信息自动化专业外文翻译.docx
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信息自动化专业外文翻译
外文资料原文
Transmittingandreceivingserialdata
TheUniversalAsynchronousReceiver/Transmitter(UART)controlleristhekeycomponentoftheserialcommunicationssubsystemofacomputer.TheUARTtakesbytesofdataandtransmitstheindividualbitsinasequentialfashion.Atthedestination,asecondUARTre-assemblesthebitsintocompletebytes.Serialtransmissionofdigitalinformation(bits)throughasinglewireorothermediumismuchmorecosteffectivethanparalleltransmissionthroughmultiplewires.AUARTisusedtoconvertthetransmittedinformationbetweenitssequentialandparallelformateachendofthelink.EachUARTcontainsashiftregisterwhichisthefundamentalmethodofconversionbetweenserialandparallelforms.
TheUARTusuallydoesnotdirectlygenerateorreceivetheexternalsignalsusedbetweendifferentitemsofequipment.Typically,separateinterfacedevicesareusedtoconvertthelogiclevelsignalsoftheUARTtoandfromtheexternalsignalinglevels.
Externalsignalsmaybeofmanydifferentforms.ExamplesofstandardsforvoltagesignalingareRS-232,RS-422andRS-485fromtheEIA.Historically,thepresenceorabsenceofcurrent(incurrentloops)wasusedintelegraphcircuits.Somesignalingschemesdonotuseelectricalwires.Examplesofsuchareopticalfiber,IrDA(infrared),and(wireless)BluetoothinitsSerialPortProfile(SPP).Somesignalingschemesusemodulationofacarriersignal(withorwithoutwires).Examplesaremodulationofaudiosignalswithphonelinemodems,RFmodulationwithdataradios,andtheDC-LINforpowerlinecommunication.
Communicationmaybe"fullduplex"(bothsendandreceiveatthesametime)or"halfduplex"(devicestaketurnstransmittingandreceiving).
Asof2008,UARTsarecommonlyusedwithRS-232forembeddedsystemscommunications.ItisusefultocommunicatebetweenmicrocontrollersandalsowithPCs.ManychipsprovideUARTfunctionalityinsilicon,andlow-costchipsexisttoconvertlogiclevelsignals(suchasTTLvoltages)toRS-232levelsignals(forexample,Maxim'sMAX232).
Asynchronousreceiveandtransmit
Inasynchronoustransmitting,teletype-styleUARTssenda"start"bit,fiveto
eightdatabits,least-significant-bitfirst,anoptional"parity"bit,andthenone,oneandahalf,ortwo"stop"bits.Thestartbitistheoppositepolarityofthedata-line'sidlestate.Thestopbitisthedata-line'sidlestate,andprovidesadelaybeforethenextcharactercanstart.(Thisiscalledasynchronousstart-stoptransmission).Inmechanicalteletypes,the"stop"bitwasoftenstretchedtotwobittimestogivethemechanismmoretimetofinishprintingacharacter.Astretched"stop"bitalsohelpsresynchronization.
Theparitybitcaneithermakethenumberof"one"bitsbetweenanystart/stoppairodd,oreven,oritcanbeomitted.Oddparityismorereliablebecauseitassuresthattherewillalwaysbeatleastonedatatransition,andthispermitsmanyUARTstoresynchronize.
Insynchronoustransmission,theclockdataisrecoveredseparatelyfromthedatastreamandnostart/stopbitsareused.Thisimprovestheefficiencyoftransmissiononsuitablechannelssincemoreofthebitssentareusabledataandnotcharacterframing.Anasynchronoustransmissionsendsnocharactersovertheinterconnectionwhenthetransmittingdevicehasnothingtosend--onlyidlestopbits;butasynchronousinterfacemustsend"pad"characterstomaintainsynchronismbetweenthereceiverandtransmitter.TheusualfilleristheASCII"SYN"character.Thismaybedoneautomaticallybythetransmittingdevice.
USARTchipshavebothsynchronousandasynchronousmodes.
SerialtoParallelAlgorithm
Asynchronoustransmissionallowsdatatobetransmittedwithoutthesenderhavingtosendaclocksignaltothereceiver.Instead,thesenderandreceivermustagreeontimingparametersinadvanceandspecialbitsareaddedtoeachwordwhichareusedtosynchronizethesendingandreceivingunits.
WhenawordisgiventotheUARTforAsynchronoustransmissions,abitcalledthe"StartBit"isaddedtothebeginningofeachwordthatistobetransmitted.TheStartBitisusedtoalertthereceiverthatawordofdataisabouttobesent,andtoforcetheclockinthereceiverintosynchronizationwiththeclockinthetransmitter.Thesetwoclocksmustbeaccurateenoughtonothavethefrequencydriftbymorethan10%duringthetransmissionoftheremainingbitsintheword.(Thisrequirementwassetinthedaysofmechanicalteleprintersandiseasilymetbymodernelectronicequipment.)
