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1、附 录Programmable logic controllerFrom Wikipedia, the free encyclopedia PLC & input/output arrangements Aprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactor
2、yassemblylines,controlofamusementrides,orcontroloflightingfixtures.PLCsareusedinmanydifferentindustriesandmachinessuchaspackagingandsemiconductormachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatur
3、eranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboun
4、dedtime,otherwiseunintendedoperationwillresult.FeaturesControlpanelwithPLC(greyelementsinthecenter).Theunitconsistsofseparateelements,fromlefttoright;powersupply,controller,relayunitsforin-andoutput.ThemaindifferencefromothercomputersisthatPLCsarearmoredfors
5、evereconditions(dust,moisture,heat,cold,etc)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsyst
6、ems.Someevenusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/Omodulesattachedtoacomputernetworkth
7、atplugsintothePLC.18/18SystemscaleAsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodeldoesnothaveenoughI/O.ModularPLCshaveachassis(alsocalledarack)intowhichareplacedmoduleswithdifferentfunctions.
8、TheprocessorandselectionofI/Omodulesiscustomisedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkis
