in particular to techniques for creating programs for such controllers BACKGROUND OF THE INVENTION Programmable logic controllers PLCs are special ized data processors that are in widespread use in a variety of fields for example as controllers for machine tools material handling and assembling systems for molding and casting machines and for robotics systems FIG 1 presents a simplified diagram of a PLC 12 that includes a plurality of input terminals 14 a plurality of output terminals 16 microprocessor 20 and memory 22 Input terminals 14 and output terminals 16 are col lectively referred to as IO points PLC 12 operates under the control of a program stored in memory 22 When incorporated into an appli cation each input terminal 14 will be connected to a switch a relay contact or some other device capable of providing an electrical signal indicating one of two States such as open vs closed in the case of a switch or relay contact or high vs low in the case of a voltage signal Each output terminal 16 is connected to a device such as a status ind
12. United States Patent 115 McManus 54 PROGRAMMING SYSTEM FOR PROGRAMMABLE LOGIC CONTROLLER 76 Inventor Kirby L McManus 15216 NE 110th Pi Redmond Wash 98052 21 Appl No 620 640 22 Filed Nov 30 1990 51 Int CLS oc cccseseees GOS5B 9 02 GO6F 3 00 52 U S Chl secicsscsecsncuscotececosccetessicees 395 155 364 147 58 Field of Search 364 DIG 1 DIG 2 364 147 395 500 162 140 155 56 References Cited U S PATENT DOCUMENTS 4 038 562 7 1977 Kintner ccssssscsseeesseeees 307 203 4 326 207 4 1982 Suda etal sssesesssssssssosssssss 364 900 4 608 628 8 1986 Saito et al occ 364 141 4 703 414 10 1987 Inoue et al oo 364 147 4 792 918 12 1988 Hirase et al 364 900 4 845 627 7 1989 Nadolski et al 364 468 4 991 076 2 1991 Zifferer et al 364 147 5 062 052 10 1991 Sparer et al wuss 364 473 FOREIGN PATENT DOCUMENTS 971252 7 1975 Canada ccssecssersssssesssseeees 342 21 985396 3 1976 Canada oc sccsesesesessesesesees 340 162 3808135 9 1989 Fed Rep of Germany a 1 00 89 06010 6 1989 PCT Int l Appl ws 9 06 89 09961 10 1989 PCT Intl Appl uu 7 00 89 09976 10 1989 PCT Intl Appl cscs 15 60 2099667 3 1981 United Kingdom 08 3 153 2112974 11 1981 United Kingdom 6000 11 32 OTHER PUBLICATIONS SLC 150 Programmable Controler Product data 80 INPUT EDIT PLC PROGRAM SELECT LADDER LOGIC COMMAN
13. and wherein the line like elements con nect a pair of input symbols in parallel to signify a logi cal OR operation and interconnect a pair of input sym bols in series to signify a logical AND operation 5 The system of claim 1 wherein each input and output symbol includes an identifier for the symbol in an alphanumeric format 6 The system of claim 1 wherein the ladder logic graph comprises a two dimensional array of grid ele ments and wherein each input and each output symbol occupies one grid element 7 A method of generating a display useful for creat ing a program for a programmable logic controller PLC the program comprising one or more alphanu meric ladder logic commands the PLC being adapted to execute said ladder logic commands each ladder 5 15 20 25 30 35 45 50 35 65 10 logic command comprising an output variable and logic specifications indicating the manner in which the output variable is to be determined the method comprising the steps of accepting user input data specifying one or more ladder logic commands in analphanumeric format and converting the one or more specified ladder logic commands in alphanumeric format into a corre sponding ladder logic graph the ladder logic graph including an output symbol corresponding to the output variable and one or more connection sym bols that graphically indicate the logic specifica tions of the selected ladder logic format a
14. connection provided by line seg ments 50 and 52 indicates the logical operation OR As with FIG 2 the graph of FIG 3 can be interpreted by noting that a signal present on line segment 50 will reach output Cifeitherswitch A or switch B is closed In general two prior methods have been used to permit a user to create ladder logic programs for PLCs In a first method the PLC includes an input module that includes a small keyboard The keyboard typically includes numeric keys function keys and a small num ber of other keys representing different types of vari ables The program is entered directly into memory 22 via such a keyboard For example to enter the ladder logic statement corresponding to FIG 3 a user might enter an identifier for input variable A then press a function key specifying a logical OR operation then enter a symbol for input variable B and finally a symbol for output variable C A second prior technique for programming PLCs involves creating the ladder logic program on a sepa rate device such a personal computer and then trans ferring the program to memory 22 via a serial data link or the like In such systems the operator of personal computer draws a ladder logic graph directly on the computer display screen one graphical element at a time When a given ladder logic graph is completed the PC software converts the graph into the corresponding ladder logic command The ladder logic commands are then transferre
