RS-422-485协议英文介绍.doc

RS-422andRS-485applicationnoteChapter1:OverviewIntroductionThepurposeofthisapplicationnoteistodescribethemainelementsofanRS-422andRS-485system.Thisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesign.SincebothRS-422andRS-485aredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriatetodiscussbothsystemsinthesameapplicationnote.ThroughoutthisapplicationnotethegenerictermsofRS-422andRS-485willbeusedtorepresenttheEIA/TIA-422andEIA/TIA-485Standards.DataTransmissionSignalsUnbalancedLineDriversEachsignalthattransmitsinanRS-232unbalanceddatatransmissionsystemappearsontheinterfaceconnectorasavoltagewithreferencetoasignalground.Forexample,thetransmitteddata(TD)fromaDTEdeviceappearsonpin2withrespecttopin7(signalground)onaDB-25connector.Thisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof5to15volts.TheRS-232receivertypicallyoperateswithinthevoltagerangeof+3to+12and-3to-12voltsasshowninFigure1.1.BalancedLineDriversInabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignal.Figure1.2showsaschematicsymbolforabalancedlinedriverandthevoltagesthatexist.Abalancedlinedriverwillproduceavoltagefrom2to6voltsacrossitsAandBoutputterminalsandwillhaveasignalground(C)connection.Althoughproperconnectiontothesignalgroundisimportant,itisntusedbyabalancedlinereceiverindeterminingthelogicstateofthedataline.Abalancedlinedrivercanalsohaveaninputsignalcalledan“Enable”signal.Thepurposeofthissignalistoconnectthedrivertoitsoutputterminals,AandB.Ifthe“Enable”signalisOFF,onecanconsiderthedriverasdisconnectedfromthetransmissionline.AnRS-485drivermusthavethe“Enable”controlsignal.AnRS-422drivermayhavethissignal,butitisnotalwaysrequired.Thedisconnectedordisabledconditionofthelinedriverusuallyisreferredtoasthe“tristate1”conditionofthedriver.1Theterm“tristate”comesfromthefactthatthereisathirdoutputstateofanRS-485driver,inadditiontotheoutputstatesof“1”and“0.”BalancedLineReceiversAbalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandB.Itwillalsohaveasignalground(C)thatisnecessaryinmakingtheproperinterfaceconnection.Figure1.3isaschematicsymbolforabalanceddifferentiallinereceiver.Figure1.3alsoshowsthevoltagesthatareimportanttothebalancedlinereceiver.IfthedifferentialinputvoltageVabisgreaterthan+200mVthereceiverwillhaveaspecificlogicstateonitsoutputterminal.Iftheinputvoltageisreversedtolessthan-200mVthereceiverwillcreatetheoppositelogicstateonitsoutputterminal.TheinputvoltagesthatabalancedlinereceivermustsenseareshowninFigure1.3.The200mVto6Vrangeisrequiredtoallowforattenuationonthetransmissionline.EIAStandardRS-422DataTransmissionTheEIAStandardRS-422-Aentitled“ElectricalCharacteristicsofBalancedVoltageDigitalInterfaceCircuits”definesthecharacteristicsofRS-422interfacecircuits.Figure1.4isatypicalRS-422four-wireinterface.Noticethatfiveconductorsareused.Eachgeneratorordrivercandriveuptoten(10)receivers.Thetwosignalingstatesofthelinearedefinedasfollows:a.Whenthe“A”terminalofthedriverisnegativewithrespecttothe“B”terminal,thelineisinabinary1(MARKorOFF)state.b.Whenthe“A”terminalofthedriverispositivewithrespecttothe“B”terminal,thelineisinabinary0(SPACEorON)state.Figure1.5showstheconditionofthevoltageofthebalancedlineforanRS-232toRS-422converterwhenthelineisinthe“idle”conditionorOFFstate.Italsoshowstherelationshipofthe“A” and“B”terminalsofanRS-422systemandthe“-”and“+”terminalmarkingsusedonmanytypesofequipment.The“A”terminalisequivalenttothe“-”designation,andthe“B”terminalequivalenttothe“+”designation.ThesamerelationshipshowninFigure1.5alsoappliesforRS-485systems.RS-422canwithstandacommonmodevoltage(Vcm)of7volts.CommonmodevoltageisdefinedasthemeanvoltageoftheAandBterminalswithrespecttosignalground.EIAStandardRS-485DataTransmissionTheRS-485Standardpermitsabalancedtransmissionlinetobesharedinapartylineormultidropmode.Asmanyas32driver/receiverpairscanshareamultidropnetwork.ManycharacteristicsofthedriversandreceiversarethesameasRS-422.TherangeofthecommonmodevoltageVcmthatthedriverandreceivercantolerateisexpandedto+12to-7volts.Sincethedrivercanbedisconnectedortristatedfromtheline,itmustwithstandthiscommonmodevoltagerangewhileinthetristatecondition.SomeRS-422drivers,evenwithtristatecapability,willnotwithstandthefullVcmvoltagerangeof+12to-7volts.Figure1.6showsatypicaltwo-wiremultidropnetwork.Notethatthetransmissionlineisterminatedonbothendsofthelinebutnotatdroppointsinthemiddleoftheline.Terminationshouldonlybeusedwithhighdataratesandlongwiringruns.AdetaileddiscussionofterminationcanbefoundinChapter2ofthisapplicationnote.ThesignalgroundlineisalsorecommendedinanRS-485systemtokeepthecommonmodevoltagethatthereceivermustacceptwithinthe-7to+12voltrange.FurtherdiscussionofgroundingcanbefoundinChapter3ofthisapplicationnote.AnRS-485networkcanalsobeconnectedinafour-wiremodeasshowninFigure1.7.Notethatfourdatawiresandanadditionalsignalgroundwireareusedina“four-wire”connection.Inafour-wirenetworkitisnecessarythatonenodebeamasternodeandallothersbeslaves.Thenetworkisconnectedsothatthemasternodecommunicatestoallslavenodes.Allslavenodescommunicateonlywiththemasternode.Thisnetworkhassomeadvantageswithequipmentwithmixedprotocolcommunications.Sincetheslavenodesneverlistentoanotherslaveresponsetothemaster,aslavenodecannotreplyincorrectlytoanotherslavenode.TristateControlofanRS-485DeviceusingRTSAsdiscussedpreviously,anRS-485systemmusthaveadriverthatcanbedisconnectedfromthetransmissionlinewhenaparticularnodeisnottransmitting.InanRS-232toRS-485converteroranRS-485serialcard,thismaybeimplementedusingtheRTScontrolsignalfromanasynchronousserialporttoenabletheRS-485driver.TheRTSlineisconnectedtotheRS-485driverenablesuchthatsettingtheRTSlinetoahigh(logic1)stateenablestheRS-485driver.SettingtheRTSlinelow(logic0)putsthedriverintothetristatecondition.Thisineffectdisconnectsthedriverfromthebus,allowingothernodestotransmitoverthesamewirepair.Figure1.8showsatimingdiagramforatypicalRS-232toRS-485converter.ThewaveformsshowwhathappensiftheVRTSwaveformisnarrowerthanthedataVSD.Thisisnotthenormalsituation,butisshownheretoillustratethelossofaportionofthedatawaveform.WhenRTScontrolisused,itisimportanttobecertainthatRTSissethighbeforedataissent.Also,theRTSlinemustthenbesetlowafterthelastdatabitissent.Thistimingisdonebythesoftwareusedtocontroltheserialportandnotbytheconverter.WhenanRS-485networkisconnectedinatwo-wiremultidroppartylinemode,thereceiverateachnodewillbeconnectedtotheline(seeFigure1.6).Thereceivercanoftenbeconfiguredtoreceiveanechoofitsowndatatransmission.Thisisdesirableinsomesystems,andtroublesomeinothers.Besuretocheckthedatasheetforyourconvertertodeterminehowthereceiver“enable”functionisconnected.Send Data Control of an rs-485 DeviceMany of B&B Electronics RS-232 to RS-485 converters and RS-485 serial cards include special circuitry ,which is triggered from the data signal to enable the RS-485 driver.Figure 1.9 is a timing diagram of the important signals used to control a converter of this type.It is important to note that the transmit data line is “disabled”at a fixed interval after the last bit ,typically one character length.If this interval after the is too short,you can miss parts of each character being sent .If this time is too long,your system may try to turn the data lone around from transmit to receive before the node (with the Send Data converter) is ready to receive data.If the latter is the case,you will miss portions (or complete characters) at the beginning of a