(完整版)基于51单片机的无线数据收发系统设计(带电路图和代码)

英文资料及中文翻译

CommunicatingwithDatalDatasignalsaretransmittedovervarioustypesoftelephonecircuits.Theytravelonwirefromtelephonepoletotelephonepole,throughundergroundcables,frommountaintoptomountaintopovermicrowavefacilities,ontheoceanfloorinsubmarinecables,andviacommunicationssatellitesfromcontinenttocontinent.Sometypeofdataconversionequipmentisrequiredtochangethedigitalmachinesignalstoaformsuitablefortransmissionoverthesefacilities.Thedatamachinewhichprovidesaninputtothetransmitsectionoftheconversionequipment,ormodulator,canbeakeyboard,printer,cardreader,papertapeterminalcomputerormagnetictapeterminal.Theoutputfromthereceivesectionoftheconverter,ordemodulator,canbeappliedtoatapepunch,printer,cardpunch,magnetictapeunit,computer,orvisualdisplayterminal.Typically,boththemodulatoranddemodulatorsectionsoftheconverterarecombinedintoatwo-waydatatransmitter-receiver,commonlycalledadatamodemordataset.Thetypicalfull-duplexdatatransmissionsystemincludingtheoriginatingdataprocessingequipmentandtheinterfaceassemblywhichconsistsofbufferandcontrolunits.Theinterfaceassemblyatthetransmitteracceptsdataattheratedeterminedbytheoperatingspeedofthedataprocessor.storesthedatatemporarily,andregeneratesitataratecompatiblewiththatofthedatamodem.Atthereceivingterminaltheinterfaceassemblyacceptsthereceiveddata,storesit,thenfeedsittothedataprocessorattheappropriaterate.Timingsignalsfromtheinterfaceassemblyatthetransmitterareappliedtothedatamodemtosynchronizethecomputerandthedataset.Atthereceiver,synchronizationpulsesarederivedfromthedatastreamtosynchronizethecomputer.Whenmorethanonedatasetfeedsintoacomputer,thecapacityoftheinterfaceequipmentisofmajorconcernsinceitmustdeterminethetimeslotallocationforeachline.Varioustypesofinterfaceassembliesareemployed,suchasmagneticcorememories,shiftregisters,anddelaylines.Notalldatacommunicationsterminalsemployaninterfacebetweenthedataprocessorandthedatamodem.Withoutaninterface,theinput,datatransmission,andoutputfunctionsproceedsimultaneouslyandatthesamerateofspeed.Sincedatasignalsarerarelyinsuitableformfortransmissionoverthevarioustypesoftransmissionfacilities,asignalcodingprocessisnormallyperformed.Ideally,thetransmissionmediumshouldhavelinearattenuationanddelaycharacteristics,butthisisneversoinpractice,andtransmissionimpairmentsarealwayspresenttodisturbthedatasignals.Asacomparison,invoicecommunicationsahighdegreeoftransmissionirregularitiescanbetolerated.Ifavoicecircuithasaheavylossorisnoisy,thespeakerscompensateautomaticallybyincreasingtheintensityoftheirvoices.Ifwordsaremissedbecauseoftransmissiondifficulties,theyareoftenunderstoodanywaybecauseoftheredundantnatureofspeech.Incontrast,thereisnoinherentredundancyindatasignalsunlesspurposelyinsertedand,therefore,transmissionvariationscaronlybecompensatedforoveraverysmallrange.Inaddition,datasignalsaresensitivetoothertransmissionimpairmentswhichhavelittleeffectonspeech.Codingisundertakentoalleviatetransmissionirregularities,toincreasetheinformationcapacityofthesystem,toenableerrordetection,andtoprovidemessagesecurity.Thecodingprocessinthedatatransmittersimplyrearrangestheapplieddatamachinesignalsintosomeotherformat.Atthereceivingendthereverseprocessingisperformedtorecovertheoriginalmachinesignals.Thediagramsshowthetwotypesofinformationsignalsthatareappliedindigitalformtoadatamodem.ShowninAisabinarynon-returntozerosignal.InBthesamesignalisshowninthereturntozeroformat.ThedifferencebetweenAandBisthatinAsuccessivemarksorspacesfollowoneanother,whereasinBtheremustbeareturntothespacelevelbetweensuccessivemarks.Thevoltagevaluesofmarksandspacesarearbitraryandmaybepositive,negative,orboth.Ofprimaryconcernwhenconsideringthetransmissionofdatafromonedevicetoanotheriswiring.Andofprimaryconcernwhenconsideringthewiringisthedatastream.Dowesendonebitatatime,ordowegroupbitsintolargergroupsand.ifso,how?Thetransmissionofbinarydataacrossalinkcanbeaccomplishedeitherinparallelmodeorserialmode.Inparallelmode,multiplebitsaresentwitheachclockpulse.Inserialmode,onebitissentwitheachclockpulse.Whilethereisonlyone-waytosendparalleldata,therearetwosubclassesofserialtransmission:synchronousandasynchronous.Asynchronoustransmissionissonamedbecausethetimingofasignalisunimportant.Instead,informationisreceivedandtranslatedbyagreed-uponpatterns.Aslongasthosepatternsarefollowed,thereceivingdevicecanretrievetheinformationwithoutregardtotherhythminwhichitissent.Patternsarebasedongroupingthebitstreamintobytes.Eachgroup,usuallyeightbits,issentalongthelinkasaunit.Thesendingsystemhandleseachgroupindependently,relayingittothelinkwheneverready,withoutregardtoatimer.Withoutasynchronizingpulse,thereceivercannotusetimingtopredictwhenthenextgroupwillarrive.Toalertthereceivertothearrivalofanewgroup,therefore,anextrabitisaddedtothebeginningofeachbyte.Thisbit,usuallya0,iscalledthestartbit.Toletthereceiverknowthatthebyteisfinished,oneormoreadditionalbitsareappendedtotheendofthebyte.Thesebits,usually1s,arecalledstopbits.Bythismethod,eachbyteisincreasedinsizetoatleast10bits,ofwhich8areinformationand2ormorearesignalstothereceiver.Inaddition,thetransmissionofeachbytemaythenbefollowedbyagapofvaryingduration.Thisgapcanberepresentedeitherbyanidlechannelorbyastreamofadditionalstopbits.Thestartandstopbitsandthegapalertthereceivertothebeginningandendofeachbyteandallowittosynchronizewiththedatastream.Thismechanismiscalledasynchronousbecause,atthebytelevel,senderandreceiverdonothavetobesynchronized.Butwithineachbyte,thereceivermuststillbesynchronizedwiththeincomingbitstream.Thatis,somesynchronizationisrequired,butonlyforthedurationofasinglebyte.Thereceivingdeviceresynchronizesattheonsetofeachnewbyte.Whenthereceiverdetectsastartbit,itsetsatimerandbeginscountingbitsastheycomein.Afternbitsthereceiverlooksforastopbit.Assoonasitdetectsthestopbit,itignoresanyreceivedpulsesuntilitdetectsthenextstartbit.Theadditionofstopandstartbitsandtheinsertionofgapsintothebitstreammakeasynchronoustransmissionslowerthanformsoftransmissionthatcanoperatewithouttheadditionofcontrolinformation.Butitischeapandeffective,twoadvantagesthatmakeitanattractivechoiceforsituationslikelow-speedcommunication.Forexample,theconnectionofaterminaltoacomputerisana1uralapplicationforasynchronoustransmission.AusertypesonJyonecharacteratatime,typesextremelyslowlyindataprocessingterms,andleavesunpredictablegapsoftimebetweeneachcharacter.Insynchronoustransmission,thebitstreamiscombinedintolonger"frames",whichmaycontainmultiplebytes.Eachbyte,however,isintroducedontothetransmissionlinkwithoutagapbetweenitandthenextone.Itislefttothereceivertoseparatethebitstreamintobytesfordecodingpurposes.Inotherwords,dataaretransmittedasanunbrokenstringof1sand0s,andthereceiverseparatesthatstringintothebytes,orcharacters,itneedstoreconstructtheinformation.Itgivesaschematicillustrationofsynchronoustransmission.Wehavedrawninthedivisionsbetweenbytes.Inreality,thosedivisionsdonotexist;thesenderputsasdataontothelineasonelongstring.Ifthesenderwishestosenddatainseparatebursts,thegapsbetweenburstsmustbefilledwithaspecialsequenceof0sand1sthatmeansidle.Thereceivercountsthebitsastheyarriveandgroupsthemineight-bitunits.Withoutgapsandstart/stopbits,thereisnobuilt-inmechanismtohelpthereceivingdeviceadjustitsbitsynchronizationinmidstream.Timingbecomesveryimportant,therefore,becausetheaccuracyofthereceivedinformationiscompletelydependentontheabilityofthereceivingdevicetokeepanaccuratecountofthebitsastheycomein.Theadvantageofsynchronoustransmissionisspeed.Withnoextrabitsorgapstointroduceatthesendingendandremoveatthereceivingendand,byextension,withfewerbitstomoveacrossthelink,synchronoustransmissionisfasterthanasynchronoustransmission.Forthisreason,itismoreusefulforhigh-speedapplicationslikethetransmissionofdatafromonecomputertoanother.Bytesynchronizationisaccomplishedinthedatalinklayer.ByfarthemostpopularserialinterfacebetweenacomputeranditsCRTterminalistheasynchronousserialinterface.Thisinterfaceissocalledbecausethetransmitteddataandthereceiveddataarenotsynchronizedoveranyextendedperiodandthereforenospecialmeansofsynchronizingtheclocksatthetransmitterandreceiverisnecessary.Infact,theasynchronousserialdatalinkisaveryoldformofdatatransmissionsystemandhasitsoriginintheeraofteleprinter.Serialdatatransmissionsystemshavebeenaroundforalongtimeandarefoundinthetelephone(humanspeech),Morsecode,semaphore,andeventhesmokesignalsonceusedbynativeAmericans.Thefundamentalproblemencounteredbyallserialdatatransmissionsystemsishowtosplittheincomingdatastreamintoindividualunits(i.e.,bits)andhowtogrouptheseunitsintocharacters.Forexample,inMorsecodethedotsanddashesofacharacterareseparatedbyanintersymbolspace,whiletheindividualcharactersareseparatedbyanintercharacterspace,whichisthreetimesthedurationofanintersymbolspace.Firstweexaminehowthedatastreamisdividedintoindividualbitsandthebitsgroupedintocharactersinanasynchronousserialdatalink.Thekeytotheoperationofthistypeoffinkisbothsimpleandingenious.Anasynchronousserialdatalinkissaidtobecharacteroriented,asinformationistransmittedintheformofgroupsofbitscalledcharacters.Thesecharactersareinvariableunitscomprising7or8bitsof"information"plus2to4controlbitsandfrequentlycorrespondtoASCII-encodedcharacters.Initially,whennoinformationisbeingtransmitted,thelineisinanidlestate.Traditionally,theidlestateisreferredtoasthemarklevel.Byconventionthiscorrespondstoalogical1level.Whenthetransmitterwishestosenddata,itfirstplacesthelineinaspacelevel(i.e.,thecomplementofamark)foroneelementperiod.ThiselementiscalledthestartbitandhasadurationofTseconds.Thetransmitterthensendsthecharacter,1bitatatime,byplacingeachsuccessivebitonthefineforadurationofTseconds,untilallbitshavebeentransmitted.Thenasingleparitybitiscalculatedbythetransmitterandsentafterthedatabits.Finally,thetransmittersendsastopbitatamarklevel(i.e.,thesamelevelastheidlestate)foroneortwobitperiods.Nowthetransmittermaysendanothercharacterwheneveritwishes.Atthereceivingendofanasynchronousserialdatalink,thereceivercontinuallymonitorsthelinelookingforastartbit.Oncethestartbithasbeendetected,thereceiverwaitsuntiltheendofthestartbitandthensamplesthenextNbitsattheircenters,usingaclockgeneratedlocallybythereceiver.Aseachincomingbitissampled,itisusedtoconstructanewcharacter.Whenthereceivedcharacterhasbeenassembled,itsparityiscalculatedandcomparedwiththereceivedparitybitfollowingthecharacter.Iftheyarenotequal,aparityerrorflagissettoindicateatransmissionerror.Themostcriticalaspectofthesystemisthereceivertiming.Thefallingedgeofthestartbittriggersthereceiver'slocalclock,whichsampleseachincoinmailngbitatitcenter.SupposethereceiverclockwaitsT/2secondsfromthefallingedgeofthestartbitandsamplestheincomingdataeveryTsecondsthereafteruntilthestopbithasbeensampled.Asthereceiver'sclockisnotsynchronizedwiththetransmitterclock,thesamplingisnotexact.Themostobviousdisadvantageofasynchronousdatatransmissionistheneedforastart,parity,andstopbitforeachtransmittedcharacter.If7bitcharactersareused,theoverallefficiencyisonly70%.Alessobviousdisadvantageisduetothecharacter-orientednatureofthedatalink.WheneverthedatalinkconnectsaCRTterminaltoacomputer,fewproblemsarise,astheterminalisitselfcharacteroriented.However,ifthedatalinkisbeingusedto,say,dumpbinarydatatoamagnetictape,problemsarise.yk/.t|—t、工数据通信数据信号在各种各样的话路上传输：它们通过导线从一根电杆传到另一根电杆；它们经过地下电缆传送；它们通过微波设备从一个山头传到另一个山头；它们通过海底电缆，通过通信卫星，从一个洲传到另一个洲。