RS-232 - Wikipedia, The Free Encyclopedia

25/3/2015 RS232Wikipedia,thefreeencyclopedia

http://en.wikipedia.org/wiki/RS232 1/12

ADB25connectorasdescribedintheRS232standard

RS232FromWikipedia,thefreeencyclopedia

Intelecommunications,RS232isastandardforserialcommunicationtransmissionofdata.ItformallydefinesthesignalsconnectingbetweenaDTE(dataterminalequipment)suchasacomputerterminal,andaDCE(datacircuitterminatingequipment,originallydefinedasdatacommunicationequipment[1]),suchasamodem.TheRS232standardiscommonlyusedincomputerserialports.Thestandarddefinestheelectricalcharacteristicsandtimingofsignals,themeaningofsignals,andthephysicalsizeandpinoutofconnectors.ThecurrentversionofthestandardisTIA232FInterfaceBetweenDataTerminalEquipmentandDataCircuitTerminatingEquipmentEmployingSerialBinaryDataInterchange,issuedin1997.

AnRS232serialportwasonceastandardfeatureofapersonalcomputer,usedforconnectionstomodems,printers,mice,datastorage,uninterruptiblepowersupplies,andotherperipheraldevices.However,RS232ishamperedbylowtransmissionspeed,largevoltageswing,andlargestandardconnectors.Inmodernpersonalcomputers,USBhasdisplacedRS232frommostofitsperipheralinterfaceroles.ManycomputersdonotcomeequippedwithRS232portsandmustuseeitheranexternalUSBtoRS232converteroraninternalexpansioncardwithoneormoreserialportstoconnecttoRS232peripherals.RS232devicesarewidelyused,especiallyinindustrialmachines,networkingequipmentandscientificinstruments.

Contents

1Scopeofthestandard2History3Limitationsofthestandard4Roleinmodernpersonalcomputers5Standarddetails

5.1Voltagelevels5.2Connectors5.3Signals5.4Cables

6Conventions6.1RTS/CTShandshaking6.23wireand5wireRS232

7Seldomusedfeatures7.1Signalrateselection7.2Loopbacktesting7.3Timingsignals7.4Secondarychannel

8Relatedstandards



9Developmenttools10References11Furtherreading12Externallinks

Scopeofthestandard

TheElectronicIndustriesAssociation(EIA)standardRS232C[1]asof1969defines:

Electricalsignalcharacteristicssuchasvoltagelevels,signalingrate,timingandslewrateofsignals,voltagewithstandlevel,shortcircuitbehavior,andmaximumloadcapacitance.Interfacemechanicalcharacteristics,pluggableconnectorsandpinidentification.Functionsofeachcircuitintheinterfaceconnector.Standardsubsetsofinterfacecircuitsforselectedtelecomapplications.

Thestandarddoesnotdefinesuchelementsasthecharacterencodingortheframingofcharacters,orerrordetectionprotocols.ThecharacterformatandtransmissionbitratearesetbytheserialporthardwarewhichmayalsocontaincircuitstoconverttheinternallogiclevelstoRS232compatiblesignallevels.Thestandarddoesnotdefinebitratesfortransmission,exceptthatitsaysitisintendedforbitrateslowerthan20,000bitspersecond.

History

RS232wasfirstintroducedin1962bytheRadioSectoroftheEIA.[2][3]TheoriginalDTEswereelectromechanicalteletypewriters,andtheoriginalDCEswere(usually)modems.Whenelectronicterminals(smartanddumb)begantobeused,theywereoftendesignedtobeinterchangeablewithteletypewriters,andsosupportedRS232.TheCrevisionofthestandardwasissuedin1969inparttoaccommodatetheelectricalcharacteristicsofthesedevices.

Sincetherequirementsofdevicessuchascomputers,printers,testinstruments,POSterminalsandsoonwerenotforeseenbythestandard,designersimplementinganRS232compatibleinterfaceontheirequipmentofteninterpretedthestandardidiosyncratically.Theresultingcommonproblemswerenonstandardpinassignmentofcircuitsonconnectors,andincorrectormissingcontrolsignals.Thelackofadherencetothestandardsproducedathrivingindustryofbreakoutboxes,patchboxes,testequipment,books,andotheraidsfortheconnectionofdisparateequipment.Acommondeviationfromthestandardwastodrivethesignalsatareducedvoltage.Somemanufacturersthereforebuilttransmittersthatsupplied+5Vand5Vandlabeledthemas"RS232compatible".

