Plant control system and method of specifying primary-factor for interlock

There is provided a plant control system comprising: program editing means for creating a control program for determining an s interlock condition of at least one interlock circuit of a plant and for diagnosis of the at least one interlock circuit; plant control means for executing the control program; and interlock diagnosing means, having a logical operation unit and display unit, for updating and displaying whether an interlock condition of the at least one interlock circuit has 10 been met at a predetermined point in time before the present, determining a factor variable of the at least one interlock circuit as a variable of the at least one interlock circuit that has changed state, determining a trigger variable of the at least one interlock circuit as a factor variable that has changed state at a time after the interlock 15 condition has not been met, specifying a source factor variable from the trigger variable of the interlock condition not being met even if plural stages of the at least one interlock circuit are formed, and automatically displaying the source factor variable in a graphic language. DISPLAY E DEVICE KEYBOARD MOUSE] 10 CONTROL-PROGRAM INTERLOCK DIAGNOSING -20 EDITING MEANS MEANS PROGRAM COMPILER LOGICAL-EXPRESSION 21 30 DATABASEUNITACONVETN UNIT 2 12'~ LADDER-TO-LOGICAL-EXPRESSIOSSONIT CONVERTING UNIT _INTERLOCK-STATE PROCESIEN T 50 PROGRAM-LOAD CLIENT UNIT DIAGNOSIS DBDT FCTOR-SPECIFIED PRO G -EXUT SION 5 30 DATABASE UNIT MOP OCGLERI N -23 31 -- -CRE INTERLOCK OICAL-EXPRESSION F IT DIAGNOSIS FB CORNING UNIT 32 - - U NIT _ _I SO URC E I C M I I G U I I OBJECT FILE INTERLOCK LOGICAL-EXPRESSION 33'- OGCA-EPRSSIO DIAGNOSIS FB3 DATA ~~ TO-LADDER -25 IE UNIT CONVERTING UNIT INT ROCK DIAGNOSIS FB3 CODE 40 OBJECT MEMORY STANDARD-5 41- DATA MEMORYPRCSIGUT ~PROGRAM-LOAD -51 INTERLOCK ~-SERVER UNIT DIAGNOSIS FB DATA - ERE I I PROGRAM-EXECUTION -52 42- CODE MEMORY] PROCESSING UNIT L INTERLOCK PROCESS-INPUT/OUTPUT -53 DIAGNOSIS FB CODE PROCESSING UNIT _INTERLOCK-STATE 54 READOUT SERVER UNIT