1、附录二 文献翻译 CANprotocol M .J .Schofield The 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 exchange
2、CAN 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 a
3、lso 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 ma
4、king 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 availa
5、bility of specific types of stations, which allows simple servicing and upgrading of the network. 2. Real-time data transmission In 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
6、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 cor
7、responding 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 wit
8、h 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
9、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 guarante
10、e low individual latency times in real-time systems. 3. Message frame formats The 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” s
11、upports a length of 29 bits for the identifier. 4. CAN extended frame format The 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 exten
12、sion (“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
13、, 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