Laterpersonalcomputers(andotherdevices)startedtomakeuseofthestandardsothattheycouldconnecttoexistingequipment.Formanyyears,anRS232compatibleportwasastandardfeatureforserialcommunications,suchasmodemconnections,onmanycomputers.Itremainedinwidespreaduseintothelate1990s.Inpersonalcomputerperipherals,ithaslargelybeensupplantedbyotherinterfacestandards,suchasUSB.RS232isstillusedtoconnectolderdesignsofperipherals,industrialequipment(suchasPLCs),consoleportsandspecialpurposeequipment.



Thestandardhasbeenrenamedseveraltimesduringitshistoryasthesponsoringorganizationchangeditsname,andhasbeenvariouslyknownasEIARS232,EIA232,andmostrecentlyasTIA232.ThestandardcontinuedtoberevisedandupdatedbytheElectronicIndustriesAllianceandsince1988bytheTelecommunicationsIndustryAssociation(TIA).[4]RevisionCwasissuedinadocumentdatedAugust1969.RevisionDwasissuedin1986.ThecurrentrevisionisTIA232FInterfaceBetweenDataTerminalEquipmentandDataCircuitTerminatingEquipmentEmployingSerialBinaryDataInterchange,issuedin1997.ChangessinceRevisionChavebeenintiminganddetailsintendedtoimproveharmonizationwiththeCCITTstandardV.24,butequipmentbuilttothecurrentstandardwillinteroperatewitholderversions.

RelatedITUTstandardsincludeV.24(circuitidentification)andV.28(signalvoltageandtimingcharacteristics).

InrevisionDofEIA232,theDsubminiatureconnectorwasformallyincludedaspartofthestandard(itwasonlyreferencedintheappendixofRS232C).Thevoltagerangewasextendedto+/25volts,andthecircuitcapacitancelimitwasexpresslystatedas2500pF.RevisionEofEIA232introducedanew,smaller,standardDshell26pin"AltA"connector,andmadeotherchangestoimprovecompatibilitywithCCITTstandardsV.24,V.28andISO2110.[5]

Limitationsofthestandard

BecauseRS232isusedbeyondtheoriginalpurposeofinterconnectingaterminalwithamodem,successorstandardshavebeendevelopedtoaddressthelimitations.IssueswiththeRS232standardinclude:[6]

Thelargevoltageswingsandrequirementforpositiveandnegativesuppliesincreasespowerconsumptionoftheinterfaceandcomplicatespowersupplydesign.Thevoltageswingrequirementalsolimitstheupperspeedofacompatibleinterface.Singleendedsignalingreferredtoacommonsignalgroundlimitsthenoiseimmunityandtransmissiondistance.Multidropconnectionamongmorethantwodevicesisnotdefined.Whilemultidrop"workarounds"havebeendevised,theyhavelimitationsinspeedandcompatibility.AsymmetricaldefinitionsofthetwoendsofthelinkmaketheassignmentoftheroleofanewlydevelopeddeviceproblematicthedesignermustdecideoneitheraDTElikeorDCElikeinterfaceandwhichconnectorpinassignmentstouse.Thehandshakingandcontrollinesoftheinterfaceareintendedforthesetupandtakedownofadialupcommunicationcircuitinparticular,theuseofhandshakelinesforflowcontrolisnotreliablyimplementedinmanydevices.Nomethodisspecifiedforsendingpowertoadevice.WhileasmallamountofcurrentcanbeextractedfromtheDTRandRTSlines,thisisonlysuitableforlowpowerdevicessuchasmice.The25wayconnectorrecommendedinthestandardislargecomparedtocurrentpractice.ThestandarddoesnotaddressthepossibilityofconnectingaDTEdirectlytoaDTE,oraDCEtoaDCE.