response.Chapter2:SystemConfigurationNetworkTopologiesNetworkconfigurationisntdefinedintheRS-422orRS-485specification.Inmostcasesthedesignercanuseaconfigurationthatbestfitsthephysicalrequirementsofthesystem.TwoWireorFourWireSystemsRS-422systemsrequireadedicatedpairofwiresforeachsignal,atransmitpair,areceivepairandanadditionalpairforeachhandshake/controlsignalused(ifrequired).ThetristatecapabilitiesofRS-485allowasinglepairofwirestosharetransmitandreceivesignalsforhalf-duplexcommunications.This“twowire”configuration(notethatanadditionalgroundconductorshouldbeused)reducescablingcost.RS-485devicesmaybeinternallyorexternallyconfiguredfortwowiresystems.InternallyconfiguredRS-485devicessimplyprovideAandBconnections(sometimeslabeled“-“and“+”).DevicesconfiguredforfourwirecommunicationsbringoutAandBconnectionsforboththetransmitandthereceivepairs.Theusercanconnectthetransmitlinestothereceivelinestocreateatwowireconfiguration.Thelattertypedeviceprovidesthesystemdesignerwiththemostconfigurationflexibility.Notethatthesignalgroundlineshouldalsobeconnectedinthesystem.ThisconnectionisnecessarytokeeptheVcmcommonmodevoltageatthereceiverwithinasaferange.Theinterfacecircuitmayoperatewithoutthesignalgroundconnection,butmaysacrificereliabilityandnoiseimmunity.Figures2.0and2.1illustrateconnectionsoftwoandfourwiresystems.TerminationTerminationisusedtomatchimpedanceofanodetotheimpedanceofthetransmissionlinebeingused.Whenimpedancearemismatched,thetransmittedsignalisnotcompletelyabsorbedbytheloadandaportionisreflectedbackintothetransmissionline.Ifthesource,transmissionlineandloadimpedanceareequalthesereflectionsareeliminated.Therearedisadvantagesofterminationaswell.Terminationincreasesloadonthedrivers,increasesinstallationcomplexity,changesbiasingrequirementsandmakessystemmodificationmoredifficult.Thedecisionwhetherornottouseterminationshouldbebasedonthecablelengthanddatarateusedbythesystem.Agoodruleofthumbisifthepropagationdelayofthedatalineismuchlessthanonebitwidth,terminationisnotneeded.Thisrulemakestheassumptionthatreflectionswilldampoutinseveraltripsupanddownthedataline.SincethereceivingUARTwillsamplethedatainthemiddleofthebit,itisimportantthatthesignallevelbesolidatthatpoint.Forexample,inasystemwith2000feetofdatalinethepropagationdelaycanbecalculatedbymultiplyingthecablelengthbythepropagationvelocityofthecable.Thisvalue,typically66to75%ofthespeedoflight(c),isspecifiedbythecablemanufacture.Forourexample,aroundtripcovers4000feetofcable.Usingpropagationvelocity of0.66c,oneroundtripiscompletedinapproximately6.2s.Ifweassumethereflectionswilldampoutinthree“roundtrips”upanddownthecablelength,thesignalwillstabilize18.6saftertheleadingedgeofabit.At9600baudonebitis104swide.Sincethereflectionsaredampedoutmuchbeforethecenterofthebit,terminationisnotrequired.Thereareseveralmethodsofterminatingdatalines.ThemethodrecommendedbyB&Bisparalleltermination.Aresistorisaddedinparallelwiththereceivers“A”and“B”linesinordertomatchthedatalinecharacteristicimpedancespecifiedbythecablemanufacture(120isacommonvalue).Thisvaluedescribestheintrinsicimpedanceofthetransmissionlineandisnotafunctionofthelinelength.Aterminatingresistoroflessthan90shouldnotbeused.Terminationresistorsshouldbeplacedonlyattheextremeendsofthedataline,andnomorethantwoterminationsshouldbeplacedinanysystemthatdoesnotuserepeaters.ThistypeofterminationclearlyaddsheavyDCloadingtoasystemandmayoverloadportpoweredRS-232toRS-485converters.Anothertypeoftermination,ACcoupledtermination,addsasmallcapacitorinserieswiththeterminationresistortoeliminatetheDCloadingeffect.AlthoughthismethodeliminatesDCloading,capacitorselectionishighlydependentonthesystemproperties.SystemdesignersinterestedinACterminationareencouragedtoreadNationalSemiconductorsApplicationNote9032forfurtherinformation.Figure2.2illustratesbothparallelandACterminationonanRS-485two-wirenode.Infour-wiresystems,theterminationisplacedacrossthereceiverof