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《机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 28.ppt》由会员上传分享，免费在线阅读，更多相关内容在应用文档-天天文库。
1、Lesson28ProgrammableLogicControllersHardwareTextIntroductionAprogrammablelogiccontroller（PLC）isasmall,self-contained,ruggedcomputerdesignedtocontrolprocessesandeventsinanindustrialenvironment—thatis,totakeoverthejobpreviouslydonewithrelaylogiccontrollers①.Figure28.1showsanumberofdi
2、fferenttypesofPLCs.Physically,theyrangeinsizefromthatofacameratothatofashoebox（andsometimeslarger）.Wiresfromswitches,sensors,andotherinputdevicesareattacheddirectlytothePLC;wiresdrivinglights,smallmotors,motorstarters,andotheroutputdevicesarealsoconnecteddirectlytothePLC（Figure28.2
3、）.EachPLCcontainsamicroprocessorthathasbeenprogrammedtodrivetheoutputterminalsinaspecifiedmanner,basedonthesignalsfromtheinputterminals②.ThePLCprogramisusuallydevelopedonaseparatecomputersuchasapersonalcomputer（PC）,usingspecialsoftwareprovidedbythePLCmanufacturer.Oncetheprogramhasb
4、eenwritten,itistransferred,ordownloaded,intothePLC.Fromthispointon,thePLCmayoperateonitsown,asacompletelyindependentcontroller.PLCHardwareFigure28.3showstheblockdiagramforaPLCandincludesthefundamentalpartsfoundinanymicroprocessorsystem.Thesemajorfunctionalblocksareexplainednext.Pow
5、erSupplyPLCsareusuallypowereddirectlyfrom120or240Vac.ThepowersupplyconvertstheACintoDCvoltagesfortheinternalmicroprocessorcomponents.Itmayalsoprovidetheuserwithasourceofreducedvoltagetodriveswitches,smallrelays,indicatorlamps,andthelike.ProcessorTheprocessorisamicroprocessor-basedC
6、PUandisthepartofthePLCthatiscapableofreadingandexecutingtheprograminstructions,one-by-one（suchastherungsofaladderlogicprogram）.ProgramMemoryTheprogrammemoryreceivesandholdsthedownloadedprograminstructionsfromtheprogrammingdevice.IfthismemoryisstandardRAM,theprogramwillbelosteveryti
7、methepoweristurnedoff,requiringittobereloaded.Toavoidthisbother,theprogrammemorymayuseanEEPROM（electricallyerasableprogrammableROM）orabattery-backupRAM,bothofwhicharecapableofretainingdataevenwhenthepowerisoff③.AnEPROM（UVerasablePROM）canalsostoretheprogram,butthisdevicerequiresaspe
8、cialprogrammingunitthatisn
