CAN网络如何工作外文翻译.docx

CAN网络如何工作外文翻译.docx

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CAN网络如何工作外文翻译

HowtheCANnetworkfunctions

Principlesofdataexchange.

WhendataaretransmittedbyCAN,nostationsareaddressed,butinstead,thecontentofthemessage（e.g.rpmorenginetemperature）isdesignatedbyanidentifierthatisuniquethroughoutthenetwork.Theidentifierdefinesnotonlythecontentbutalsothepriorityofthemessage.Thisisimportantforbusallocationwhenseveralstationsarecompetingforbusaccess.

IftheCPUofagivenstationwishestosendamessagetooneormorestations,itpassesthedatatobetransmittedandtheiridentifierstotheassignedCANchip（”Makeready”）.ThisisalltheCPUhastodotoinitiatedataexchange.ThemessageisconstructedandtransmittedbytheCANchip.AssoonastheCANchipreceivesthebusallocation（”SendMessage”）allotherstationsontheCANnetworkbecomereceiversofthismessage（”ReceiveMessage”）.EachstationintheCANnetwork,havingreceivedthemessagecorrectly,performsanacceptancetesttodeterminewhetherthedatareceivedarerelevantforthatstation（”Select”）.Ifthedataareofsignificanceforthestationconcernedtheyareprocessed（”Accept”）,otherwisetheyareignored.

Ahighdegreeofsystemandconfigurationflexibilityisachievedasaresultofthecontent-orientedaddressingscheme.ItisveryeasytoaddstationstotheexistingCANnetworkwithoutmakinganyhardwareorsoftwaremodificationstotheexistingstations,providedthatthenewstationsarepurelyreceivers.Becausethedatatransmissionprotocoldoesnotrequirephysicaldestinationaddressesfortheindividualcomponents,itsupportstheconceptofmodularelectronicsandalsopermitsmultiplereception（broadcast,multicast）andthesynchronizationofdistributedprocesses:

measurementsneededasinformationbyseveralcontrollerscanbetransmittedviathenetwork,insuchawaythatitisunnecessaryforeachcontrollertohaveitsownsensor.

BroadcasttransmissionandacceptancefilteringbyCANnodes

Non-destructivebitwisearbitration.

Forthedatatobeprocessedinrealtimetheymustbetransmittedrapidly.Thisnotonlyrequiresaphysicaldatatransferpathwithupto1Mbit/sbutalsocallsforrapidbusallocationwhenseveralstationswishtosendmessagessimultaneously.Inreal-timeprocessingtheurgencyofmessagestobeexchangedoverthenetworkcandiffergreatly:

arapidlychangingdimension（e.g.engineload）hastobetransmittedmorefrequentlyandthereforewithlessdelaysthanotherdimensions（e.g.enginetemperature）whichchangerelativelyslowly.Thepriorityatwhichamessageistransmittedcomparedwithanotherlessurgentmessageisspecifiedbytheidentifierofthemessageconcerned.Theprioritiesarelaiddownduringsystemdesignintheformofcorrespondingbinaryvaluesandcannotbechangeddynamically.Theidentifierwiththelowestbinarynumberhasthehighestpriority.

Busaccessconflictsareresolvedbybitwisearbitrationontheidentifiersinvolvedbyeachstationobservingthebuslevelbitforbit.Inaccordancewiththe”wiredand”mechanism,bywhichthedominantstate（logical0）overwritestherecessivestate（logical1）,thecompetitionforbusallocationislostbyallthosestationswithrecessivetransmissionanddominantobservation.All”losers”automaticallybecomereceiversofthemessagewiththehighestpriorityanddonotreattempttransmissionuntilthebusisavailableagain.

Efficiencyofbusallocation.

Theefficiencyofthebusallocationsystemisdeterminedmainlybythepossibleapplicationforaserialbussystem.Inordertojudgeassimplyaspossiblywhichbussystemsaresuitableforwhichapplicationstheliteratureincludesamethodofclassifyingbusallocationprocedures.Generallywedistinguishbetweenthefollowingclasses:

●Allocationonafixedtimeschedule.

Allocationismadesequentiallytoeachparticipantforamaximumdurationregardlessofwhetherthisparticipantneedsthebusatthismomentornot（examples:

tokenslotortokenpassing）.

●Busallocationonthebasisofneed.

Thebusisallocatedtooneparticipantonthebasisoftransmissionrequestsoutstanding,i.e.theallocationsystemonlyconsidersparticipantswishingtotransmit（examples:

CSMA,CSMA/CD,flyingmaster,roundrobinorbitwisearbitration）.ForCAN,busallocationisnegotiatedpurelyamongthemessageswaitingtobetransmitted.ThismeansthattheprocedurespecifiedbyCANisclassifiedasallocationonthebasisofneed.

Anothermeansofassessingtheefficiencyofbusarbitrationsystemsisthebusaccessmethod:

●Non-destructivebusaccess.

Withmethodsofthistypethebusisallocatedtooneandonlyonestationeitherimmediatelyorwithinaspecifiedtimefollowingasinglebusaccess（byoneormorestations）.Thisensuresthateachbusaccessbyoneormorestationsleadstoanunambiguousbusallocation（examples:

tokenslot,tokenpassing,roundrobin,bitwisearbitration）

●Destructivebusallocation.

Simultaneousbusaccessbymorethanonestationcausesalltransmissionattemptstobeabortedandthereforethereisnosuccessfulbusallocation.Morethanonebusaccessmaybenecessaryinordertoallocatethebusatall,thenumberofattemptsbeforebusallocationissuccessfulbeingapurelystatisticalquantity（examples:

CSMA/CD,Ethernet）.

InordertoprocessalltransmissionrequestsofaCANnetworkwhilecomplyingwithlatencyconstraintsataslowadatatransferrateaspossible,theCANprotocolmustimplementabusallocationmethodthatguaranteesthatthereisalwaysunambiguousbusallocationevenwhentherearesimultaneousbusaccessesfromdifferentstations.

Themethodofbitwisearbitrationusingtheidentifierofthemessagestobetransmitteduniquelyresolvesanycollisionbetweenanumberofstationswantingtotransmit,anditdoesthisatthelatestwithin13（standardformat）or33（extendedformat）bitperiodsforanybusaccessperiod.Unlikethemessage-wisearbitrationemployedbytheCSMA/CDmethodthisnondestructivemethodofconflictresolutionensuresthatnobuscapacityisusedwithouttransmittingusefulinformation.

EveninsituationswherethebusisoverloadedthelinkageofthebusaccessprioritytothecontentofthemessageprovestobeabeneficialsystemattributecomparedwithexistingCSMA/CDortokenprotocols:

inspiteoftheinsufficientbustransportcapacity,alloutstandingtransmissionrequestsareprocessedinorderoftheirimportancetotheoverallsystem（asdeterminedbythemessagepriority）.

Theavailabletransmissioncapacityisutilizedefficientlyforthetransmissionofusefuldatasince”gaps”inbusallocationarekeptverysmall.Thecollapseofthewholetransmissionsystemduetooverload,ascanoccurwiththeCSMA/CDprotocol,isnotpossiblewithCAN.Thus,CANpermitsimplementationoffast,traffic-dependentbusaccesswhichisnon-destructivebecauseofbitwisearbitrationbasedonthemessagepriorityemployed.

Non-destructivebusaccesscanbefurtherclassifiedinto

●centralizedbusaccesscontroland

●decentralizedbusaccesscontrol

dependingonwhetherthecontrolmechanismsarepresentinthesystemonlyonce（centralized）ormorethanonce（decentralized）.

Acommunicationsystemwithadesignatedstation（interaliaforcentralizedbusaccesscontrol）mustprovideastrategytotakeeffectintheeventofafailureofthemasterstation.Thisconcepthasthedisadvantagethatthestrategyforfailuremanagementisdifficultandcostlytoimplementandalsothatthetakeoverofthecentralstationbyaredundantstationcanbeverytime-consuming.

Forthesereasonsandtocircumventtheproblemofthereliabilityofthemasterstation（andthusofthewholecommunicationsystem）,theCANprotocolimplementsdecentralizedbuscontrol.Allmajorcommunicationmechanisms,includingbusaccesscontrol,areimplementedseveraltimesinthesystem,becausethisistheonlywaytofulfilthehighrequirementsfortheavailabilityofthecommunicationsystem.

InsummaryitcanbesaidthatCANimplementsatraffic-dependentbusallocationsystemthatpermits,bymeansofanon-destructivebusaccesswithdecentralizedbusaccesscontrol,ahighusefuldatarateatthelowestpossiblebusdatarateintermsofthebusbusyrateforallstations.Theefficiencyofthebusarbitrationprocedureisincreasedbythefactthatthebusisutilizedonlybythosestationswithpendingtransmissionrequests.

Theserequestsarehandledintheorderoftheimportanceofthemessagesforthesystemasawhole.Thisprovesespeciallyadvantageousinoverloadsituations.Sincebusaccessisprioritizedonthebasisofthemessages,itispossibletoguaranteelowindividuallatencytimesinreal-timesystems.

Messageframeformats.

TheCANprotocolsupportstwomessageframeformats,theonlyessentialdifferencebeinginthelengthoftheidentifier（ID）.InthestandardformatthelengthoftheIDis11bitsandintheextendedformatthelengthis29bits.Themessageframefortransmittingmessagesonthebuscomprisessevenmainfields.

Amessageinthestandardformatbeginswiththestartbit”startofframe”,thisisfollowedbythe”arbitrationfield”,whichcontainstheidentifierandthe”RTR”（remotetransmissionrequest）bit,whichindicateswhetheritisadataframeorarequestframewithoutanydatabytes（remoteframe）.

The”controlfield”containstheIDE（identifierextension）bit,whichindicateseitherstandardformatorextendedformat,abitreservedforfutureextensionsand-inthelast4bits-acountofthedatabytesinthedatafield.

The”datafield”rangesfrom0to8bytesinlengthandisfollowedbythe”CRCfield”,whichisusedasaframesecuritycheckfordet