PDF《BOSCH》

2 or more units start transmitting messages at the same time, the bus access conflict is resolved by bitwise arbitration using the IDENTIFIER. The mechanism of arbitration guarantees that neither information nor time is lost. If a DATA FRAME and a REMOTE FRAME with the same IDENTIFIER are initiated at the same time, the DATA FRAME prevails over the REMOTE FRAME. During arbitration every transmitter compares the level of the bit transmitted with the level that is monitored on the bus. If these levels are equal the unit may continue to send. When a '

recessive'

level is sent and a '

dominant'

level is monitored (see Bus Values), the unit has lost arbitration and must withdraw without sending one more bit. Safety In order to achieve the utmost safety of data transfer, powerful measures for error detection, signalling and self-checking are implemented in every CAN node. Error Detection For detecting errors the following measures have been taken: - Monitoring (transmitters compare the bit levels to be transmitted with the bit levels detected on the bus) - Cyclic Redundancy Check - Bit Stuffing - Message Frame Check Basic Concepts BOSCH ROBERT BOSCH GmbH, Postfach

30 02 40, D-70442 Stuttgart Sep.

1991 Part A - page

8 Performance of Error Detection The error detection mechanisms have the following properties: - all global errors are detected. - all local errors at transmitters are detected. - up to

5 randomly distributed errors in a message are detected. - burst errors of length less than

15 in a message are detected. - errors of any odd number in a message are detected. Total residual error probability for undetected corrupted messages: less than message error rate * 4.7 * 10-11 . Error Signalling and Recovery Time Corrupted messages are flagged by any node detecting an error. Such messages are aborted and will be retransmitted automatically. The recovery time from detecting an error until the start of the next message is at most

29 bit times, if there is no further error. Fault Confinement CAN nodes are able to distinguish short disturbances from permanent failures. Defective nodes are switched off. Connections The CAN serial communication link is a bus to which a number of units may be connected. This number has no theoretical........