课件应用笔记

TableofCHAPTER1:TableofCHAPTER1: DATATRANSMISSION UnbalancedLine BalancedLine BalancedLine EIASTANDARDRS-422DATA EIASTANDARDRS-485DATA TRISTATECONTROLOFANRS-485DEVICEUSING SENDDATACONTROLOFANRS-485 CHAPTER2:SYSTEM NETWORK TWOWIREORFOURWIRE EXTENDINGTHE :SELECTINGS-485 NUMBEROF CABLE CHAPTER4:TRANSIENTPROTECTIONOFRS-422ANDRS-485 WHATDOESASURGELOOK Surge CommonModevs.Differential GROUND TRANSIENTPROTECTIONUSING Isolation Isolation Shunting ConnectingSignal Shunting SPECIALCONSIDERATIONFORFAULT CHOOSINGTHERIGHTPROTECTIONFORYOUR RS-422/485ApplicationTableofiB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81CHAPTER5: CHAPTER5: RS-422 MASTER-SLAVE FourWireMaster-Slave TwoWireMaster-Slave SYSTEMSWITHPORTPOWERED CHAPTER6:SELECTINGRS-485 CHAPTER7:SOURCESOFFURTHER APPENDIXA:EIASPECIFICATION APPENDIXEIASTANDARDRS-423DATA TableofRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Chapter1:ThepurposeofthisapplicationnoteistodescribethemainelementsofanRS-422andRS-485system.Thisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesign.SincebothRS-422andRS-485aredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriateChapter1:ThepurposeofthisapplicationnoteistodescribethemainelementsofanRS-422andRS-485system.Thisapplicationnoteattemptstocoverenoughtechnicaldetailssothatthesystemdesignerwillhaveconsideredalltheimportantaspectsinhisdatasystemdesign.SincebothRS-422andRS-485aredatatransmissionsystemsthatusebalanceddifferentialsignals,itisappropriatetodiscussbothsystemsinthesameapplicationnote.ThroughoutthisapplicationnotethegenerictermsofRS-422andRS-485willbeusedtorepresenttheEIA/TIA-422andEIA/TIA-485Standards.DataTransmissionUnbalancedLineEachsignalthattransmitsinanRS-232unbalanceddatatransmissionsystemappearsontheinterfaceconnectorasavoltagewithreferencetoasignalground.Forexample,thetransmitteddata(TD)fromaDTEdeviceappearsonpin2withrespecttopin7(signalground)onaDB-25connector.Thisvoltagewillbenegativeifthelineisidleandalternatebetweenthatnegativelevelandapositivelevelwhendataissentwithamagnitudeof5toeRS-232receiver-3-12voltsasshowninFigureBalancedLineateswithinthevoltagerangeInabalanceddifferentialsystemthevoltageproducedbythedriverappearsacrossapairofsignallinesthattransmitonlyonesignal.Figure1.2showsaschematicsymbolforabalancedlinedriverandthevoltagesthatexist.Abalancedlinedriverwillproduceavoltagefrom2to6voltsacrossitsAandBoutputterminalsandwillhaveasignalground(C)connection.Althoughproperconnectiontothesignalgroundisimportant,itisn'tusedbyabalancedlinereceiverindeterminingthelogicstateofthedataline.Abalancedlinedrivercanalsohaveaninputsignalcalledan“Enable”signal.Thepurposeofthissignalistoconnectthedrivertoitsoutputterminals,AandB.Ifthe“Enable”signalisOFF,onecanconsiderthedriverasdisconnectedfromthetransmissionline.AnRS-485drivermusthavethe“Enable”controlsignal.AnRS-422drivermayhavethissignal,butitisnotalwaysrequired.Thedisconnectedor"disabled"conditionofthelinedriverusuallyisreferredtoasthe“tristate1”conditionofthedriver.1Theterm“tristate”comesfromthefactthatthereisathirdoutputstateofanRS-485driver,inadditiontotheoutputstatesof“1”and“0.”RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(811FigureFigure2RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureFigure2RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81BalancedLineAbalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandB.Itwillalsohaveasignalground(C)thatisnecessaryinmakingtheproperinterfaceconnection.FigureBalancedLineAbalanceddifferentiallinereceiversensesthevoltagestateofthetransmissionlineacrosstwosignalinputlines,AandB.Itwillalsohaveasignalground(C)thatisnecessaryinmakingtheproperinterfaceconnection.Figure1.3isaschematicsymbolforabalanceddifferentiallinereceiver.Figure1.3alsoshowsthevoltagesthatareimportanttothebalancedlinereceiver.IfthedifferentialinputvoltageVabisgreaterthan+200mVthereceiverwillhaveaspecificlogicstateonitsoutputterminal.Iftheinputvoltageisreversedtolessthan-200mVthereceiverwillcreatetheoppositelogicstateonitsoutputterminal.TheinputvoltagesthatabalancedlinereceivermustsenseareshowninFigure1.3.The200mVto6Vrangeisrequiredtoallowforattenuationonthetransmissionline.EIAStandardRS-422DataTheEIAStandardRS-422-Aentitled“ElectricalCharacteristicsofBalancedVoltageDigitalInterfaceCircuits”definesthecharacteristicsofRS-422interfacecircuits.Figure1.4isatypicalRS-422four-wireinterface.Noticethatfiveconductorsareused.Eachgeneratorordrivercandriveupeivers.Thetwoofthelinearedefineda.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)of±7volts.CommonmodevoltageisdefinedasthemeanvoltageoftheAandBterminalswithrespecttosignalRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(813FigureFigure4RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureFigure4RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(815FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(815EIAStandardRS-485EIAStandardRS-485DataTheRS-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.6RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureRS-422/485ApplicationB&BElectronics--POBox1040--FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(817Figure8RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Figure8RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81AnRS-485networkcanalsobeAnRS-485networkcanalsobeconnectedinafour-wiremodeasshowninFigure1.7.Notethatfourdatawiresandanadditionalsignalgroundwireareusedina“four-wire”connection.Inafour-wirenetworkitisnecessarythatonenodebeamasternodeandallothersbeslaves.Thenetworkisconnectedsothatthemasternodecommunicatestoallslavenodes.Allslavenodescommunicateonlywiththemasternode.Thisnetworkhassomeadvantageswithequipmentwithmixedprotocolcommunications.Sincetheslavenodesneverlistentoanotherslaveresponsetothemaster,aslavenodecannotreplyincorrectlytoanotherslavenode.TristateControlofanRS-485DeviceusingAsdiscussedpreviously,anRS-485systemmusthaveadriverthatcanbedisconnectedfromthetransmissionlinewhenaparticularnodeisnottransmitting.InanRS-232toRS-485converteroranRS-485serialcard,thismaybeimplementedusingtheRTScontrolsignalfromanasynchronousserialporttoenabletheRS-485driver.TheRTSlineisconnectedtotheRS-485driverenablesuchthatsettingtheRTSlinetoahigh(logic1)stateenablestheRS-485driver.SettingtheRTSlinelow(logic0)putsthedriverintothetristatecondition.Thisineffectdisconnectsthedriverfromthernodestotransmitoverthesamewirepair.Figure1.8showsmforatypicalRS-converter.TheshowwhathappensiftheVRTSwaveformisnarrowerthanthedataVSD.Thisisnotthenormalsituation,butisshownheretoillustratethelossofportionofthedatawaveform.WhenRTScontrolisused,itisimportanttobecertainthatRTSissethighbeforedataissent.Also,theRTSlinemustthenbesetlowafterthelastdatabitissent.Thistimingisdonebythesoftwareusedtocontroltheserialportandnotbytheconverter.WhenanRS-485networkisconnectedinatwo-wiremultidroppartylinemode,thereceiverateachnodewillbeconnectedtotheline(seeFigure1.6).Thereceivercanoftenbeconfiguredtoreceiveanechoofitsowndatatransmission.Thisisdesirableinsomesystems,andtroublesomeinothers.Besuretocheckthedatasheetforyourconvertertodeterminehowthereceiver“enable”functionisconnected.