422 and 485 Overview and System Configurations

1 ApplicationReportSLLA070D June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations MannySoltero,JingZhang, IndustrialInterfaceUpdatedbyKevinZhang,C larkKinnaird, ,commonlyknownasRS-422andRS-485,respecti vely, , severalapplicationsarepresented, June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010, June2002 RevisedMay2010 Copyright 2002 2010, ,astheyareknowntoday, ,respectively, (TIA) , theofficialANSI documents,onlythemostprevalentarediscuss edin tonotduplicatetheofficialdocuments, animportantaspectofdifferentialdatatrans missionin minimizinglinereflectionsduetotransmissi on-lineeffects, , ,theRS-422standard'stitleisElectricalCha racteristicsofBalancedVoltageDigitalInte rfaceCircuits, andis publishedbytheANSIT elecommunicationIndustryAssociation/Elec tronicIndustriesAssociation(TIA/EIA).

2 Intheindustry,thetermRS-422is commonlyusedratherthantheofficialname, specifiedasa simplexmultidropstandard,whichmeansonlyo nedriveranduptotenreceiverscanbeattached toa 'stitleisElectricalCharacteristicsofGene ratorsandReceiversforUsein BalancedDigitalMultipointSystems. RS-485is commonlyused, morethanonedriverisrequired, time,andupto32unitloads( ) thisapplicationreportin ,butaredistinct, ,a condenseddescriptionofeachstandardis presentedin shownin Figure1. Thedriver(orgenerator)is labeledD,thereceiveris labeledR,andtheterminationimpedanceis ZT. Theterminationimpedanceshouldbeequaltoth echaracteristicimpedanceofthecable,Zo, andis difficulttoachieveandis applicationdependent,typically, 20%is ,uptonineadditionalreceiverscanbeplaceda longthecablefrompointsA andBtopointsA'andB', ,systemsofupto1 kmarenotuncommon, ,thelongerthecable,theslowerthesignaling ratemustbe, ruleofthumb,thedatasignalingrate(inbps)m ultipliedbythecablelength(inmeters)shoul dnotexceed108.

3 Forexample,a systemwitha cablemeasuring500mshouldnottransmitdataa tspeedsgreaterthan200kbps(108/500).3 SLLA070D June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010,TexasInstrumentsIncorporatedZTA B ABLogic(1 or 0)RDS0450-01 Logic(1 or 0)DAB+ VOAS0451-01 VOD+ VOB+ = driver(orgenerator)BR = receiverCZT= (ANSITIA/EIA-423standard),theRS-423speci fiesanunbalancedsignalingscheme,whichis (VOD, VOA, andVOBM easured)Theoutputvoltageshallnotexceed 6 V underunloadedconditions,andthedifferenti alvoltage[measuredasthedifferencebetween anoutputvoltage,VOA(VOB), anditscomplementaryoutputvoltage,VOB(VOA )] is nogreaterthan |VOD| 10V,|VOA| 6 V,and|VOB| 6 VBVOA= voltageonA outputCVOB= voltageonB outputDVOD= (VODandVOCM easured)Toensureproperdrivestrength,a minimumof 2-VVOD anda maximumof 3-VVOS aremeasured(seeFigure3).

4 Furthermore,a checkondriveroutput-voltagebalancebetwee nthedifferentialoutputvoltagesis putin placetomeasurethechangein thesevoltages(nottoexceed400mV).Themaxim umlimitof400mVmostoftenis June2002 RevisedMay2010 Copyright 2002 2010,TexasInstrumentsIncorporatedLogic(1 or 0)DAB50 WVOD+ + VOC50WA. VOD| 2 V, |V| 3 VB. | V| = ||V| - V|| | V| = ||V| - V|| VDOCOCOC Logic(1 or 0)DABIOCS0453-01 IOCL ogic(1 or 0) (IOSM easured)Withthedrivershortedtoground,the magnitudeoftheoutputcurrentshallnotexcee d150mA,regardlessofthestateofthedriverou tput(highorlow) |IOC| toground (IOFFM easured;VOApplied)Asshownin Figure5, withthedriverpowereddown,themagnitudeoft heoutputleakagecurrentshallnotexceed100 A foroutputvoltagesrangingfrom to6 A |Ioff| 100 A for VO 6 June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010, VSS0-V DifferentialVODL ogic(1 or 0)DAB100WS0455-01 VOD+ VSSA.