thenode.BiasinganRS-485NetworkWhenanRS-485networkisinanidlestate,allnodesareinlisten(receive)mode.Underthisconditiontherearenoactivedriversonthenetwork,alldriversaretristated.Withoutanythingdrivingthenetwork,thestateofthelineisunknown.Ifthevoltage level atthereceiversAandBinputsis less than200mVthelogiclevelattheoutputofthereceiverswillbethevalueofthelastbitreceived.Inordertomaintaintheproperidlevoltagestate,biasresistorsmustbeappliedtoforcethedatalinestotheidlecondition.BiasresistorsarenothingmorethanapullupresistoronthedataBline(typicallyto5volts)andapulldown(toground)onthedataAline.Figure2.3illustratestheplacementofbiasresistorsonatransceiverinatwo-wireconfiguration.NotethatinanRS-485four-wireconfiguration,thebiasresistorsshouldbeplacedonthereceiverlines.Thevalueofthebiasresistorsisdependentonterminationandnumberofnodesinthesystem.ThegoalistogenerateenoughDCbiascurrentinthenetworktomaintainaminimumof200mVbetweentheBandAdataline.Considerthefollowingtwoexamplesofbiasresistorcalculation.Example1.10node,RS-485networkwithtwo120terminationresistorsEachRS-485nodehasaloadimpedanceof12K.10nodesinparallelgivealoadof1200.Additionally,thetwo120terminationresistorsresultinanother60Wload,foratotalloadof57.Clearlytheterminationresistorsareresponsibleforamajorityoftheloading.Inordertomaintainatleast200mVbetweentheBandAline,weneedabiascurrentof3.5mAtoflowthroughtheload.Tocreatethisbiasfroma5Vsupplyatotalseriesresistanceof1428orlessisrequired.Subtractthe57thatisalreadypartoftheload,andweareleftwith1371.Placinghalfofthisvalueasapullupto5Vandhalfasapulldowntogroundgivesamaximumbiasresistorvalueof685foreachofthetwobiasingresistors.Example2.32node,RS-485networkwithoutterminationEachRS-485nodehasaloadimpedanceof12K.32nodesinparallelgivesatotalloadof375.Inordertomaintainatleast200mVacross375weneedacurrentof0.53mA.Togeneratethiscurrentfroma5Vsupplyrequiresatotalresistance of9375maximum.Since375ofthistotalisinthereceiverload,ourbiasresistorsmustaddto9Korless.Noticethatverylittlebiascurrentisrequiredinsystemswithouttermination.Biasresistorscanbeplacedanywhereinthenetworkorcanbesplitamongmultiplenodes.Theparallelcombinationofallbiasresistorsinasystemmustbeequaltoorlessthanthecalculatedbiasingrequirements.B&BElectronicsuses4.7KbiasresistorsinallRS-485products.Thisvalueisadequateformostsystemswithouttermination.Thesystemdesignershouldalwayscalculatethebiasingrequirementsofthenetwork.Symptomsofunderbiasingrangefromdecreasednoiseimmunitytocompletedatafailure.Overbiasinghaslesseffectonasystem,theprimaryresultisincreasedloadonthedrivers.SystemsusingportpoweredRS-232toRS-485converterscanbesensitivetooverbiasing.ExtendingtheSpecificationSomesystemsrequirelongerdistancesorhighernumbersofnodesthansupportedbyRS-422orRS-485.Repeatersarecommonlyusedtoovercomethesebarriers.AnRS-485repeatersuchasB&BElectronics485OPcanbeplacedinasystemtodividetheloadintomultiplesegments.Each“refreshed”signaliscapableofdrivinganother4000feetofcableandanadditional31RS-485loads.AnothermethodofincreasingthenumberofRS-485nodesistouselowloadtypeRS-485receivers.ThesereceiversuseahigherinputimpedancetoreducetheloadontheRS-485driverstoincreasethetotalnumberofnodes.Therearecurrentlyhalfandquarterloadintegratedcircuitreceiversavailable,extendingthetotalallowablenumberofnodesto64and128.RS-422和RS-485应用第一章概论引言本文的目的是描述RS-422和RS-485系统的主要构成。本文尽量包含足够的技术细节以便系统设计师在数据系统设计时能考虑到所有的重要方面。因为RS-422和RS-485都是使用平衡差分信号的数据传输系统,所以将二者放在一起讨论是合适的。在本文中，RS-422和RS-485通用术语被用于代表EIA/TIA-422和EIA/TIA-485标准。数据传输信号非平衡线驱动器在不平衡数据传输系统RS-232中传输的各个信号，其出现在接口连接器的电压是以信号地为参考的。例如,对于DB-25连接器，从DTE设备传送的数据(TD)是2脚上的电压，它以信号地7脚为参考电平，如果总线为空闲状态，其电压为负，如果正在发送数据，其电压则在515伏的正电平和负电平之间反复切换。如图1.1所示，RS-232接收器的典型工作电压为+3+