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(819FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81SendDataControlSendDataControlofanRS-485ManyofB&BElectronics’RS-232toRS-485convertersandRS-485serialcardsincludespecialcircuitry,whichistriggeredfromthedatasignaltoenabletheRS-485driver.Figure1.9isatimingdiagramoftheimportantsignalsusedtocontrolaconverterofthistype.Itisimportanttonotethatthetransmitdatalineis“disabled”atafixedintervalafterthelastbit,typicallyonecharacterlength.Ifthisintervalistooshort,youcanmisspartsofeachcharacterbeingsent.Ifthistimeistoolong,yoursystemmaytrytoturnthedatalinearoundfromtransmittoreceivebeforethenode(withtheSendDataconverter)isreadytoreceivedata.Ifthelatteristhecase,youwillmissportions(orcompletecharacters)atthebeginningofaresponse.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Chapter2:SystemNetworkNetworkChapter2:SystemNetworkNetworkconfigurationisn’tdefinedintheRS-422orRS-485Inmostcasesthedesignercanuseaconfigurationthatbestfitsthephysicalrequirementsofthesystem.TwoWireorFourWireRS-422systemsrequireadedicatedpairofwiresforeachsignal,atransmitpair,areceivepairandanadditionalpairforeachhandshake/controlsignalused(ifrequired).ThetristatecapabilitiesofRS-485allowasinglepairofwirestosharetransmitandreceivesignalsforhalf-duplexcommunications.This“twowire”configuration(notethatanadditionalgroundconductorshouldbeused)reducescablingcost.RS-485devicesmaybeinternallyorexternallyconfiguredfortwowiresystems.InternallyconfiguredRS-485devicessimplyprovideAandBconnections(sometimeslabeled“-“and“+”).onfiguredforcationsbringoutAandconnectionsforboththetransmitandthereceivepairs.Theusercanconnectthetransmitlinestothereceivelinestocreateatwowireconfiguration.Thelattertypedeviceprovidesthesystemdesignerwiththemostconfigurationflexibility.Notethatthesignalgroundlineshouldalsobeconnectedinthesystem.ThisconnectionisnecessarytokeeptheVcmcommonmodevoltageatthereceiverwithinasaferange.Theinterfacecircuitmayoperatewithoutthesignalgroundconnection,butmaysacrificereliabilityandnoiseimmunity.Figures2.1and2.2illustrateconnectionsoftwoandfourwireRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Figure2.1TypicalRS-485FourWireMultidropRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,ILPH(815)433-5100--Figure2.1TypicalRS-485FourWireMultidropRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,ILPH(815)433-5100--Figure2.2TypicalRS-485TwoWireMultidropRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Figure2.2TypicalRS-485TwoWireMultidropRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81TerminationisusedtoTerminationisusedtomatchimpedanceofanodetotheimpedanceofthetransmissionlinebeingused.Whenimpedancearemismatched,thetransmittedsignalisnotcompletelyabsorbedbytheloadandaportionisreflectedbackintothetransmissionline.Ifthesource,transmissionlineandloadimpedanceareequalthesereflectionsareeliminated.Therearedisadvantagesofterminationaswell.Terminationincreasesloadonthedrivers,increasesinstallationcomplexity,changesbiasingrequirementsandmakessystemmodificationmoredifficult.Thedecisionwhetherornottouseterminationshouldbebasedonthecablelengthanddatarateusedbythesystem.Agoodruleofthumbisifthepropagationdelayofthedatalineismuchlessthanonebitwidth,terminationisnotneeded.Thisrulemakestheassumptionthatreflectionswilldampoutinseveraltripsupanddownthedataline.SincethereceivingUARTwillsamplethedatainthemiddleofthebit,itisimportantthatthesignallevelbesolidatthatpoint.Forexample,inasystemwith2000feetofdatalinethepropagationdelaycanbecalculatedbymultiplyingthecablelengthbythepropagationvelocityofthecable.Thisvalue,typically66to75%ofthespeedoflight(c),ishecableForourexample,aroundtripcovers4000feetofcable.Usingapropagationvelocityof0.66c,oneroundtripiscompletedinapproximately6.2s.Ifweassumethereflectionswilldampoutinthree“roundtrips”upanddownthecablelength,thesignalwillstabilize18.6saftertheleadingedgeofabit.At9600baudonebitis104swide.Sincethereflectionsaredampedoutmuchbeforethecenterofthebit,terminationisnotrequired.Thereareseveralmethodsofterminatingdatalines.ThemethodrecommendedbyB&Bisparalleltermination.Aresistorisaddedinparallelwiththereceiver’s“A”and“B”linesinordertomatchthedatalinecharacteristicimpedancespecifiedbythecablemanufacture(120isacommonvalue).Thisvaluedescribestheintrinsicimpedanceofthetransmissionlineandisnotafunctionofthelinelength.Aterminatingresistoroflessthan90shouldnotbeused.Terminationresistorsshouldbeplacedonlyattheextremeendsofthedataline,andnomorethantwoterminationsshouldbeplacedinanysystemthatdoesnotuserepeaters.ThistypeofterminationclearlyaddsheavyDCloadingtoasystemandmayoverloadportpoweredRS-232toRS-485converters.Anothertypeoftermination,ACcoupledtermination,addsasmallcapacitorinserieswiththeRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81terminationresistortoeliminatetheDCloadingeffect.AlthoughthismethodeliminatesDCloading,capacitorselectionishighlydependentonthesystemproperties.SystemdesignersinterestedinACterminationareencouragedtoreadNationalSemiconductorsApplicationNote9032forfurtherinformation.Figure2.3terminationresistortoeliminatetheDCloadingeffect.AlthoughthismethodeliminatesDCloading,capacitorselectionishighlydependentonthesystemproperties.SystemdesignersinterestedinACterminationareencouragedtoreadNationalSemiconductorsApplicationNote9032forfurtherinformation.Figure2.3illustratesbothparallelandACterminationonanRS-485two-wireInfour-wiresystems,theterminationisplacedacrossthereceiverofFigure2.3ParallelandACnRS-485WhenanRS-485networkisinanidlestate,allnodesareinlisten(receive)mode.Underthisconditiontherearenoactivedriversonthenetwork,alldriversaretristated.Withoutanythingdrivingthenetwork,thestateofthelineisunknown.Ifthevoltagelevelatthereceiver’sAandBinputsislessthan200mVthelogiclevelattheoutputofthereceiverswillbethevalueofthelastbitreceived.Inordertomaintaintheproperidlevoltagestate,biasresistorsmustbeappliedtoforcethedatalinestotheidlecondition.BiasresistorsarenothingmorethanapullupresistoronthedataBline(typicallyto5volts)andapulldown(toground)onthedataAline.Figure2.4illustratestheplacementofbiasresistorsonatransceiverinatwo-wireconfiguration.NotethatinanRS-485four-wireconfiguration,thebiasresistorsshouldbeplacedonthereceiverlines.Thevalueofthebiasresistorsisdependentonterminationandnumberofnodesinthesystem.ThegoalistogenerateenoughDCbiascurrentinthenetworktomaintainaminimumof200mVbetweentheBandAdataline.Considerthefollowingtwoexamplesofbiasresistorcalculation.2RefertoChapter7forinformationonNationalSemiconductorsApplicationRS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81BiasBiasFigure2.4TransceiverwithBiasExample1.10node,RS-485BiasBiasFigure2.4TransceiverwithBiasExample1.10node,RS-485networkwithtwo120terminationEachRS-485