CAN 協(xié)議畢業(yè)論文外文翻譯

1、附錄二 文獻翻譯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 協(xié)議 斯科菲爾德can協(xié)議是在iso 11898中定義的國際標準。除了can協(xié)議本身，can協(xié)議的一致性測試在iso 16845中定義

26、，用于保證can芯片的可互換性。1數(shù)據(jù)交換原理can基于“廣播通訊機制”，該機制又建立在面向消息的傳送協(xié)議的基礎(chǔ)之上。它定義消息內(nèi)容而不是站和站地址。每條消息都有一個消息標識符，且該消息標識符在整個網(wǎng)絡內(nèi)是獨一無二的，因為它定義了消息內(nèi)容及消息優(yōu)先權(quán)。這在多個站爭搶總線訪問(總線仲裁)的情況下很重要。 面向內(nèi)容的尋址方案帶來的結(jié)果是高度的系統(tǒng)和組態(tài)靈活性。只要新站只是接收器，可以很輕松地將站添加到現(xiàn)有的can網(wǎng)絡，無需對目前站的任何硬件和軟件進行改動。這允許運用模塊化概念，也允許接收多重數(shù)據(jù)并實現(xiàn)分布式過程的同步。同時，數(shù)據(jù)傳輸并非基于站的特定類型的可用性。2. 實時數(shù)據(jù)傳輸在實時過程中，通過

27、網(wǎng)絡進行交換的消息的緊急程度可以有很大差別：快速變化的單位（例如發(fā)動機載荷）需要更頻繁地進行傳輸，因此比其它單位(例如發(fā)動機溫度)的延遲更少。 使用每個消息的標識符來指定用于衡量消息發(fā)送的緊急程度的優(yōu)先級。優(yōu)先級在系統(tǒng)設計時就已確定下來了，它表現(xiàn)為相應的二進制值，并且無法動態(tài)地更改。二進制數(shù)最小的標識符具有最高的優(yōu)先級。 通過逐位觀測總線電平的每個站所含標識符的逐位仲裁機制來解決總線訪問沖突。這種情況按照顯性狀態(tài)覆蓋隱性狀態(tài)發(fā)生。所有那些帶有隱性傳輸和顯性觀測的站(節(jié)點)會失去爭搶總線訪問的機會。所有那些“爭搶失敗者”會自動成為優(yōu)先級最高的消息的接收器，且在總線再次可用之前不會重新嘗試傳輸。

28、按照傳輸請求對于整個系統(tǒng)的重要程度，對傳輸請求進行處理。這點在過載情況下更能證明其優(yōu)勢。由于是基于消息區(qū)分總線訪問的優(yōu)先等級，從而可以保證實時系統(tǒng)中個體的等待時間很短。3. 消息幀格式can協(xié)議支持兩種消息幀格式，二者唯一的重大差別在于標識符的長度?！癱an基本幀/支持長度為11位的標識符，而“can擴展幀”支持長度為29位的標識符。4. can擴展幀格式擴展幀格式消息與基本幀格式消息的不同在于所使用的標識符長度。29位的標識符由11位標識符(“基本標識符”)和18位擴展符(“標識符擴展”)組成。can基本幀格式和can擴展幀格式之間的區(qū)別在于ide位的使用：在11位幀的情況下，該ide位作為顯性位傳輸，在29位幀的情況下，該ide位作為隱性位傳輸。由于兩種格式必須共存于一根總線中，在總線訪問沖突的情況下，利用不同的格式和相同的標識符/基本標識符規(guī)定總線上的哪個信息具有較高的優(yōu)先權(quán)：11位消息始終比29位消息的優(yōu)先級高。擴展格式要付出一些代價：總線等待時間較長(最小20個位時間)，具有擴展格式的消息要求更多的帶寬(大約20 %)，且錯誤檢測的性能較低(因為15位crc所選用的多項式