Introduction
Serialcommunicationtechnologyreferstoacommunicationmethodinwhichbothsidesofthecommunicationarecarriedoutonabit-by-bitbasisandinaccordancewiththetimesequence.Inserialcommunication,thedataistransmittedbitbybit,andeachbitofdataoccupiesafixedlengthoftime.Youcanuseafewcommunicationlinestocompletetheexchangeofinformationbetweensystems.Itisespeciallysuitableforcomputersandcomputers,andbetweencomputersandperipherals.Long-distancecommunication.Serialcommunicationismostlyusedforserialtransmissionofdatabetweensystems(multi-mastercontrolsystem),equipment(maincontrolequipmentandauxiliaryequipment),andbetweendevices(maincontrolCPUandfunctionalchip)torealizedatatransmissionandsharing.
Thesalientfeaturesoftheserialbuscommunicationprocessare:fewercommunicationlines,simpleandeasywiring,convenientconstruction,flexiblestructure,inter-systemnegotiationagreement,highdegreeoffreedomandflexibility,sointhedesignofelectroniccircuitsTherearemoreandmoreapplicationsinmanyaspects,suchasinformationtransmission.
Serialcommunicationreferstotheserialtransmissionofdatabetweenthecomputerhostandperipheralsandbetweenthehostsystemandthehostsystem.Usingadataline,thedataistransmittedonebyone,andeachbitofdataoccupiesafixedlengthoftime.Itonlyneedsafewlinestoexchangeinformationbetweensystems,whichisespeciallysuitableforlong-distancecommunicationbetweencomputersandcomputers,andbetweencomputersandperipherals.
Classification
Synchronouscommunication
Synchronouscommunicationisacommunicationmethodthatcontinuouslytransmitsdataserially.Onlyoneframeofinformationistransmittedatatime.Theinformationframehereisdifferentfromthecharacterframeinasynchronouscommunication,andusuallycontainsseveraldatacharacters.
Theyareallcomposedofsynchronizationcharacters,datacharactersandcheckcharacters(CRC).Thesynchronizationcharacterislocatedatthebeginningoftheframeandisusedtoconfirmthebeginningofthedatacharacter.Afterthesynchronizationcharacter,thenumberofdatacharactersisnotlimitedandisdeterminedbythelengthofthedatablocktobetransmitted;thereare1to2checkcharacters,whichareusedbythereceivingendtoverifythecorrectnessofthereceivedcharactersequence.Thedisadvantageofsynchronouscommunicationisthatthesendingclockandthereceivingclockarerequiredtomaintainstrictsynchronization.
Asynchronouscommunication
Inasynchronouscommunication,therearetwoimportantindicatorsinasynchronouscommunication:characterframeformatandbaudrate.Dataisusuallytransmittedinacharacterframeinunitsofcharactersorbytes.Thecharacterframeissentframebyframebythesender,andreceivedbythereceivingdeviceframebyframethroughthetransmissionline.Thesendingendandthereceivingendcancontrolthesendingandreceivingofdatabytheirrespectiveclocks.Thesetwoclocksourcesareindependentofeachotherandarenotsynchronizedwitheachother.
Whenthereceivingenddetectsthelow-levellogic"0"(thatis,thecharacterframestartbit)sentoverthetransmissionline,itdeterminesthatthesendingendhasbeguntosenddata.WheneverthereceivingendreceivesthecharacterframeWhenthestopbitisdisplayed,itisknownthataframeofcharactershasbeensent.
Features
Dataistransmittedinsequenceonasinglebit-widetransmissionline,onebitafteranother,calledserialcommunication.Inparallelcommunication,onebyte(8bits)ofdataissimultaneouslytransmittedfromthesourcetothedestinationon8paralleltransmissionlines;whileinserialcommunication,dataistransmittedonebyoneonasingle1-bitwidetransmissionline.Sequentialtransferbitbybit.Suchabyteofdatamustbetransmittedonebyonesequentiallyfromlowtohighin8times.Itcanbeseenthatthecharacteristicsofserialcommunicationareasfollows:
1.Itisobvioustosavetransmissionlines.Especiallyinremotecommunication,thisfeatureisparticularlyimportant.Thisisalsothemainadvantageofserialcommunication.
