外文翻译-CAN

外文原文IntroductionControllerAreaNetwork(CAN)wasinitiallycreatedbyGermanautomotivesystemsupplierRobertBoschinthemid-1980sforautomotiveapplicationsasamethodforenablingrobustserialcommunication.Thegoalwastomakeautomobilesmorereliable,safeandfuel-efficientwhiledecreasingwiringharnessweightandcomplexity.Sinceitsinception,theCANprotocolhasgainedwidespreadpopularityinindustrialautomationandautomotive/truckapplications.Othermarketswherenetworkedsolutionscanbringattractivebenefitslikemedicalequipment,testequipmentandmobilemachinesarealsostartingtoutilizethebenefitsofCAN.ThegoalofthisapplicationnoteistoexplainsomeofthebasicsofCANandshowthebenefitsofchoosingCANforembeddedsystemsnetworkedapplications.CANOverviewMostnetworkapplicationsfollowalayeredapproachtosystemimplementation.Thissystemeticapproachenablessinteroperabilitybetweenproductsfromdifferentmanufacturers.AstandardwascreatedbytheInternationalStandardsOrganization(ISO)asatemplatetofollowforthislayeredapproach.ItiscalledtheISOOpenSystemsInterconnection(OSI)NetworkLayeringReferenceModel.TheCANprotocolitselfimplementsmostofthelowertwolayersofthisreferencemodel.ThecommunicationmediumportionofthemodelwaspurposelyleftoutoftheBoschCANspecificationtoenablesystemdesignerstoadaptandoptimizethecommunicationprotocolonmultiplemediaformaximumflexibility(twistedpair,singlewire,opticallyisolated,RF,IR,etc.).Withthisflexibility,however,comesthepossibilityofinteroperabilityconcerns.Toeasesomeoftheseconcerns,theInternationalStandardsOrganizationandSocietyofAutomotiveEngineers(SAE)havedefinedsomeprotocolsbasedonCANthatincludetheMediaDependentsInterfacedefinitionsuchthatallofthelowertwolayersarespecified.ISO11898isastandardforhigh-speedapplications,ISO11519isastandardforlow-speedapplications,andJ1939(fromSAE)istargetedfortruckandbusapplications.Allthreeoftheseprotocolsspecifya5Vdifferentialelectricalbusasthephysicalinterface.TherestofthelayersoftheISO/OSIprotocolstackarelefttobeimplementedbythesystemsoftwaredeveloper.HigherLayerProtocols(HLPs)aregenerallyusedtoimplementtheupperfivelayersoftheOSIReferenceModel.HLPsareusedto:1)Standardizestartupproceduresincludingbitratesused,2)Distributeaddressesamongparticipatingnodesortypesofmessages,3)Determinethestructureofthemessages,and4)Providesystem-levelerrorhandlingroutines.ThisisbynomeansafulllistofthefunctionsHLPsperform;howeveritdoesdescribesomeoftheirbasicfunctionality.CANProtocolBasicsCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD)TheCANcommunicationprotocolisaCSMA/CDprotocol.TheCSMAstandsforCarrierSenseMultipleAccess.Whatthismeansisthateverynodeonthenetworkmustmonitorthebusforaperiodofnoactivitybeforetryingtosendamessageonthebus(CarrierSense).Also,oncethisperiodofnoactivityoccurs,everynodeonthebushasanequalopportunitytotransmitamessage(MultipleAccess).TheCDstandsforCollisionDetection.Iftwonodesonthenetworkstarttransmittingatthesametime,thenodeswilldetectthecollisionandtaketheappropriateaction.InCANprotocol,anondestructivebitwisearbitrationmethodisutilized.Thismeansthatmessagesremainintactafterarbitrationiscompletedevenifcollisionsaredetected.Allofthisarbitrationtakesplacewithoutcorruptionordelayofthehigherprioritymessage.Thereareacoupleofthingsthatarerequiredtosupportnon-destructivebitwisearbitration