AftertheStartBit,theindividualbitsofthewordofdataaresent,withtheLeastSignificantBit(LSB)beingsentfirst.Eachbitinthetransmissionistransmitted
forexactlythesameamountoftimeasalloftheotherbits,andthereceiver“looks”atthewireatapproximatelyhalfwaythroughtheperiodassignedtoeachbittodetermineifthebitisa1ora0.Forexample,ifittakestwosecondstosendeachbit,thereceiverwillexaminethesignaltodetermineifitisa1ora0afteronesecondhaspassed,thenitwillwaittwosecondsandthenexaminethevalueofthenextbit,andsoon.
Thesenderdoesnotknowwhenthereceiverhas“looked”atthevalueofthebit.Thesenderonlyknowswhentheclocksaystobegintransmittingthenextbitoftheword.
Whentheentiredatawordhasbeensent,thetransmittermayaddaParityBitthatthetransmittergenerates.TheParityBitmaybeusedbythereceivertoperformsimpleerrorchecking.ThenatleastoneStopBitissentbythetransmitter.
Whenthereceiverhasreceivedallofthebitsinthedataword,itmaycheckfortheParityBits(bothsenderandreceivermustagreeonwhetheraParityBitistobeused),andthenthereceiverlooksforaStopBit.IftheStopBitdoesnotappearwhenitissupposedto,theUARTconsiderstheentirewordtobegarbledandwillreportaFramingErrortothehostprocessorwhenthedatawordisread.TheusualcauseofaFramingErroristhatthesenderandreceiverclockswerenotrunningatthesamespeed,orthatthesignalwasinterrupted.
Regardlessofwhetherthedatawasreceivedcorrectlyornot,theUARTautomaticallydiscardstheStart,ParityandStopbits.Ifthesenderandreceiverareconfiguredidentically,thesebitsarenotpassedtothehost.Ifanotherwordisreadyfortransmission,theStartBitforthenewwordcanbesentassoonastheStopBitforthepreviouswordhasbeensent.Becauseasynchronousdatais“selfsynchronizing”,ifthereisnodatatotransmit,thetransmissionlinecanbeidle.Adatacommunicationpulsecanonlybeinoneoftwostatesbuttherearemanynamesforthetwostates.Whenon,circuitclosed,lowvoltage,currentflowing,oralogicalzero,thepulseissaidtobeinthe"space"condition.Whenoff,circuitopen,highvoltage,currentstopped,oralogicalone,thepulseissaidtobeinthe"mark"condition.Acharactercodebeginswiththedatacommunicationcircuitinthespacecondition.Ifthemarkconditionappears,alogicaloneisrecordedotherwisealogicalzero.
Figure1showsthisformat.
Thestartbitisalwaysa0(logiclow),whichisalsocalledaspace.ThestartbitsignalsthereceivingDTEthatacharactercodeiscoming.Thenextfivetoeightbits,dependingonthecodesetemployed,representthecharacter.IntheASCIIcodesettheeighthdatabitmaybeaparitybit.Thenextoneortwobitsarealwaysinthe
mark(logichigh,i.e.,'1')conditionandcalledthestopbit(s).Theyprovidea"rest"intervalforthereceivingDTEsothatitmayprepareforthenextcharacterwhichmaybeafterthestopbit(s).TherestintervalwasrequiredbymechanicalTeletypeswhichusedamotordrivencamshafttodecodeeachcharacter.Attheendofeachcharacterthemotorneededtimetostrikethecharacterbail(printthecharacter)andresetthecamshaft.
AlloperationsoftheUARThardwarearecontrolledbyaclocksignalwhichrunsatamultiple(say,16)ofthedatarate-eachdatabitisaslongas16clockpulses.Thereceiverteststhestateoftheincomingsignaloneachclockpulse,lookingforthebeginningofthestartbit.Iftheapparentstartbitlastsatleastone-halfofthebittime,itisvalidandsignalsthestartofanewcharacter.Ifnot,thespuriouspulseisignored.Afterwaitingafurtherbittime,thestateofthelineisagainsampledandtheresultinglevelclockedintoashiftregister.Aftertherequirednumberofbitperiodsforthecharacterlength(5to8bits,typically)haveelapsed,thecontentsoftheshiftregisterismadeavailable(inparallelfashion)tothereceivingsystem.TheUARTwillsetaflagindicatingnewdataisavailable,andmayalsogenerateaprocessorinterrupttorequestthatthehostprocessortransfersthereceiveddata.InsomecommontypesofUART,asmallfirst-in,first-out(FIFO)buffermemoryisinsertedbetweenthereceivershiftregisterandthehostsysteminterface.ThisallowsthehostprocessormoretimetohandleaninterruptfromtheUARTandpreventslossofreceiveddataathighrates.
Transmissionoperationissimplersinceitisunderthecontrolofthetransmittingsystem.Assoonasdataisdepositedintheshiftregister,theUARThardwaregeneratesastartbit,shiftstherequirednumberofdatabitsouttotheline,generatesandappendstheparitybit(ifused),andappendsthestopbits.SincetransmissionofasinglecharactermaytakealongtimerelativetoCPUspeeds,theUARTwillmaintainaflagshowingbusystatussothatthehostsystemdoesnotdepositanewcharacterfortransmissionuntilthepreviousonehasbeencompleted;thismayalsobedonewithaninterrupt.Sincefull-duplexoperationrequi