为了把数字化机器信号变换为适合在这些设备中传输的信号形式,需要使用某种类型的数据变换设备。向变换设备发送部分（即调制器）提供输入的数据设备可以是键盘、打印机、卡片阅读器、纸带终端计算机或磁带终端机。变换器接收部分（即解调器）的输出可以适用于纸带凿孔机、打印机、卡片凿孔机、磁带机、计算机或视频显示终端。一般地说，变换器的调制部分和解调部分合并成为一个双向数据发送接收机，通常称之为数据调制解调器或数据传输机。典型的全双工数据传输系统，包括始发端数据处理设备和由缓冲器和控制单元组成的接口部件。发端的接口部件以数据处理机的处理速度所确定的速率接收数据，将它们暂时存储起来，并以与数据调制解调器兼容的速率予以转发。在接收端，接口部件接受所收到的数据，将它们存储起来，再以适当的速率送到数据处理机中去。来自发端接口部件的定时信号被加到数据调制解调器上，以使计算机与数传机同步。在接收端，从数据流中取出同步脉冲使计算机同步。当有一台以上数传机接至一台计算机时，接口设备的容量是主要问题，因为它必须确定分配给每条线路的时隙。有各种类型的接口部件可以使用，如磁芯存储器、移位寄存器和时延线。然而并不是所有的数据通信终端在数据处理机和数据调制解调器之间都使用接口。如果没有接口，那么输入、数据传输和输出这三个操作过程同时进行，而且速率相同。由于数据信号的形式一般不适宜在各种传输设备上传送，通常对信号要进行编码。在理想情况下，传输媒介应当具有线性衰减和线性时延的特性。但实际情况根本不是这样，传输损伤总是存在，干扰了数据信号。相比之下，语声通信可以容忍极不规则的传输情况。如果电话电路的衰耗严重或噪声大，说话人就会提高嗓音，自动予以弥补。如果讲的某些单词因传输困难而没听见，双方往往仍可听懂，因为语言有冗余度。数据信号则与之相反，除非有意加入，它本身没有冗余度，所以传输质量的不稳定只能得到非常有限的补偿。另外，数据信号对基本上不影响话音的其他传输质量下降很敏感。为了减少不正常的传输情况，增加系统的信息容量，实现差错检测和消息保密，就要采用编码手段。数据发送端的编码仅仅是将所输入的数据信号重新排列成其他形式。在接收端则进行相反的过程（译码），恢复原来的数据信号。所给的波形表示以数字形式输入到数据调制解调器的两类信息信号。波形A是二进制不归零（NRZ）信号，波形B是同一信号的归零（RZ）形式。波形A与波形B的区别是：波形A中传号或空号连续不新地出现，而波形B中脉冲幅度必须在两个连续信号之间回到空号电平上来。传号和空号的电压值是任意的，可以是正值或负值，也可以是正负值兼而有之。当研究数据从一个设备向另一个设备传输时,我们关心的主要问题之一是连线。而考虑连线时，数据流又是我们所关心的问题。我们是一次发送一个比特呢,或者是成组发送它们呢？如果要成组发送，又如何做到这一点呢？通过链路来发送二进制数据的方法可以这样实现：要么采用并行方式，要么使用串行的模式。在并行模式中，在每一个时钟脉冲到来时,可同时发送多个比特。而在串行方式里，伴随每个时钟只发送一个比特。虽然只有一种并行发送数据的方法,但串行传输却有两类：同步传输和异步传输。异步传输被如此称呼，是因为信号的定时并不重要。不同的是，信息是按事先约定的方式来接收和翻译的。只要遵照这些约定，接收器件就能够恢复信息，而不理会它们在发送时的节拍。约定的基础是将比特流组合成字节。每一个组合通常含有8个比特，它被作为一个单元在链路上发送。发送系统单独处理每个组合，当将组合准备停当就将它放到链路上，且与定时器没有关系。没有了同步脉冲，接收机就不能利用定时信号去预测下一个组合什么时候到达。因此，为了通知接收机有新的组合到达，就得在每个字节的开始加上一个额外的比特。这个比特通常为0，并被称为起始位。为了让接收机知道字节的结束，在字节的尾部又另加了一个或多个比特。这些比特通常为1，被人们称为停止住。运用这种方法，每一个字节的长度至少增加到10个比特，其中有8个比特的信息，以及2个或更多的比特，作为向接收机打"招呼"的信号。此外，在每个字节传送之后，可能会有一段变化的间隙。这个时隙可用空闲信道或另加停止位来表示。起始位、停止位和时隙告诉接收机每一个字节的开始和结束，并让接收机按照数据流进行同步。这种机制被称为是异步的，因为在字节级上，发送器和接收器不需要同步。但是在每个字节内部，接收器仍需与流入的比特流同步。这就是说，某种同步还是需要的，但仅限于在一个字节持续的期间内。在每一个新的字节开始时，接收机又重新进行同步。当接收机检测到一个起始位，它就将定时器置位，并在比特流入时开始记数。在接收了n比特之后，接收器就寻找停止住。一旦它检测到停止住，它就忽略以后收到的脉冲，直到检测到下一个起始位为止。比起不添加控制信息就能运行的传输形式，异步传输由于增加了停止住、起始位和在比特流中插入时隙而显得慢一些。但由于具有便宜和高效两大优点，这使得它在如低速通信的一些场合成为一项诱人的选择。例如，终端与计算机的连接就是异步传输方式很自然的应用。用户每次只能敲一个字符，这在数据处理领域里是极慢的，而且在每个字符阔的间隙长短也难以预测。在同步传输中，比特流合并成较长的“帧”，而帧可能含有多个字节。然而当字节被引入到传输链路的时候，在字节之间却没有间隙。将比特流分成字节的任务是由接收器在解码过程中完成的。换句话说，数据是以1和0组成的无间断码串来传输的，而接受器将这个码串分离成字节或字符，它需要将信息重新恢复。对同步传输做了简要的说明。在字节之间我们画上了分隔线。事实上，这些分隔线并不存在，而发送器是将一长串的数据放到线路上的。如果发送器想以分离突发的形式发送数据，则突发数据群之间的间隙就必须用一种由0和