Roleinmodernpersonalcomputers



PCIExpressx1cardwithoneRS232port

InthebookPC97HardwareDesignGuide,[7]MicrosoftdeprecatedsupportfortheRS232compatibleserialportoftheoriginalIBMPCdesign.Today,RS232hasmostlybeenreplacedinpersonalcomputersbyUSBforlocalcommunications.ComparedwithRS232,USBisfaster,useslowervoltages,andhasconnectorsthataresimplertoconnectanduse.However,USBislimitedbystandardtonomorethan5metersofcable,thusfavoringRS232whenlongerdistancesareneeded.Bothstandardshavesoftwaresupportinpopularoperatingsystems.

USBisdesignedtomakeiteasyfordevicedriverstocommunicatewithhardware.USBismorecomplexthantheRS232standardbecauseitincludesaprotocolfortransferringdatatodevices.Thisrequiresmoresoftwaretosupporttheprotocolused.Thereisnodirectanalogtotheterminalprogramsusedtoletuserscommunicatedirectlywithserialports.

Serialportsofpersonalcomputersarealsosometimesusedtodirectlycontrolvarioushardwaredevices,suchasrelaysorlamps.Personalcomputersmayuseaserialporttointerfacetodevicessuchasuninterruptiblepowersupplies.Insomecases,serialdataisnotexchanged,butthecontrollinesareusedtosignalconditionssuchaslossofpowerorlowbatteryalarms.AnapplicationprogramcandetectorchangethestateofRS232controllinesintheregistersoftheserialhardwareusingonlyafewinput/outputinstructionsbycontrast,aUSBinterfacerequiressoftwaretodecodetheserialdata.

DevicesthatconvertbetweenUSBandRS232donotworkwithallsoftwareoronallpersonalcomputers.

Infieldssuchaslaboratoryautomationorsurveying,RS232devicesmaycontinuetobeused.PLCs,VFDs,servodrives,andCNCequipmentareprogrammableviaRS232.Somemanufacturershaverespondedtothisdemand:ToshibareintroducedtheDE9MconnectorontheTecralaptop.

SerialportswithRS232arealsocommonlyusedtocommunicatetoheadlesssystemssuchasservers,wherenomonitororkeyboardisinstalled,duringbootwhenoperatingsystemisnotrunningyetandthereforenonetworkconnectionispossible.AnRS232serialportcancommunicatetosomeembeddedsystemssuchasroutersasanalternativetonetworkmodeofmonitoring.

Standarddetails

InRS232,userdataissentasatimeseriesofbits.Bothsynchronousandasynchronoustransmissionsaresupportedbythestandard.Inadditiontothedatacircuits,thestandarddefinesanumberofcontrolcircuitsusedtomanagetheconnectionbetweentheDTEandDCE.Eachdataorcontrolcircuitonlyoperatesinonedirection,thatis,signalingfromaDTEtotheattachedDCEorthereverse.Sincetransmitdataandreceivedataareseparatecircuits,theinterfacecanoperateinafullduplexmanner,supportingconcurrentdataflowinbothdirections.Thestandarddoesnotdefinecharacterframingwithinthedatastream,orcharacterencoding.

Voltagelevels

TheRS232standarddefinesthevoltagelevelsthatcorrespondtologicaloneandlogicalzerolevelsforthedatatransmissionandthecontrolsignallines.Validsignalsareeitherintherangeof+3to+15voltsortherange3to15voltswithrespecttotheground/commonpinconsequently,therangebetween3to+3voltsisnotavalidRS232level.Fordatatransmissionlines(TxD,RxDandtheirsecondarychannelequivalents)logiconeisdefinedasanegativevoltage,thesignalconditioniscalled"mark".Logiczeroispositiveandthesignalconditionistermed"space".Controlsignalshavetheoppositepolarity:theassertedoractivestateispositive



DiagrammaticoscilloscopetraceofvoltagelevelsforanASCII"K"character(0x4B)with1startbit,8databits,1stopbit.Thisistypicalforstartstopcommunications,butthestandarddoesnotdictateacharacterformatorbitorder.

