(完整版)基于51單片機的無線數(shù)據(jù)收發(fā)系統(tǒng)設計(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)信號在各種各樣的話路上傳輸：它們通過導線從一根電桿傳到另一根電桿；它們經(jīng)過地下電纜傳送；它們通過微波設備從一個山頭傳到另一個山頭；它們通過海底電纜，通過通信衛(wèi)星，從一個洲傳到另一個洲。為了把數(shù)字化機器信號變換為適合在這些設備中傳輸?shù)男盘栃问?需要使用某種類型的數(shù)據(jù)變換設備。向變換設備發(fā)送部分（即調(diào)制器）提供輸入的數(shù)據(jù)設備可以是鍵盤、打印機、卡片閱讀器、紙帶終端計算機或磁帶終端機。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、打印機、卡片鑿孔機、磁帶機、計算機或視頻顯示終端。一般地說，變換器的調(diào)制部分和解調(diào)部分合并成為一個雙向數(shù)據(jù)發(fā)送接收機，通常稱之為數(shù)據(jù)調(diào)制解調(diào)器或數(shù)據(jù)傳輸機。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)處理機的處理速度所確定的速率接收數(shù)據(jù)，將它們暫時存儲起來，并以與數(shù)據(jù)調(diào)制解調(diào)器兼容的速率予以轉發(fā)。在接收端，接口部件接受所收到的數(shù)據(jù)，將它們存儲起來，再以適當?shù)乃俾仕偷綌?shù)據(jù)處理機中去。來自發(fā)端接口部件的定時信號被加到數(shù)據(jù)調(diào)制解調(diào)器上，以使計算機與數(shù)傳機同步。在接收端，從數(shù)據(jù)流中取出同步脈沖使計算機同步。當有一臺以上數(shù)傳機接至一臺計算機時，接口設備的容量是主要問題，因為它必須確定分配給每條線路的時隙。有各種類型的接口部件可以使用，如磁芯存儲器、移位寄存器和時延線。然而并不是所有的數(shù)據(jù)通信終端在數(shù)據(jù)處理機和數(shù)據(jù)調(diào)制解調(diào)器之間都使用接口。如果沒有接口，那么輸入、數(shù)據(jù)傳輸和輸出這三個操作過程同時進行，而且速率相同。由于數(shù)據(jù)信號的形式一般不適宜在各種傳輸設備上傳送，通常對信號要進行編碼。在理想情況下，傳輸媒介應當具有線性衰減和線性時延的特性。但實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。相比之下，語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重或噪聲大，說話人就會提高嗓音，自動予以彌補。如果講的某些單詞因傳輸困難而沒聽見，雙方往往仍可聽懂，因為語言有冗余度。數(shù)據(jù)信號則與之相反，除非有意加入，它本身沒有冗余度，所以傳輸質(zhì)量的不穩(wěn)定只能得到非常有限的補償。另外，數(shù)據(jù)信號對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，實現(xiàn)差錯檢測和消息保密，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)信號重新排列成其他形式。在接收端則進行相反的過程（譯碼），恢復原來的數(shù)據(jù)信號。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。波形A是二進制不歸零（NRZ）信號，波形B是同一信號的歸零（RZ）形式。波形A與波形B的區(qū)別是：波形A中傳號或空號連續(xù)不新地出現(xiàn)，而波形B中脈沖幅度必須在兩個連續(xù)信號之間回到空號電平上來。傳號和空號的電壓值是任意的，可以是正值或負值，也可以是正負值兼而有之。當研究數(shù)據(jù)從一個設備向另一個設備傳輸時,我們關心的主要問題之一是連線。而考慮連線時，數(shù)據(jù)流又是我們所關心的問題。我們是一次發(fā)送一個比特呢,或者是成組發(fā)送它們呢？如果要成組發(fā)送，又如何做到這一點呢？通過鏈路來發(fā)送二進制數(shù)據(jù)的方法可以這樣實現(xiàn)：要么采用并行方式，要么使用串行的模式。在并行模式中，在每一個時鐘脈沖到來時,可同時發(fā)送多個比特。而在串行方式里，伴隨每個時鐘只發(fā)送一個比特。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，是因為信號的定時并不重要。不同的是，信息是按事先約定的方式來接收和翻譯的。只要遵照這些約定，接收器件就能夠恢復信息，而不理會它們在發(fā)送時的節(jié)拍。約定的基礎是將比特流組合成字節(jié)。每一個組合通常含有8個比特，它被作為一個單元在鏈路上發(fā)送。發(fā)送系統(tǒng)單獨處理每個組合，當將組合準備停當就將它放到鏈路上，且與定時器沒有關系。沒有了同步脈沖，接收機就不能利用定時信號去預測下一個組合什么時候到達。因此，為了通知接收機有新的組合到達，就得在每個字節(jié)的開始加上一個額外的比特。這個比特通常為0，并被稱為起始位。為了讓接收機知道字節(jié)的結束，在字節(jié)的尾部又另加了一個或多個比特。這些比特通常為1，被人們稱為停止住。運用這種方法，每一個字節(jié)的長度至少增加到10個比特，其中有8個比特的信息，以及2個或更多的比特，作為向接收機打"招呼"的信號。此外，在每個字節(jié)傳送之后，可能會有一段變化的間隙。這個時隙可用空閑信道或另加停止位來表示。起始位、停止位和時隙告訴接收機每一個字節(jié)的開始和結束，并讓接收機按照數(shù)據(jù)流進行同步。這種機制被稱為是異步的，因為在字節(jié)級上，發(fā)送器和接收器不需要同步。但是在每個字節(jié)內(nèi)部，接收器仍需與流入的比特流同步。這就是說，某種同步還是需要的，但僅限于在一個字節(jié)持續(xù)的期間內(nèi)。在每一個新的字節(jié)開始時，接收機又重新進行同步。當接收機檢測到一個起始位，它就將定時器置位，并在比特流入時開始記數(shù)。在接收了n比特之后，接收器就尋找停止住。一旦它檢測到停止住，它就忽略以后收到的脈沖，直到檢測到下一個起始位為止。比起不添加控制信息就能運行的傳輸形式，異步傳輸由于增加了停止住、起始位和在比特流中插入時隙而顯得慢一些。但由于具有便宜和高效兩大優(yōu)點，這使得它在如低速通信的一些場合成為一項誘人的選擇。例如，終端與計算機的連接就是異步傳輸方式很自然的應用。用戶每次只能敲一個字符，這在數(shù)據(jù)處理領域里是極慢的，而且在每個字符闊的間隙長短也難以預測。在同步傳輸中，比特流合并成較長的“幀”，而幀可能含有多個字節(jié)。然而當字節(jié)被引入到傳輸鏈路的時候，在字節(jié)之間卻沒有間隙。將比特流分成字節(jié)的任務是由接收器在解碼過程中完成的。換句話說，數(shù)據(jù)是以1和0組成的無間斷碼串來傳輸?shù)?，而接受器將這個碼串分離成字節(jié)或字符，它需要將信息重新恢復。對同步傳輸做了簡要的說明。在字節(jié)之間我們畫上了分隔線。事實上，這些分隔線并不存在，而發(fā)送器是將一長串的數(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