Error detection

There are 5 error detection mechanisms: -

1. Cyclic redundancy check. Each message contains a 15 bit CRC code computed by sender and checked by receivers, who will flag any errors. More in the spec (in black binder)
2. Frame check. At certain points in the frame, the correct value is predefined.
3. ACK (acknowledgement) Error Check. If transmitter determines an error has not been acknowledged, an ACK error is flagged.
4. Bit Monitoring. A transmitter checks the network and flags a bit error if the value on the bus is not that sent. This does not happen during transmission of the identifier field, of course, as that is how a collision is detected.
5. Bit stuffing After 5 consecutive bits of the same value, a bit of the opposite value is added to the frame.

If an error is detected, an error frame is sent, aborting the transmission.

Error confinement (unique to CAN?) provides a mechanism for distinguishing between temporary and permanent errors. Each node has two error counters (for transmit and receive) which are incremented when errors are found. It is covered in more detail in the spececification, but briefly each receive error increments its counter by one, and each transmit error increments its counter by 8. If either counter goes above 127 the node concerned goes into “error passive” mode. In this mode it can still transmit and receive messages, but is restricted in flagging errors. If a device’s transmit error counter goes above 255, the device will go into “bus off” mode and will cease to be active. This condition will clearly need to be modelled in simulating CANbus systems for FMEA. This seems to imply that we must allow for the modelling of repeat errors or for modelling the network as though the counter(s) had reached a level such that devices were going into “bus off” mode.

Error detection is thorough. Omegas stuff suggests that the undetected error probability is 10 to the power of –11. Of course, detected errors will result in loss or delay to messages, which effects might well need modelling.

Author: Jon Bell