RS232datalineontheterminalsofthereceiverside(RxD)probedbyanoscilloscope(foranASCII"K"character(0x4B)with1startbit,8databits,1stopbitandnoparitybits).

voltageandthedeassertedorinactivestateisnegativevoltage.Examplesofcontrollinesincluderequesttosend(RTS),cleartosend(CTS),dataterminalready(DTR),anddatasetready(DSR).

Thestandardspecifiesamaximumopencircuitvoltageof25volts:signallevelsof5V,10V,12V,and15Vareallcommonlyseendependingonthevoltagesavailabletothelinedrivercircuit.SomeRS232driverchipshaveinbuiltcircuitrytoproducetherequiredvoltagesfroma3or5voltsupply.RS232driversandreceiversmustbeabletowithstandindefiniteshortcircuittogroundortoanyvoltagelevelupto25volts.Theslewrate,orhowfastthesignalchangesbetweenlevels,isalsocontrolled.

Becausethevoltagelevelsarehigherthanlogiclevelstypicallyusedbyintegratedcircuits,specialinterveningdrivercircuitsarerequiredtotranslatelogiclevels.Thesealsoprotectthedevice'sinternalcircuitryfromshortcircuitsortransientsthatmayappearontheRS232interface,andprovidesufficientcurrenttocomplywiththeslewraterequirementsfordatatransmission.

BecausebothendsoftheRS232circuitdependonthegroundpinbeingzerovolts,problemswilloccurwhenconnectingmachineryandcomputerswherethevoltagebetweenthegroundpinononeend,andthegroundpinontheotherisnotzero.Thismayalsocauseahazardousgroundloop.UseofacommongroundlimitsRS232toapplicationswithrelativelyshortcables.Ifthetwodevicesarefarenoughapartoronseparatepowersystems,thelocalgroundconnectionsateitherendofthecablewillhavedifferingvoltagesthisdifferencewillreducethenoisemarginofthesignals.Balanced,differential,serialconnectionssuchasUSB,RS422andRS485cantoleratelargergroundvoltagedifferencesbecauseofthedifferentialsignaling.[8]

Unusedinterfacesignalsterminatedtogroundwillhaveanundefinedlogicstate.Whereitisnecessarytopermanentlysetacontrolsignaltoadefinedstate,itmustbeconnectedtoavoltagesourcethatassertsthelogic1orlogic0level,forexamplewithapullupresistor.Somedevicesprovidetestvoltagesontheirinterfaceconnectorsforthispurpose.

Connectors

RS232devicesmaybeclassifiedasDataTerminalEquipment(DTE)orDataCommunicationEquipment(DCE)thisdefinesateachdevicewhichwireswillbesendingandreceivingeachsignal.ThestandardrecommendedbutdidnotmakemandatorytheDsubminiature25pinconnector.Accordingtothestandard,maleconnectorshaveDTEpinfunctions,andfemaleconnectorshaveDCEpinfunctions.Otherdevicesmayhaveanycombinationofconnectorgenderandpindefinitions.Manyterminalsweremanufacturedwithfemaleconnectorsbutweresoldwithacablewithmaleconnectorsateachendtheterminalwithitscablesatisfiedtherecommendationsinthestandard.Thestandardspecifies20differentsignalconnections.Sincemostdevicesuseonlyafewsignals,smallerconnectorscanoftenbeused.

PersonalcomputermanufacturersreplacedtheDB25MconnectorbythesmallerDE9Mconnector.Differentpinnumberswereusedforthesignals(forthisseeserialport).Thisconnector,withvaryingpinouts,becamecommonforpersonalcomputersandrelateddevices.



Presenceofa25pinDsubconnectordoesnotnecessarilyindicateanRS232Ccompliantinterface.Forexample,ontheoriginalIBMPC,amaleDsubwasanRS232CDTEport(withanonstandardcurrentloopinterfaceonreservedpins),butthefemaleDsubconnectoronthesamePCmodelwasusedfortheparallelCentronicsprinterport.Somepersonalcomputersputnonstandardvoltagesorsignalsonsomepinsoftheirserialports.

Signals

ThefollowingtablelistscommonlyusedRS232signalsandpinassignments.[9]Seeserialport(pinouts)fornonstandardvariationsincludingthepopularDE9connector.

Signal Origin DB25pinName Typicalpurpose Abbreviation DTE DCE

DataTerminalReady

IndicatespresenceofDTEtoDCE. DTR 20

DataCarrierDetect

DCEisconnectedtothetelephoneline. DCD 8

DataSetReady DCEisreadytoreceivecommandsordata. DSR 6

RingIndicator

DCEhasdetectedanincomingringsignalonthetelephoneline. RI 22

RequestToSend DTErequeststheDCEpreparetoreceivedata. RTS 4

ClearToSend IndicatesDCEisreadytoacceptdata. CTS 5

TransmittedData CarriesdatafromDTEtoDCE. TxD 2

ReceivedData CarriesdatafromDCEtoDTE. RxD 3

CommonGround GND common 7

ProtectiveGround PG common 1

ThesignalsarenamedfromthestandpointoftheDTE.Thegroundsignalisacommonreturnfortheotherconnections.TheDB25connectorincludesasecond"protectiveground"onpin1.

Datacanbesentoverasecondarychannel(whenimplementedbytheDTEandDCEdevices),whichisequivalenttotheprimarychannel.Pinassignmentsaredescribedinfollowingtable:



Signal Pin

CommonGround 7(sameasprimary)

SecondaryTransmittedData(STD) 14

SecondaryReceivedData(SRD) 16

SecondaryRequestToSend(SRTS) 19

SecondaryClearToSend(SCTS) 13

SecondaryCarrierDetect(SDCD) 12

RingIndicator'(RI),isasignalsentfromthemodemtotheterminaldevice.Itindicatestotheterminaldevicethatthephonelineisringing.Inmanycomputerserialports,ahardwareinterruptisgeneratedwhentheRIsignalchangesstate.HavingsupportforthishardwareinterruptmeansthataprogramoroperatingsystemcanbeinformedofachangeinstateoftheRIpin,withoutrequiringthesoftwaretoconstantly"poll"thestateofthepin.RIisaonewaysignalfromthemodemtotheterminal(ormoregenerally,theDCEtotheDTE)thatdoesnotcorrespondtoanothersignalthatcarriessimilarinformationtheoppositeway.

OnanexternalmodemthestatusoftheRingIndicatorpinisoftencoupledtothe"AA"(autoanswer)light,whichflashesiftheRIsignalhasdetectedaring.TheassertedRIsignalfollowstheringingpatternclosely,whichcanpermitsoftwaretodetectdistinctiveringpatterns.

TheRingIndicatorsignalisusedbysomeolderuninterruptiblepowersupplies(UPS's)tosignalapowerfailurestatetothecomputer.

Certainpersonalcomputerscanbeconfiguredforwakeonring,allowingacomputerthatissuspendedtoansweraphonecall.

Cables

Thestandarddoesnotdefineamaximumcablelengthbutinsteaddefinesthemaximumcapacitancethatacompliantdrivecircuitmusttolerate.Awidelyusedruleofthumbindicatesthatcablesmorethan50feet(15m)longwillhavetoomuchcapacitance,unlessspecialcablesareused.Byusinglowcapacitancecables,fullspeedcommunicationcanbemaintainedoverlargerdistancesuptoabout1,000feet(300m).[10]Forlongerdistances,othersignalstandardsarebettersuitedtomaintainhighspeed.

Sincethestandarddefinitionsarenotalwayscorrectlyapplied,itisoftennecessarytoconsultdocumentation,testconnectionswithabreakoutbox,orusetrialanderrortofindacablethatworkswheninterconnectingtwodevices.ConnectingafullystandardcompliantDCEdeviceandDTEdevicewoulduseacablethatconnectsidenticalpinnumbersineachconnector(asocalled"straightcable")."Genderchangers"areavailabletosolvegendermismatchesbetweencablesandconnectors.Connectingdeviceswithdifferenttypesofconnectorsrequiresacablethatconnectsthecorrespondingpinsaccordingtothetableabove.Cableswith9pinsononeendand25ontheotherarecommon.Manufacturersofequipmentwith8P8CconnectorsusuallyprovideacablewitheitheraDB25orDE9connector(orsometimesinterchangeableconnectorssotheycanworkwithmultipledevices).Poorqualitycablescancausefalsesignalsbycrosstalkbetweendataandcontrollines(suchasRingIndicator).

Ifagivencablewillnotallowadataconnection,especiallyifagenderchangerisinuse,anullmodemcablemaybenecessary.Genderchangersandnullmodemcablesarenotmentionedinthestandard,sothereisnoofficiallysanctioneddesignforthem.



Conventions

Forfunctionalcommunicationthroughaserialportinterface,conventionsofbitrate,characterframing,communicationsprotocol,characterencoding,datacompression,anderrordetection,notdefinedinRS232,mustbeagreedtobybothsendingandreceivingequipment.Forexample,considertheserialportsoftheoriginalIBMPC.Thisimplementationusedan8250UARTusingasynchronousstartstopcharacterformattingwith7or8databitsperframe,usuallyASCIIcharactercoding,anddataratesprogrammablebetween75bitspersecondand115,200bitspersecond.Dataratesabove20,000bitspersecondareoutofthescopeofthestandard,althoughhigherdataratesaresometimesusedbycommerciallymanufacturedequipment.SincemostRS232devicesdonothaveautomaticbaudratedetection,usersmustmanuallysetthebaudrate(andallotherparameters)atbothendsoftheRS232connection.

Intheparticularcaseofthe8250UARTusedbytheIBMPCandothers,baudrateswereprogrammablebywritingintegervaluestoadividerregisterandbyselectingoneofseveralclockprescalersforthedivider.ThisallowedaPCtobeconnectedtodevicesusingratesotherthanthosestandardizedformodems.Notallbaudratescanbeprogrammed,duetotheclockfrequencyofthe8250UARTinthePC,andthegranularityofthebaudratesetting.ThisincludesthebaudrateofMIDI,31,250bitspersecond,whichisnotachievablebyastandardIBMPCserialport.[11]MIDItoRS232interfacesdesignedfortheIBMPCincludebaudratetranslationhardwaretoadjustthebaudrateoftheMIDIdatatosomethingthattheIBMPCcansupport,forexample19,200or38,400bitspersecond.

RTS/CTShandshaking

Intypicalapplication,theDTEassertsRTStoindicatethatitisreadytoreceivedatafromtheDCE,andtheDCEassertsCTStoindicatethatitisreadytoreceivedatafromtheDTE.Thisiscalled"RTS/CTShandshaking"andisanexampleofhardwareflowcontrol.However,"hardwareflowcontrol"inthedescriptionoftheoptionsavailableonanRS232equippeddevicedoesnotalwaysmeanRTS/CTShandshaking.

Notethatequipmentusingthisprotocolmustbepreparedtobuffersomeextradata,sinceatransmissionmayhavebegunjustbeforethecontrollinestatechange.

ThisisnottheoriginalmeaningofRTS,whichisconsistentwithitsname("requesttosend").

Inolderversionsofthespecification,RS232'suseoftheRTSandCTSlinesisasymmetric:TheDTEassertsRTStoindicateadesiretotransmittotheDCE,andtheDCEassertsCTSinresponsetograntpermission.ThereisnowayfortheDTEtoindicatethatitisunabletoacceptdatafromtheDCE.ThisuseofRTS/CTSallowsforhalfduplex(onedirectionatatime)modemsthatdisabletheirtransmitterswhennotrequired,andmusttransmitasynchronizationpreambletothereceiverwhentheyarereenabled.Suchmodemsarenolongerincommonuse.

ThisschemeisalsoemployedonpresentdayRS232toRS485converters.RS485isamultipleaccessbusonwhichonlyonedevicecantransmitatatime,aconceptthatisnotprovidedforinRS232.TheRS232deviceassertsRTStotelltheconvertertotakecontroloftheRS485bussothattheconverter,andthustheRS232device,cansenddataontothebus.

