Theory Of Operation

The purpose of the diagnostic is to monitor the CAN bus, the transmitted messages on the CAN bus and the validity of the CAN signals. Several control units are connected to the CAN bus which send their messages cyclically in a defined time window. The messages contain multiple signals representing vehicle information.

CAN Bus Monitoring:  To distinguish general problems with the CAN bus (e.g. electrical problems) from problems with certain messages or signals, the general state of the CAN bus is monitored.

CAN Message Monitoring:  All messages received and used by the TCU are monitored for timeout and validity of the DLC (Data Length Code) information in the message header. If available, the message is also monitored for the validity of a checksum and for the correct behavior of an alive-counter that the sending control unit is expected to increment on each transmittal.

CAN Signal Monitoring:  If the sending control unit does not have valid information to send, it may send a defined failure value indicating an invalid signal. Incoming signals with a defined failure value are monitored for signal validity.