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FoundationofPLC1.1THECENTRALPROCESSINGUNITAlthoughreferredtoasthebrainofthesystem,theCentralProcessingUnitinanormalinstallationistheunsunghero,buriedinacontrolcabinet,allbutforgotten.1.1.1BasicFunctionalityInaprogrammablecontrollersystem,thecentralprocessingunit(CPU)providesboththeheartandthebrainrequiredforsuccessfulandtimelycontrolexecution.Itrapidlyandefficientlyscansallofthesysteminputs,examinesandsolvestheapplicationlogic,andupdatesallofthesystemoutputs.Inaddition,italsogivesitselfacheckupeachscantoensurethatitsstructureisstillintact.Inthischapterwewillexaminethecentralprocessingunitasitrelatestotheentiresystem.IncludedwillbethevariousfunctionalblocksintheCPU,typicalscantechniques,I/Ointerfaceandmemoryusers,powersupplies,andsystemdiagnostics.1.1.2TypicalFunctionBlockInteractionsInpractice,thecentralprocessingunitcanvaryinitsarchitecture,butconsistsofthebasicbuildingblockstructureillustratedinFig.1.1.Theprocessingsectionconsistsofoneormoremicroprocessorsandtheirassociatedcircuitry.Whileitistruethatsomeoftheluxuryofusingmicroprocessors,mostmodernsystemsuseeitherasinglemicroprocessorssuchastheAMD2903,usedinabitslicearchitecture.Thismultiplemicroprocessorsystemtobreakthecontrolsystemtasksintomanysmallcomponentswhichcanbeexecutedinparallel.Theresultofthisapproachistoachieveexecutionspeedsthatareordersofmagnitudefasterthantheirsingle-taskingcounterparts.InadditiontoefficientlyprocessingdirectI/Ocontrolinformationandbeingprogrammable,therealadvantagethatmicroprocessor-basedsystemhaveovertheirhardwiredrelaycounterpartsistheabilitytoacquireandmanipulatenumericaldataeasily.Itisthisattributethatmakesprogrammablecontrollersthepowerhousesthattheyaretodayinsolvingtoughfactoryautomationproblems.Thefactoryoftomorrowwillrunefficientlyonlyifqualityinformationaboutprocessneedsandstatusoftheprocessequipmentareknownonarealtimesbasis.Thiscanandwillcomeaboutonlyiftheunitlevelcontrollers,includingprogrammablecontrollers,areempoweredwiththeabilitytocollect,analyze,concentrate,and
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