【文章內(nèi)容簡介】 Errors detected by frame checks are designated ”format errors”.●ACK errors As mentioned above, frames received are acknowledged by all recipients through positive acknowledgement. If no acknowledgement is received by the transmitter of the message (ACK error) this may mean that there is a transmission error which has been detected only by the recipients, that the ACK field has been corrupted or that there are no receivers. The CAN protocol also implements two mechanisms for error detection at the bit level.●Monitoring The ability of the transmitter to detect errors is based on the monitoring of bus signals: each node which transmits also observes the bus level and thus detects differences between the bit sent and the bit received. This permits reliable detection of all global errors and errors local to the transmitter.●Bit stuffingThe coding of the individual bits is tested at bit level. The bit representation used by CAN is NRZ (nonreturntozero) coding, which guarantees maximum efficiency in bit coding. The synchronisation edges are generated by means of bit stuffing, . after five consecutive equal bits the sender inserts into the bit stream a stuff bit with the plementary value, which is removed by the receivers. The code check is limited to checking adherence to the stuffing rule. If one or more errors are discovered by at least one station (any station) using the above mechanisms, the current transmission is aborted by sending an ”error flag”. This prevents other stations accepting the message and thus ensures the consistency of data throughout the network. After transmission of an erroneous message has been aborted, the sender automatically reattempts transmission (automatic repeat request). There may again be petition for bus allocation. As a rule, retransmission will be begun within 23 bit periods after error detection。 in special cases the system recovery time is 31 bit periods. 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, thus blocking the bus system if no measures for selfmonitoring were taken. The CAN protocol therefore provides a mechanism for distinguishing sporadic errors from permanent errors and localizing station failures (fault confinement). This is done by statistical assessment of station error situations with the aim of recognizing a station‘s own defects and possibly entering an operating mode where the rest of the CAN network is not negatively affected. This may go as far as the station switching itself off to prevent messages erroneously recognized as incorrect from being aborted.Data reliability of the CAN protocol.The introduction of safetyrelated systems in automobiles brought with it high requirements for the reliability of data transmission. The objective is frequently formulated as not permitting any dangerous situations for the driver to occur as a result of data exchange throughout the whole life of a vehicle. This goal is achieved if the reliability of the data is sufficiently high or the residual error probability is sufficiently low. In the context of bus systems data, reliability is understood as the capability to identify data corrupted by transmission faults. The residual error probability is a statistical measure of the impairment of data reliability: it specifies the probability that data will be corrupted and that this corruption will remain undetected. The residual error probability should be so small that on average no corrupted data will go undetected throughout the whole life of a system.Residual error probability as a function of bit error probability Calculation of the residual error probability requires that the errors which occur be classified and that the whole transmission path be described by a model. If we determine the residual error probability of CAN as a function of the bit error probability for message lengths of 80 to 90 bits, for system configurations of, for instance, five or ten nodes and with an error rate of 1/1000 (an error in one message in every thousand), then maximum bit error probability is approximately in the order of 1013. Based on this it is possible to calculate the maximum number of undetectable errors for a diven CAN network. For example, if a CAN network operates at a data rate of 1 Mbit/s, at an average bus capacity utilization of 50 percent, for a total operating life of 4000 hours and with an average message length of 80 bits, then the total number of messages transmitted is 9 x 1010. The statistical number of undetected transmission errors during the operating life is thus in the order of less than 102. Or to put it another way, with an operating time of eight hours per day on 365 days per year and an error rate of s, one undetected error occurs every thousand years (statistical average).Extended format CAN messages The SAE ”Truck and Bus” submittee standardized signals and messages as well as data transmission protocols for various data rates. lt became apparent that stanardization of this kind is easier to implement when a longer identification field is available. To support these efforts, the CAN protocol was extended by the introduction of a 29bit identifier. This identifier is made up of the existing 11bit identifier (base ID) and an 18bit extension (ID extension). Thus the CAN protocol allows the use of two message formats: StandardCAN (Version ) and ExtendedCAN (Version ). As the two formats have to coexist on one bus it is laid down which message has higher priority on the bus in the case of bus access collisions with dithering formats and the same base ident