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Programmablelogiccontroller
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Aprogrammablelogiccontroller(PLCorprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,controlofamusementrides,orcontroloflightingfixtures.PLCsareusedinmanydifferentindustriesandmachinessuchaspackagingandsemiconductormachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.Features
ControlpanelwithPLC(greyelementsinthecenter.Theunitconsistsofseparateelements,fromlefttoright;powersupply,controller,relayunitsforin-andoutput
ThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditions(dust,moisture,heat,cold,etcandhavethefacilityforextensive
input/output
(I/O
arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure,andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticor
hydraulic
cylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/output
arrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.Systemscale
AsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodeldoesnothaveenoughI/O.

ModularPLCshaveachassis(alsocalledarackintowhichareplacedmoduleswithdifferentfunctions.TheprocessorandselectionofI/Omodulesiscustomisedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkisusedsothatrackscanbedistributedawayfromtheprocessor,reducingthewiringcostsforlargeplants.Userinterface
Seealso:Listofhuman-computerinteractiontopics
PLCsmayneedtointeractwithpeopleforthepurposeofconfiguration,alarmreportingoreverydaycontrol.
AHuman-MachineInterface(HMIisemployedforthispurpose.HMIsarealsoreferredtoasMMIs(ManMachineInterfaceandGUI(GraphicalUserInterface.
Asimplesystemmayusebuttonsandlightstointeractwiththeuser.Textdisplaysareavailableaswellasgraphicaltouchscreens.Morecomplexsystemsuseaprogrammingandmonitoringsoftwareinstalledonacomputer,withthePLCconnectedviaacommunicationinterface.Communications
PLCshavebuiltincommunicationsportsusually9-PinRS232,andoptionallyforRS485andEthernet.ModbusorDF1isusuallyincludedasoneofthecommunicationsprotocols.Others'optionsincludevariousfieldbusessuchasDeviceNetorProfibus.OthercommunicationsprotocolsthatmaybeusedarelistedintheListofautomationprotocols.
MostmodernPLCscancommunicateoveranetworktosomeothersystem,suchasacomputerrunningaSCADA(SupervisoryControlAndDataAcquisitionsystemorwebbrowser.
PLCsusedinlargerI/Osystemsmayhavepeer-to-peer(P2Pcommunicationbetweenprocessors.Thisallowsseparatepartsofacomplexprocesstohaveindividualcontrolwhileallowingthesubsystemstoco-ordinateoverthecommunicationlink.ThesecommunicationlinksarealsooftenusedforHMI(Human-MachineInterfacedevicessuchaskeypadsor
PC-typeworkstations.Someoftoday'sPLCscancommunicateoverawiderangeofmedia
includingRS-485,Coaxial,andevenEthernetforI/Ocontrolatnetworkspeedsupto100Mbit/s.

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