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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-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.Thisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof±5to±15volts.TheRS-232receivertypicallyoperateswithinthevoltagerangeof+3to+12and-3to-12voltsasshowninFigure1.1.Figure1.1:RS-232InterfaceCircuitBalancedLineDriversInabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignal.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"conditionoftheline
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