Controller Area Network (CAN BUS) Protocol

Every CAN controller along a bus will try to detect the errors outlined above within each message. If an error is found, the discovering node will transmit an Error Flag, thus destroying the bus traffic. The other nodes will detect the error caused by the Error Flag (if they haven’t already detected the original error) and take appropriate action, i.e. discard the current message.

Each node maintains two error counters: the Transmit Error Counter and the Receive Error Counter. There are several rules governing how these counters are incremented and/or decremented. In essence, a transmitter detecting a fault increments its Transmit Error Counter faster than the listening nodes will increment their Receive Error Counter. This is because there is a good chance that it is the transmitter who is at fault!

A node starts out in Error Active mode. When any one of the two Error Counters raises above 127, the node will enter a state known as Error Passive and when the Transmit Error Counter raises above 255, the node will enter the Bus Off state.

• An Error Active node will transmit Active Error Flags when it detects errors.
• An Error Passive node will transmit Passive Error Flags when it detects errors.
• A node which is Bus Off will not transmit anything on the bus at all.

The rules for increasing and decreasing the error counters are somewhat complex, but the principle is simple: transmit errors give 8 error points, and receive errors give 1 error point. Correctly transmitted and/or received messages causes the counter(s) to decrease.

Example (slightly simplified): Let’s assume that node A on a bus has a bad day. Whenever A tries to transmit a message, it fails (for whatever reason). Each time this happens, it increases its Transmit Error Counter by 8 and transmits an Active Error Flag. Then it will attempt to retransmit the message.. and the same thing happens.

When the Transmit Error Counter raises above 127 (i.e. after 16 attempts), node A goes Error Passive. The difference is that it will now transmit Passive Error Flags on the bus. A Passive Error Flag comprises 6 recessive bits, and will not destroy other bus traffic – so the other nodes will not hear A complaining about bus errors. However, A continues to increase its Transmit Error Counter. When it raises above 255, node A finally gives in and goes Bus Off.

What does the other nodes think about node A? – For every active error flag that A transmitted, the other nodes will increase their Receive Error Counters by 1. By the time that A goes Bus Off, the other nodes will have a count in their Receive Error Counters that is well below the limit for Error Passive, i.e. 127. This count will decrease by one for every correctly received message. However, node A will stay bus off.

Most CAN controllers will provide status bits (and corresponding interrupts) for two states:

• “Error Warning” – one or both error counters are above 96
• Bus Off, as described above.

Some – but not all! – controllers also provide a bit for the Error Passive state. A few controllers also provide direct access to the error counters.

The CAN controller’s habit of automatically retransmitting messages when errors have occurred can be annoying at times. There is at least one controller on the market (the SJA1000 from Philips) that allows for full manual control of the error handling.