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RS485协议.pdf

RS485协议

马马虎虎
2009-10-26 0人阅读 举报 0 0 暂无简介

简介:本文档为《RS485协议pdf》,可适用于IT/计算机领域

RSApplicationNoteCoverPageBBElectronicsPOBoxOttawa,ILPH()FAX()RSandRSApplicationNoteBBElectronicsMfgCoIncPOBoxOttawa,ILPH()FAX()InternetAddresses:BBHomePage:http:wwwbbeleccomCustomerService:salesbbeleccomTechnicalSupport:techsuptbbeleccomÓBBElectronicsRevisedOctoberRSApplicationNoteTableofContentsiBBElectronicsPOBoxOttawa,ILPH()FAX()TableofContentsCHAPTER:OVERVIEWINTRODUCTIONDATATRANSMISSIONSIGNALSUnbalancedLineDriversBalancedLineDriversBalancedLineReceiversEIASTANDARDRSDATATRANSMISSIONEIASTANDARDRSDATATRANSMISSIONTRISTATECONTROLOFANRSDEVICEUSINGRTSSENDDATACONTROLOFANRSDEVICECHAPTER:SYSTEMCONFIGURATIONNETWORKTOPOLOGIESTWOWIREORFOURWIRESYSTEMSTERMINATIONBIASINGANRSNETWORKEXTENDINGTHESPECIFICATIONCHAPTER:SELECTINGRSANDRSCABLINGNUMBEROFCONDUCTORSSHIELDINGCABLECHARACTERISTICSCHAPTER:TRANSIENTPROTECTIONOFRSANDRSSYSTEMSWHATDOESASURGELOOKLIKESurgeSpecificationsCommonModevsDifferentialModeGROUND¹GROUNDTRANSIENTPROTECTIONUSINGISOLATIONIsolationTheoryIsolationDevicesTRANSIENTPROTECTIONUSINGSHUNTINGShuntingTheoryConnectingSignalGroundsShuntingDevicesCOMBININGISOLATIONANDSHUNTINGSPECIALCONSIDERATIONFORFAULTCONDITIONSCHOOSINGTHERIGHTPROTECTIONFORYOURSYSTEMiiTableofContentsRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()CHAPTER:SOFTWAREINTRODUCTIONRSSYSTEMSRSDRIVERCONTROLRSRECEIVERCONTROLMASTERSLAVESYSTEMSFourWireMasterSlaveSystemsTwoWireMasterSlaveSystemsMULTIMASTERRSSYSTEMSSYSTEMSWITHPORTPOWEREDCONVERTERSCHAPTER:SELECTINGRSDEVICESCHAPTER:SOURCESOFFURTHERINFORMATIONAPPENDIXA:EIASPECIFICATIONSUMMARYAPPENDIXB:EIASTANDARDRSDATATRANSMISSIONRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()Chapter:OverviewIntroductionThepurposeofthisapplicationnoteistodescribethemainelementsofanRSandRSsystemThisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesignSincebothRSandRSaredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriatetodiscussbothsystemsinthesameapplicationnoteThroughoutthisapplicationnotethegenerictermsofRSandRSwillbeusedtorepresenttheEIATIAandEIATIAStandardsDataTransmissionSignalsUnbalancedLineDriversEachsignalthattransmitsinanRSunbalanceddatatransmissionsystemappearsontheinterfaceconnectorasavoltagewithreferencetoasignalgroundForexample,thetransmitteddata(TD)fromaDTEdeviceappearsonpinwithrespecttopin(signalground)onaDBconnectorThisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof±to±voltsTheRSreceivertypicallyoperateswithinthevoltagerangeoftoandtovoltsasshowninFigureBalancedLineDriversInabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignalFigureshowsaschematicsymbolforabalancedlinedriverandthevoltagesthatexistAbalancedlinedriverwillproduceavoltagefromtovoltsacrossitsAandBoutputterminalsandwillhaveasignalground(C)connectionAlthoughproperconnectiontothesignalgroundisimportant,itisn'tusedbyabalancedlinereceiverindeterminingthelogicstateofthedatalineAbalancedlinedrivercanalsohaveaninputsignalcalledan“Enable”signalThepurposeofthissignalistoconnectthedrivertoitsoutputterminals,AandBIfthe“Enable”signalisOFF,onecanconsiderthedriverasdisconnectedfromthetransmissionlineAnRSdrivermusthavethe“Enable”controlsignalAnRSdrivermayhavethissignal,butitisnotalwaysrequiredThedisconnectedor"disabled"conditionofthelinedriverusuallyisreferredtoasthe“tristate”conditionofthedriverTheterm“tristate”comesfromthefactthatthereisathirdoutputstateofanRSdriver,inadditiontotheoutputstatesof“”and“”RSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureFigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()BalancedLineReceiversAbalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandBItwillalsohaveasignalground(C)thatisnecessaryinmakingtheproperinterfaceconnectionFigureisaschematicsymbolforabalanceddifferentiallinereceiverFigurealsoshowsthevoltagesthatareimportanttothebalancedlinereceiverIfthedifferentialinputvoltageVabisgreaterthanmVthereceiverwillhaveaspecificlogicstateonitsoutputterminalIftheinputvoltageisreversedtolessthanmVthereceiverwillcreatetheoppositelogicstateonitsoutputterminalTheinputvoltagesthatabalancedlinereceivermustsenseareshowninFigureThemVtoVrangeisrequiredtoallowforattenuationonthetransmissionlineEIAStandardRSDataTransmissionTheEIAStandardRSAentitled“ElectricalCharacteristicsofBalancedVoltageDigitalInterfaceCircuits”definesthecharacteristicsofRSinterfacecircuitsFigureisatypicalRSfourwireinterfaceNoticethatfiveconductorsareusedEachgeneratorordrivercandriveuptoten()receiversThetwosignalingstatesofthelinearedefinedasfollows:aWhenthe“A”terminalofthedriverisnegativewithrespecttothe“B”terminal,thelineisinabinary(MARKorOFF)statebWhenthe“A”terminalofthedriverispositivewithrespecttothe“B”terminal,thelineisinabinary(SPACEorON)stateFigureshowstheconditionofthevoltageofthebalancedlineforanRStoRSconverterwhenthelineisinthe“idle”conditionorOFFstateItalsoshowstherelationshipofthe“A”and“B”terminalsofanRSsystemandthe““and“”terminalmarkingsusedonmanytypesofequipmentThe“A”terminalisequivalenttothe““designation,andthe“B”terminalequivalenttothe“”designationThesamerelationshipshowninFigurealsoappliesforRSsystemsRScanwithstandacommonmodevoltage(Vcm)of±voltsCommonmodevoltageisdefinedasthemeanvoltageoftheAandBterminalswithrespecttosignalgroundRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureFigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()EIAStandardRSDataTransmissionTheRSStandardpermitsabalancedtransmissionlinetobesharedinapartylineormultidropmodeAsmanyasdriverreceiverpairscanshareamultidropnetworkManycharacteristicsofthedriversandreceiversarethesameasRSTherangeofthecommonmodevoltageVcmthatthedriverandreceivercantolerateisexpandedtotovoltsSincethedrivercanbedisconnectedortristatedfromtheline,itmustwithstandthiscommonmodevoltagerangewhileinthetristateconditionSomeRSdrivers,evenwithtristatecapability,willnotwithstandthefullVcmvoltagerangeoftovoltsFigureshowsatypicaltwowiremultidropnetworkNotethatthetransmissionlineisterminatedonbothendsofthelinebutnotatdroppointsinthemiddleofthelineTerminationshouldonlybeusedwithhighdataratesandlongwiringrunsAdetaileddiscussionofterminationcanbefoundinChapterofthisapplicationnoteThesignalgroundlineisalsorecommendedinanRSsystemtokeepthecommonmodevoltagethatthereceivermustacceptwithinthetovoltrangeFurtherdiscussionofgroundingcanbefoundinChapterofthisapplicationnoteRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()AnRSnetworkcanalsobeconnectedinafourwiremodeasshowninFigureNotethatfourdatawiresandanadditionalsignalgroundwireareusedina“fourwire”connectionInafourwirenetworkitisnecessarythatonenodebeamasternodeandallothersbeslavesThenetworkisconnectedsothatthemasternodecommunicatestoallslavenodesAllslavenodescommunicateonlywiththemasternodeThisnetworkhassomeadvantageswithequipmentwithmixedprotocolcommunicationsSincetheslavenodesneverlistentoanotherslaveresponsetothemaster,aslavenodecannotreplyincorrectlytoanotherslavenodeTristateControlofanRSDeviceusingRTSAsdiscussedpreviously,anRSsystemmusthaveadriverthatcanbedisconnectedfromthetransmissionlinewhenaparticularnodeisnottransmittingInanRStoRSconverteroranRSserialcard,thismaybeimplementedusingtheRTScontrolsignalfromanasynchronousserialporttoenabletheRSdriverTheRTSlineisconnectedtotheRSdriverenablesuchthatsettingtheRTSlinetoahigh(logic)stateenablestheRSdriverSettingtheRTSlinelow(logic)putsthedriverintothetristateconditionThisineffectdisconnectsthedriverfromthebus,allowingothernodestotransmitoverthesamewirepairFigureshowsatimingdiagramforatypicalRStoRSconverterThewaveformsshowwhathappensiftheVRTSwaveformisnarrowerthanthedataVSDThisisnotthenormalsituation,butisshownheretoillustratethelossofaportionofthedatawaveformWhenRTScontrolisused,itisimportanttobecertainthatRTSissethighbeforedataissentAlso,theRTSlinemustthenbesetlowafterthelastdatabitissentThistimingisdonebythesoftwareusedtocontroltheserialportandnotbytheconverterWhenanRSnetworkisconnectedinatwowiremultidroppartylinemode,thereceiverateachnodewillbeconnectedtotheline(seeFigure)ThereceivercanoftenbeconfiguredtoreceiveanechoofitsowndatatransmissionThisisdesirableinsomesystems,andtroublesomeinothersBesuretocheckthedatasheetforyourconvertertodeterminehowthereceiver“enable”functionisconnectedRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()SendDataControlofanRSDeviceManyofBBElectronics’RStoRSconvertersandRSserialcardsincludespecialcircuitry,whichistriggeredfromthedatasignaltoenabletheRSdriverFigureisatimingdiagramoftheimportantsignalsusedtocontrolaconverterofthistypeItisimportanttonotethatthetransmitdatalineis“disabled”atafixedintervalafterthelastbit,typicallyonecharacterlengthIfthisintervalistooshort,youcanmisspartsofeachcharacterbeingsentIfthistimeistoolong,yoursystemmaytrytoturnthedatalinearoundfromtransmittoreceivebeforethenode(withtheSendDataconverter)isreadytoreceivedataIfthelatteristhecase,youwillmissportions(orcompletecharacters)atthebeginningofaresponseRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()Chapter:SystemConfigurationNetworkTopologiesNetworkconfigurationisn’tdefinedintheRSorRSspecificationInmostcasesthedesignercanuseaconfigurationthatbestfitsthephysicalrequirementsofthesystemTwoWireorFourWireSystemsRSsystemsrequireadedicatedpairofwiresforeachsignal,atransmitpair,areceivepairandanadditionalpairforeachhandshakecontrolsignalused(ifrequired)ThetristatecapabilitiesofRSallowasinglepairofwirestosharetransmitandreceivesignalsforhalfduplexcommunicationsThis“twowire”configuration(notethatanadditionalgroundconductorshouldbeused)reducescablingcostRSdevicesmaybeinternallyorexternallyconfiguredfortwowiresystemsInternallyconfiguredRSdevicessimplyprovideAandBconnections(sometimeslabeled““and“”)DevicesconfiguredforfourwirecommunicationsbringoutAandBconnectionsforboththetransmitandthereceivepairsTheusercanconnectthetransmitlinestothereceivelinestocreateatwowireconfigurationThelattertypedeviceprovidesthesystemdesignerwiththemostconfigurationflexibilityNotethatthesignalgroundlineshouldalsobeconnectedinthesystemThisconnectionisnecessarytokeeptheVcmcommonmodevoltageatthereceiverwithinasaferangeTheinterfacecircuitmayoperatewithoutthesignalgroundconnection,butmaysacrificereliabilityandnoiseimmunityFiguresandillustrateconnectionsoftwoandfourwiresystemsRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureTypicalRSFourWireMultidropConfigurationRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()FigureTypicalRSTwoWireMultidropNetworkRSApplicationNoteBBElectronicsPOBoxOttawa,ILPH()FAX()TerminationTerminationisusedtomatchimpedanceofanodetotheimpedanceofthetransmissionlinebeingusedWhenimpedancearemismatched,thetransmittedsignalisnotcompletelyabsorbedbytheloadandaportionisreflectedbackintothetransmissionlineIfthesource,transmissionlineandloadimpedanceareequalthesereflectionsareeliminatedTherearedisadvantagesofterminationaswellTerminationincreasesloadonthedrivers,increasesinstallationcomplexity,changesbiasingrequirementsandmakessystemmodificationmoredifficultThedecisionwhetherornottouseterminationshouldbebasedonthecablelengthanddatarateusedbythesystemAgoodruleofthumbisifthepropagationdelayofthedatalineismuchlessthanonebitwidth,terminationisnotneededThisrulemakestheassumptionthatreflectionswilldampoutinseveraltripsupanddownthedatalineSincethereceivingUARTwillsamplethedatainthemiddleofthebit,itisimportantthatthesignallevelbesolidatthatpointForexample,inasystemwithfeetofdatalinethepropagationdelaycanbecalculatedbymultiplyingthecablelengthbythepropagationvelocityofthecableThisvalue,typicallytoofthespeedoflight(c),isspecifiedbythecablemanufactureForourexample,aroundtripcoversfeetofcableUsingapropagationvelocityof´c,oneroundtripiscompletedinapproximatelymsIfweassumethereflectionswil

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