In bus off condition of CAN protocol, how does ECU monitors the CAN bus?

[SKUND.11]

In Bus -off condition , the ECU(node) disconnects from the CAN bus.So how it monitors the transfer and receiving of the message signal in CAN bus until Bus -off recovery stage is received.

[Jaroslav JANOS]

Hi @SKUND.1​,

the bus-off condition of the ECU occurs only when the transmitting error counter exceeds 255. The device is disconnected from the bus (but only logically, not physically) and cannot transmit any message or anyhow affect the bus. However, the reception is still possible and so the ECU can monitor the transfer of messages on the bus.

BR,

Jaroslav

[SKUND.11]

Thanks for answering my doubt @Jaroslav JANOS​ but i have another doubt,you said "reception is still possible "how would ECU know that he have received a message/byte.

[Jaroslav JANOS · Accepted solution]

Sorry, I think I don't understand your question - could you explain it more thoroughly, maybe on some example?

Because the reception is (by the CAN standard) unaffected, node sees/receives messages the same way it did before in the error active/passive mode. Node cannot transmit its own messages, error flags and acknowledge received messages.

Of course, in your implementation, you can ignore received messages when the ECU is in bus off state (because the node or the bus are probably corrupted), but this is not mandatory.

Even the recovery sequence requires the ECU to sense 128 repetitions of 11 consecutive recessive bits (typically delimiters of the CAN messages) on the bus to get back from the bus off mode to the normal mode.

BR,

Jaroslav