SerialcommunicationinterfaceRS-232andRS-485isthenormallyusedinterfacesatanytime,itnotonlybuiltineveryPC-set,butalsobuiltinanytypesofcomputerswhicharefrommicrocontrollertohostcomputers,andequipmentconnectedwiththem.ThegeneralusageofRS-232istoconnectamodern,theotherarrangementswithRS-232interfacesincludeprinter,datacollectionmodule,testingdeviceandcontrolloop.RS-232canbealsousedinthesimplelinkofanytypeofcomputer.But,RS-485canbelinkwithseveraldevicesbymeansofalonglead,ithavesamefunction.RS-232isdesignedtodealwiththecommunicationbetweentwoarrangementswithdistancelimitationfrom50to100ft,whichdependonPotterrateandtypeofelectriccable.BecauseofwideapplicationofRS-232port,itsotherusageistolinkanothertypeofadapterconvertedformtheport.Forexample,asimpleloopchangesaRS-232intoaRS-485port.Theportcanbelinkwithseveraldevicesbymeansofalonglead.Non-equilibriumleadisusedtolinkedtoRS-232.Althoughnon-equilibriumshouldsomethingavoidable,itonlyreferstotheelectricpropertyofthesignalinlead.Inanon-equilibriumlead,signalvoltageisappliedtoanotherlead,allhasanothertechnicalterm“signalend”.AdvantagesinRS-232RS-232isusedeverywhere.EveryPC-sethasoneormoreRS-232ports.TherenewedcomputerssustainotherserialinterfaceslikeUSB.ButRS-232candomanythingswhichUSBcantdo.Inthemicro-controllerinterfacechipcanbeeasilyusedtochangeintoRS-232.Thejointingdistancecanreach50-100ft.Manyexternalinterfacesarenotusedfortoolongdistance.ThelongestdistancejointofUSBcanreach16ft.ThedistancebetweenparallelprinterinterfaceonthePC-setandthehostcomputercanreach10to15ft,or30ftbyusingTypeTEEE-1284Bdriver.ButRS-232canusethelongerelectriccable.IfeveryRS-232portislinkedwithamodern,Telephonenetworkcanusedtosenddataintherageoftheworld.Foradoublelink,3leadsareneeded.Aparallelcircuitgenerallyneeds8datalines,twoormorecontrolsignallinesandgroundwires。Itismoreexpensiveifmoreleadsandbiggerconnectorsareused.DisadvantagesinRS-232Ifonesideoflinkneedsparalleldata,ithastotransformserialintoparalleldata.ButifitiseasilyrealizedaUARTisused.Becauseoftheserialportsouseful,itismaybehardertolookforafreeserialportPC-setmayhavemanyserialports,butasystemmaybecantdistributeasolebreakingenquiringsignaltoeveryserialport.Mostmicro-controlleronlyhasonehardwareserialport.Nomorethan2setinonelink.Thebiggestdesignatedtransmittingrateofdatais2000bits/s.Butmanyinterfacechipcansurpassthisnumericalvalue,especiallyintheshortdistancelink.Alongerlinkneedsadifferentinterface.3basicsignalsofthedoubleRS-232communicationasfollows:TD.TransformdataDTEtoDCEItsalsocalledTXandTXD.RD.TransformdataDCEtoDTEItsalsocalledRXandRXD.SG.Signalland.ItsalsocalledGNDandSGND.VoltageLogic0isapositiveRS-232voltage,thelogicelectriclevelofRS-232isindicatedwithpositive-negative,andnotonlyusethepositivevoltagesignalofthe5VTTLandCMOSlogictoindicate.Inthedatalogger(TD)ofRS-232，alogic0isdefinedequaltoorhigherthan+5V,andaLogic1definedequalto-5Vorlowerthan-5V.Inanotherword,signalusethenegativelogic,inthislogic,thenegativevoltageisLogic1.Controlsignalusesthesamevoltage,butitusespositivelogic.Apositivevoltageindicatesthisfunctionisonandanegativevoltageindicatesthisfunctionisoff.RS-232interfacechiptransformthesesignalsinreverse.InoutputpinofoneUART,Alogic1datumoraoffcontrolsignaliscloseto5V,itproducesnegativevoltageontheinterfaceofRS-232.Alogic0datumoranon-controlsignaliscloseto0V,itproducesapositivevoltageontheinterfaceofRS-232.BecauseoftheRS-232receptormaybeattheendofalongelectricalcable,whenthesignalreachesthereceiver,itsvoltageisprobablyweakerorwiththenoisesignal.Inconsiderationofthiscircumstanceminimumatthereceivershouldbelowerthanthevoltageatthedriver.Aninputhigherthan+3VontheRDareaLogic0,oronacontrolinputisopen.Aninputlowerthan-3VontheRDareaLogic1,oronacontrolinputisoff.Accordingtothisstandard,inputlogicelectricallevelbetween-3Vand+3VhavenodefinitionNoisetolerablelimitationorvoltagetolerablelimitationisdifferenceoutputvoltageandinputvoltage.ThebigvoltageundulationfromRS-232leadtoanoisetolerablelimitationwiderthanlogic5TTL.Forexample