Example: biology

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

Conductor Insulation Cable Shield Sheath Cable : Type : Impedance : Capacitance : Velocity : Belden 3109A 4 - pair, 22 AWG PLCT/CM 120 W 11 pF/ft 78% (1.3 ns/ft)

Information

Domain:

Source:

Link to this page:

Please notify us if you found a problem with this document:

Other abuse

Text of The RS-485 Design Guide (Rev. C) - Texas - TI.com

1SLLA272C Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuideAll trademarksare the propertyof Febuary2008 RevisedOctober2016The - InterfaceABSTRACTAs a shortcompendiumfor successfuldatatransmissiondesign,this applicationreportdiscussestheimportantas pectsof the 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,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)3NetworkTopologyThe RS-485standardssuggeststhat its nodesbe networkedin a daisy-chain,also knownas partyline orbus topology(seeFigure1. In this topology,the participatingdrivers,receivers,and transceiversconnectto a maincabletrunkvia interfacebus can be designedfor full-duplexor half-duplextransmission(seeFigure2).Figu re1. RS-485Bus StructureConductorInsulationCable ShieldSheathCable :Type :Impedance :Capacitance :Velocity :Belden 3109A4 - pair, 22 AWG PLCT /CM120W11 pF/ft78% ( ns /ft)+ 200 mV- 200 mV+ V- VDRRTRTfrom Masterto Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuideThe 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,whereasstandardconformreceiversdete cta differentialinputdownto 200 mV. The two valuesprovidesufficientmarginfor a reliabledatatransmissionevenunderseveres ignaldegradationacrossthe cableand robustnessis the mainreasonwhy RS-485is well suitedfor long-distancenetworkingin RS-485SpecifiedMinimumBus SignalLevels5CableTypeRS-485applications benefitfromdifferentialsignalingovertwis ted-paircable,becausenoisefromexternalso urcescoupleequallyinto 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 Exampleof RS-485CommunicationCablerStubtLx v x c10 Terminationand StubLength3SLLA272C Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuideBeyondthe 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).Figure5. ProperRS-485TerminationsApplicationsin 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).Table1 lists the maximumstub lengthsof the cablein Figure4, (78%velocity),for StubLengthVersusRiseTimeDEVICESIGNALRATE [kbps]RISETIMEtr[ns]MAXIMUMSTUBLENGTH[ft ]SN65HVD1210001007SN65LBC18425025019SN65 HVD3082E20050038NOTE:Driverswith long rise timesare well suitedfor applicationsrequiringlong stub lengthsandreduced, UL20 ULNUL per transceiver-=RT120WRB523WRBVBusRT523W120 WBUS-mimBAB0VRV x (1/ 375 4 / Z )=+ Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuide7FailsafeFailsafeoperat ionis 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,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-BusFailsafeBiasing8Bus 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. Standard-compliantdriversmustbe able to drive32 of s transceiversoftenprovidereducedunit loading,suchas 1/8 UL, thus allowingtheconnectionof up to 256 transceiverson the 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 x C '> Rate [Mbps]CableLength [m] Length5SLLA272C Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuide9DataRateVersusBus 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.(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.(3)Section3 presentsthe lowerfrequencyrangewherethe line resistance,and not the switching, ,the cableresistanceapproachesthe valuefo the voltagedividerdiminishesthe signalby -6 dB. For a 22 AWGcable,120 W, UTP,this occursat CableLengthVersusDataRate10MinimumNodeSpacingThe RS-485bus is a distributedparametercircuitwhoseelectricalcharacteristicsare primarilydefinedbythe distributedinductanceand capacitancealongthe physicalmedia,whichincludesthe interconnectingcablesand the bus in the formof devicesand theirinterconnectionslowersthe bus impedanceand causesimpedancemismatchesbetweenthe mediaand the loadedsectionof the bus. Inputsignalsarrivingat thesemismatchesare partiallyreflectedbackto the signalsourcedistortingthe validreceiverinputvoltagelevelduringthe first signaltransitionfroman outputdriveranywhereon the bus requiresa minimumloadedbus impedanceof Z > x Z0, whichcan be achievedby keepingthe minimumdistance,d, betweenbus nodes:(4)WhereCLis the lumpedload capacitanceand C, the mediacapacitance(cableor PCBtrace)per Installation123VnVcc1Vcc2Electrical Installation123VnVcc1Vcc2Electrical Installation123Vnhigh loop currentlow