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1、PLC1.AboutProgrammableLogicControllers(PLCs)PLCs(programmablelogiccontrollers)arethecontrolhubsforawidevarietyofautomatedsystemsandprocesses.Theycontainmultipleinputsandoutputsthatusetransistorsandothercircuitrytosimulateswitchesandrelaystocontrolequipment.Theyareprogrammabl
2、eviasoftwareinterfacedviastandardcomputerinterfacesandproprietarylanguagesandnetworkoptions.ProgrammablelogiccontrollersI/Ochannelspecificationsincludetotalnumberofpoints,numberofinputsandoutputs,abilitytoexpand,andmaximumnumberofchannels. Numberofpointsisthesumoftheinputsan
3、dtheoutputs.PLCsmaybespecifiedbyanypossiblecombinationofthesevalues. Expandableunitsmaybestackedorlinkedtogethertoincreasetotalcontrolcapacity. Maximumnumberofchannelsreferstothemaximumtotalnumberofinputandoutputchannelsinanexpandedsystem. PLCsystemspecificationstoconsiderin
4、cludescantime,numberofinstructions,datamemory,andprogrammemory. ScantimeisthetimerequiredbythePLCtocheckthestatesofitsinputsandoutputs. Instructionsarestandardoperations(suchasmathfunctions)availabletoPLCsoftware. Datamemoryisthecapacityfordatastorage. Programmemoryisthecapa
5、cityforcontrolsoftware.AvailableinputsforprogrammablelogiccontrollersincludeDC,AC,analog,thermocouple,RTD,frequencyorpulse,transistor,andinterruptinputs. OutputsforPLCsincludeDC,AC,relay,analog,frequencyorpulse,ProgrammingoptionsforPLCsincludefrontpanel,handheld,andcomputer.
6、Programmablelogiccontrollersuseavarietyofsoftwareprogramminglanguagesforcontrol. TheseincludeIEC61131-3,sequentialfunctionchart(SFC),functionblockdiagram(FBD),ladderdiagram(LD),structuredtext(ST),instructionlist(IL),relayladderlogic(RLL),flowchart,C,andBasic. TheIEC61131-3pr
7、ogrammingenvironmentprovidessupportforfivelanguagesspecifiedbytheglobalstandard:SequentialFunctionChart,FunctionBlockDiagram,LadderDiagram,StructuredText,andInstructionList.Thisallowsformulti-vendorcompatibilityandmulti-languageprogramming. SFCisagraphicallanguagethatprovide
8、scoordinationofprogram16/16sequences,supportingalternativesequenceselection
