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1、ProgrammableLogicControllers:ProgrammingMethodsandApplicationsbyJohnR.HackworthandFrederickD.Hackworth,Jr.PLCConfigurationsProgrammablecontrollers(theshortenednameusedforprogrammablelogiccontrollers)aremuchlikepersonalcomputersinthattheusercanbeoverwhelmedbythevastarrayofoptionsand
2、configurationsavailable.Also,likepersonalcomputers,thebestteacherofwhichonetoselectisexperience.Asonegainsexperiencewiththevariousoptionsandconfigurationsavailable,itbecomeslessconfusingtobeabletoselecttheunitthatwillbestperforminaparticularapplication.BasicPLCsareavailableonasingl
3、eprintedcircuitboardasshowninFigure2-1.TheyaresometimescalledsingleboardPLCsoropenframePLCs.Thesearetotallyselfcontained(withtheexceptionofapowersupply)and,wheninstalledinasystem,theyaresimplymountedinsideacontrolscabinetonthreadedstandoffs.Screwterminalsontheprintedcircuitboardall
4、owfortheconnectionoftheinput,output,andpowersupplywires.Theseunitsaregenerallynotexpandable,meaningthatextrainputs,outputs,andmemorycannotbeaddedtothebasicunit.However,someofthemoresophisticatedmodelscanbelinkedbycabletoexpansionboardsthatcanprovideextraI/O.Therefore,withfewexcepti
5、ons,whenusingthistypeofPLC,thesystemdesignermusttakecaretospecifyaunitthathasenoughinputs,outputs,andprogrammingcapabilitytohandleboththepresentneedofthesystemandanyfuturemodificationsthatmayberequired.SingleboardPLCsareveryinexpensive(somelessthan$100),easytoprogram,small,andconsu
6、melittlepower,but,generallyspeaking,theydonothavealargenumberofinputsandoutputs,andhaveasomewhatlimitedinstructionset.Theyarebestsuitedtosmall,relativelysimplecontrolapplications.PLCsarealsoavailablehousedinasinglecase(sometimesreferredtoasashoebox)withallinputandoutput,powerandcon
7、trolconnectionpointslocatedonthesingleunit,asshowninFigure2-2.Thesearegenerallychosenaccordingtoavailableprogrammemoryandrequirednumberandvoltageofinputsandoutputstosuittheapplication.Thesesystemsgenerallyhaveanexpansionport(aninterconnectionsocket)whichwillallowtheadditionofspecia
8、lizedunitssuchashighspeedc