ThesymmetricRTS/CTShandshakingalternativewasdevelopedandmarketedbyvariousequipmentmanufacturersbeforeitwasincorporatedinRS232.ItwaseventuallycodifiedinversionRS232E(actuallyTIA232Ebythattime)bydefininganewsignal,"RTR(ReadytoReceive),"whichisCCITTV.24circuit133.TIA232Eandthecorrespondinginternationalstandardswereupdatedtoshowthatcircuit133,whenimplemented,sharesthesamepinasRTS(RequesttoSend),andthatwhen133isinuse,RTSisassumedbytheDCEtobeONatalltimes.[12]



3wireand5wireRS232

Aminimal"3wire"RS232connectionconsistingonlyoftransmitdata,receivedata,andground,iscommonlyusedwhenthefullfacilitiesofRS232arenotrequired.Evenatwowireconnection(dataandground)canbeusedifthedataflowisoneway(forexample,adigitalpostalscalethatperiodicallysendsaweightreading,oraGPSreceiverthatperiodicallysendsposition,ifnoconfigurationviaRS232isnecessary).Whenonlyhardwareflowcontrolisrequiredinadditiontotwowaydata,theRTSandCTSlinesareaddedina5wireversion.

Seldomusedfeatures

TheEIA232standardspecifiesconnectionsforseveralfeaturesthatarenotusedinmostimplementations.Theiruserequires25pinconnectorsandcables.

Signalrateselection

TheDTEorDCEcanspecifyuseofa"high"or"low"signalingrate.TheratesaswellaswhichdevicewillselecttheratemustbeconfiguredinboththeDTEandDCE.Theprearrangeddeviceselectsthehighratebysettingpin23toON.

Loopbacktesting

ManyDCEdeviceshavealoopbackcapabilityusedfortesting.Whenenabled,signalsareechoedbacktothesenderratherthanbeingsentontothereceiver.Ifsupported,theDTEcansignalthelocalDCE(theoneitisconnectedto)toenterloopbackmodebysettingpin18toON,ortheremoteDCE(theonethelocalDCEisconnectedto)toenterloopbackmodebysettingpin21toON.ThelatterteststhecommunicationslinkaswellasbothDCE's.WhentheDCEisintestmodeitsignalstheDTEbysettingpin25toON.

Acommonlyusedversionofloopbacktestingdoesnotinvolveanyspecialcapabilityofeitherend.Ahardwareloopbackissimplyawireconnectingcomplementarypinstogetherinthesameconnector(seeloopback).

LoopbacktestingisoftenperformedwithaspecializedDTEcalledabiterrorratetester(orBERT).

Timingsignals

Somesynchronousdevicesprovideaclocksignaltosynchronizedatatransmission,especiallyathigherdatarates.TwotimingsignalsareprovidedbytheDCEonpins15and17.Pin15isthetransmitterclock,orsendtiming(ST)theDTEputsthenextbitonthedataline(pin2)whenthisclocktransitionsfromOFFtoON(soitisstableduringtheONtoOFFtransitionwhentheDCEregistersthebit).Pin17isthereceiverclock,orreceivetiming(RT)theDTEreadsthenextbitfromthedataline(pin3)whenthisclocktransitionsfromONtoOFF.

Alternatively,theDTEcanprovideaclocksignal,calledtransmittertiming(TT),onpin24fortransmitteddata.DataischangedwhentheclocktransitionsfromOFFtoONandreadduringtheONtoOFFtransition.TTcanbeusedtoovercometheissuewhereSTmusttraverseacableofunknownlengthanddelay,clockabitoutoftheDTEafteranotherunknowndelay,andreturnittotheDCEoverthesameunknowncabledelay.SincetherelationbetweenthetransmittedbitandTTcanbefixedintheDTEdesign,andsincebothsignalstraversethesamecablelength,usingTTeliminatestheissue.TTmaybegeneratedbyloopingSTbackwithanappropriatephasechangetoalignitwiththetransmitteddata.STloopbacktoTTletstheDTEusetheDCEasthefrequencyreference,andcorrecttheclocktodatatiming.



SynchronousclockingisrequiredforsuchprotocolsasSDLC,HDLC,andX.25.

