The RS-485 Design Guide (Rev. C) - Texas Instruments

1 1 SLLA272C Febuary2008 RevisedOctober2016 SubmitDocumentationFeedbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485 DesignGuideApplicationReportSLLA272C Febuary2008 RevisedOctober2016 The - InterfaceABSTRACTAs a shortcompendiumfor successfuldatatransmissiondesign,this applicationreportdiscussestheimportantas pectsof the trademarksare the propertyof 1983,the ElectronicsIndustriesAssociation(EIA)app roveda new usagein industrial,medical,and consumerapplications,RS-485has becomethe industry s applicationreportpresentsdesignguideline sfor engineersnew to the RS-485standardthat can helpthemaccomplisha robustand reliabledatatransmissiondesignin the shortesttime applicationreportpresentsdesignguideline sfor engineersnew to the RS-485standardthat can helpthemaccomplisha robustand reliabledatatransmissiondesignin the shortesttime FeaturesRS-485is an contrastto completeinterfacestandards,whichdefineth efunctional,mechanical,and electricalspecifications.

2 RS-485only definesthe electricalcharacteristicsofdriversand receiversthat couldbe usedto implementa standard,however,is intendedto be referencedby higherlevelstandards,suchas DL/T645,forexample,whichdefinesthe communicationprotocolfor electronicenergy-metersin China,specifyingRS-485 as the featuresof RS-485are: Balancedinterface Multipointoperationfroma single5-V supply 7-V to +12-Vbus common-moderange Up to 32 unit loads 10-Mbpsmaximumdatarate (at 40 feet) 4000-footmaximumcablelength(at 100 kbps)3 NetworkTopologyThe RS-485standardssuggeststhat its nodesbe networkedin a daisy-chain,also knownas partyline orbus topology(seeFigure1.)

3 In this topology,the participatingdrivers,receivers,and transceiversconnectto a maincabletrunkvia interfacebus can be designedfor full-duplexor half-duplextransmission(seeFigure2).+ 200 mV- 200 mV+ V- VDRRTRT from Masterto Febuary2008 RevisedOctober2016 SubmitDocumentationFeedbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485 DesignGuideFigure1. RS-485 Bus StructureThe full-dupleximplementationrequirestwo signalpairs,(fourwires),and full-duplextransceiverswithseparatebus accesslinesfor transmitterand nodeto simultaneouslytransmitdataon one pair whilereceivingdataon the Full-Duplexand Half-DuplexBus Structuresin RS-485In half-duplex,only one signalpair is used,requiringthe drivingand receivingof datato occurat controlledoperationof all nodesvia directioncontrolsignals,suchas Driver/ReceiverEnablesignals,to ensurethat only one driveris activeon the bus at any driveraccessingthe bus at the sametime leadsto bus contention,which,at alltimes,mustbe differentialoutputof a V acrossa 54- load.

4 Whereasstandardconformreceiversdetecta differentialinputdownto 200 mV. The two valuesprovidesufficientmarginfor a reliabledatatransmissionevenunderseveres ignaldegradationacrossthe cableand robustnessis the mainreasonwhy RS-485is well suitedfor long-distancenetworkingin RS-485 SpecifiedMinimumBus SignalLevels5 CableTypeRS-485applicationsbenefitfromdi fferentialsignalingovertwisted-paircable ,becausenoisefromexternalsourcescoupleeq uallyinto bothsignallinesas common-modenoise,whichis rejectedby the of the sheathed,unshielded,twisted-pairtype,(UT P),with a characteristicimpedanceof 120 W and 22 24 showsthe cross-sectionof a four-pair,UTPcabletypicallyusedfor two ,in two-pairand single-pairversions,are availabletoaccommodatethe low-costdesignof v x c10 RTRT120W220pF60W220pF120W60W60W60 WConductorInsulationCable ShieldSheathCable :Type :Impedance :Capacitance :Velocity :Belden 3109A4 - pair, 22 AWG PLCT /CM120W11 pF/ft78% ( ns /ft) Terminationand StubLength3 SLLA272C Febuary2008 RevisedOctober2016 SubmitDocumentationFeedbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485 DesignGuideFigure4.

5 Exampleof RS-485 CommunicationCableBeyondthe networkcabling,it is mandatorythat the layoutof printed-circuitboardsand the connectorpinassignmentsof RS-485equipmentmaintainthe electricalcharacteristicsof the networkby keepingbothsignallinescloseand equidistantto Terminationand StubLengthDatatransmissionlinesshouldalw aysbe terminatedand stubsshouldbe as shortas possibleto avoidsignalreflectionson the line. Properterminationrequiresthe matchingof the terminatingresistors,RT, tothe characteristicimpedance,Z0, of the RS-485standardrecommendscableswith Z0= 120 W, the cabletrunkis commonlyterminatedwith 120-Wresistors,one at eachcableend (seeFigure5, left).