2.Thedatatransmissionefficiencyislow.Comparedwithparallelcommunication,thisisalsoobvious.Thisisalsothemaindisadvantageofserialcommunication.
Forexample:totransmitonebyte,ifthetimerequiredforparallelcommunicationis1T,thetimerequiredforserialcommunicationisatleast8T.Itcanbeseenthatserialcommunicationissuitableforlong-distancetransmission,whichcanrangefromafewmeterstothousandsofkilometers.Forlong-distance,low-speedcommunication,serialcommunicationisoftentheonlyoption.Parallelcommunicationissuitableforshort-distance,high-speeddatatransmission,andthetransmissiondistanceisusuallylessthan30meters.Itisparticularlyworthmentioningthattheexistingpublictelephonenetworkisauniversallong-distancecommunicationmedium.Althoughitisdesignedforthetransmissionofsoundsignals,theuseofmodulationanddemodulationtechnologycanmaketheexistingpublictelephonenetworksystemserialdatacommunication.Provideconvenientandpracticalcommunicationlines.
Serialcommunicationinterferencesource
Serialcommunicationworkplacesaremostlyincomplexenvironmentssuchasstrongcurrent/outdoors,andthedistancebetweencommunicationpartiesisgenerallylong,sotheyaresusceptibletointerference.Inserialcommunication,whenthebaudrateisconstant,thedatabittransmissiontimeisrelativelyshort.Duetothedatabitsampling/acquisitioncharacteristicsofserialcommunication,thebitinformationisdisturbed,andtheentirebytedataistheerrorinformation.
Inreality,factorsthatarelikelytobringserialcommunicationinterferenceinclude:
(1)Environmentalelectromagneticinterferenceisneartheserialcommunicationworkingequipment,andthereisunavoidablestrongcurrentequipment,Powertransmitter,etc.Inthestrongelectromagneticfieldinductionzoneemitted/inducedbythesedevices,thereisstrongenvironmentalelectromagneticinterference.Serialcommunicationequipmentworksinthisenvironment,duetothesuperimpositionofnoise(interference)onthesignallevel,causingdataerrorsonbothsidesofthecommunication.
(2)Systemnoise
Serialcommunicationreliesonserialcommunicationchip.Duetothedesignprocessandproductionlevelofthechip,thenoisecontroloftheoutputlevelisuneven.Thenoisethatproducestheoutputlevelincludesthepowersupplyindigitallogicandthestabilityofthedeviceitself.Incommunication,therippleofthepowersupplywillinevitablybeloadedintothecommunicationline.Whentherippleislarge,itiseasytocauseserialcommunicationerrors.
(3)Bitrateerror
Thetwopartiesofserialcommunicationhaveagreedinadvancewithafixedbaudrateasthepaceofdatatransmission.Theconsistencyofthebaudrateisthebasisforthestabilityandreliabilityofserialcommunicationdata.Sincethebaudrateofthetwocommunicationpartiesisgeneratedlocally,thebaudratewiththeerrorratecausesthecoderateerrorofthecommunicationparties.Thegreaterthebaudrateerror,thegreatertheprobabilityofcommunicationdataerrors.
(4)Groundloopandreferencegroundpotential
Inapplicationswherebothcommunicationpartiessharethesameground,duetotheinconsistencyofthehighandlowlevelsofthereferencegroundsignalbetweensystems,thetransmittedsignalhasagroundvoltageCertainerror.Inthelow-voltagepowersupplyapplicationsystem,theerrorofthereferencegroundpotentialonbothsidesistoolarge,whichwillcausethedataerroroftheserialcommunication.Theaboveinterferencesources,underdifferenthardwareoptimizationmeasuressuchascommunicationlineshielding,lineisolation,andbaudratecalibration,canreduceoreliminatesomeoftheinterference,butthereisstillthepossibilityofdataerrors.Therefore,inadditiontotheprotectionofhardwareanti-interference,itisespeciallynecessarytoaddasoftwareerrordetectionmechanism.
Serialcommunicationisolationmethod
Theactualneedofisolation
Serialcommunicationduetoitsworkingcharacteristics(bitwisetransmissionissusceptibletointerference,long-distanceinformationexchange),Applications(industrialcontrolinharshenvironments,outdoor,etc.),levelmatchingbetweendevices(theworkinglevelsofthedevicesonbothsidesareinconsistent,etc.),correspondingisolationprotectionisrequired.Throughisolation,thefollowingobjectivesareachieved.(1)Deviceprotection,protectiveisolationIntoday'srapiddevelopmentofelectronicdevices,low-power,high-packagechipsarewidelyused.Thelow-voltageworkingconditionsofmicroprocessorsandthehigh-voltageworkingenvironmentofperipheraldeviceshavedifferentdevelopmentprocesses.Currentmicroprocessorchiplevelsaremostlylow-voltagedevicessuchas1.8V,3.3V,and5.0V.WiththecontinuousemergenceofdigitalICswithdifferentoperatingvoltages,thenecessityoflogiclevelconversionhasbecomemoreprominent.Forexample,the3.3VinputandoutputI/OoftheSTM32controllerandthetraditionalserialcommunicationinterfacecoreenvironmenthavedifferentdevelopmentprocesses.Therefore,inordertoachievethelevelmatchingbetweenthecontrollerandthecommunicationinterfacechip,itisespeciallynecessarytoaddisolationdevicestoprotectthepinsofthecontrollerfromdamageduetoexcessivelyhighorlowoperatingvoltages.
(2)Shieldinginterferenceandlineisolation.Becausemanyserialcommunicationequipmentworkintheharshenvironmentoftheindustrialsiteorthelong-distancetransmissionofthepowerdistributionsystem,thereareoftenObviousinterferencesignalsareinduced,causingoccasionalerrorsinthecommunicationprocess,whichinturnaffectsthereliableoperationoftheentiresystem.Thesourcesofinterferingsignalsincludespaceradiation,crosstalk,systemnoise,etc.Forexample,inRS-232Ccommunication,duetoitssingle-endedsignaltransmissionmode,whenthegroundpotentialsbetweenthedifferentgroundwiresofthetwocommunicationpartiesareinconsistent,commonmodeinterferencevoltagewillbeintroduced,causingcommunicationinstability.
Inserialcommunication,shieldingofcommunicationlinescanreducetheinfluenceofradiationinterference,andtheinfluenceofcommonmodeinterferencevoltagecanbereducedthroughdifferentialsignaltransmission,butthelevelconversionandLineisolationdesignedtoreduceinterferenceisstillessential.
Applicationofisolationmethod
(1)Discretedeviceisolationtechnology
Intheneedofisolationdesign,thelevelconversionisolationmethodbetweendevicescanbepurelydiscreteThedeviceiscomplete.Theultimategoaloflevelconversionistorealizethatthelevelsonbothsidesoftheworkingunitaredeterminedaccordingtotheirneeds.Discretedeviceisolationmethodsmainlyusethereasonablematchingofresistorsandtransistorstomatchthelevelbetweeninputandoutput.ItisaconventionalandeffectivemethodtousetheswitchingfunctionoftheMOStubetorealizedouble-sidedlevelconversion.Thiskindofisolationmethodisgenerallycommongroundisolation,whichonlycompleteslevelconversiontoachievethefunctionofprotectingthedevice,andisnotelectricallyisolatedbetweensystems.
(2)Optocouplerisolationtechnology
Opticalcoupler,abbreviatedasoptocoupler,isatypeofdevicethatuseslightasamediumtorealizeelectricalsignaltransmission.Itsworkingprincipleistopackagethelightemitter(lightemittingdevice)andthephotoreceptor(photosensitivedevice)insidethechip,andcontrolthelightemittingbytheelectricalsignalappliedtotheinputend.Thephotoreceptorgeneratesanelectricalsignalanddrivestheoutputwhenthelightisinternallyilluminated.Attheend,the"electricity-optical-electricity"conversionhasbeenrealized.Sincetheelectricalsignalsonbothsidesoftheoptocouplerarecompletelyisolated,andlightisusedasthetransmissionmediuminside,theinput/outputoftheoptocouplerisinsulatedtocompletetheisolatedtransmissionofunidirectionalsignals,whichiswidelyusedindigitalcircuits.Theapplicationofordinaryoptocoupler(TLP521)inisolationcircuitislimitedbythecharacteristicsofthedevice,itstransmissioncharacteristicsarebetteratlowfrequency,andhigh-frequencysignaltransmissionisseverelydistorted.Intheactualcircuittest,the115kbpserialcommunicationfrequencycanbebasicallyadaptedthroughcircuitdeviceparametermatchingandcircuitstructureoptimization.FromToshibaSemiconductor'soptocouplerproductline,itcanbeknownthatitscommunicationratecovers20kbps~50Mbps.Therefore,inhigh-speedcommunicationtransmission,high-speedoptocouplersshouldbeselectedaccordingtodesignneeds.
