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1、ModbusCommunicationProtocolM.D.KotsovosDepartmentofMechnicalEngineering,ImperialCollegeofScienceandTecgnology,London(U.K.)ModbusprotocolisaprotocolusedforcommunicationbetweenthePLCandModiconcontrollersinventedin1978.Modbuscommunicationprotocolparticipantisdefinedas"maste
2、r”(MASTER)and"Slave”(SLAVE).Modbusregulatethatonlythemainstationwiththeinitiative,thesmartpassiveresponsefromthestation,includingtheanswertotheerrormessage.ThisarticlewillsetthemasterstationPLC,high-voltageswitchgearprotectiondeviceissetto1,theslave;thecapacitorfeedercab
3、inetprotectiondeviceissettothe2ndslave.Modbusprotocolhastwotransmissionmodes,ASCIIandRTU.InASCIImode,eachmessagerequirestwo8BitbyteASCIIcharacters.Theadvantageistoallowthetransmissionofcharacterstoreach1sintervalswithouterror.InRTUmode,eachbytecontainstwo4Bitof8Bithexade
4、cimalcharacters.Theadvantageisinthesamebaudrate,sendingmorethanASCIImodedata.Buteachmessagemustbetransmittedinacontinuousstreamofdata.Inthispaper,themodelisRTUmode.Itdoesnotstartandstopbits,butatleast3.5characterintervals(T1-T2-T3-T4)markthebeginningandtheend.RTUmodeuseC
5、RC16redundancycheck.Theinformationframefromtheaddressfield,andCRCcheckdomainfunctionaldomainstructure.Allofthecharactersfromthe16-bithexadecimalnumbercomponents.Bitsofeachbyteare:1startbit,8databits,aparitybit,1or2stopbits(whennoparity).Modbusprotocolisusedinacommonlangu
6、ageelectroniccontroller.Throughthisagreement,betweenthecontrollers,thecontrollerviathenetwork(suchasEthernet)andotherdevicescancommunicate.Ithasbecomeacommonindustrystandard.Modbusprotocolisawidelyinindustrialapplicationsistrulyopen,standardnetworkprotocol,bymostSCADA(Su
7、pervisorControlAndDataAcquisitionSupervisoryControlandDataAcquisitionSystem)HMEngineering(HumanMachineInterfaceHMI)softwaresupport,soitactuallyhasbecomeacommonindustrystandard.Withitdifferentmanufacturersofindustrialcontroldevicescanbeconnectedintoanetwork,centralizedmon
8、itoring.Thisprotocoldefinesamessagestructureusedbythecontrollertorecognize,whethertheyaretocommunicatew
