Baud rate mismatch on CAN network

[gator]

We are using the onboard MSCAN module in a MC9S12DP512 and have a question about CAN baud rate mismatches on a CAN network. As an example, assume we have a CAN network in an automotive setting (OBD bus) that is running at a baud rate of 500k. What would happen if we connect a CAN device to the CAN network (OBD bus) that is operating at a baud rate of 250k but is not making any OBD requests - just listening only? Would one of the nodes on the network start generating error frames?

Thanks,

[danielmartynek]

Hi,

If the node with 250k baud rate is in listen-only mode, the error frame is not generated, because it cannot send any messages. However, its error counter will be increasing. Other nodes that run at 500k will not be affected.

There is an article which deals with Automatic Baudrate Detection.

Regards,

Daniel

[gator]

Thanks a lot,

What if the node with the 250k baud rate is NOT in listen-only mode? Will it transmit error frames on the bus or will the other nodes running at 500k transmit error frames instead?

[danielmartynek]

Hi,

If the node with the 250k baud rate is NOT in listen-only mode, it will detect an error within each message and will send Error Flag, destroying the bus traffic. Because the other nodes (500k) detect the error caused by the Error Flag, they discard the current message and increase their error counters.

Regards,

Daniel

[gator]

Thanks, that makes sense. Say we have a node that is in listen-only mode and it is able to successfully detect the baud rate of the network. Its only purpose is to passively monitor bus traffic. If we choose to leave that node in listen-only mode even after the baud rate has been detected, will that have any negative consequences?

Thanks again.

[danielmartynek]

Hi,

You can leave that node in listen-only mode. I don’t see any problem with that.

Regards,

Daniel