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 ).

2 StandardRS-422driversareassuredtosourcea ndsinka minimum20mAacrossa100 minimumdifferentialoutputvoltage,VOD, of2 V acrosstheload(seeFigure1). representedbytheslopeoftheshadedregionin Figure2. 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.

3 Whichincludetheoutputstageofthedriver,th ecommonmoderangeoftheinterface,theinputr esistanceofthereceiver, ,seeAN-759 ComparingEIA-485andEIA-422-ALineDriversa ndReceiversin MultipointApplications(SNLA023). , ,it is notpossibletooperateat4000feetwhenoperat ingat10 January2000 RevisedApril2013 SubmitDocumentationFeedbackCopyright 2000 2013, chartdisplayingtherecommendedoperational regionofa typicalRS-422standardinterfaceis showninFigure3. 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).

4 ,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).

5 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.

6 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).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.

7 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).ESDis notlimitedtoa onetimeoccurencein thelifeofanIC,unlessit is maybeanongoingtranspirationthatcanweardo wnanICuntilit ,theindustryhasdevelopeda defactominimumstandardof2,000V ESDprotectionunderhumanbodymodel(HBM)

8 , ,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).

9 Thus, to,one,beabletodrainoffverylargeamountso fcurrentveryquickly,andtwo, ,causedbya verylargeelectricfield(potential), ,clampingvoltagesthatareabovetheTransZor b'sspecifiedreversestand-offvoltage(Vrs) . 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.

10 , (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.