AN-1031 TIA/EIA-422-B Overview (Rev. B) - TI.com

1 ApplicationReportSNLA044B January2000 RevisedApril2013AN-1031 balancedinterfacecircuitutilizingtheTIA/ EIA-422-B(formerlyRS-422-A) a registeredtrademarkofGeneralSemiconducto rIndustries! January2000 RevisedApril2013AN-1031 TIA/EIA-422-BOverviewSubmitDocumentation FeedbackCopyright 2000 2013, (RS-422)is anindustrystandardspecifyingtheelectrica lcharacteristicsofa ,EIA/TIA,andRS;althoughpreviouslycorrect ,todayTIA/EIAis ,forsimplicity, ,a ; , RS-422drivercandriveuptotenunitloads(4k tocircuitcommonis oneunitload).Thedriveriscapableoftransmi ttingdataacross4000feet(recommendedlimit )ofcable;butnotatmaximumdatarates(seeFig ure3 ). StandardRS-422driversareassuredtosourcea ndsinka minimum20mAacrossa100 minimumdifferentialoutputvoltage,VOD, of2 V acrosstheload(seeFigure1). representedbytheslopeoftheshadedregionin Figure2.

2 Theoperatingrangeofthereceiveris definedbetween 10V andis representedbytheshadedareain ,RS-422receivershavea 200mVthresholdovertheentirecommonmoderan geof 7 is assuredbetweenthedriver'sdifferentialout putswingandthereceiver' ,a daisychainis ,atlongdistancesorhighdatarates,terminat ionis recommendedtoreducereflectionscausedbya mismatchin theimpedanceofthecableandtheimpedanceoft hereceiver' , ,theRS-485differentialinterfacestandardi s ,therearedifferencesthatdistinguishthetw ostandardsfromoneanother;whichincludethe outputstageofthedriver,thecommonmoderang eoftheinterface,theinputresistanceofther eceiver, ,seeAN-759 ComparingEIA-485andEIA-422-ALineDriversa ndReceiversin MultipointApplications(SNLA023). , ,it is notpossibletooperateat4000feetwhenoperat ingat10 January2000 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2000 2013, chartdisplayingtherecommendedoperational regionofa typicalRS-422standardinterfaceis showninFigure3.

3 24 AWG,copper,16pF/ft,twisted-paircablepara llelterminatedwitha 100 recommendedforusewhendesigninga RS-422interfacethatis consideredtobea transmissionlineif thepropagationdelayofthecableis greaterthan thetransition(riseorfall) a ,thetransitiontimeforthetime-domainanaly sisis ,whicheveredgeis thisapplicationreportbutis ,seeAN-806 DataTransmissionLinesandTheirCharacteris tics(SNLA026),AN-807 Reflections:ComputationsandWaveforms(SNL A027), andAN-808 LongTransmissionLinesandDataSignalQualit y(SNLA028). ,a verypopularformoftermination, possiblebecausetheterminationresistor(Rt )is chosentocloselymatchthecableimpedance(ZO ) (seeFigure4). ,timedomainreflectometry, ,becauseoftheminimizedaffectofthetransmi ssionline,multi-dropconfigurationswithgo odsignalqualityarealsopossible,aslongast hestubs(discussedlater) disadvantageofparallelterminationis smallerdifferentialoutputvoltageandlower DCnoisemarginsthanwitha ,if thereceiverhasbuilt-infailsafecircuitry, a knownoutputcannotbeensuredwiththistypeof terminationschemeunlessthereceiverspecif icallysaysthatit supports terminated ,seeAN-903A ComparisonofDifferentialTerminationTechn iques(SNLA034).