nodehasaloadimpedanceof12K.10nodesinparallelgivealoadof1200.Additionally,thetwo120terminationresistorsresultinanother60load,foratotalloadof57.Clearlytheterminationresistorsareresponsibleforamajorityoftheloading.Inordertomaintainatleast200mVbetweentheBandAline,weneedabiascurrentof3.5mAtoflowthroughtheload.Tocreatethisbiasfroma5Vsupplyatotalseriesresistanceof1428orlessisrequired.Subtractthe57thatisalreadypartoftheload,andweareleftwith1371.Placinghalfofthisvalueasapullupto5Vandhalfasapulldowntogroundgivesamaximumbiasresistorvalueof685foreachofthetwobiasingresistors.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Example2.32node,RS-485networkwithoutExample2.32node,RS-485networkwithoutEachRS-485nodehasaloadimpedanceof12K.32nodesinparallelgivesatotalloadof375.Inordertomaintainatleast200mVacross375weneedacurrentof0.53mA.Togeneratethiscurrentfroma5Vsupplyrequiresatotalresistanceof9375maximum.Since375ofthistotalisinthereceiverload,ourbiasresistorsmustaddto9Korless.NoticethatverylittlebiascurrentisrequiredinsystemswithoutBiasresistorscanbeplacedanywhereinthenetworkorcanbesplitamongmultiplenodes.Theparallelcombinationofallbiasresistorsinasystemmustbeequaltoorlessthanthecalculatedbiasingrequirements.B&BElectronicsuses4.7KbiasresistorsinallRS-485products.Thisvalueisadequateformostsystemswithouttermination.Thesystemdesignershouldalwayscalculatethebiasingrequirementsofthenetwork.Symptomsofunderbiasingrangefromdecreasednoiseimmunitytocompletedatafailure.Overbiasinghaslesseffectonasystem,theprimaryresultisincreasedloadonthedrivers.SystemsusingportpoweredRS-232toRS-485converterscanbesensitivetooverbiasing.ExtendingtheSomesystemsrequirelongerdistancesorhighernumbersofnodesthansupportedbyRS-422orRS-485.Repeatersarecommonlyusedtoovercomethesebarriers.AnRS-485repeatersuchasB&BElectronics’485OPcanbeplacedinasystemtodividetheloadintomultiplesegments.Each“refreshed”signaliscapableofdrivinganother4000feetofcableandanadditional31RS-485loads.AnothermethodofincreasingthenumberofRS-485nodesistouselowloadtypeRS-485receivers.ThesereceiversuseahigherinputimpedancetoreducetheloadontheRS-485driverstoincreasethetotalnumberofnodes.Therearecurrentlyhalfandquarterloadintegratedcircuitreceiversavailable,extendingthetotalallowablenumberofnodesto64and128.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Chapter3:SelectingRS-422andRS-485Chapter3:SelectingRS-422andRS-485CableselectionforRS-422andRS-485systemsisoftenneglected.Attentiontoafewdetailsintheselectionprocesscanpreventthecostlyprospectofre-pullingthousandsoffeetofcable.NumberofThesignalgroundconductorisoftenoverlookedwhenorderingcable.Anextratwistedpairmustbespecifiedtohaveenoughconductorstorunasignalground.Atwowiresystemthenrequirestwotwistedpair,andafourwiresystemrequiresthreetwistedpair.Itisoftenhardtoquantifyifshieldedcableisrequiredinanapplicationornot.Sincetheaddedcostofshieldedcableisusuallyminimalitisworthinstallingthefirsttime.CableWhenchoosingatransmissionlineforRS-422orRS-485,itisnecessarytoexaminetherequireddistanceofthecableandthedatarateofthesystem.TheAppendixtoEIARS-422-AStandardpresentsanempiricalcurvethatrelatesCableLengthtoDataRatefor24AWGtwisted-pairtelephonecablethathasashuntcapacitanceof16pF/ft.andisterminatedin100ohms(seeFigure3.1).Thiscurveisbasedonsignalqualityrequirementsof:Signalriseandfalltimeequalto,orlessthan,one-halfunitintervalattheapplicablemodulationrate.Themaximumvoltagelossbetweendriverandloadof6RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureatransmissionlineareacombinationofAClosses(skineffect),DCconductorloss,leakage,andAClossesinthedielectric.Inhighqualitycable,theconductorlossesandthedielectriclossesareonthesameorderofmagnitude.Figure3.2isincludedinthisapplicationnotetopointoutthesignificantdifferenceinperformanceofdifferentcables.ThischartshowsAttenuationversusFrequencyforthreedifferentBeldencables.NotethatthepolyethylenecablesoffermuchlowerattenuationthanPVCcables.