CAN 协议 毕业论文外文翻译.doc

山东科技大学学士学位论文 文献翻译附录二 文献翻译can protocol m .j .schofieldthe can protocol is an international standard defined in the iso 11898. beside the can protocol itself the conformance test for the can protocol is defined in the iso 16845, which guarantees the interchangeability of the can chips.1. principles of data exchangecan is based on the “broadcast communication mechanism”, which is based on a message-oriented transmission protocol. it defines message contents rather than stations and station addresses. every message has a message identifier, which is unique within the whole network since it defines content and also the priority of the message. this is important when several stations compete for bus access (bus arbitration). as a result of the content-oriented addressing scheme a high degree of system and configuration flexibility is achieved. it is easy to add stations to an existing can network without making any hardware or software modifications to the present stations as long as the new stations are purely receivers. this allows for a modular concept and also permits the reception of multiple data and the synchronization of distributed processes. also, data transmission is not based on the availability of specific types of stations, which allows simple servicing and upgrading of the network.2. real-time data transmissionin real-time processing the urgency of messages to be exchanged over the network can differ greatly: a rapidly changing dimension, e.g. engine load, has to be transmitted more frequently and therefore with less delays than other dimensions, e.g. engine temperature. the priority, at which a message is transmitted compared to another less urgent message, is specified by the identifier of each message. the priorities are laid down during system design in the form of corresponding binary values and cannot be changed dynamically. the identifier with the lowest binary number has the highest priority. bus access conflicts are resolved by bit-wise arbitration of the identifiers involved by each station observing the bus level bit for bit. this happens in accordance with the wired-and-mechanism, by which the dominant state overwrites the recessive state. all those stations (nodes) with recessive transmission and dominant observation lose the competition for bus access. all those losers automatically become receivers of the message with the highest priority and do not re-attempt transmission until the bus is available again. transmission requests are handled in order of their importance for the system as a whole. this proves especially advantageous in overload situations. since bus access is prioritized on the basis of the messages, it is possible to guarantee low individual latency times in real-time systems.3. message frame formatsthe can protocol supports two message frame formats, the only essential difference being in the length of the identifier. the “can base frame” supports a length of 11 bits for the identifier, and the “can extended frame” supports a length of 29 bits for the identifier. 4. can extended frame formatthe difference between an extended frame format message and a base frame format message is the length of the identifier used. the 29-bit identifier is made up of the 11-bit identifier (“base identifier”) and an 18-bit extension (“identifier extension”). the distinction between can base frame format and can extended frame format is made by using the ide bit, which is transmitted as dominant in case of an 11-bit frame, and transmitted as recessive in case of a 29-bit frame. as the two formats have to co-exist on one bus, it is laid down which message has higher priority on the bus in the case of bus access collision with different formats and the same identifier / base identifier: the 11-bit message always has priority over the 29-bit message. the extended format has some trade-offs: the bus latency time is longer (in minimum 20 bit-times), messages in extended format require more bandwidth (about 20 %), and the error detection performance is lower (because the chosen polynomial for the 15-bit crc is optimized for frame length up to 112 bits). can controllers, which support extended frame format messages are also able to send and receive messages in can base frame format. can controllers that