英文资料与中文翻译.docx

英文资料与中文翻译.docx

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英文资料与中文翻译

英文资料与中文翻译

INTRODUCTION

ControllerAreaNetwork（CAN）wasinitiallycreatedbyGermanautomotivesystemsupplierRobertBoschinthemid-1980sforautomotiveapplicationsasamethodforenablingrobustserialcommunication.Thegoalwastomakeautomobilesmorereliable,safeandfuel-efficientwhiledecreasingwiringharnessweightandcomplexity.Sinceitsinception,theCANprotocolhasgainedwidespreadpopularityinindustrialautomationandautomotive/truckapplications.Othermarketswherenetworkedsolutionscanbringattractivebenefitslikemedicalequipment,testequipmentandmobilemachinesarealsostartingtoutilizethebenefitsofCAN.ThegoalofthisapplicationnoteistoexplainsomeofthebasicsofCANandshowthebenefitsofchoosingCANforembeddedsystemsnetworkedapplications.

CANOVERVIEW

Mostnetworkapplicationsfollowalayeredapproachtosystemimplementation.Thissystematicapproachenablesinteroperabilitybetweenproductfromdifferentmanufacturers.AstandardwascreatedbytheInternationalStandardsOrganization（ISO）asatemplatetofollowforthislayeredapproach.ItiscalledtheISOOpenSystemsInterconnection（OSI）NetworkLayeringReferenceModelandisshowninFigure1forreference.TheCANprotocolitselfimplementsmostofthelowertwolayersofthisreferencemodel.ThecommunicationmediumportionofthemodelwaspurposelyleftoutoftheBoschCANspecificationtoenablesystemdesignerstoadaptandoptimizethecommunicationprotocolonmultiplemediaformaximumflexibility（twistedpair,singlewire,opticallyisolated,RF,IR,etc.）.Withthisflexibility,however,comesthepossibilityofinteroperabilityconcerns.Toeasesomeoftheseconcerns,theInternationalStandardsOrganizationandSocietyofAutomotiveEngineers（SAE）havedefinedsomeprotocolsbasedonCANthatincludetheMediaDependantInterfacedefinitionsuchthatallofthelowertwolayersarespecified.

ISO11898isastandardforhigh-speedapplications,ISO11519isastandardforlow-speedapplications,andJ1939（fromSAE）istargetedfortruckandbusapplications.Allthreeoftheseprotocolsspecifya5Vdifferentialelectricalbusasthephysicalinterface.TherestofthelayersoftheISO/OSIprotocolstackarelefttobeimplementedbythesystemsoftwaredeveloper.HigherLayerProtocols（HLPs）aregenerallyusedtoimplementtheupperfivelayersoftheOSIReferenceModel.

HLPsareusedto:

1）standardizestartupproceduresincludingbitratesused,

2）distributeaddressesamongparticipatingnodesortypesofmessages,

3）determinethestructureofthemessages,and

4）providesystem-levelerrorhandlingroutines.ThisisbynomeansafulllistofthefunctionsHLPsperform,howeveritdoesdescribesomeoftheirbasicfunctionality.

CANPROTOCOLBASICS

CarrierSenseMultipleAccesswithCollision

Detection（CSMA/CD）

TheCANcommunicationprotocolisaCSMA/CDprotocol.TheCSMAstandsforCarrierSenseMultipleAccess.Whatthismeansisthateverynodeonthenetworkmustmonitorthebusforaperiodofnoactivitybeforetryingtosendamessageonthebus（CarrierSense）.Also,oncethisperiodofnoactivityoccurs,everynodeonthebushasanequalopportunitytotransmitamessage（MultipleAccess）.TheCDstandsforCollisionDetection.Iftwonodesonthenetworkstarttransmittingatthesametime,thenodeswilldetectthe‘collision’andtaketheappropriateaction.InCANprotocol,anondestructivebitwisearbitrationmethodisutilized.Thismeansthatmessagesremainintactafterarbitrationiscompletedevenifcollisionsaredetected.Allofthisarbitrationtakesplacewithoutcorruptionordelayofthehigherprioritymessage.

Thereareacoupleofthingsthatarerequiredtosupportnon-destructivebitwisearbitration.First,logicstatesneedtobedefinedasdominantorrecessive.Second,thetransmittingnodemustmonitorthestateofthebustoseeifthelogicstateitistryingtosendactuallyappearsonthebus.CANdefinesalogicbit0asadominantbitandalogicbit1asarecessivebit.

Adominantbitstatewillalwayswinarbitrationoverarecessivebitstate,thereforethelowerthevalueintheMessageIdentifier（thefieldusedinthemessagearbitrationprocess）,thehigherthepriorityofthemessage.Asanexample,supposetwonodesaretryingtotransmitamessageatthesametime.Eachnodewillmonitorthebustomakesurethebitthatitistryingtosendactuallyappearsonthebus.Thelowerprioritymessagewillatsomepointtrytosendarecessivebitandthemonitoredstateonthebuswillbeadominant.Atthatpointthisnodelosesarbitrationandimmediatelystopstransmitting.Thehigherprioritymessagewillcontinueuntilcompletionandthenodethatlostarbitrationwillwaitforthenextperiodofnoactivityonthebusandtrytotransmititsmessageagain.

Message-BasedCommunication

CANprotocolisamessage-basedprotocol,notanaddressbasedprotocol