5 T = time duration of the unit intervalB. V= |V- V|C. 2 V |V| 10 VuiSSODODOD?? (VODM easured)Basically, 100- resistoracrossthedifferentialoutput,thev oltagemonotonicallychangesbetween10%and9 0%ofVSSwithina tenthoftheunitinterval,tui, or20ns,whicheveris ,theresultantvoltageshallnotchangemoreth an10%ofVSSaftera transitionhasoccurred(limitsovershootsan dundershoots). (VIA, andVIB, Applied;IIA, andIIB, Measured)A maximumlimitontheinputcharacteristicmust beplacedonthereceivertoensurea maximumloadonthebuswhenalltenreceiversar eplacedonit. Withthecommon-modevoltageVIA, (VIB) rangingfrom+10V to 10V whileVIB, (VIA,) is heldto0 V,theresultantinputcurrentshouldremainwi thintheshadedregion(seeFigure7) in devicewithinputcharacteristicswithinthes hadedregionrevealsthattheinputimpedancei s nosmallerthan4 k , June2002 RevisedMay2010 Copyright 2002 2010,TexasInstrumentsIncorporatedS0456-0 1RA B IIB IIA vIA vIB +3V 3V+10V 10V = V/ I = 13 4 k (VCM, VIA, andVIB, Applied.)

6 VIDM easured)Figure8 showsthetestcircuitusedtodeterminea receiver' ,suggestedtestvoltagesforbothinputsandth epurposeofthemeasurementsaregivenin June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010,TexasInstrumentsIncorporatedS0457-0 1RA B IIB IIA vIA vIB +10 10+200 mV 200 < |VID| < common-modevoltagevaryingfrom 7 V to7 V,VIDneednotbegreaterthan thestandard,themagnitudeofthedifferentia linputvoltage,VID, varyingfrom200mVto10V,is (1)VCM(2)VIA'(3)VIB'(3)STATE+10V 2 V+12V+4 VQEnsurescorrectoperationwithmaximumdiff erentialvoltagesupply 10V+2V 12V 4 VQ+10V+4V+6V+7 VQEnsurescorrectoperationwithmaximumcomm onmodevoltagesupply 10V 4 V 6 V 7 VQ+100mV 100mV+200mV0 VQ 100mV+100mV 200mV0 VQ200mVthresholdtestacrosscommonmodevolt agesupply+ + +200V+7VQ 200V 7 VQ(1)|VID| < 12V (maximuminputdifferentialvoltagewithoutd amagingdevice)(2)VCMis measuredasthearithmeticaverageofVIA'andV IB', or(VIA'+ VIB')/2.

7 (3)|VIA'| < 10V,|VIB'| < 10V (maximuminputvoltagestoensurecorrectoper ation) required,unlessthedatarateoftheapplicati onis lessthan200kbpsorthesignalrise/falltimea ttheloadendofthecableis usedtodescribea systemthatdoesnotbehavelikea ,cableterminationis RS-422-compliantsystem8RS-422andRS-485 StandardsOverviewandSystemConfigurations SLLA070D June2002 RevisedMay2010 Copyright 2002 2010,TexasInstrumentsIncorporatedD1D3 ZTZTS0458-01 ABA B A/A B/B tomatchtheterminationresistancetothechar acteristicimpedanceofthecable,Zo, whilethesecondoptionis toplaceanadditionalcapacitorin andFigure9, it is balanceddigitalmultipointsystems,whereas theRS-422cansupportonlyonedriverperbusli ne(multidrop).Parametervaluesspecifiedin 485aresimilartothosespecifiedin , driverBD3/R3= transceiverCR2= receiverDZT= [receiver,driver(offstate),transceiver] (half-duplexoperation),faultconditionsmi ghtoccur(causedbyinadvertentshortsonoutp utdriversorlinecontention).