4 3 SNLA044B January2000 RevisedApril2013AN-1031 TIA/EIA-422-BOverviewSubmitDocumentation FeedbackCopyright 2000 2013, ,open,terminated,andshortedinputaretheth reetypesoffailsafetoconsiderasshownin Figure5. A receiverwithfullfailsafeprotectionassure sa theconditionwhenthereceiver'soutputis notconnectedbutthereceiveris receiverthatprovidesfailsafeforanopeninp utcondition,mayprovidefailsafeforidle(dr iver'soutputidle)andTRI-STATE(driver'sou tputdisabled)conditionsif thebusis ,theoutputstateofthereceivermaybeknownfo rbothidleandTRI-STATE busconditionsin receiveris abletoprovideopeninputfailsafewithintern alpull-upandpull-downresistors,typically >50k . Sometimesonlyonebiasresistoris usedononeinputandtheotherinputbiasedtoa , ZOwhere:Rt= terminationresistorZO= ; , , a receiverdoesnothavethisfeature, ,seeAN-847 FAILSAFEB iasingofDifferentialBuses(SNLA031) thatis January2000 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2000 2013, whenthereceiveroutputis areceiverdoesnothavethistypeoffailsafe, , ,seeAN-903A ComparisonofDifferentialTerminationTechn iques(SNLA034).

5 TI' ,thethresholdpointofthereceiverwasshifte d,violatingtheRS-422 , ,point-to-point,is a ,differentialstandardsarenotrestrictedfr omusein typicalpoint-to-pointsystemis shownin ,multi-drop,is onedriverwithtwoormorereceiversnormallyc onnectedin ,themaximumnumberofreceiversis 10if thereceiver'sinputimpedance(RIN) is equalto4 k a receiver'sRINis equalto8 k thenthatreceiveris equalto ,a RS-422driverthatcandrive10unitloadscandr ive20receiverswitha RIN= 8 k .Anexampleofa multi-dropapplicationis shownin multi-point,whichusestwoormoredriverscon nectedtooneormorereceivers(seeFigure6 ). ,a multi-pointsystemcanbeaccomplishedif ,contentionbetweendrivers, , ,seeAN-759 ComparingEIA-485andEIA-422-ALineDriversa ndReceiversin MultipointApplications(SNLA023).6 ESDP rotectionElectrostaticdischarge(ESD)is normallyaneventofveryhighpotentialfora (IC).

6 ESDis notlimitedtoa onetimeoccurencein thelifeofanIC,unlessit is maybeanongoingtranspirationthatcanweardo wnanICuntilit ,theindustryhasdevelopeda defactominimumstandardof2,000V ESDprotectionunderhumanbodymodel(HBM) , ,TransZorbs , andprotectedconnectorsarethreepossibleso lutionsforincreasinganIC' theresultofthemanufacturer' ,ESDwasnotaswellpublicizedorstandardized asit is ,ICdesignersstrivetoachievefive,ten, January2000 RevisedApril2013AN-1031 TIA/EIA-422-BOverviewSubmitDocumentation FeedbackCopyright 2000 2013, Point-to-PointRS-422 SystemWithTransZorbsThelatertwosolutions ,TransZorbsandprotectedconnectors,arebot hexternaltotheICandusuallyimplemented,by thesystemmanufacturer,toprovideadditiona lprotection,if needed, (viacablesorothermedia).Thus, to,one,beabletodrainoffverylargeamountso fcurrentveryquickly,andtwo, ,causedbya verylargeelectricfield(potential), ,clampingvoltagesthatareabovetheTransZor b'sspecifiedreversestand-offvoltage(Vrs) .

7 Takecarewhenselectinga TransZorb,if theVrsis toohigh, showsanexampleofa ,anothersolutionforincreasingESDprotecti onofanIC, ,onlyonedeviceis ,if youhavea DB-25pinconnector,all25linesreceivethesa meincreasein ,designersdonotneedtoworryaboutlackofadd itionalPCboardspace,sincethedeviceis ,theprotectedconnectoronlyaddsanaddition al5 pFor6 a showsa January2000 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2000 2013, , ,systemmanufacturerswouldliketheICmanufa cturerstoprovidealltheESDprotectionrequi redfortheirsystems, , (driverorreceiver)intoa ,cableconnectors, a largerconcern,sinceremovinganIC,froma liveinterface,normallyis , ,theRS-422devicemayreceivebuspotentialst hatexceedthepowersupplyvoltage(VCC). First,thismaycausebiasingofdiodesandbusc lamping,orresultin , ,theICmaytransmitsignalswhenthesupplyvol tageis belowminimumoperatinglevelwhichmayresult in ,whenremovinganICfromanactiveinterface, ,thepotentialdifferenceattheinstanceofth edisconnectis minimizedsincethepotentialis liveinterfaceare,one, thedevice' ,it willnotalwaysappeartheresobesuretoaskthe chipmanufacturer'stechnicalsupportgroup, busvoltagesexceedVCC.

