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1、PLC1.AboutProgrammableLogicControllers(PLCs)PLCs(programmablelogiccontrollers)arethecontrolhubsforawidevarietyofautomatedsystemsandprocesses.Theycontainmultipleinputsandoutputsthatusetransistorsandothercircuitrytosimulateswitchesandrelaystocontrolequip
2、ment.Theyareprogrammableviasoftwareinterfacedviastandardcomputerinterfacesandproprietarylanguagesandnetworkoptions.ProgrammablelogiccontrollersI/Ochannelspecificationsincludetotalnumberofpoints,numberofinputsandoutputs,abilitytoexpand,andmaximumnumbero
3、fchannels. Numberofpointsisthesumoftheinputsandtheoutputs.PLCsmaybespecifiedbyanypossiblecombinationofthesevalues. Expandableunitsmaybestackedorlinkedtogethertoincreasetotalcontrolcapacity. Maximumnumberofchannelsreferstothemaximumtotalnumberofinputand
4、outputchannelsinanexpandedsystem. PLCsystemspecificationstoconsiderincludescantime,numberofinstructions,datamemory,andprogrammemory. ScantimeisthetimerequiredbythePLCtocheckthestatesofitsinputsandoutputs. Instructionsarestandardoperations(suchasmathfun
5、ctions)availabletoPLCsoftware. Datamemoryisthecapacityfordatastorage. Programmemoryisthecapacityforcontrolsoftware.AvailableinputsforprogrammablelogiccontrollersincludeDC,AC,analog,thermocouple,RTD,frequencyorpulse,transistor,andinterruptinputs. Output
6、sforPLCsincludeDC,AC,relay,analog,frequencyorpulse,ProgrammingoptionsforPLCsincludefrontpanel,handheld,andcomputer.Programmablelogiccontrollersuseavarietyofsoftwareprogramminglanguagesforcontrol. TheseincludeIEC61131-3,sequentialfunctionchart(SFC),func
7、tionblockdiagram(FBD),ladderdiagram(LD),structuredtext(ST),instructionlist(IL),relayladderlogic(RLL),flowchart,C,andBasic. TheIEC61131-3programmingenvironmentprovidessupportforfivelanguagesspecifiedbytheglobalstandard:SequentialFunctionChart,FunctionBl
8、ockDiagram,LadderDiagram,StructuredText,andInstructionList.Thisallowsformulti-vendorcompatibilityandmulti-languageprogramming. SFCisagraphicallanguagethatprovidescoordinationofprogram16/16sequences,supportingalternativesequenceselection