8 Therefore, ,considerthecasewhendriverD1inFigure9 is intendedtosenda signaltoreceiverR2,butdriverD3stillis thedesignerdidnotdisabledriverD3beforein itiatingthetransmission,a knownaslinecontention(seeSummaryComparis onoftheStandardssection).Themaximumrecom mendedcablelengthis ,theamountofnoisea designeriswillingtotolerateis thedecidingfactorin , (VIAandVIBA pplied;IIAandIIBM easured)AswiththeRS-422,a maximumlimitontheI/Vcharacteristicmustbe placedonthereceiver,driver(offstate),and transceivertoensurea (VIB) rangingfrom 7 V to12V,whileVIB(VIA) is grounded,theresultinginputcurrentIIA(IIB )shouldremainwithintheshadedregionin bothpower-onandpower-offconditions(seeFi gure10). A devicewithinputcharacteristicsthatfallwi thintheshadedregionconformstohavinga June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010,TexasInstrumentsIncorporatedS0459-0 1 LoadA/A B/B IIBIIAvIAvIB+5V 3V+12V mA1 mAVIIIL ogic(1 or 0)DAB+ VOAS0451-01 VOD+ VOB+ thoughtofasa12-k dependentonloading,a designthatrequiresa largenumberofstations(drivers,receivers, ortransceivers)attachedtothebusneedsa ,a bus, (VOD,VOA, andVOBM easured)Theoutputvoltageshallnotexceed 6 V underunloadedconditions,andthedifferenti alvoltagegeneratedwillbenosmallerthan andnogreaterthan 6 |VOD| 6 VB|VOA| 6 V;|VOB| 6 June2002 RevisedMay2010 Copyright 2002 2010,TexasInstrumentsIncorporatedLogic(1 or 0)DAB27 WVOD+ + VOC27WA.

9 V |V| 5 V, and | V| = ||V- |V|| VB. -ODODODODD 1 V |V| 3 V, and | V| = ||V- |V|| VOCOCOCOCD Logic(1 or 0)DAB375WS0460-01 VOD+ 7 VV |CM| 12 V375W60 WLogic(1 or 0) (VODandVOCM easured)SimilartotheRS-422,RS-485alsospe cifiesa minimumoutputvoltagetoensureproperdrives trength,butalsoplacesa alsoshowsthemagnitudein thechangein (VODM easured;VCMA pplied)Withthetestcircuitshownin Figure13, |VOD| 5 (IOSM easured;VOApplied)Withthedrivershortedto a voltagesourcethatis variedfrom 7 V to12V,currentshallnotexceed250mA,andthed rivershallnotbedamaged(seeFigure14). TexasInstrumentsincorporatesintoallitsRS -485-compliantdevicesa ,complyingwiththefullrangefrom 7 V to12V foranindefinitetimeis a June2002 RevisedMay2010RS-422andRS-485 StandardsOverviewandSystemConfigurations Copyright 2002 2010,TexasInstrumentsIncorporatedLogic(1 or 0)DAB54W50 pFS0462-01 VOD+ ,TIoffersproductsthatcomplyfully(indeter minatetimeperiodandfullvoltagerange) ,butcannotsustainthecurrentovera thecapabilitytokeepthecurrentwithinspeci ficationforlongperiods,butata (VODM easured)Toensuresignalquality,RS-485also placesa identicaltotheonedescribedfortheRS-422,b uta differenttestcircuitisnecessary(seeFigur e15).

10 Thereafter,theoutputvoltageshallnotchang emorethan10%ofVSS, and|VOD| 5 (VCM, VIA, andVIB, Applied;VIDM easured)Usingthesametestcircuitasin RS-422,Table2 'VIB' 7 V 200V VCM+12V+ +200V+ + VCM 7 V 2 V 5 V VCM+12V+7V+5V+ + VCM3 FailsafeOperationThefeatureofreceiverfai lsafeis a benefitin manyRS-422andRS-485applications;however, , knownaslineidleorbusidleandoccurswhenall driversplacetheiroutputsintoa ,thedifferentialbusvoltageis leftfloating( ,indeterminate:neitherlogic-highnorlogic -lowstate)if thereis noterminationresistors,andthedifferentia lbusvoltageis closetozeroin ,asa result,thereceivercanbefalselytriggeredi ntoeithera logic-highorlogic-lowstate, ,thisis undesirable, is besttodetectsucha situationandplacethereceiveroutputsintoa ,desirablefeaturethata failsafeprovidesis toprotectthereceiverfromshortedlinecondi tions, June2002 RevisedMay2010 Copyright 2002 2010, ,short-circuit,andidle-busfailsafecircui trywithintheintegratedcircuits, foundin definedvoltageacrossthereceiver'sinput,r egardlessofwhetherthesignalpairis shortedtogetheroris isrecommendedtousewhenthereis nointernalfailsafecircuit,ortoreinforcet heeffectofinternalfailsafecircuitsin foundin systemthatusesdrivers,receivers,andtrans ceiversthatcomplywithRS-422orRS-485.