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1TheProgrammableLogicControllerJohnR.Hackworth,FrederickD.Hackworth,Jr.ProgrammableLogicControllers:ProgrammingMethodsandApplications.2.5~2.8,TheProgrammableLogicController,JohnR.Hackworth,FrederickD.Hackworth,Jr.FirstEdition.London:PrenticeHall,2003.35~502.5SystemBlockDiagramAProgrammableControllerisaspecializedcomputer.Sinceitisacomputer,ithasallthebasiccomponentpartsthatanyothercomputerhas;aCentralProcessingUnit,Memory,InputInterfacingandOutputInterfacing.AtypicalprogrammablecontrollerblockdiagramisshowninFigure2.5.Figure2.5ProgrammableControllerBlockDiagramTheCentralProcessingUnit(CPU)isthecontrolportionofthePLC.Itinterpretstheprogramcommandsretrievedfrommemoryandactsonthosecommands.InpresentdayPLC'sthisunitisamicroprocessorbasedsystem.TheCPUishousedintheprocessormoduleofmodularizedsystems.Memoryinthesystemisgenerallyoftwotypes;ROMandRAM.TheROMmemorycontainstheprograminformationthatallowstheCPUtointerpretandactontheLadderLogicprogramstoredintheRAMmemory.RAMmemoryisgenerallykeptalivewithanon-boardbatterysothatladderprogrammingisnotlostwhenthesystempowerisremoved.Thisbatterycanbeastandarddrycellorrechargeablenickel-cadmiumtype.NewerPLCunitsarenowavailablewithElectricallyErasableProgrammableReadOnlyMemory(EEPROM)whichdoesnotrequireabattery.Memoryisalsohousedintheprocessormoduleinmodularsystems.Inputunitscanbeanyofseveraldifferenttypesdependingoninputsignalsexpectedasdescribedabove.Theinputsectioncanacceptdiscreteoranalogsignalsofvariousvoltageandcurrentlevels.PresentdaycontrollersofferdiscretesignalinputsofbothACandDCvoltagesfromTTLto250VDCandfrom5to250VAC.Analoginputunitscanacceptinputlevelssuchas±10VDC,±5VDCand4-20ma.currentloopvalues.DiscreteinputunitspresenteachinputtotheCPUasasingle1or0whileanaloginputunitscontainanalogtodigitalconversioncircuitryandpresenttheinputvoltagetotheCPUasbinarynumbernormalizedtothemaximumcountavailablefromtheunit.Thenumberofbitsrepresentingtheinputvoltageorcurrentdepends23upontheresolutionoftheunit.Thisnumbergenerallycontainsadefinednumberofmagnitudebitsandasignbit.RegisterinputunitspresentthewordinputtotheCPUasitisreceived(BinaryorBCD).Outputunitsoperatemuchthesameastheinputunitswiththeexceptionthattheunitiseithersinking(supplyingaground)orsourcing(providingavoltage)discretevoltagesorsourcinganalogvoltageorcurrent.TheseoutputsignalsarepresentedasdirectedbytheCPU.TheoutputcircuitofdiscreteunitscanbetransistorsforTTLandhigherDCvoltageorTriacsforACvoltageoutputs.Forhighercurrentapplicationsandsituationswhereaphysicalcontactclosureisrequired,mechanicalrelaycontactsareavailable.Thesehighercurrents,however,aregenerallylimitedtoabout2-3amperes.Theanalogoutputunitshaveinternalcircuitrywhichperformsthedigitaltoanalogconversionandgeneratesthevariablevoltageorcurrentoutput.2.6Update-SolvetheLadder-UpdateWhenpowerisappliedtoaprogrammablelogiccontroller,thePLC’soperationconsistsoftwosteps:(1)updateinputsandoutputsand(2)solvetheladder.Thismayseemlikeaverysimplisticapproachtosomethingthathastobemorecomplicatedbuttheretrulyareonlythesetwosteps.Ifthesetwostepsarethoroughlyunderstood,writingandmodifyingprogramsandgettingthemostfromthedeviceismucheasiertoaccomplish.Withthisunderstanding,thethingsthatcanbeundertakenarethenuptotheimaginationoftheprogrammer.Youwillnoticethatthe“update-solvetheladder”sequencebeginsafterstartup.TheactualstartupsequenceincludessomeoperationstransparenttotheuserorprogrammerthatoccurbeforeactualPLCoperationontheuserprogrambegins.Duringthisstartuptheremaybeextensivediagnosticchecksperformedbytheprocessoronthingslikememory,I/Odevices,communicationwithotherdevices(ifpresent)andprogramintegrity.Insophisticatedmodularsystems,theprocessorisabletoidentifythevariousmoduletypes,theirlocationinthesystemandaddress.Thistypeofsystemanalysisandtestinggenerallyoccursduringstartupbeforeactualprogramexecution.2.7UpdateThefirstthingthePLCdoeswhenitbeginstofunctionisupdateI/O.Thismeansthatalldiscreteinputstatesarerecordedfromtheinputunitandalldiscretestatestobeoutputaretransferredtotheoutputunit.Registerdatagenerallyhasspecificaddressesassociatedwithitforbothinputandoutputdatareferredtoasinputandoutputregisters.Theseregistersareavailabletotheinputandoutputmodulesrequiringthemandareupdatedwiththediscretedata.Sincethisisinput/outputupdating,itisreferredtoasI/OUpdate.TheupdatingofdiscreteinputandoutputinformationisaccomplishedwiththeuseofinputandoutputimageregisterssetasideinthePLCmemory.Eachdiscreteinputpointhasassociatedwithitonebitofaninputimageregister.Likewise,eachdiscreteoutputpointhasonebitofanoutputimageregisterassociatedwithit.WhenI/Oupdatingoccurs,eachinputpointthatisONatthattimewillcausea1tobesetatthebitaddressassociatedwiththatparticularinput.Iftheinputisoff,a0willbesetintothebitaddress.Memoryintoday'sPLC'sisgenerallyconfiguredin16bitwords.Thismeansthatonewordofmemorycanstorethestatesof16discreteinputpoints.Therefore,theremaybeanumberofwordsofmemorysetasideastheinputandoutputimageregisters.AtI/Oupdate,thestatusoftheinputimageregisterissetaccordingtothestateofalldiscreteinputsa