信息自动化专业外文翻译.docx

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