15. d to the PLC and stored in memory 22 SUMMARY OF THE INVENTION The present invention provides a system for creating a program for a programmable logic controller PLC Using the invention a user inputs a ladder logic com mand in an alphanumeric format and the system then creates a corresponding ladder logic graph on a suitable display means The user can then verify the relationship between input and output variables depicted in the graph and if necessary edit the alphanumeric command until the graph depicts the desired relationship Thus both program entry and program verification take place in the most convenient respective formats i e alphanu meric for program entry and graphical for program verification In a preferred embodiment the system creates a pro gram comprising one or more alphanumeric ladder logic commands Each command includes an output variable and logic specifications indicating the manner in which the output variable is to be determined The system comprises means for accepting user input data specifying one or more ladder logic commands in an alphanumeric format and means for converting one or more selected ladder logic commands into a corre 5 237 652 3 sponding ladder logic graph The ladder logic graphs are displayed and permit verification of the ladder logic commands The display is preferably via a computer display screen although other display means such as printers may also be used Each lad
16. der logic graph 30 is shown in FIG 2 The ladder logic graph comprises input symbols 32 and 34 and output symbol 36 interconnected by line segments 40 and 42 Input symbols 32 and 34 are as signed labels or variable names A and B respectively while variable name C is assigned to output symbol 36 In the convention used in ladder logic graphs the graph of FIG 2 is equivalent to the statement that output C is on if and only if inputs A and B are both on i e the equivalent of a logical AND operation between inputs A and B to produce output C The fact that inputs A 10 15 20 25 30 35 40 45 50 55 60 65 2 and B are ANDed together is indicated by the fact that their input symbols are connected in series by line seg ment 40 Line segment 42 indicates that the result of A AND B controls output C Input symbols A and B can be regarded as normally open i e off switches and the ladder logic graph can be interpreted by noting that a voltage or other signal present on line segment 44 to the left of the input symbols will reach output point C if and only if switches A and B are both closed i e on FIG 3 represents a second ladder logic graph with inputs A B and output C except that now inputs A and B are connected in parallel by line segments 50 and 52 This ladder logic graph is equivalent to the statement that output C will be on if either input A or input B is on Thus the parallel
17. der logic graph includes an output symbol corresponding to the output variable and one or more connection symbols that graphically indicate the logic specifications of the selected command Typically each ladder logic command also includes one or more input variables and each ladder logic graph includes an input symbol corresponding to each input variable In a pre ferred embodiment the connection symbols include line like elements that connect a pair of input symbols in series to indicate a logical AND operation and that connect a pair of input symbols in parallel to indicate a logical OR operation BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a simplified diagram of a programmable logic controller FIGS 2 and 3 show examples of ladder logic graphs FIG 4 is a block diagram illustrating the use of a separate computer to produce the PLC program FIG 5 is a block diagram of a preferred embodiment of the programming system of the invention FIGS 6a 6d illustrate ladder logic graphs corre sponding to a particular PLC program FIG 7 is a flow chart showing the overall operation of a preferred embodiment of the invention FIG 8 is a flow chart of the punctuation processing module FIG 9 illustrates a ladder logic graph derived from a ladder logic command using parentheses and FIG 10 is a ladder logic graph that includes a timer symbol DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The programming system of the prese