loop current100O100OGround loopGround loopa)b)c)CircuitgroundCircuitground4050 6070800 .50 .40 .30 .20 .10Dis tan ce (m)Media - Distributed Capacitance - (pF /m )10060402010CL( pF )Groundingand Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuideFigure8. MinimumNodeSpacingWithDeviceand MediaCapacitanceEquation4 presentsthe relationshipfor the minimumdevicespacingas a functionof the distributedmediaand lumped-loadcapacitance;Figure8 showsthis line circuitbus pins,connectorcontacts,printed-circuitbo ardtraces,protectiondevices,and any otherphysicalconnectionsto the trunkline as long as thedistancefromthe bus to the transceiver(the stub)is the individualcapacitancecontributions:5-V transceiverstypicallypossessa capacitanceof 7 pF, whereas3-V transceivershaveapproximatelytwicethat capacitanceat 16 pF. Boardtracesadd to pF/cmdependingon suppressiondevicecapacitancecan vary pF/mfor low capacitance,unshielded,twisted-paircable to 70 pF/mfor IsolationWhendesigninga remotedatalink, the designermustassumethat largegroundpotentialdifferences(GPD) as common-modenoise,Vn, to the the totalsuperimposedsignalis withinthe receiver s inputcommon-moderange,relyingon the localearthgroundas a reliablepathfor the returncurrentis dangerous(seeFigure9a).Figure9. DesignPitfallsto be Awareoff: a) HighGPD,b) HighLoopCurrent,c) ReducedLoopCurrent,Yet HighlySensitiveto InducedNoiseDue to LargeGroundLoopIsolatedXCVRNon-isolatedX CVRIsolatedXCVRIsolatedXCVRVcc4RDGND4Vcc 1RDGND1Vcc2R DGND2Vcc3R Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedThe RS-485DesignGuideBecauseremotenodesare likelyto drawtheirpowerfromdifferentsectionsof the electricalinstallation,modificationto the installation,( ,duringmaintenancework),can increasethe GPDto the extentthatthe receiver s inputcommon-moderangeis ,a datalink workingtodaymightceaseoperationsometimei n the directconnectionof remotegroundsthroughgroundwire also is not recommended(seeFigure9b),as this causeslargegroundloop currentsto coupleinto the datalinesas allowfor a directconnectionof remotegrounds,the RS485standardrecommendsthe separationofdevicegroundand localsystemgroundvia the insertionof resistors(Figure9c). Althoughthis approachreducesloop current,the existenceof a largegroundloop keepsthe datalink sensitiveto noisegeneratedsomewhereelse alongthe ,a robustdatalink has not approachto tolerateGPDsup to severalkilovoltsacrossa robustRS-485datalink and overlongdistanceis the galvanicisolationof the signaland supplylinesof a bus transceiverfromits localsignaland supplysources(seeFigure10).Figure10. Isolationof TwoRemoteTransceiverStationsWithSingle-G roundReferenceIn this case,supplyisolators,suchas isolatedDC/DCconverters,and signalisolators,suchas digital,capacitiveisolators,preventcurre ntflow betweenremotesystemgroundsand avoidthe showsthe detailedconnectionof only two transceivernodes,Figure11 givesanexamplefor multiple, transceiversbut one connectto the bus via the left providesthe single-groundreferencefor the Isolationof MultipleFieldbusTransceiverStations12Con clusionThe objectiveof this applicationreportis to coverthe mainaspectsof an enormousamountof technicalliteratureon the subject,this document s intentis to providesystemdesignersnew to RS-485with designguidelinesin a very discussionspresentedin this documentand consultingthe detailedapplicationreportsin thereferencesectioncan help accomplishinga robust,RS-485-compliantsystemdesignin the Febuary2008 RevisedOctober2016SubmitDocumentationFee dbackCopyright 2008 2016,TexasInstrumentsIncorporatedRevisio nHistorySupportingthe designeffort,TexasInstrumentsprovidesan extensiveproductrangeof EMI,low-power(1/8 UL), high ESDprotection(from16 kV up to30 kV), and integratedfailsafefunctionsfor open-,short-and long-distanceapplicationsrequiringisolat ion,the productrangeextendsto unidirectionaland bidirectional,digitalisolatorsin dual,tripleand quadversions(fromDC to 150 Mbps),and isolatedDC/DCconverters(with3-V and 5-V regulatedoutputs),to providethe powersupplyacrossthe availableat enteringthe blue literaturenumbersthat followinto DataTransmissionSystemsapplicationreport (SLLA268) TIA/EIA-485(RS-485)designnotes(SLLA036) RS-485SignalLoss, TI AnalogApplicationJournal,4Q 2006(SLYT257) RS-485Line Circuitsapplicationreport(SLLA200) RS-485Buses, TI AnalogApplicationJournal,2Q 2006(SLYT241) (SLLA177) StatisticalSurveyof Common-ModeNoise, TI AnalogApplicationJournal,Nov 2000(SLYT153) RS-485DataBuses, TI AnalogApplicationJournal,3Q 2004(SLYT080) RS-485Unit Loadand