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1、IntroductionsofPLCandMCUAPLCisadevicethatwasinventedtoreplacethenecessarysequentialrelaycircuitsformachinecontrol.ThePLCworksbylookingatitsinputsanddependingupontheirstate,turningon/offitsoutputs.Theuserentersaprogram,usuallyviasoftwareorprogrammerthatgivesthedesiredresults.PLCareusedinmany“realwor
2、ld”applications.Ifthereisindustrypresent,chancesaregoodthatthereisaPLCpresent.Ifyouareinvolvedinmachining,packaging,materialhandling,automatedassemblyorcountlessotherindustries,youareprobablyalreadyusingthem.Ifyouarenot,youarewastingmoneyandtime.Almostanyapplicationthatneedssometypeofelectricalcont
3、rolhasneedforPLC.Forexample,let’sassumethatwhenaswitchturnsonwewanttoturnasolenoidonfor5secondsandthenturnitoffregardlessofhowlongtheswitchisonfor.Wecandothiswithasimpleexternaltimer.Whatiftheprocessalsoneededtocounthowmanytimestheswitchindividuallyturnedon?Weneedalotofexternalcounters.Asyoucansee,
4、thebiggertheprocessthemoreofaneedwehaveforaPLC.WecansimplyprogramthePLCtocountitsinputsandturnthesolenoidsonforthespecifiedtime.Wewilltakealookatwhatisconsideredtobethe“top20”PLCinstructions.Itcanbesafelyestimatedthatwithafirmunderstandingofthereinstructionsonecansolvemorethan80%oftheapplicationsin
5、existence.That‘sright,morethan80%!Ofcoursewe’lllearnmorethanjusttheseinstructionstohelpyousolvealmostALLyourpotentialPLCapplications.ThePLCmainlyconsistsofaCPU,memoryareas,andappropriatecircuitstoreceiveinput/outputdata,asshowninFig.19.1WecanactuallyconsiderthePLCtobeaboxfullofhundredsorthousandsof
6、separaterelays,counters,timeranddatestoragelocations.Dothesecounters,timers,etc.reallyexist?No,theydon’t“physically”existbutrathertheyaresimulatedandcanbeconsideredsoftwarecounters,timers,etc.Theseinternalrelaysaresimulatedthroughbitlocationsinregisters.Whatdoeseachpartdo?INPUTRELAYS-(contacts)Thes
7、eareconnectedtotheoutsideworld.Theyphysicallyexistandreceivesignalsfromswitches,sensors,etc...Typicallytheyarenotrelaysbutrathertheyaretransistors.INTERNALUTILITYRELAYS-(contacts)Thesedonotreceivesignalsfro