just cover the base frame format do not interpret extended frames correctly. however there are can controllers, which only support the base frame format but recognize extended messages and ignore them.5. detecting and signaling errorsunlike other bus systems, the can protocol does not use acknowledgement messages but instead signals errors immediately as they occur. for error detection the can protocol implements three mechanisms at the message level (data link layer: osi layer 2): cyclic redundancy check (crc): the crc safeguards the information in the frame by adding a frame check sequence (fcs) at the transmission end. at the receiver this fcs is re-computed and tested against the received fcs. if they do not match, there has been a crc error. frame check: this mechanism verifies the structure of the transmitted frame by checking the bit fields against the fixed format and the frame size. errors detected by frame checks are designated format errors. ack errors: receivers of a message acknowledge the received frames. if the transmitter does not receive an acknowledgement an ack error is indicated. the can protocol also implements two mechanisms for error detection at the bit level (physical layer: osi layer 1): monitoring: the ability of the transmitter to detect errors is based on the monitoring of bus signals. each station that transmits also observes the bus level and thus detects differences between the bit sent and the bit received. this permits reliable detection of global errors and errors local to the transmitter. bit stuffing: the coding of the individual bits is tested at bit level. the bit representation used by can is non return to zero (nrz) coding. the synchronization edges are generated by means of bit stuffing. that means after five consecutive equal bits the transmitter inserts a stuff bit into the bit stream. this stuff bit has a complementary value, which is removed by the receivers. if one or more errors are discovered by at least one station using the above mechanisms, the current transmission is aborted by sending an error frame. this prevents other stations from accepting the message and thus ensures the consistency of data throughout the network. after transmission of an erroneous message that has been aborted, the sender automatically re-attempts transmission (automatic re-transmission). nodes may again compete for bus access. however effective and efficient the method described may be, in the event of a defective station it might lead to all messages (including correct ones) being aborted. if no measures for self-monitoring were taken, the bus system would be blocked by this. the can protocol therefore provides a mechanism to distinguish sporadic errors from permanent errors and local failures at the station. this is done by statistical assessment of station error situations with the aim of recognizing a stations own defects and possibly entering an operation mode in which the rest of the can network is not negatively affected. this may continue as far as the station switching itself off to prevent other nodes messages erroneously from being recognized as incorrect.can 协议 斯科菲尔德can协议是在iso 11898中定义的国际标准。除了can协议本身，can协议的一致性测试在iso 16845中定义，用于保证can芯片的可互换性。1数据交换原理can基于“广播通讯机制”，该机制又建立在面向消息的传送协议的基础之上。它定义消息内容而不是站和站地址。每条消息都有一个消息标识符，且该消息标识符在整个网络内是独一无二的，因为它定义了消息内容及消息优先权。这在多个站争抢总线访问(总线仲裁)的情况下很重要。 面向内容的寻址方案带来的结果是高度的系统和组态灵活性。只要新站只是接收器，可以很轻松地将站添加到现有的can网络，无需对目前站的任何硬件和软件进行改动。这允许运用模块化概念，也允许接收多重数据并实现分布式过程的同步。同时，数据传输并非基于站的特定类型的可用性。2. 实时数据传输在实时过程中，通过网络进行交换的消息的紧急程度可以有很大差别：快速变化的单位（例如发动机载荷）需要更频繁地进行传输，因此比其它单位(例如发动机温度)的延迟更少。 使用每个消息的标识符来指定用于衡量消息发送的紧急程度的优先级。优先级在系统设计时就已确定下来了，它表现为相应的二进制值，并且无法动态地更改。二进制数最小的标识符具有最高的优先级。 通过逐位观测总线电平的每个站所含标识符的逐位仲裁机制来解决总线访问冲突。这种情况按照显性状态覆盖隐性状态发生。所有那些带有隐性传输和显性观测的站(节点)会失去争抢总线访问的机会。所有那些“争抢失败者”会自动成为优先级最高的消息的接收器，且在总线再次可用之前不会重新尝试传输。 按照传输请求对于整个系统的重要程度，对传输请求进行处理。这点在过载情况下更能证明其优势。由于是基于消息区分总线访问的优先等级，从而可以保证实时系统中个体的等待时间很短。3. 消息帧格式can协议支持两种消息帧格式，二者唯一的重大差别在于标识符的长度。“can基本帧/支持长度为11位的标识符，而“can扩展帧”支持长度为29位的标识符。4. can扩展帧格式扩展帧格式消息与基本帧格式消息的不同在于所使用的标识符长度。29位的标识符由11位标识符(“基本标识符”)和18位扩展符(“标识符扩展”)组成。can基本帧格式和can扩展帧格式之间的区别在于ide位的使用：在11位帧的情况下，该ide位作为显性位传输，在29位帧的情况下，该ide位作为隐性位传输。由于两种格式必须共存于一根总线中，在总线访问冲突的情况下，利用不同的格式和相同的标识符/基本标识符规定总线上的哪个信息具有较高的优先权：11位消息始终比29位消息的优先级高。扩展格式要付出一些代价：总线等待时间较长(最小20个位时间)，具有扩展格式的消息要求更多的带宽(大约20 %)，且错误检测的性能较低(因为15位crc所选用的多