(完整版)基于51單片機(jī)的無(wú)線數(shù)據(jù)收發(fā)系統(tǒng)設(shè)計(jì)(帶電路圖和代碼)

英文資料及中文翻譯

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、工數(shù)據(jù)通信數(shù)據(jù)信號(hào)在各種各樣的話路上傳輸：它們通過(guò)導(dǎo)線從一根電桿傳到另一根電桿；它們經(jīng)過(guò)地下電纜傳送；它們通過(guò)微波設(shè)備從一個(gè)山頭傳到另一個(gè)山頭；它們通過(guò)海底電纜，通過(guò)通信衛(wèi)星，從一個(gè)洲傳到另一個(gè)洲。為了把數(shù)字化機(jī)器信號(hào)變換為適合在這些設(shè)備中傳輸?shù)男盘?hào)形式,需要使用某種類型的數(shù)據(jù)變換設(shè)備。向變換設(shè)備發(fā)送部分（即調(diào)制器）提供輸入的數(shù)據(jù)設(shè)備可以是鍵盤(pán)、打印機(jī)、卡片閱讀器、紙帶終端計(jì)算機(jī)或磁帶終端機(jī)。變換器接收部分（即解調(diào)器）的輸出可以適用于紙帶鑿孔機(jī)、打印機(jī)、卡片鑿孔機(jī)、磁帶機(jī)、計(jì)算機(jī)或視頻顯示終端。一般地說(shuō)，變換器的調(diào)制部分和解調(diào)部分合并成為一個(gè)雙向數(shù)據(jù)發(fā)送接收機(jī)，通常稱之為數(shù)據(jù)調(diào)制解調(diào)器或數(shù)據(jù)傳輸機(jī)。典型的全雙工數(shù)據(jù)傳輸系統(tǒng)，包括始發(fā)端數(shù)據(jù)處理設(shè)備和由緩沖器和控制單元組成的接口部件。發(fā)端的接口部件以數(shù)據(jù)處理機(jī)的處理速度所確定的速率接收數(shù)據(jù)，將它們暫時(shí)存儲(chǔ)起來(lái)，并以與數(shù)據(jù)調(diào)制解調(diào)器兼容的速率予以轉(zhuǎn)發(fā)。在接收端，接口部件接受所收到的數(shù)據(jù)，將它們存儲(chǔ)起來(lái)，再以適當(dāng)?shù)乃俾仕偷綌?shù)據(jù)處理機(jī)中去。來(lái)自發(fā)端接口部件的定時(shí)信號(hào)被加到數(shù)據(jù)調(diào)制解調(diào)器上，以使計(jì)算機(jī)與數(shù)傳機(jī)同步。在接收端，從數(shù)據(jù)流中取出同步脈沖使計(jì)算機(jī)同步。當(dāng)有一臺(tái)以上數(shù)傳機(jī)接至一臺(tái)計(jì)算機(jī)時(shí)，接口設(shè)備的容量是主要問(wèn)題，因?yàn)樗仨毚_定分配給每條線路的時(shí)隙。有各種類型的接口部件可以使用，如磁芯存儲(chǔ)器、移位寄存器和時(shí)延線。然而并不是所有的數(shù)據(jù)通信終端在數(shù)據(jù)處理機(jī)和數(shù)據(jù)調(diào)制解調(diào)器之間都使用接口。如果沒(méi)有接口，那么輸入、數(shù)據(jù)傳輸和輸出這三個(gè)操作過(guò)程同時(shí)進(jìn)行，而且速率相同。由于數(shù)據(jù)信號(hào)的形式一般不適宜在各種傳輸設(shè)備上傳送，通常對(duì)信號(hào)要進(jìn)行編碼。在理想情況下，傳輸媒介應(yīng)當(dāng)具有線性衰減和線性時(shí)延的特性。但實(shí)際情況根本不是這樣，傳輸損傷總是存在，干擾了數(shù)據(jù)信號(hào)。相比之下，語(yǔ)聲通信可以容忍極不規(guī)則的傳輸情況。如果電話電路的衰耗嚴(yán)重或噪聲大，說(shuō)話人就會(huì)提高嗓音，自動(dòng)予以彌補(bǔ)。如果講的某些單詞因傳輸困難而沒(méi)聽(tīng)見(jiàn)，雙方往往仍可聽(tīng)懂，因?yàn)檎Z(yǔ)言有冗余度。數(shù)據(jù)信號(hào)則與之相反，除非有意加入，它本身沒(méi)有冗余度，所以傳輸質(zhì)量的不穩(wěn)定只能得到非常有限的補(bǔ)償。另外，數(shù)據(jù)信號(hào)對(duì)基本上不影響話音的其他傳輸質(zhì)量下降很敏感。為了減少不正常的傳輸情況，增加系統(tǒng)的信息容量，實(shí)現(xiàn)差錯(cuò)檢測(cè)和消息保密，就要采用編碼手段。數(shù)據(jù)發(fā)送端的編碼僅僅是將所輸入的數(shù)據(jù)信號(hào)重新排列成其他形式。在接收端則進(jìn)行相反的過(guò)程（譯碼），恢復(fù)原來(lái)的數(shù)據(jù)信號(hào)。所給的波形表示以數(shù)字形式輸入到數(shù)據(jù)調(diào)制解調(diào)器的兩類信息信號(hào)。波形A是二進(jìn)制不歸零（NRZ）信號(hào)，波形B是同一信號(hào)的歸零（RZ）形式。波形A與波形B的區(qū)別是：波形A中傳號(hào)或空號(hào)連續(xù)不新地出現(xiàn)，而波形B中脈沖幅度必須在兩個(gè)連續(xù)信號(hào)之間回到空號(hào)電平上來(lái)。