(3)Newisolationtechnology
Withtheever-changingproducts,newdevicesemergeinendlessly.MainstreamchipvendorsTexasInstruments(TI),ArnoldSemiconductor(ADI)andSiliconLabshavedevelopeddifferenttypesofdigitalisolatorssuchascapacitiveisolation,magneticcouplingisolation,andradiofrequencyisolation.
Capacitiveisolation
Capacitiveisolationusestheinsulatingmaterialbetweencapacitorplatesasanisolationlayer,andcompletessignaltransmissionthroughchangesintheinternalelectricfield.TI'sISO72xseriesareapplicationsoftypicalcapacitiveisolationtechnology.Inthecapacitiveisolationfunction,thesignaltransmissionchannelisdividedinto"lowfrequencychannel"and"highfrequencychannel".Thelow-frequencysignalismodulatedbythehigh-frequencycarriergeneratedbythebuilt-inoscillatorandPWM,andismodulatedandtransmittedinadifferentialmode.Thelow-passfilterattheoutputremovesthehigh-frequencycarrier.Thehigh-frequencysignalisnotmodulatedandcoded,andisdirectlytransmittedthroughtheisolationlayerafterdifferentialconversion.Theoutputterminalmakeslogicaldecisionsthroughthetimerelationship,therebycontrollingtheoutputmultiplexertooutputcorrectly.
Magneticcouplingisolation
Magneticcouplingisolationusestheprincipleofatransformer,andusesamagneticcouplingmethodforsignaltransmissionbetweentheprimarycoilandthesecondarycoilofthetransformertoachievetheisolationeffect.ADI'siCouplerpatentedtechnologyisbasedonthemagneticisolationtechnologyoftheaircoretransformerinthechip.TheADUMseriesisatypicalapplicationofmagneticcouplingisolationtechnology.
iCouplermagneticisolationtechnologyrealizessignalisolationthroughthemagneticcouplingbetweentheprimaryandsecondarycoilsofanair-coretransformerbasedontheinternalfeaturesizeofthechip.Thesignaltransmissionusesaspecificshortpulsecombinationtoindicatehighandlowlevels.Twoconsecutiveshortpulsesrepresentahighlevel,andasingleshortpulserepresentsalowlevel.Theoutputterminaldeterminestheoutputlevelstateaccordingtothenumberofdetectionpulses.Therefreshercircuitandthewatchdogcircuitprovideprotectionforthelevelstateoftheinputterminalandthefail-safestateoftheoutputterminal.
Radiofrequencyisolation
Radiofrequencyisolationusestheprincipleofradiofrequencytransmission.Atthetransmittingend,theoriginalsignalmodulationbasedonthehigh-frequencysignaliscompletedandsentthroughthetransmittingantenna.Atthereceivingend,thedemodulatorcompletesthedemodulationofthemodulatedsignalandrestorestheoriginalsignal.Throughsuchmodulationanddemodulation,theeffectofisolationisachieved.SiliconLabs'RFisolationisradiofrequencyisolation,andSi84xxseriesaretypicalapplicationsofradiofrequencyisolationtechnology.
RFisolationadoptsISOprotypeRFradiofrequencyisolationprinciple.ThechipconsistsofasemiconductorRFradiofrequencytransmitter,areceiverandadifferentialcapacitiveisolationbarrierbetweenthetwo.Duringwork,usethebasicON/OFFbutton(OOKfunction).Whentheinputdataishighvoltage,thetransmittergeneratesanRFradiofrequencymodulationsignal;whentheinputdataislow,thegeneratorhasnoRFradiofrequencymodulationsignal.Themodulatedsignalissenttothereceiverthroughtheisolationbarrier.Whenthereceiverdetectsamodulationsignalinthesamefrequencyband,itisdemodulatedbythedemodulatorandoutputsahighlevel;whenthereisnomodulationsignal,itoutputsalowlevel.