,eventhoughtaoutputfromRS-232driveristhesmallest+5V,itsalsobeweakeninthereceiverorthenoisepeakriseto2V,andstillisaeffectiveLogic0.ManyoutputsfromRS-232havethewidervoltageundulation:9Vand12Vareverycommonaffairs.Alltheseaffairscanleadtoanoiselimitation.Thebiggestpermittingvoltageundulationis15V,eventhoughthereceivemusthavenoharmtodealwiththevoltagehigherto25V.TheothertwotechnicaltermsconcernwithRS-232tobeusedisMarkandSpace.SpaceisLogic0andMarkisLogic1.Thesetwotechnicaltermswhichwasusedbythemachinerecordinginstrumenttorecordbinarydatainmanyyearsago,eachofthemarerepresentationphysicalandblank.ClockingLimitation:TIA/EIA-232includingsmallestandbiggestclockingstipulate.AllouttheRS-232interfacechipsallaccordthisstipulate.AppointedrevolvingspeedlimitedthebiggestPotterrateofinterface.Therevolvingrateismeasuretotherateofspeedchangewhenoutputcut-overvoltage,anddescribethechangeofinstantaneousvelocityinTheadvantageinlimitedtherevolvingisforimprovesignalqualitybyinthematteroffacttolimitedtheproblemwhichisduetovoltagedeflection,voltagedeflectionoccurthesignalinthelongdistancelinkwhichhashigh-speedriseanddescend.Theremoredetailednarrationstothisproblemareincapter10.Butrevolvingvelocityalsolimitedtothebiggestspeedofalink.Underspeedof30V/s,aoutputneed0.3stocut-overtimein166times.Actually,becausetheUARTtoreadinputdatainnearthemiddleofthedigit,andalsobecausemanytimesconsultclockinveryaccurate,usuallycanmayusethedigitwidthforcut-overtimeasshort5to10timesafety.Consideredtheseconditions,someinterfacechipspermitting115kbpsormorehigherPotterrate,eventhoughitsinfringethesuggestionthisstandard.Exceptforhasabiggestcut-overvelocity,RS-232driveralsomustaccordingtheloweststandardtoensurethesignalnostopoverintheundefinedareabetweenthelogicalcondition.Tocontrolsignalother40bpsandthelowersignal.Signallinesmustspendtimenotsurpass1sinthetransmitareabetweentheeffectiveLogic1andLogic0.Tootherdataandtimesignalthislimitationplacewideis4%orunder20bpsis2s.Frequenciesofsignalriseanddescendalsoprobablyclosetobeequal.Transformationbetween5VlogicalandRS-232Manymicro-controllershaveasynchronousserialports,buttheirinputandoutputrathertouse5VlogicthantoRS-232voltage.Tolink5VlogictoaRS-232portneedtotransformintoRS-232electricallevelandtransformfromRS-232electricallevelinto5Vlogic.Touseof5Vlogic,IsupposedtotheuseofelectricallevelofTTLorCMOSlogicchips,thesechipsaresupplyelectricitybyasingle+5Velectrical,signalelectricallevelrefertogroundingelectricallevel.Tables6-2indicatethis.Ialsosupposedtopositivelogic,inhere,thelogicalhighelectriclevelislogic1.ToTTLlogic,alogicallowoutputmustbelow0.4V,alogicallowinputmustbelow0.8V.Alogicalhighoutputmustbe2.4Vatleastandalogicalhighinputmustbe2Vatleast.Touseoftheselogicalelectricallevels,aninterfacemayhavenoisesignalat0.4Vandnotnecessarilytomakemistake.TheselogicalelectricallevelsareusedintoTTLlogicofearlydaysstandard7400seriesandotherdriversseries,including74LS74Fand74ALSTTL.TheNMOStechnologyearlyisusedtomakemicro-controllersalsousethesehelicalelectricallevels.MostCMOSchipsdefinitedifferentlogicalelectricallevelsandhavewidernoisetolerable.AlogicallowCMOSoutputbelow0.1Vandlogicallowinputmayhigherto20%electricalvoltage,ortoelectricalvoltagein5Vcanreachto1V.Alogicalhighoutputtobe4.9Vatleastandlogicalhighinputmustreachtoelectronicallyvoltages70%atleast,orto5Velectricalinputreachto3.5V.Touseofthisseriesoflogicalelectricallevel,including4000series,74HCand74AClogic.SomeofCMOSchipshaveTTLcompatibleinputandCMOScompatibleoutput.ItsmakethemcanlinkwithCMOSorTTLlogicdirectly.Thechipsoffollowtothisusualpracticeincluding74HClogicseriesandmostofmicro-controllers。串行通信接口RS-232和RS-485是在任何時(shí)候都常用的接口。它們不僅已經(jīng)被內(nèi)置于每臺(tái)PC，而且已被內(nèi)置于從微控制器到主機(jī)的多種類型的電腦和與它們連接的設(shè)備。RS-232的最通常得用處是連接到一個(gè)Modem，其他擁有RS-232接口的設(shè)備包括打印機(jī)，數(shù)據(jù)采集模塊，測試裝置和控制回路。你也可以將RS-232用在任何類型得計(jì)算機(jī)之間的簡單連接中。RS-485則是用于線路比較長的情況下使用，其作用和RS-232一樣。RS-232是設(shè)計(jì)來處理兩臺(tái)設(shè)備之間的通信的，距離限制為50到100f，這決定于波特率和電纜類型。因?yàn)镽S-232端口被廣泛應(yīng)用，它的另