.First,logicstatesneedtobedefinedasdominantorrecessive.Second,thetransmittingnodemustmonitorthestateofthebustoseeifthelogicstateitistryingtosendactuallyappearsonthebus.CANdefinealogicbit0asadominantbitandalogicbit1asarecessivebit.Adominantbitstatewillalwayswinarbitrationoverarecessivebitstate,thereforethelowerthevalueintheMessageIdentifier(thefieldusedinthemessagearbitrationprocess),thehigherthepriorityofthemessage.Asanexample,supposetwonodesaretryingtotransmitamessageatthesametime.Eachnodewillmonitorthebustomakesurethebitthatitistryingtosendactuallyappearsonthebus.Thelowerprioritymessagewillatsomepointtrytosendarecessivebitandthemonitoredstateonthebuswillbeadominant.Atthatpointthisnodelosesarbitrationandimmediatelystopstransmitting.Thehigherprioritymessagewillcontinueuntilcompletionandthenodethatlostarbitrationwillwaitforthenextperiodofnoactivityonthebusandtrytotransmititsmessageagain.Message-BasedCommunicationCANprotocolisamessage-basedprotocol,notanaddressbasedprotocol.Thismeansthatmessagesarenottransmittedfromonenodetoanothernodebasedonaddresses.EmbeddedintheCANmessageitselfisthepriorityandthecontentsofthedatabeingtransmitted.Allnodesinthesystemreceiveeverymessagetransmittedonthebus(andwillacknowledgeifthemessagewasproperlyreceived).Itisuptoeachnodeinthesystemtodecidewhetherthemessagereceivedshouldbeimmediatelydiscardedorkepttobeprocessed.Asinglemessagecanbedestinedforoneparticularnodetoreceive,ormanynodesbasedonthewaythenetworkandsystemaredesigned.Forexample,anautomotiveairbagsensorcanbeconnectedviaCANtoasafetysystemrouternodeonly.Thisrouternodetakesinothersafetysysteminformationandroutesittoallothernodesonthesafetysystemnetwork.Thenalltheothernodesonthesafetysystemnetworkcanreceivethelatestairbagsensorinformationfromtherouteratthesametime,acknowledgeifthemessagewasreceivedproperly,anddecidewhethertoutilizethisinformationordiscardit.AnotherusefulfeaturebuiltintotheCANprotocolistheabilityforanodetorequestinformationfromothernodes.ThisiscalledaRemoteTransmitRequest(RTR).Thisisdifferentfromtheexampleinthepreviousparagraphbecauseinsteadofwaitingforinformationtobesentbyaparticularnode,thisnodespecificallyrequestsdatatobesenttoit.Oneadditionalbenefitofthismessage-basedprotocolisthatadditionalnodescanbeaddedtothesystemwithoutthenecessitytoreprogramallothernodestorecognizethisaddition.Thisnewnodewillstartreceivingmessagesfromthenetworkand,basedonthemessageID,decidewhethertoprocessordiscardthereceivedinformation.CANMessageFrameDescriptionCANprotocoldefinefourdifferenttypesofmessages(orFrames).ThefirstandmostcommontypeofframeisaDataFrame.Thisisusedwhenanodetransmitsinformationtoanyorallothernodesinthesystem.SecondisaRemoteFrame,whichisbasicallyaDataFramewiththeRTRbitsettosignifyitisaRemoteTransmitRequest.Theothertwoframetypesareforhandlingerrors.OneiscalledanErrorFrameandoneiscalledanOverloadFrame.ErrorFramesaregeneratedbynodesthatdetectanyoneofthemanyprotocolerrorsdefinedbyCAN.Overloaderrorsaregeneratedbynodesthatrequiremoretimetoprocessmessagesalreadyreceived.DataFramesconsistoffieldsthatprovideadditionalinformationaboutthemessageasdefinedbytheCANspecification.EmbeddedintheDataFramesareArbitrationFields,ControlFields,DataFields,CRCFields,a2-bitAcknowledgeFieldandanEndofFrame.TheArbitrationFieldisusedtoprioritizemessagesonthebus.SincetheCANprotocoldefinesalogical0asthedominantstate,thelowerthenumberinthearbitrationfield,thehigherprioritythemessagehasonthebus.Thearbitrationfieldconsistsof12-bits(11identifierbitsandoneRTRbit)or32-bits(29identifierbits,1-bittodefinethemessageasanextendeddataframe,anSRRbitwhichisunused,andanRTRbit),dependingonwhetherStandardFramesorExtendedFramesarebeingutilized.ThecurrentversionoftheCANspecification,version2.0B,defines29-bitidentifiersandcallsthemExtendedFrames.PreviousversionsoftheCANspecificationdefined11-bitidentifierswhicharecalledStandardFrames.Asdescribedintheprecedingsection,theRemoteTransmitRequest(RTR)isusedbyanodewhenitrequiresinformationtobesenttoitfromanothernode.ToaccomplishanRTR,aRemoteFrameissentwiththeidentifieroftherequiredDataFrame.TheRTRbitintheArbitrationFieldisutilizedtodifferentiatebetweenaRemoteFrameandaDataFrame.IftheRTRbitisrecessive,thenthemessageisaRemoteFrame.IftheRTRbitisdominant,themessageisaDataFrame.TheControlFieldconsistsofsixbits.TheMSBistheIDEbit(signifiesExtendedFrame)whichshouldbedominantforStandardDataFrames.ThisbitdeterminesifthemessageisaStandardorExtendedFrame.InExtendedFrames,thisbitisRB1anditisreserved.ThenextbitisRB0anditisalsoreserved.ThefourLSBsaretheDataLengthCode(DLC)bits.TheDataLengthCodebitsdeterminehowmanydatabytesareincludedinthemessage.ItshouldbenotedthataRemoteFramehasnodatafield,regardlessofthevalueoftheDLCbits.TheDataFieldconsistsofthenumberofdatabytesdescribedintheDataLengthCodeoftheControlField.TheCRCFieldconsistsofa15-bitCRCfieldandaCRCdelimiter,andisusedbyreceivingnodestodetermineiftransmissionerrorshaveoccurred.TheAcknowledgeFieldisutilizedtoindicateifthemessagewasreceivedcorrectly.Anynodethathascorrectlyreceivedthemessage,regardlessofwhetherthenodeprocessesordiscardsthedata,putsadominantbitonthebusintheACKSlotbittimeThelasttwomessagetypesareErrorFramesandOverloadFrames.WhenanodedetectsoneofthemanytypesoferrorsdefinedbytheCANprotocol,anErrorFrameoccurs.OverloadFramestellthenetworkthatthenodesendingtheOverloadFrameisnotreadytoreceiveadditionalmessagesatthistime,orthatintermissionhasbeenviolated.Theseerrorswillbediscussedinmoredetailinthenextsection.Fast,RobustCommunicationBecauseCANwasinitiallydesignedforuseinautomobiles,aprotocolthatefficientlyhandlederrorswascriticalifitwastogainmarketacceptance.Withthereleaseofversion2.0BoftheCANspecification,themaximumcommunicationratewasincreased8xovertheversion1.0specificationto1Mbit/sec.Atthisrate,eventhemosttime-criticalparameterscanbetransmittedseriallywithoutlatencyconcerns.Inadditiontothis,theCANprotocolhasacomprehensivelistoferrorsitcandetectthatensurestheintegrityofmessages.CANnodeshavetheabilitytodeterminefaultconditionsandtransitiontodifferentmodesbasedontheseverityofproblemsbeingencountered.Theyalsohavetheabilitytodetectshortdisturbancesfrompermanentfailuresandmodifytheirfunctionalityaccordingly.CANnodescantransitionfromfunctioninglikeanormalnode(beingabletotransmitandreceivemessagesnormally),toshuttingdowncompletely(bus-off)basedontheseverityoftheerrorsdetected.ThisfeatureiscalledFaultConfinement.NofaultyCANnodeornodeswillbeabletomonopolizeallofthebandwidthonthenetworkbecausefaultswillbeconfinedtothefaultynodesandthesefaultynodeswillshutoffbeforebringingthenetworkdown.ThisisverypowerfulbecauseFaultConfinementguaranteesbandwidthforcriticalsysteminformation.ConclusionTheCANprotocolwasoptimizedforsystemsthatneedtotransmitandreceiverelativelysmallamountsofinformation(ascomparedtoEthernetorUSB,whicharedesignedtomovemuchlargerblocksofdata)reliablytoanyorallothernodesonthenetwork.CSMA/CDallowseverynodetohaveanequalchancetogainaccesstothebus,andallowsforsmoothhandlingofcollisions.Sincetheprotocolismessage-based,notaddressbased,allmessagesonthebusreceiveeverymessageandacknowledgeeverymessage,regardlessofwhetherinneedsthedataornot.Thisallowsthebustooperateinnode-to-nodeormulticastmessagingformatswithouthavingtosenddifferenttypesofmessages.Fast,robustmessagetransmissionwithfaultconfinementisalsoabigplusforCANbecausefaultynodeswillautomaticallydropoffthebusnotallowinganyonenodefrombringinganetworkdown.Thiseffectivelyguaranteesthatbandwidthwillalwaysbeavailableforcriticalmessagestobetransmitted.WithallofthesebenefitsbuiltintotheCANprotocolanditsmomentumintheautomotiveworld,othermarketswillbegintoseeandimplementCANintotheirsystems.中文翻译简介控制器区域网络（CAN）的最初创建者是80年代中期的德国汽车系统供应商罗伯特博世，作为汽车应用启用强大的串行通信的方法。其目标是让汽车更可靠，安全，省油同时减少线束的重量和复杂性。公司自成立以来，已取得了CAN协议广泛普及和其在工业自动化汽车/卡车方面的应用。该总线在其他市场网络解决方案一样能够带来强有力的利润，例如医疗设备，测试设备和移动机器也开始利用CAN总线的优势。本文的目的是解释一些CAN的基本知识，和选择CAN通信作为嵌入式系统网络应用的好处。CAN总线概述大多数网络应用程序遵循分层方法实施。这一系统的方法使不同产品之间的制造商共同创建一个标准的国际标准化组织（ISO）的为模板遵循这种分层方法。这就是所谓的国际标准组织开放系统互连（OSI）网络分层参考模型。CAN协议本身实现了较低的最基本参考模型层的通信模型中的一部分，是故意忽略CAN规范，使系统设计调整和优化的通信协议最大程度的灵活性（双绞线多种媒体，单丝，光隔离，射频，红外线等）。有了这个灵活性，但是，随之而来的互操作的可能性成为关注。为了缓解这些问题一些国际标准组织与国际汽车工程师学会（SAE）在原来的基础上定义了一些在CAN协议基础上的定义，这包括在指定两个底层媒体独立接口的定义。ISO11898是一个用于高速应用的标准，ISO11519是一个低速应用标准，J1939的（从SAE）对象是卡车和总线的应用。所有这些协议的物理接口指定为一个5V的差分电气总线。其余剩下ISO/OSI协议栈层由系统软件开发商来实现，更高一层的协议（HLPs）一般用来实现上面五层的OSI参考模型。HLPs用于：1）规范程序，包括比特率启动使用时，2）参与节点之间分配地址消息或类型，3）确定的消息结构，4）提供系统级的错误处理例程。这绝不是一个执行的职能HLPs完整列表，但它确实描述了他们的基本的一些功能。CAN总线协议基础载波侦听多路访问冲突检测（CSMA/CD）CAN通讯协议是CSMA/CD协议，CSMA代表载波侦听多路访问。这意味着，每一个网络节点在尝试往总线上发送一个消息期间，必须监测总线并保证总线处于不活动状态（载波监听）；另外，当总线处于空闲状态，总线上的每一个节点传输消息的机会是平等的（多址接入）。CD代表冲突检测。如果两个网络上的节点在同一时间开始发送数据，节点将检测到冲突并采取适当的行动。在CAN协议中，利用无损逐位仲裁的方法。这也就是说，即使完成冲突检测，仲裁后消息仍然保持完整，所有这一切仲裁均无损坏或延误的最高优先级的消息。无损逐位仲裁的实现需要两个条件。第一，逻辑状态必须定义为显性或隐性。第二，发送节点必须监视总线状态，以此观察正在试图发送到总线上的实际逻辑状态。CAN总线定义为一个逻辑显性位0和逻辑隐性位1。一个显性位的状态总能仲裁隐性位的状态，因此，较低的消息标识符值（消息中的仲裁过程中使用的字段），消息的优先级越高。例如，假设两个节点试图在同一时间发送消息。每个节点会监察总线，以保证它正试图发送的位确确实实出现在总线上。较低的优先级的消息将在某个点尝试发送一个隐性位，并监视总线状态使自己成为主节点。此时，这个节点失去仲裁，并立即停止发送。更高的优先级消息将持续发送直到完成，失去仲裁的节点将等待总线的下一个空闲周期，并尝试发送数据。基于消息的通信CAN协议是一个基于消息的协议，而不是一个基于地址的协议，这意味着消息不能从一个节点地址传送到另一个节点地址上。嵌入式CAN消息本身就是优先和正在传输数据的内容。所有系统中的节点将接收每个传递在总线上的消息（如果消息正确接收并确认）。它是由系统的每个节点决定是否将接收到的数据立即丢弃还是保存处理。一个单一的消息可以设计成指定接受一个特定的节点，或许多建立在网络和系统上的节点。例如，汽车安全气囊传感器只能通过CAN连接到安全系统的路由器节点，此路由器节点接收其他系统安全信息并转发到安全系统网络的其他节点上。安全系统网络上的所有其他节点可以在同一时间从路由器上收到最新的安全气囊传感器的信息，确认是否被正确接收后，决定是否利用这些信息，或遗弃它。另外一个内置在CAN协议中的有用的特性是，可以要求一个节点向另一个节点发送消息，这就是所谓的远程发送请求（RTR）。这个与前面的例子不同，因为，此时节点在等待一个特定的节点的消息，这个特定节点发出的消息是被动的。这种基于消息的协议的一个额外的好处是，可以添加额外的节点，系统没有必要对所有其他节点进行重新编程，也可以识别这个节点。这个新的节点将开始接收来自网络的消息，根据消息ID，决定是否要处理或丢弃收到的信息。CAN消息帧描述CAN协议定义了四种不同类型的信息（或帧）。第一个也是最常见的类型是数据帧，这是用来当一个节点发送信息系统中的任何或所有其他节点；二是远程帧，这基本上是一个用来设置表示，它是一个远程发送请求RTR位的数据帧。其他两个错误处理的帧，一个为错误帧和一个为过载帧。检测出CAN协议定义的错误都将产生错误帧，产生超载错误的节点，需要更多的时间来处理已经接收的数据。数据帧中包括的有关数据信息的额外字段由CAN定义规范，数据帧中嵌入