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1、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
2、 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
3、 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 makin
4、g 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 availabil
5、ity 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
6、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 correspo
7、nding 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
8、 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 r
9、e-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
10、 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
11、 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 (“i
12、dentifier 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
13、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 min
14、imum 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 ab
15、le 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 sig
16、naling 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
17、 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 transm
18、itted 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
19、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
20、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 ar
21、e 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 mechan
22、isms, 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 transmi
23、ssion (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 sys
24、tem 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 ent
25、ering 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
26、，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
27、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。
28、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
29、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所选用的多项式是针对112位的帧长进行优化的)。 支持扩展帧格式消息的CAN控制器也能够发送和接收具有CAN基本帧格式的消息。只包括基本帧格式的CAN基础框架。而CAN控制器仅支持基本帧格式，对于扩展消息只能识别和忽略
30、。5. 检测错误并发信号通知与其它总线系统不同，CAN协议不使用确认消息，而是在错误发生时发信号通知。对于错误检测，CAN协议在消息级别执行三种机制(数据链路层：OSI层2)： 周期性的冗余检查(CRC)：CRC通过在传输端添加帧检查序列(FCS)来保护帧中的信息。在接收器处，参照接收到的FCS对该FCS进行验算和测试。如果它们不匹配，表示已发生CRC错误。 帧检查：该机制通过参照固定格式和帧大小来检查位字段，以验证所传输帧的结构。通过帧检查检测到的错误被命名为“格式错误”。 ACK错误：消息的接收器确认所接收到的帧。如果发送器没有接收到确认，表示有ACK错误。 对于位级别的错误检测，CAN协
31、议也执行两种机制(物理层：OSI层1)： 监视：发送器的错误检测能力基于对总线信号的监视。传输消息的每个站也观测总线电平，这样可以检测到发送的位与接收的位之间的差别。这能实现可靠地检测全局错误以及发送器的本地错误。 位填充：在位级别测试单个位的编码。CA采用的位表达法是“不归零制(NRZ)”编码。同步沿通过位填充产生。这表示在5个连续的相等位之后，发送器将一个填充位插入到位流中。该填充位带有被接收器删除的互补值。 如果至少一个站利用上述机制发现了一个或者多个错误，那么通过发送“错误帧”。 这防止其他站接受信息,从而保证了整个网络的数据的一致性。一个错误的讯息传递被中止后,发送端会自动re-at
32、tempts传输(自动传输)。节点可以再次争抢总线访问。 然而，尽管这里所描述的方法切实有效，但是在站出现故障的情况下，该方法会导致所有消息(包括正确的消息)被取消。如果没有采取自我监视的措施，总线系统会因此而受阻。因此，CAN协议提供一种机制将偶发性错误与永久性错误和站的本地故障区分开来。可以通过对站错误情况的统计评估来达到这一目的，评估的目的是识别站自身的故障或者引进一种工作模式，在该模式下CAN网络的其它部分不受负面影响。这种情况可能一直持续到站将自己关闭，以防止其它节点的消息被误认为是错误的。6a*CZ7H$dq8KqqfHVZFedswSyXTy#&QA9wkxFyeQ!djs#Xu
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