傳號(hào)和空號(hào)的電壓值是任意的，可以是正值或負(fù)值，也可以是正負(fù)值兼而有之。當(dāng)研究數(shù)據(jù)從一個(gè)設(shè)備向另一個(gè)設(shè)備傳輸時(shí),我們關(guān)心的主要問(wèn)題之一是連線。而考慮連線時(shí)，數(shù)據(jù)流又是我們所關(guān)心的問(wèn)題。我們是一次發(fā)送一個(gè)比特呢,或者是成組發(fā)送它們呢？如果要成組發(fā)送，又如何做到這一點(diǎn)呢？通過(guò)鏈路來(lái)發(fā)送二進(jìn)制數(shù)據(jù)的方法可以這樣實(shí)現(xiàn)：要么采用并行方式，要么使用串行的模式。在并行模式中，在每一個(gè)時(shí)鐘脈沖到來(lái)時(shí),可同時(shí)發(fā)送多個(gè)比特。而在串行方式里，伴隨每個(gè)時(shí)鐘只發(fā)送一個(gè)比特。雖然只有一種并行發(fā)送數(shù)據(jù)的方法,但串行傳輸卻有兩類：同步傳輸和異步傳輸。異步傳輸被如此稱呼，是因?yàn)樾盘?hào)的定時(shí)并不重要。不同的是，信息是按事先約定的方式來(lái)接收和翻譯的。只要遵照這些約定，接收器件就能夠恢復(fù)信息，而不理會(huì)它們?cè)诎l(fā)送時(shí)的節(jié)拍。約定的基礎(chǔ)是將比特流組合成字節(jié)。每一個(gè)組合通常含有8個(gè)比特，它被作為一個(gè)單元在鏈路上發(fā)送。發(fā)送系統(tǒng)單獨(dú)處理每個(gè)組合，當(dāng)將組合準(zhǔn)備停當(dāng)就將它放到鏈路上，且與定時(shí)器沒(méi)有關(guān)系。沒(méi)有了同步脈沖，接收機(jī)就不能利用定時(shí)信號(hào)去預(yù)測(cè)下一個(gè)組合什么時(shí)候到達(dá)。因此，為了通知接收機(jī)有新的組合到達(dá)，就得在每個(gè)字節(jié)的開(kāi)始加上一個(gè)額外的比特。這個(gè)比特通常為0，并被稱為起始位。為了讓接收機(jī)知道字節(jié)的結(jié)束，在字節(jié)的尾部又另加了一個(gè)或多個(gè)比特。這些比特通常為1，被人們稱為停止住。運(yùn)用這種方法，每一個(gè)字節(jié)的長(zhǎng)度至少增加到10個(gè)比特，其中有8個(gè)比特的信息，以及2個(gè)或更多的比特，作為向接收機(jī)打"招呼"的信號(hào)。此外，在每個(gè)字節(jié)傳送之后，可能會(huì)有一段變化的間隙。這個(gè)時(shí)隙可用空閑信道或另加停止位來(lái)表示。起始位、停止位和時(shí)隙告訴接收機(jī)每一個(gè)字節(jié)的開(kāi)始和結(jié)束，并讓接收機(jī)按照數(shù)據(jù)流進(jìn)行同步。這種機(jī)制被稱為是異步的，因?yàn)樵谧止?jié)級(jí)上，發(fā)送器和接收器不需要同步。但是在每個(gè)字節(jié)內(nèi)部，接收器仍需與流入的比特流同步。這就是說(shuō)，某種同步還是需要的，但僅限于在一個(gè)字節(jié)持續(xù)的期間內(nèi)。在每一個(gè)新的字節(jié)開(kāi)始時(shí)，接收機(jī)又重新進(jìn)行同步。當(dāng)接收機(jī)檢測(cè)到一個(gè)起始位，它就將定時(shí)器置位，并在比特流入時(shí)開(kāi)始記數(shù)。在接收了n比特之后，接收器就尋找停止住。一旦它檢測(cè)到停止住，它就忽略以后收到的脈沖，直到檢測(cè)到下一個(gè)起始位為止。比起不添加控制信息就能運(yùn)行的傳輸形式，異步傳輸由于增加了停止住、起始位和在比特流中插入時(shí)隙而顯得慢一些。但由于具有便宜和高效兩大優(yōu)點(diǎn)，這使得它在如低速通信的一些場(chǎng)合成為一項(xiàng)誘人的選擇。例如，終端與計(jì)算機(jī)的連接就是異步傳輸方式很自然的應(yīng)用。用戶每次只能敲一個(gè)字符，這在數(shù)據(jù)處理領(lǐng)域里是極慢的，而且在每個(gè)字符闊的間隙長(zhǎng)短也難以預(yù)測(cè)。在同步傳輸中，比特流合并成較長(zhǎng)的“幀”，而幀可能含有多個(gè)字節(jié)。然而當(dāng)字節(jié)被引入到傳輸鏈路的時(shí)候，在字節(jié)之間卻沒(méi)有間隙。將比特流分成字節(jié)的任務(wù)是由接收器在解碼過(guò)程中完成的。換句話說(shuō)，數(shù)據(jù)是以1和0組成的無(wú)間斷碼串來(lái)傳輸?shù)?，而接受器將這個(gè)碼串分離成字節(jié)或字符，它需要將信息重新恢復(fù)。對(duì)同步傳輸做了簡(jiǎn)要的說(shuō)明。在字節(jié)之間我們畫(huà)上了分隔線。事實(shí)上，這些分隔線并不存在，而發(fā)送器是將一長(zhǎng)串的數(shù)據(jù)放到線路上的。如果發(fā)送器想以分離突發(fā)的形式發(fā)送數(shù)據(jù)，則突發(fā)數(shù)據(jù)群之間的間隙就必須用一種由0和