controller area network , low priority nodes

[aswin p ajayan]

hi
i am a total beginner in CAN .

i was just going through CAN basics and i got stuck on some thing. an app note on CAN said about the arbitration , my question is how the mesage of a low priority node transmitted through bus as it always fails in arbitration . does a case of not servicing a low priority node occur in case of can

PS: sorry for my bad english

 

[electroRF]

CAN is quite similar to I2C, so my answer would be based on I2C.

In I2C, Arbitration is supported as well.

If 2 Masters try to send a sequence at the same time, the one with the higher priority, call it Master A, would take over the bus.

At this case, Master B, would need to wait for the Bus to become Idle again, and re-send its sequence, hoping that no higher priority Master would try to send data at the same time.

You need to repeat the latter, until Master B wins the arbitration.

 

[kubeek]

If master B fails to win arbitration, or it gets it so late that it doesnt matter anymore, than the network is obviously overloaded.
In a system that needs the messages to get through in time, the priority can be composed of the base priority and a how much time remains to the deadline. This allows for better predictability of the arrival time and less dropped packets in complex networks.