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1、Thischaptercontinuesfromthepreviouschaptersonprogrammingandintroducesinternalrelays.Avarietyofothertermsareoftenusedtodescribetheseelements,suchasauxiliaryrelays,markers,flags,coils,andbitstorage.Theseareoneoftheelementsincludedamongthespecialbuilt-infunction
2、swithPLCsandareverywidelyusedinprogramming.AsmallPLCmighthaveahundredormoreinternalrelays,someofthembatterybackedsothattheycanbeusedinsituationswhereitisnecessarytoensuresafeshutdownofaplantintheeventofpowerfailure.Laterchaptersconsiderothercommonbuilt-inelem
3、ents.7.1InternalRelaysInPLCsthereareelementsthatareusedtoholddata,thatis,bits,andbehavelikerelays,beingabletobeswitchedonoroffandtoswitchotherdevicesonoroff.Hencetheterminternalrelay.Suchinternalrelaysdonotexistasreal-worldswitchingdevicesbutaremerelybitsinth
4、estoragememorythatbehaveinthesamewayasrelays.Forprogramming,theycanbetreatedinthesamewayasanexternalrelayoutputandinput.Thusinputstoexternalswitchescanbeusedtogiveanoutputfromaninternalrelay.Thisthenresultsintheinternalrelaycontactsbeingused,inconjunctionwith
5、otherexternalinputswitches,togiveanoutput,suchasactivatingamotor.Thuswemighthave(Figure7.1)Ononerungoftheprogram:Inputstoexternalinputsactivatetheinternalrelayoutput.Onalaterrungoftheprogram:Asaconsequenceoftheinternalrelayoutput,internalrelaycontactsareactiv
6、atedandsocontrolsomeoutput.Inusinganinternalrelay,ithastobeactivatedononerungofaprogramandthenitsoutputusedtooperateswitchingcontactsonanotherrung,orrungs,oftheprogram.Internalrelayscanbeprogrammedwithasmanysetsofassociatedcontactsasdesired.Todistinguishinter
7、nalrelayoutputsfromexternalrelayoutputs,theyaregivendifferenttypesofaddresses.Differentmanufacturerstendtousedifferenttermsforinternalrelaysandhavedifferentwaysofexpressingtheiraddresses.Forexample,MitsubishiusesthetermauxiliaryrelayormarkerandthenotationM100
8、,M101,andsoon.SiemensusesthetermflagandthenotationF0.0,F0.1,andsoon.TelemecaniqueusesthetermbitandthenotationB0,B1,andsoon.ToshibausestheterminternalrelayandthenotationR000,R001,andsoon.A
