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1、ProgrammableLogicControllers(PLCs)1。AboutProgrammableLogicControllers(PLCs)PLCs(programmablelogiccontrollers)arethecontrolhubsforawidevarietyofautomatedsystemsandprocesses.Theycontainmultipleinputsandoutputsthatusetransistorsandothercircuitrytosimulateswitches
2、andrelaystocontrolequipment.Theyareprogrammableviasoftwareinterfacedviastandardcomputerinterfacesandproprietarylanguagesandnetworkoptions.ProgrammablelogiccontrollersI/Ochannelspecificationsincludetotalnumberofpoints,numberofinputsandoutputs,abilitytoexpand,an
3、dmaximumnumberofchannels. Numberofpointsisthesumoftheinputsandtheoutputs.PLCsmaybespecifiedbyanypossiblecombinationofthesevalues. Expandableunitsmaybestackedorlinkedtogethertoincreasetotalcontrolcapacity. Maximumnumberofchannelsreferstothemaximumtotalnumberofi
4、nputandoutputchannelsinanexpandedsystem. PLCsystemspecificationstoconsiderincludescantime,numberofinstructions,datamemory,andprogrammemory. ScantimeisthetimerequiredbythePLCtocheckthestatesofitsinputsandoutputs. Instructionsarestandardoperations(suchasmathfunc
5、tions)availabletoPLCsoftware. Datamemoryisthecapacityfordatastorage. Programmemoryisthecapacityforcontrolsoftware.AvailableinputsforprogrammablelogiccontrollersincludeDC,AC,analog,thermocouple,RTD,frequencyorpulse,transistor,andinterruptinputs. OutputsforPLCsi
6、ncludeDC,AC,relay,analog,frequencyorpulse,transistor,andtriac. ProgrammingoptionsforPLCsincludefrontpanel,handheld,andcomputer.Programmablelogiccontrollersuseavarietyofsoftwareprogramminglanguagesforcontrol. TheseincludeIEC61131-3,sequentialfunctionchart(SFC),
7、functionblockdiagram(FBD),ladderdiagram(LD),structuredtext(ST),instructionlist(IL),relayladderlogic(RLL),flowchart,C,andBasic. TheIEC61131-3programmingenvironmentprovidessupportforfivelanguagesspecifiedbytheglobalstandard:SequentialFunctionChart,FunctionBlockD
8、iagram,LadderDiagram,StructuredText,andInstructionList.Thisallowsformulti-vendorcompatibilityandmulti-languageprogramming. SFCisagraphicallanguagethatprovidescoordinationofprograms