18. he ladder logic graph shown in FIG 6a in which the ladder logic command has been translated into an equivalent ladder logic graph The ladder logic graph comprises input symbols 102 and 104 output symbol 106 and line seg ments 108 110 and 112 Each input symbol comprises a pair of spaced vertical bars the name of the correspond ing input variable above the bars and the horizontal lines to either side of the vertical bars Output symbol 106 is similar except that it includes parentheses rather than vertical bars Input symbol 104 includes a back slash that reflects the logical NOT symbol appearing with this in front of the AtTemp variable in program line 10 Line segment 108 simply indicates the beginning of the ladder logic graph line segment 110 indicates a logical AND operation corresponding to the period in program line 10 and line segment 112 connects the input symbols and their associated logic with output symbol 106 For many if not most PLC programmers the graphi cal display shown in FIG 6a can be more readily com prehended than the textual command shown in program line 10 above However the programming system of the present invention does not require the user to painstak ingly create a ladder logic graph on the computer screen Instead the invention requires the user to per form the far simpler task of typing in an alphanumeric command and the corresponding ladder logic graph may then be created automatically In a
19. iable to be on when its input is on and off when its input is off Latch mode causes the output variable to remain on even after its input is off For an output variable that has been latched the vari able is turned off using the R symbol Finally one shot mode indicates that when its input is on the output variable remains on for one complete scan and then is turned off As indicated in FIG 6d the microwave output will be reset R whenever the door is open or whenever the oven reaches the preset temperature in temperature mode or the end of a preset interval in timer mode Comparing program lines 10 13 with FIGS 6a 6d readily indicates the improvement in program writing and debugging that is made possible by the present invention FIGS 7 and 8 set forth a flow chart for a preferred embodiment of the display function The operation of these flow charts will be illustrated by program line 12 above and the corresponding ladder logic graph shown in FIG 6c The display function of FIG 7 begins with an initialization step in block 200 Preferably the dis play area on which the ladder logic graph will be drawn is divided into a two dimensional array of grid elements with each grid element being large enough to display an input or output symbol On a conventional 80 x 25 com puter monitor a suitable size for each grid element is eight characters wide and two characters high Block 200 initializes a current grid pointer preferably
20. icator a relay coil etc that is to be controlled by the PLC The PLC operates by repetitively executing what is termed a scan During each scan the PLC reads the signals present at input terminals 14 to determine what will here be referred to as input data Next the PLC combines the input data with the program stored in memory 22 to determine the corresponding output data Finally the PLC uses the output data to set the values of the signals at output terminals 16 and then proceeds to start the next scan The program stored in memory 22 and used by the PLC to control its operation is typically expressed in what is termed ladder logic format Each ladder logic program comprises one or more ladder logic state ments In the PLC art these ladder logic statements are often termed rungs Each ladder logic statement de fines the relationship between an output variable and in most cases One or more input variables Input variables include variables corresponding to the signals at input terminals 14 while output variables include variables corresponding to the signals at output terminals 16 Other types of input and output variables are described below A simple ladder logic statement might indicate that a particular output variable is on if and only if input variables 1 and 2 are both on For easier comprehension ladder logic statements and programs are often expressed in terms of ladder logic graphs A simple lad
21. ing Sheets 82 LOAD PROGRAM SAVE PROGRAM 84 DOWNLOAD PROGRAM TO PLC US Patent Aug 17 1993 Sheet 1 of 6 5 237 652 12 PROGRAMMABLE LOGIC 14 CONTROLLER 20 Soe 16 FIG 1 po 9 44 A AA c gt 327 ve 42 96 FIG 2 j C Ar i 50 52 US Patent Aug 17 1993 Sheet 2 of 6 5 237 652 COMPUTER S 70 62 64 66 80 INPUT EDIT PLC PROGRAM 86 SELECT LADDER LOGIC COMMAND S DOWNLOAD PROGRAM TO PLC 88 DISPLAY CORRESPONDING LADDER LOGIC GRAPH S FIG 5 US Patent Aug 17 1993 Sheet 3 of 6 5 237 652 110 108 SwTemp C AtTemp 112 Warming eae cae ai 1024 104 ae FIC Ga SwTime TimeZero TimerRunning P 122 136 TimerRunning l 134 1 32 Warming Door Start MicroWaves al be t ic ae 124 126 128 130 FIG 6c Warming TimerRunning Nea Door MicroWaves NS R FIC 6d U S Patent Aug 17 1993 Sheet 4 of 6 5 237 652 DISPLAY FUNCTION INITIALIZATION DRAW _ENTRY POINT GET NEXT TOKEN PUNCTUATION VARIABLE NAME DRAW SYMBOL RESET FLAGS PROCESS PUNCTUATION FIG 7 US Patent Aug 17 1993 Sheet 5 of 6 5 237 652 PROCESS _ FIG 8 220 YES YES ERROR NO 222 SAVE POINTER MOVE UP MOVE DOWN SET LOGIC AND NO SET LOGIC OR MOVE POINTER NO SET SENSE FLAG NO RESET FLAGS TO DEFAULTS NO DRAW OUTPUT SYMBOL ERRO
22. kslash token causes a sense flag to be set which flag is used by draw symbol block 208 to place a backslash in the input symbol In a preferred embodiment the present invention provides for the use of parentheses to specify the order in which logical operations are to be carried out For example a ladder logic statement of the following form lt a b c gt d would normally be evaluated by ANDing variables a and b and then ORing the result with variable c to produce variable d However the sequence of opera tions can be modified using parentheses as follows lt a b c gt d In this case variables b and c are ORed and the result is then ANDed with variable a to produce variable d In a preferred embodiment of the present invention this above statement with parenthesis would produce the ladder logic graph shown in FIG 9 Referring to the flow chart of FIGS 7 and 8 the entry point symbol would be drawn in block 200 and input symbol 250 is then drawn in block 208 In this graph the entry point symbol merges with the horizontal line at the left of the input symbol The following token is a period and block 226 therefore sets the logic flag equal to AND The next token is a left parentheses so the control is then passed to block 220 that tests the logic flag Since the logic flag is not equal to OR block 222 causes the current grid pointer position 252 to be saved and then moves the grid pointer up one grid element drawing
23. nd displaying said ladder logic graph 8 The method of claim 7 wherein each ladder logic command comprises one or more input variables and wherein the ladder logic graph includes an input sym bol corresponding to each input variable 9 The method of claim 8 wherein each connection symbol comprises one or more line like elements inter connecting the input and output symbols 10 The method of claim 9 wherein the logic specifi cations include an AND specifier specifying a logical AND operation an OR specifier specifying a logical OR condition and wherein the line like elements con nect a pair of input symbols in parallel to signify a logi cal OR operation and interconnect a pair of input sym bols in series to signify a logical AND operation 11 The method of claim 7 wherein each input and output symbol includes an identifier for the symbol in an alphanumeric format 12 The method of claim 7 wherein the ladder logic graph comprises a two dimensional array of grid ele ments and wherein each input and each output symbol occupies one grid element k g
24. nt invention is outlined in FIGS 4 and 5 Referring initially to FIG 4 the PLC program is initially created on computer 60 that includes display e g CRT monitor 62 input sys tem e g keyboard 64 and memory 66 An operator uses computer 60 to create a suitable PLC program optionally stores the program in memory 66 and then downloads the program to PLC 70 via link 72 that may comprise a standard RS 232 serial line FIG 5 outlines a preferred embodiment of a program for controlling computer 60 in accordance with the present invention The program comprises input edit module 80 that may essentially comprise a text editor or the like The input edit module permits a user to input a new PLC program or to edit a preexisting PLC pro gram in an alphanumeric format A preexisting pro gram to be edited may be loaded from memory 66 via load function 82 and a new or edited program may be saved in memory 66 by save function 84 During the input or editing process the user may select one or more of the ladder logic commands via select function 86 When a given ladder logic command is selected display function 88 determines and displays the corresponding ladder logic graph on display 62 This display function is described in detail below Dis play function 88 essentially allows the user to visualize each ladder logic command so that the program input and editing process becomes more intuitive and reliable Exiting from display function
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