RS-422andRS-485applicationnoteChapter1:OverviewIntroductionThepurposeofthisapplicationnoteistodescribethemainelementsofanRS-422andRS-485system.Thisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesign.SincebothRS-422andRS-485aredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriatetodiscussbothsystemsinthesameapplicationnote.ThroughoutthisapplicationnotethegenerictermsofRS-422andRS-485willbeusedtorepresenttheEIA/TIA-422andEIA/TIA-485Standards.DataTransmissionSignalsUnbalancedLineDriversEachsignalthattransmitsinanRS-232unbalanceddatatransmissionsystemappearsontheinterfaceconnectorasavoltagewithreferencetoasignalground.Forexample,thetransmitteddata(TD)fromaDTEdeviceappearsonpin2withrespecttopin7(signalground)onaDB-25connector.Thisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof5to15volts.TheRS-232receivertypicallyoperateswithinthevoltagerangeof+3to+12and-3to-12voltsasshowninFigure1.1.BalancedLineDriversInabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignal.Figure1.2showsaschematicsymbolforabalancedlinedriverandthevoltagesthatexist.Abalancedlinedriverwillproduceavoltagefrom2to6voltsacrossitsAandBoutputterminalsandwillhaveasignalground(C)connection.Althoughproperconnectiontothesignalgroundisimportant,itisntusedbyabalancedlinereceiverindeterminingthelogicstateofthedataline.Abalancedlinedrivercanalsohaveaninputsignalcalledan“Enable”signal.Thepurposeofthissignalistoconnectthedrivertoitsoutputterminals,AandB.Ifthe“Enable”signalisOFF,onecanconsiderthedriverasdisconnectedfromthetransmissionline.AnRS-485drivermusthavethe“Enable”controlsignal.AnRS-422drivermayhavethissignal,butitisnotalwaysrequired.Thedisconnectedordisabledconditionofthelinedriverusuallyisreferredtoasthe“tristate1”conditionofthedriver.1Theterm“tristate”comesfromthefactthatthereisathirdoutputstateofanRS-485driver,inadditiontotheoutputstatesof“1”and“0.”BalancedLineReceiversAbalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandB.Itwillalsohaveasignalground(C)thatisnecessaryinmakingtheproperinterfaceconnection.Figure1.3isaschematicsymbolforabalanceddifferentiallinereceiver.Figure1.3alsoshowsthevoltagesthatareimportanttothebalancedlinereceiver.IfthedifferentialinputvoltageVabisgreaterthan+200mVthereceiverwillhaveaspecificlogicstateonitsoutputterminal.Iftheinputvoltageisreversedtolessthan-200mVthereceiverwillcreatetheoppositelogicstateonitsoutputterminal.TheinputvoltagesthatabalancedlinereceivermustsenseareshowninFigure1.3.The200mVto6Vrangeisrequiredtoallowforattenuationonthetransmissionline.EIAStandardRS-422DataTransmissionTheEIAStandardRS-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)of7volts.CommonmodevoltageisdefinedasthemeanvoltageoftheAandBterminalswithrespecttosignalground.EIAStandardRS-485DataTransmissionTheRS-485Standardpermitsabalancedtransmissionlinetobesharedinapartylineormultidropmode.Asmanyas32driver/receiverpairscanshareamultidropnetwork.Manycharacteristicsofthedriversandreceive