Secondarychannel

Thereisasecondarydatachannel,identicalincapabilitytothefirst.Fivesignals(plusthecommongroundoftheprimarychannel)comprisethesecondarychannel:SecondaryTransmittedData(STD),SecondaryReceivedData(SRD),SecondaryRequestToSend(SRTS),SecondaryClearToSend(SCTS),andSecondaryCarrierDetect(SDCD).

Relatedstandards

OtherserialsignalingstandardsmaynotinteroperatewithstandardcompliantRS232ports.Forexample,usingtheTTLlevelsofnear+5and0Vputsthemarklevelintheundefinedareaofthestandard.SuchlevelsaresometimesusedwithNMEA0183compliantGPSreceiversanddepthfinders.

A20mAcurrentloopusestheabsenceof20mAcurrentforhigh,andthepresenceofcurrentintheloopforlowthissignalingmethodisoftenusedforlongdistanceandopticallyisolatedlinks.ConnectionofacurrentloopdevicetoacompliantRS232portrequiresaleveltranslator.Currentloopdevicescansupplyvoltagesinexcessofthewithstandvoltagelimitsofacompliantdevice.TheoriginalIBMPCserialportcardimplementeda20mAcurrentloopinterface,whichwasneveremulatedbyothersuppliersofplugcompatibleequipment.

OtherserialinterfacessimilartoRS232:

RS422(ahighspeedsystemsimilartoRS232butwithdifferentialsignaling)RS423(ahighspeedsystemsimilartoRS422butwithunbalancedsignaling)RS449(afunctionalandmechanicalinterfacethatusedRS422andRS423signalsitnevercaughtonlikeRS232andwaswithdrawnbytheEIA)RS485(adescendantofRS422thatcanbeusedasabusinmultidropconfigurations)MILSTD188(asystemlikeRS232butwithbetterimpedanceandrisetimecontrol)EIA530(ahighspeedsystemusingRS422orRS423electricalpropertiesinanEIA232pinoutconfiguration,thuscombiningthebestofbothsupersedesRS449)EIA/TIA5618PositionNonSynchronousInterfaceBetweenDataTerminalEquipmentandDataCircuitTerminatingEquipmentEmployingSerialBinaryDataInterchangeEIA/TIA562ElectricalCharacteristicsforanUnbalancedDigitalInterface(lowvoltageversionofEIA/TIA232)TIA574(standardizesthe9pinDsubminiatureconnectorpinoutforusewithEIA232electricalsignalling,asoriginatedontheIBMPC/AT)

Developmenttools

WhendevelopingortroubleshootingsystemsusingRS232,closeexaminationofhardwaresignalscanbeimportanttofindproblems.AseriallineanalyzerisadevicesimilartoalogicanalyzerbutspecializedforRS232'svoltagelevels,connectors,and,whereused,clocksignals.Theseriallineanalyzercancollect,store,anddisplaythedataandcontrolsignals,allowingdeveloperstoviewthemindetail.SomesimplydisplaythesignalsaswaveformsmoreelaborateversionsincludetheabilitytodecodecharactersinASCIIorothercommon



WikimediaCommonshasmediarelatedtoRS232.

Wikibookshasabookonthetopicof:Serial

codesandtointerpretcommonprotocolsusedoverRS232suchasSDLC,HDLC,DDCMP,andX.25.Seriallineanalyzersareavailableasstandaloneunits,assoftwareandinterfacecablesforgeneralpurposelogicanalyzersandoscilloscopes,andasprogramsthatrunoncommonpersonalcomputersanddevices.

References

Furtherreading

SerialPortComplete:COMPorts,USBVirtualCOMPorts,andPortsforEmbeddedSystems2ndEditionJanAxelsonLakeviewResearch380pages2007ISBN9781931448062.

Externallinks

Retrievedfrom"http://en.wikipedia.org/w/index.php?title=RS232&oldid=651814615"

Categories: Telecommunicationsequipment

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4. ^"TIAFactsataGlance"(http://www.tiaonline.org/about/).AboutTIA.TelecommunicationsIndustryAssociation.Retrieved28July2011.

5. ^S.Mackay,E.Wright,D.Reynders,J.Park,PracticalIndustrialDataNetworks:Design,InstallationandTroubleshooting,Newnes,2004ISBN075065807X,pages4142

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