CAN Network With Single Node (CAN protocol)

Question

I am new to CAN protocol, going through the Robert Bosch's CAN Specification ver2.0 Part B. I can't understand the following lines on page 63

" Note: Start up / Wake up: if during startup only one node is online, and if this node transmits some message, it will get no acknowledgement, detect an error and repeats the message. It can become 'error passive' but not 'bus off' due to this reason."

As far as I understand when a transmitter detects an error(like Acknowledgement error) it retransmits the message and also increments the transmit error count (TEC) by 8. So if there is only one node then its TEC should increase by 8 everytime it transmits a message and should go into 'bus off' condition once TEC goes above 255.

Can someone please explain why the specification says it can only go 'Error Passive' but not 'Bus off'?

Answer 1

I think you missed this part of the specification:

"Exception 1:

If the TRANSMITTER is 'error passive' and detects an ACKNOWLEDGEMENT ERROR because of not detecting a 'dominant' ACK and does not detect a 'dominant' bit while sending its PASSIVE ERROR FLAG. "

In this case, the TEC is not changed!

So, in your case, when the only node in the network re-transmits every time and the TEC becomes >= 128, it becomes 'error passive'. Then the above exception case becomes valid! And the TEC is no more changed! Hence no 'bus off'.

Answer 2

Yes, that is correct as if the Tx ECU is not receiving any ACK from any ECU in the CAN network then it will stay in Error passive state. Because the Networking protocol needs when more than one systems are available in CAN network and if no ECU available in the network, then it is not meaning as the ECU is having an issue, so instead of going to CAN bus off state it should stay in Error passive state. To know more about CAN error, you can follow the below link: