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1、TheCANProtocolTheCANmessagesTheCANbusisabroadcasttypeofbus.ThismeansthatallnodescanHhearnalltransmissions.Thereisnowaytosendamessagetojustaspecificnode;allnodeswillinvariablypickupalltraffic.TheCANhardware,however,provideslocalfilteringsothateachnodemayreactonlyontheinterestingme
2、ssages.CANusesshortmessages-themaximumutilityloadis94bits.Thereisnoexplicitaddressinthemessages;instead,themessagescanbesaidtobecontents-acldressed.thatis,theircontentsimplicitlydeterminestheiraddress.MessageTypesTherearefourdifferentmessagetypes(ornframesn)onaCANbus:•theDataFram
3、e,•theRemoteFrame,•theErrorFrame,and•theOverloadFrame.TheDataFrameTheDataFrameisthemostcommonmessagetype・Itcomprisesthefollowingmajorparts(afewdetailsareomittedforthesakeofbrevity):•theArbitrationField,whichdeterminesthepriorityofthemessagewhentwoormorenodesarecontendingforthebus
4、.TheArbitrationFieldcontains:oForCAN2.0A,an11-bitIdentifierandonebit,theRTRbit,whichisdominantfordataframes.oForCAN2.0B,a29-bitIdentifier(whichalsocontainstworecessivebits:SRRandIDE)andtheRTRbit.•theDataField,whichcontainszerotoeightbytesofdata.•theCRCField,whichcontainsa15-bitch
5、ecksumcalculatedonmostpartsofthemessage.Thischecksumisusedforerrordetection.•anAcknowledgementSlot;anyCANcontrollerthathasbeenabletocorrectlyreceivethemessagesendsanAcknowledgementbitattheendofeachmessage.ThetransmitterchecksforthepresenceoftheAcknowledgebitandretransmitsthemessa
6、geifnoacknowledgewasdetected.Identifier[ArbitrationFieldControlFieldDataFieldFrametStratofSlotCRCField
7、EndofMl/CRCACKDelimiterDelimiteACAN2.0A("standardCAN”)DataFrame.ArbitrationField~IdentifierIdentifierJStratofSRRIDERTRFrameControlFieldDataFieldACKSlotCRCFieldIEndofI山ICRCACKDe
8、limiterDelimiteACAN2.0B(ExtendedCAN”)DataFrame.TheRemoteFrameTheRemoteFrameisjustliketheDataFrame,withtwoimportantdifferences:•itisexplicitlymarkedasaRemoteFrame(theRTRbitintheArbitrationFieldisrecessive),and•thereisnoDataField.TheintendedpurposeoftheRemoteFrameistosolicitthetran
9、smissionofthecorrespondingDataFrame.If,s
