CAN Data Bus Specification

In order to process the data in real time, they must be transmitted quickly.

This requires not only a rapid physical data transmission line but also a rapid bus connection when several control modules wish to transmit at the same time.

There are different priorities for the individual messages to take into account the different urgencies of data transmitted by the bus.

The advance angle, for example, has a very high priority, the slip values have a medium priority and outside temperature has a lower priority.

The priority with which a message is transmitted is determined by the identifier of the respective message.

An identifier which corresponds to a low binary number has a high priority and vice versa.

The CAN protocol is based on two logical states: The bits are either "recessive" (logic 1) or "dominant" (logic 0). If a dominant bit is transmitted from at least one station, recessive bits which other stations transmit are overwritten.


Example







If several control modules want to transmit at the same time, the bus access conflict is solved by means of "bit-wise arbitration" via the respective identifiers.

When the arbitration field is transmitted, the transmitter checks after each bit whether it is still authorized to transmit, or whether another control module is transmitting data of higher priority.

If a recessive bit transmitted by the first transmitter is overwritten by a dominant bit of another transmitter, the first transmitter loses its transmission authorization (arbitration) and becomes a receiver.

The first control module (N I) loses the arbitration with bit 3.
The third control module (N III) loses the arbitration with bit 7.
The second control module (N II) retains bus access and can send its message.

The other control modules try to transmit their message when the bus is free. The transmission authorization is then again assigned depending on the priority of the message.