MaximumNumberof Bus Connections, TI AnalogApplicationJournal,1Q2004(SLYT086) TransferRateapplicationreport(SLLA098) FastSignalingRatesapplicationreport(SLLA 173) E-MeterApplicationsapplicationreport(SLL A112) RS-485DataBuses, TI AnalogApplicationJournal,3Q 2004(SLYT064) ReceiverEqualizationto ExtendRS-485DataCommunications applicationreport(SLLA169) RS-485Unit Loadand MaximumNumberof Bus Connectionsapplicationreport(SLLA166) Solutionsapplicationreport(SLLA067) DigitalMotorControlApplicationsapplicati onreport(SLLA143) and 485 StandardsOverviewand SystemConfigurationsapplicationreport(SL LA070) M-LVDS,Powerand SpeedComparisonapplicationreport(SLLA106 ) Insertionwith DifferentialInterfaceProductsapplication report(SLLA107) ISO72xFamilyof High-SpeedDigitalIsolatorsapplicationrep ort(SLLA198)RevisionHistoryNOTE:Pagenumb ersfor previousrevisionsmay differfrompagenumbersin the Revision(May2008)to C ChangedDataRate[bps]To: DataRate[Mbps}in its subsidiaries(TI) reservethe rightto makecorrections,enhancements,improvement sand otherchangesto its semiconductorproductsand servicesper JESD46,latestissue,and to discontinueany productor serviceper JESD48, latestrelevantinformationbeforeplacingor dersand shouldverifythat suchinformationis semiconductorproducts(alsoreferredto hereinas components ) are sold subjectto TI s termsand conditionsof salesuppliedat the time of warrantsperformanceof its componentsto the specificationsapplicableat the time of sale,in accordancewith the warrantyin TI s termsand conditionsof sale of otherqualitycontroltechniquesare usedto the extentTI deemsnecessaryto supportthis applicablelaw, testingof all parametersof eachcomponentis not assumesno liabilityfor applicationsassistanceor the designof Buyers responsiblefor theirproductsandapplicationsusingTI minimizethe risksassociatedwith Buyers productsand applications,Buyersshouldprovideadequate designand doesnot warrantor representthat any license,eitherexpressor implied,is grantedunderany patentright,copyright,maskworkright,orot herintellectualpropertyrightrelatingto any combination,machine,or processin whichTI componentsor servicesare TI regardingthird-partyproductsor servicesdoesnot constitutea licenseto use suchproductsor servicesor a of suchinformationmay requirea licensefroma thirdpartyunderthe patentsor otherintellectualpropertyof thethirdparty,or a licensefromTI underthe patentsor otherintellectualpropertyof significantportionsof TI informationin TI databooksor datasheetsis permissibleonly if reproductionis withoutalterationand is accompaniedby all associatedwarranties,conditions,limitati ons,and is not responsibleor liablefor thirdpartiesmay be subjectto TI componentsor serviceswith statementsdifferentfromor beyondthe parametersstatedby TI for that componentor servicevoidsall expressand any impliedwarrantiesfor the associatedTI componentor serviceand is an unfairand is not responsibleor liablefor any agreesthat it is solelyresponsiblefor compliancewith all legal,regulatoryand safety-relatedrequirementsconcerningits products,and any use of TI componentsin its applications,notwithstandingany applications-relatedinformationor supportthat may be providedby TI. Buyerrepresentsand agreesthat it has all the necessaryexpertiseto createand implementsafeguardswhichanticipatedanger ousconsequencesof failures,monitorfailuresand theirconsequences,lessenthe likelihoodof failuresthat mightcauseharmand take fully indemnifyTI and its representativesagainstany damagesarisingout of the useof any TI componentsin somecases,TI componentsmay be promotedspecificallyto ,TI s goal is tohelp enablecustomersto designand createtheirown end-productsolutionsthat ,suchcomponentsare subjectto TI componentsare authorizedfor use in FDAClassIII (or similarlife-criticalmedicalequipment)unl essauthorizedofficersof the partieshaveexecuteda componentswhichTI has specificallydesignatedas militarygradeor enhancedplastic are designedand intendedfor use inmilitary/aerospaceapplicationsor agreesthat any militaryor aerospaceuse of TI componentswhichhavenotbeenso designatedis solelyat the Buyer's risk, and that Buyeris solelyresponsiblefor compliancewith all legalandregulatoryrequirementsin connectionwith has specificallydesignatedcertaincomponentsa s meetingISO/TS16949requirements,mainlyfor automotiveuse. In any caseof use ofnon-designatedproducts,TI will not be responsiblefor any failureto ,Avionicsand E2E :TexasInstruments,PostOfficeBox 655303,Dallas,Texas75265Copyright 2016,TexasInstrumentsIncorporated

Related search results