8 Two,sinceremovalis lessharmfulthaninserting, ,it is PCboard,a staggeredconnectionwherethegroundtraceis thelongest,sothatit makescontactfirst,andtheVCCtraceis , F capacitoris recommendedforeachpoweredICin a devicehasmorethanonepowersupply(VCCandVE E) ,a 10 F capacitormaybeusednearthemainpathwhereVC Cis a systemis large, January2000 RevisedApril2013AN-1031 TIA/EIA-422-BOverviewSubmitDocumentation FeedbackCopyright 2000 2013, a multi-dropstandard,receiversmaybeconnect edtothebusviaa importantbecauseoftheaffectit increased,itscharacteristicsbegintoactas a a stubconsideredtobea transmissionline?Atypicalguidelineforstu bsandtransmissionlinesis statedhere:TimeDomain:If thepropagationdelayofthestub(onewaytrip) is greaterthan thetransitiontime,measuredfrom0%to100%of thesignaltransition,thenthestubmaybecons idereda stubbeginstoactasa , , veryimportanttorememberbecauseif thetotallengthofthecableis 1,000feet,a becausethecablecapacitanceslowsthetransi tiontimeofthedriver'soutputasitpropagate sdownthecableandthetransitiontimeis bulkcapacitanceloadora RS-422driverwithoutputwaveshapecontrolli keTI'sDS3691,DS3692, (s)closesttothedriver' a stubis toolongandcausesa noticeablereflection,it willbemeasuredpositiveonthestubbecauseth evoltagereflectioncoefficientis positivesincetheloadimpedanceZLis largerthanthestubimpedanceZo.

9 Theformulaforthereflectioncoefficientatt heloadis shownin Equation1.(1)Note,theloadimpedanceis equivalenttotheinputimpedanceofthereceiv er,whichis equaltoorgreaterthan4 k . Thestubimpedanceis typicallyabout100 130 . Thereflectioncreatedatthestubwillpropaga tein bothdirectionsawayfromthestub(seeFigure9 ). Keepin (seeFigure10).Therefore,thereceiver,when physicallyconnected, identicalforbothinputswhilethereceiveris (0V,0 mA)coordinate(seeFigure10). Whilethereceiverispoweredon,theimpedance ofbothinputsarethesamebuttheinputimpedan cecurvesdonotpassthroughthe(0V,0 mA) ,thecurvesmayormaynotcrossthex andy axisatthesamepoints(seeFigure10). Forreceiverswithbuilt-inopeninputfailsaf e, RS-422receiveris assuredfrom 10V to+10V,whichis is equaltothe7 Vcommonmodevoltageplus3 V January2000 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2000 2013, assuredtobegreaterthan 10V whetherthereceiveris RS-422driverandreceiver(s).

10 ( 7 V). rangeof22 AWGto28 AWGis ,seeSection2 in January2000 RevisedApril2013AN-1031 TIA/EIA-422-BOverviewSubmitDocumentation FeedbackCopyright 2000 2013, ,includinginformationconcerningshields,i nsulation,andcablecharacteristics,seeAN- 916A PracticalGuideToCableSelection(SNLA219). 12 SummaryRS-422is a standardizeddifferentialelectricalinterf acecapableoftransmittingdatain RS-422interface,makesuredesign-inconcern sareaddressedattheearlystagesofdesigntoe liminateproblemslaterin theapplication,whichmaybea a ,RS-422hasbeensupersededbyRS-485, ,seeAN-979 ThePracticalLimitsofRS-485(SNLA042).13 References TIA/EIA-422-B ,ElectricalCharacteristicso fBalancedDigitalInterfaceCircuits, INTERFACE:DataTransmissionDatabook AN-759 ComparingEIA-485andEIA-422-ALineDriversa ndReceiversin MultipointApplications(SNLA023) AN-806 DataTransmissionLinesandTheirCharacteris tics(SNLA026) AN-807 Reflections.