RS422和RS485应用中英文翻译.docx

RS422和RS485应用中英文翻译.docx

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RS422和RS485应用中英文翻译

毕业设计说明书

外文文献及中文翻译

学生姓名：

学号：

学院：

系名：

电子与计算机科学技术系

专业：

电子科学与技术

指导教师：

2011年6月

RS-422andRS-485applicationnote

Chapter1:

Overview

Introduction

ThepurposeofthisapplicationnoteistodescribethemainelementsofanRS-422andRS-485system.Thisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesign.SincebothRS-422andRS-485aredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriatetodiscussbothsystemsinthesameapplicationnote.ThroughoutthisapplicationnotethegenerictermsofRS-422andRS-485willbeusedtorepresenttheEIA/TIA-422andEIA/TIA-485Standards.

DataTransmissionSignals

UnbalancedLineDrivers

EachsignalthattransmitsinanRS-232unbalanceddatatransmissionsystemappearsontheinterfaceconnectorasavoltagewithreferencetoasignalground.Forexample,thetransmitteddata（TD）fromaDTEdeviceappearsonpin2withrespecttopin7（signalground）onaDB-25connector.Thisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof±5to±15volts.TheRS-232receivertypicallyoperateswithinthevoltagerangeof+3to+12and-3to-12voltsasshowninFigure1.1.

Figure1.1:

RS-232InterfaceCircuit

BalancedLineDrivers

Inabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignal.Figure1.2showsaschematicsymbolforabalancedlinedriverandthevoltagesthatexist.Abalancedlinedriverwillproduceavoltagefrom2to6voltsacrossitsAandBoutputterminalsandwillhaveasignalground（C）connection.Althoughproperconnectiontothesignalgroundisimportant,itisn'tusedbyabalancedlinereceiverindeterminingthelogicstateofthedataline.Abalancedlinedrivercanalsohaveaninputsignalcalledan“Enable”signal.Thepurposeofthissignalistoconnectthedrivertoitsoutputterminals,AandB.Ifthe“Enable”signalisOFF,onecanconsiderthedriverasdisconnectedfromthetransmissionline.AnRS-485drivermusthavethe“Enable”controlsignal.AnRS-422drivermayhavethissignal,butitisnotalwaysrequired.Thedisconnectedor"disabled"conditionofthelinedriverusuallyisreferredtoasthe“tristate1”conditionofthedriver.

1Theterm“tristate”comesfromthefactthatthereisathirdoutputstateofanRS-485driver,inadditiontotheoutputstatesof“1”and“0.”

Figure1.2:

BalancedDifferentialOutputLineDriver

BalancedLineReceivers

Abalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandB.Itwillalsohaveasignalground（C）thatisnecessaryinmakingtheproperinterfaceconnection.Figure1.3isaschematicsymbolforabalanceddifferentiallinereceiver.Figure1.3alsoshowsthevoltagesthatareimportanttothebalancedlinereceiver.IfthedifferentialinputvoltageVabisgreaterthan+200mVthereceiverwillhaveaspecificlogicstateonitsoutputterminal.Iftheinputvoltageisreversedtolessthan-200mVthereceiverwillcreatetheoppositelogicstateonitsoutputterminal.TheinputvoltagesthatabalancedlinereceivermustsenseareshowninFigure1.3.The200mVto6Vrangeisrequiredtoallowforattenuationonthetransmissionline.

Figure1.3:

BalancedDifferentialInputLineReceiver

EIAStandardRS-422DataTransmission

TheEIAStandardRS-422-Aentitled“ElectricalCharacteristicsofBalancedVoltageDigitalInterfaceCircuits”definesthecharacteristicsofRS-422interfacecircuits.Figure1.4isatypicalRS-422four-wireinterface.Noticethatfiveconductorsareused.Eachgeneratorordrivercandriveuptoten（10）receivers.Thetwosignalingstatesofthelinearedefinedasfollows:

a.Whenthe“A”terminalofthedriverisnegativewithrespecttothe“B”terminal,thelineisinabinary1（MARKorOFF）state.

b.Whenthe“A”terminalofthedriverispositivewithrespecttothe“B”terminal,thelineisinabinary0（SPACEorON）state.

Figure1.5showstheconditionofthevoltageofthebalancedlineforanRS-232toRS-422converterwhenthelineisinthe“idle”conditionorOFFstate.Italsoshowstherelationshipofthe“A”and“B”terminalsofanRS-422systemandthe“-“and“+”terminalmarkingsusedonmanytypesofequipment.The“A”terminalisequivalenttothe“-“designation,andthe“B”terminalequivalenttothe“+”designation.ThesamerelationshipshowninFigure1.5alsoappliesforRS-485systems.RS-422canwithstandacommonmodevoltage（Vcm）of±7volts.  CommonmodevoltageisdefinedasthemeanvoltageoftheAandBterminalswithrespecttosignalground.

Figure1.4:

TypicalRS-422FourWireNetwork

Figure1.5:

RelationshipbetweenEIAStandard“A”and“B”terminalsonanRS-422orRS-485Device

and“+”and“-”IdentificationConvention

NOTE：

Under“idle”conditionsitispossibletodeterminewhichterminalis“A”andwhichis“B”.

EIAStandardRS-485DataTransmission

TheRS-485Standardpermitsabalancedtransmissionlinetobesharedinapartylineormultidropmode.  Asmanyas32driver/receiverpairscanshareamultidropnetwork.ManycharacteristicsofthedriversandreceiversarethesameasRS-422.TherangeofthecommonmodevoltageVcmthatthedriverandreceivercantolerateisexpandedto+12to-7volts.Sincethedrivercanbedisconnectedortristatedfromtheline,itmustwithstandthiscommonmodevoltagerangewhileinthetristatecondition.  SomeRS-422drivers,evenwithtristatecapability,willnotwithstandthefullVcmvoltagerangeof+12to-7volts.

Figure1.6showsatypicaltwo-wiremultidropnetwork.  Notethatthetransmissionlineisterminatedonbothendsofthelinebutnotatdroppointsinthemiddleoftheline.Terminationshouldonlybeusedwithhighdataratesandlongwiringruns.AdetaileddiscussionofterminationcanbefoundinChapter2ofthisapplicationnote.ThesignalgroundlineisalsorecommendedinanRS-485systemtokeepthecommonmodevoltagethatthereceivermustacceptwithinthe-7to+12voltrange.FurtherdiscussionofgroundingcanbefoundinChapter3ofthisapplicationnote.

Figure1.6:

typicalRS-485twowiremultidropnetwork

AnRS-485networkcanalsobeconnectedinafour-wiremodeasshowninFigure1.7.Notethatfourdatawiresandanadditionalsignalgroundwireareusedina“four-wire”connection.Inafour-wirenetworkitisnecessarythatonenodebeamasternodeandallothersbeslaves.Thenetworkisconnectedsothatthemasternodecommunicatestoallslavenodes.Allslavenodescommunicateonlywiththemasternode.Thisnetworkhassomeadvantageswithequipmentwithmixedprotocolcommunications.Sincetheslavenodesneverlistentoanotherslaveresponsetothemaster,aslavenodecannotreplyincorrectlytoanotherslavenode.

Figure1.7:

typicalRS-485fourwiremultidropnetwork

TristateControlofanRS-485DeviceusingRTS

Asdiscussedpreviously,anRS-485systemmusthaveadriverthatcanbedisconnectedfromthetransmissionlinewhenaparticularnodeisnottransmitting.InanRS-232toRS-485converteroranRS-485serialcard,thismaybeimplementedusingtheRTScontrolsignalfromanasynchronousserialporttoenabletheRS-485driver.TheRTSlineisconnectedtotheRS-485driverenablesuchthatsettingtheRTSlinetoahigh（logic1）stateenablestheRS-485driver.SettingtheRTSlinelow（logic0）putsthedriverintothetristatecondition.  Thisineffectdisconnectsthedriverfromthebus,allowingothernodestotransmitoverthesamewirepair.Figure1.8showsatimingdiagramforatypicalRS-232toRS-485converter.ThewaveformsshowwhathappensiftheVRTSwaveformisnarrowerthanthedataVSD.Thisisnotthenormalsituation,butisshownheretoillustratethelossofaportionofthedatawaveform.WhenRTScontrolisused,itisimportanttobecertainthatRTSissethighbeforedataissent.Also,theRTSlinemustthenbesetlowafterthelastdatabitissent.Thistimingisdonebythesoftwareusedtocontroltheserialportandnotbytheconverter.

Figure1.8:

TimingDiagramforRS-232toRS-485ConverterwithRTSControlofRS-485DriverandReceiver

Note:

1.Voltagehereisdeterminedbyotherdevicesontheline

2.Allpeakvaluesofvoltagesareapproximate

WhenanRS-485networkisconnectedinatwo-wiremultidroppartylinemode,thereceiverateachnodewillbeconnectedtotheline（seeFigure1.6）.Thereceivercanoftenbeconfiguredtoreceiveanechoofitsowndatatransmission.Thisisdesirableinsomesystems,andtroublesomeinothers.Besuretocheckthedatasheetforyourconvertertodeterminehowthereceiver“enable”functionisconnected.

Figure1.9-TimingDiagramforRS-232toRS-485ConverterwithSendData（SD）ControlofRS-485DriverandReceiver

Note:

1.Voltagehereisdeterminedbyotherdevicesontheline.

2.Thistimingintervaldeterminedbycomponentsintimingcircuit.Thestartofthisintervalisdeterminedbytheleadingedgeofeachdatabit.

3.Allpeakvaluesofvoltagesareapproximate.

Chapter2:

SystemConfiguration

NetworkTopologies

Networkconfigurationisn’tdefinedintheRS-422orRS-485specification.Inmostcasesthedesignercanuseaconfigurationthatbestfitsthephysicalrequirementsofthesystem.

TwoWireorFourWireSystems

RS-422systemsrequireadedicatedpairofwiresforeachsignal,atransmitpair,areceivepairandanadditionalpairforeachhandshake/controlsignalused（ifrequired）.Th