Although there is a special order on the Controller Area Network Bus, all subscribers may in principal begin after successful ending of the previous message, transmitting their message. The control units do not only transmit, but at the same time receive information when they are transmitting. In pleasant contrast to human communication the control unit that receives a precedence field (Arbitration-field) from another control unit will immediately stop its less important transmission. More important data (e.g., of the ABS system) can temporarily edge out more insignificant data (e.g. cooling temperature). The identifier specifies the data packet unambiguously. The address is at the same time precedence identification number (Identifier). Thus, it contains the information as to how important the message is.
0 volts prevail over 5 volts
On top in the figure two control units (controller) in the Bus began by chance precisely at the same time with transmitting their message. Both transmit first of all a start bit and consequently the 11-bit Identifier. Up until No. 6 both are the same. At No. 7 the Bus receives from the left for a short time 0 V and from the right 5 V. Of course, the 0 V from the left overlay the 5 V from the right. Because both control units send and read at the same time the difference between the written and read bit is striking to the right control unit, and it stops its broadcasting process. The more important the message, the more dominant bits there are early on on the Identifier.
The sender of the insignificant message may try again after the other control unit's message has ended. Thereby, messages with top priority can be put on the bus without time delay. Indeed, the more insignificant messages may be hit by a traffic jam. The Identifier is programmed by the designer of the Controller Area Network Bus, and its order of importance contains gaps to be able to integrate other systems later on. 02/13