6 Figure5. ProperRS-485 TerminationsApplicationsin noisyenvironmentsoftenhavethe 120- resistorsreplacedby two 60- , low-passfilterstoprovideadditionalcommon -modenoisefiltering,(seeFigure5, right).It is importantto matchthe resistorvalues,(preferablywith 1% precisionresistors),to ensureequalrollofffrequenciesof ,( ,20%),causethe filter cornerfrequenciesto differand common-modenoiseto beconvertedinto differentialnoise,thus compromisingthe receiver s electricallengthof a stub,(the distancebetweena transceiverand cabletrunk),shouldbe shorterthan1/10of the driver s outputrise time,and is giventhrough:(1)Where: LStub= maximumstub length(ft) tr= driver(10/90)rise time (ns) v = signalvelocityof the cableas factorof c c = speedof light ( x 108ft/s).

7 Table1 lists the maximumstub lengthsof the cablein Figure4, (78%velocity),for x (1/ 375 4 / Z )=+ Febuary2008 RevisedOctober2016 SubmitDocumentationFeedbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485 DesignGuideTable1. StubLengthVersusRiseTimeDEVICESIGNALRATE [kbps]RISETIMEtr[ns]MAXIMUMSTUBLENGTH[ft ]SN65 HVD1210001007SN65 LBC18425025019SN65 HVD3082E20050038 NOTE:Driverswith long rise timesare well suitedfor applicationsrequiringlong stub lengthsandreduced, a receiver s abilityto assumea determinedoutputstatein the absenceof an lead to the loss of signal(LOS) , causedby a wire breakor by the disconnectionof a transceiverfromthe , causedby an insulationfault connectingthe wiresof a differentialpair to , occurringwhennoneof the bus driversis causeconventionalreceiversto assumerandomoutputstateswhentheinputsign alis zero,moderntransceiverdesignsincludebias ingcircuitsfor open-circuit,short-circuit,andidle-busfa ilsafe,that forcethe receiveroutputto a determinedstate,underan drawbackof thesefailsafedesignsis theirworst-casenoisemarginof 10 mV only.

8 Thus requiringexternalfailsafecircuitryto increasenoisemarginfor applicationsin externalfailsafecircuitconsistsof a resistivevoltagedividerthat generatessufficientdifferentialbusvoltag e,to drivethe receiveroutputinto a ensuresufficientnoisemargin,VABmustinclu dethe maximumdifferentialnoisemeasuredin additionto the 200-mVreceiverinputthreshold,VAB=200 mV + VNoise.(2)For a minimumbus voltageof , (5 V 5%),VAB= , and Z0= 120 W, RByields528 523-Wresistorsin seriesto RTestablishesthe failsafecircuitshownin ExternalIdle-BusFailsafeBiasing8 Bus LoadingBecausea driver'soutputdependson the currentit mustsupplyinto a load,addingtransceiversandfailsafecircui tsto the bus increasesthe total load estimatethe maximumnumberofbus loadspossible,RS-485specifiesa hypotheticaltermof a unit load (UL),whichrepresentsa loadimpedanceof approximately12 kW.

9 Standard-compliantdriversmustbe able to drive32 of s transceiversoftenprovidereducedunit loading,suchas 1/8 UL, thus allowingtheconnectionof up to 256 transceiverson the Rate [Mbps]CableLength [m]321 STANDARDFAILSAFE32 UL20 ULNUL per transceiver-= Length5 SLLA272C Febuary2008 RevisedOctober2016 SubmitDocumentationFeedbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485 DesignGuideBecausefailsafebiasingcontrib utesup to 20 unit loadsof bus loading,the maximumnumberoftransceivers,N, is reducedto:(3)Thus,whenusing1/8-ULtransce ivers,it is possibleto connectup to a maximumof 96 devicesto the LengthThe maximumbus lengthis limitedby the transmissionline lossesand the signaljitter at a a jitter of 10%or moreof the baudperiod,Figure7showsthe cablelengthversusdatarate characteristicof a conventionalRS-485cablefor a 10%signaljitter.

10 (1)Section1 of the graphpresentsthe areaof high ,the lossesof thetransmissionline can be neglectedand the datarate is mainlydeterminedby the driver s rise standardrecommends10 Mbps,today s fast interfacecircuitscan operateat dataratesof up to 40 Mbps.(2)Section2 showsthe transitionfromshortto long lossesof the transmissionlineshaveto betakeninto ,with increasingcablelength,the datarate mustbe rule of thumbstatesthat the productof the line length[m] timesthe datarate [bps]shouldbe < 107. This rule is far moreconservativethantoday's cableperformanceand will thereforeshowless lengthat a givendatarate thanthegraphpresents.