中英文外文翻译--PLC和微处理器.doc

IntroductionsofPLCandMCUAPLCisadevicethatwasinventedtoreplacethenecessarysequentialrelaycircuitsformachinecontrol.ThePLCworksbylookingatitsinputsanddependingupontheirstate,turningon/offitsoutputs.Theuserentersaprogram,usuallyviasoftwareorprogrammerthatgivesthedesiredresults.PLCareusedinmany“realworld”applications.Ifthereisindustrypresent,chancesaregoodthatthereisaPLCpresent.Ifyouareinvolvedinmachining,packaging,materialhandling,automatedassemblyorcountlessotherindustries,youareprobablyalreadyusingthem.Ifyouarenot,youarewastingmoneyandtime.AlmostanyapplicationthatneedssometypeofelectricalcontrolhasneedforPLC.Forexample,letsassumethatwhenaswitchturnsonwewanttoturnasolenoidonfor5secondsandthenturnitoffregardlessofhowlongtheswitchisonfor.Wecandothiswithasimpleexternaltimer.Whatiftheprocessalsoneededtocounthowmanytimestheswitchindividuallyturnedon?Weneedalotofexternalcounters.Asyoucansee,thebiggertheprocessthemoreofaneedwehaveforaPLC.WecansimplyprogramthePLCtocountitsinputsandturnthesolenoidsonforthespecifiedtime.Wewilltakealookatwhatisconsideredtobethe“top20”PLCinstructions.Itcanbesafelyestimatedthatwithafirmunderstandingofthereinstructionsonecansolvemorethan80%oftheapplicationsinexistence.Thatsright,morethan80%!OfcoursewelllearnmorethanjusttheseinstructionstohelpyousolvealmostALLyourpotentialPLCapplications.ThePLCmainlyconsistsofaCPU,memoryareas,andappropriatecircuitstoreceiveinput/outputdata,asshowninFig.19.1WecanactuallyconsiderthePLCtobeaboxfullofhundredsorthousandsofseparaterelays,counters,timeranddatestoragelocations.Dothesecounters,timers,etc.reallyexist?No,theydont“physically”existbutrathertheyaresimulatedandcanbeconsideredsoftwarecounters,timers,etc.Theseinternalrelaysaresimulatedthroughbitlocationsinregisters.Whatdoeseachpartdo?INPUTRELAYS-(contacts)Theseareconnectedtotheoutsideworld.Theyphysicallyexistandreceivesignalsfromswitches,sensors,etc.Typicallytheyarenotrelaysbutrathertheyaretransistors.INTERNALUTILITYRELAYS-(contacts)Thesedonotreceivesignalsfromtheoutsideworldnordotheyphysicallyexist.TheyaresimulatedrelaysandarewhatenablesaPLCtoeliminateexternalrelays.Therearealsosomespecialrelaysthatarededicatedtoperformingonlyonetask.Somearealwaysonwhilesomearealwaysoff.Someareononlyonceduringpower-onandaretypicallyuserforinitializingdatawhatwasstored.COUNTERSTheseagaindonotphysicallyexist.Theyaresimulatedcountersandtheycanbeprogrammedtocountpulses.Typicallythesecounterscancountup,downorbothupanddown.Sincetherearesimulated,theyarelimitedintheircountingspeed.Somemanufacturersalsoincludehigh-speedcountersthatarehardwarebased.Wecanthinkoftheseasphysicallyexisting.Mosttimersthesecounterscancountup,downorupanddown.TIMERSThesealsodonotphysicallyexist.Theycomeinmanyvarietiesandincrements.Themostcommontypeisanon-delaytype.Otherincludeoff-delayandbothretentiveandnon-retentivetypes.Incrementsvaryfrom1msthrough1s.OUTPUTRELAYS-(coil)Theseareconnectedtotheoutsideworld.Theyphysicallyexistandsendon/offsignalstosolenoids,lights,etcTheycanbetransistors,relays,ortriacsdependinguponthemodelchosen.DATASTORAGE-Typicallythereareregistersassignedtosimplystoredata.Thereareusuallyusedastemporarystorageformathordatamanipulation.Theycanalsotypicallybeuserpower-uptheywillstillhavethesamecontentsasbeforepowerwarremoved.Veryconvenientandnecessary!APLCworksbycontinuallyscanningaprogram.Wecanthinkofthisscancycleasconsistingof3importantsteps,asshowninFig.19.2Therearetypicallymorethan3butwecanfocusontheimportantpartsandnotworryabouttheothers.Typicallytheothersarecheckingthesystemandupdatingthecurrentandtimervalues.Step1-CHECKINPUTSTATUS-FirstthePLCtakesalookateachinputtodetermineifitisonoroff.Inotherwords,isthesensorconnectedtothefirstinputon?Howaboutthesecondinput?HowaboutthethirdItrecordsthisdataintoitsmemorytobeusedduringthenextstep.Step2-EXECUTEPROGRAM-NextthePLCexecutesyourprogramoneinstructionatatime.Maybeyourprogramsaidthatifthefirstinputwasonthenitshouldturnonthefirstoutput.Sinceisalreadyknowswhichinputsareon/offfromthepreviousstep,itwillbeabletodecidewhetherthefirstoutputshouldbeturnedonbasedonthestateofthefirstinput.Itwillstoretheexecutionresultsforuselaterduringthenextstep.Step3-UPDATEOUTPUTSTSTUS-FinallythePLCupdatesthestatusofoutputs.Itupdatestheoutputsbasedonwhichinputswereonduringthefirststepandtheresultsofexecutingyourprogramduringthesecondstep.Basedontheexampleinstep2itwouldnowturnonthefirstoutputbecausethefirstinputwasonandyourprogramsaidtoturnonthefirstoutputwhenthisconditionistrue.AfterthethirdstepthePLCgoesbacktosteponeandrepeatsthestepscontinuously.Onescantimeisdefinedasthetimeistakestoexecutethe3stepslistedabove.Thusapracticalsystemiscontrolledtoperformspecifiedoperationsasdesired.TheAT89S52isalow-power,high-performanceCMOS8-bitmicrocontrollerwith8Kbytesofin-systemprogrammableFlashmemory.ThedeviceismanufacturedusingAtmelshigh-densitynonvolatilememorytechnologyandiscompatiblewiththeindustry-standard80C51instructionsetandpin-out.Theon-chipFlashallowstheprogrammemorytobereprogrammedin-systemorbyaconventionalnonvolatilememoryprogrammer.Bycombiningaversatile8-bitCPUwithin-systemprogrammableFlashonamonolithicchip,theAtmelAT89S52isapowerfulmicrocontrollerwhichprovidesahighly-flexibleandcost-effectivesolutiontomanyembeddedcontrolapplications.TheAT89S52providesthefollowingstandardfeatures:8KbytesofFlash,256bytesofRAM,32I/Olines,Watchdogtimer,twodatapointers,three16-bittimer/counters,asix-vectortwo-levelinterruptarchitecture,afullduplexserialport,on-chiposcillator,andclockcircuitry.Inaddition,theAT89S52isdesignedwithstaticlogicforoperationdowntozerofrequencyandsupportstwosoftwareselectablepowersavingmodes.TheIdleModestopstheCPUwhileallowingtheRAM,timer/counters,serialport,andinterruptsystemtocontinuefunctioning.ThePower-downmodesavestheRAMcontentsbutfreezestheoscillator,disablingallotherchipfunctionsuntilthenextinterruptorhardwarereset.Port0isan8-bitopendrainbidirectionalI/Oport.Asanoutputport,eachpincansinkeightTTLinputs.Wheniswrittentoport0pins,thepinscanbeusedashigh-impedanceinputs.Port0canalsobeconfiguredtobethemultiplexedlo