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1、外文原文：IntroductiontoPLCInordertousetheadvantagesofallthosecontrollersandeliminatethedifficulties,theprogrammablelogiccontrollerswereinvented.programmablelogiccontrollers(PLCs),alsoreferredtoasprogrammablecontrollers,areinthecomputerfamily.APLCwasreplace
2、mentforrelaydevices.Theyareprogrammedusingaladderdiagram,whichisstandardelectricwiringdiagram.AsPLCbecomemoreflexible,high-levelaswellaslow-levellanguagesareavailabletoPLCprogrammers.PLChavetheflexibilityofcomputersaswellasastandardandersyinterfacewith
3、processesandotherdevices.Theyareusedincommercialandapplications.Theyarewidelyaccepedinindustryforcontrollingfromasingledevicetoacomplexmanufacturingfacility.programmablelogiccontrollerswerefirstintroducedin1968asasubstituteforhardwiredrelaypanels.Theor
4、iginalintentwastoreplaceamechanicalswitchingdevice(relaymodules).However,since1968,thecapabilitiesofthePLCweretoreplacerelaypanels,modernPLChavemanymorefunctions.Theiruseextendsfromsimpleprocesscontroltomanufacturingsystemcontrolsandmonitoring.Theyareu
5、sedforhigh-speeddigitalprocessing,high-speeddigitalcommunication,high-levelcomputer-languagesupport,and,ofcourse,forbasiccontrol.PLCvaryinsizeandpower.AlargePLCcanhaveupto10000I/Opointsandsupportallthefunctionsdiscussedearlier.Thereareexpansionslotstoa
6、ccommodatePCandotercommunitionsdecices.Formanyapplications,asmallPLCissufficient.ThespeedofPLCisconstantlyimproving,eventhelow-endPLCperformathighspeed.Onetotwomicrosec/kbyteofmemoryspeedisverycommon.ProgrammablelogiccontrollersArchitectureThebasiccomp
7、onentsinarelaypanelcircuitandthedesignofarelayladderdiagramhavebeendiscussed.Intheimplementationofsuchacircuit,notonlydoesthelogicproblemhavetobethoughtout,butalsetheelectricalcompatibilityandwiringlayouthavetobeconsidered.Breadboardingarelaypanelcircu
8、itisatedioustask;itrequiresalotofcarefulplanningandwork.Thedebuggingandchangingofacircuitisevenmoredifficult.Programmablelogiccontrollersreplacemostofthiswiringbysoftwareprogramming.Therefore,thetaskismademucheasier,becausethewiresandth