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureatransmissionlineareacombinationofAClosses(skineffect),DCconductorloss,leakage,andAClossesinthedielectric.Inhighqualitycable,theconductorlossesandthedielectriclossesareonthesameorderofmagnitude.Figure3.2isincludedinthisapplicationnotetopointoutthesignificantdifferenceinperformanceofdifferentcables.ThischartshowsAttenuationversusFrequencyforthreedifferentBeldencables.NotethatthepolyethylenecablesoffermuchlowerattenuationthanPVCcables.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureAnotherapproachtochoosingtransmissionlineisthe“E-GRADEProgram,”whichhasbeenestablishedbyAnixterBros.Inc.Anixterisaworldwidedistributorofwiringsystemproducts.Underthisprogram,Anixterdividesdatainterfacecablesintofourcategoriesasfollows:E-GRADEE-GRADEE-GRADEE-GRADELIMITEDDISTANCEEXTENDEDDISTANCEMAXIMUMSimplechartsareusedtohelptheuserselectthepropercablewithoutanytechnicalunderstandingofthecableparameters.ThisprogramdividestheusagecategoriesintoEIA-232-D,EIA-422-A,andEIA-423-A.Whenusingthisliterature,usetheEIA-422-AchartsforchoosingRS-485cable.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81FigureAnotherapproachtochoosingtransmissionlineisthe“E-GRADEProgram,”whichhasbeenestablishedbyAnixterBros.Inc.Anixterisaworldwidedistributorofwiringsystemproducts.Underthisprogram,Anixterdividesdatainterfacecablesintofourcategoriesasfollows:E-GRADEE-GRADEE-GRADEE-GRADELIMITEDDISTANCEEXTENDEDDISTANCEMAXIMUMSimplechartsareusedtohelptheuserselectthepropercablewithoutanytechnicalunderstandingofthecableparameters.ThisprogramdividestheusagecategoriesintoEIA-232-D,EIA-422-A,andEIA-423-A.Whenusingthisliterature,usetheEIA-422-AchartsforchoosingRS-485cable.RS-422/485ApplicationB&BElectronics--POBox1040--Ottawa,IL61350PH(815)433-5100--FAX(81Chapter4:TransientProtectionofRS-422Chapter4:TransientProtectionofRS-422andRS-485ThefirststeptowardsprotectinganRS-422orRS-485systemfromtransientsisunderstandingthenatureoftheenergyweareguardingagainst.Transientenergymaycomefromseveralsources,mosttypicallyenvironmentalconditionsorinducedbyswitchingheavyinductiveloads.WhatdoesasurgelookSurgeWhiletransientsmaynotalwaysconformtoindustryspecifications,boththeInstituteofElectricalandElectronicsEngineers(IEEE)andtheInternationalElectrotechnicalCommission(IEC)havedevelopedtransientmodelsforuseinevaluatingelectricalandelectronicequipmentforimmunitytosurges.Thesemodelscanoffersomeinsightintothetypesofenergythatmustbecontrolledtopreventsystemdamage.BothIEC1000-4-5:1995“SurgeImmunityTest”andIEEEC62.41-1991“IEEERecommendedPracticeonSurgeVoltagesinLow-VoltageACPowernea“1.2/50s-ionwave”surgewhichhasasvoltagerisetimewitha50sdecayacrossanopencircuit.Thecurrentwaveformhasan8srisetimewitha20sdecayintoashortcircuit.Opencircuitvoltageslevelsfrom1to6kVarecommonlyusedinboththepositiveandnegativepolarities,althoughundersomecircumstancesvoltagesashighas20kVmaybeapplied.Figures4.1and4.2illustratethecombinationwavecharacteristics.Inaddition,IEEEC62.41alsospecifiesa