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Priority-oriented message transmission

The arbitration process described above guarantees that the message with the highest priority in each case is always transmitted as soon as the bus is free. The priority of the message is determined by the value of the message identifier. The smaller this value is, the higher the priority of the message. The principle of priority-oriented messages enables a very efficient use of the bandwidth available for data transmission. Therefore it is possible that low-priority messages occupy the bus 100% without substantially delaying the transmission of messages with higher priority messages. For the message with the highest priority, a maximum latency time of approx. 130 µs results at a transmission rate of 1 Mbit/s. On the other hand, when designing a CAN system it must be ensured that high-priority messages do not constantly occupy the bus. This is possible, for example, by introducing so-called minimal "transmission block times" (CANopen: "Inhibit-Time").

Bit rate and bus length

The principle of bit-wise arbitration used with CAN requires a comparison of the local bit levels of all network nodes distributed over the bus within a bit-time interval. As the signal propagation time required for the signal distribution over the bus is proportional to the length of the bus, the necessary duration of a bit interval is prolonged accordingly with increasing bus length. The maximum bus length (network extension) and maximum bit rate are thus inversely proportional according to the following general formula: Bit rate (in Mbit/s) x bus length (in m) ≤60.

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