GF54.00-P-0005-03A CAN Data Bus Fault Recognition

GF54.00-P-0005-03A CAN Data Bus Fault Recognition

Faults which occur during data transfer can result in errors. These transmission errors must therefore be detected and rectified. A distinction is made between two levels of fault identification in the CAN protocol:
^ Mechanisms at the data frame level
^ Mechanisms at the bit level

Mechanisms at the data frame level

Cyclic redundancy check:
The transmitter establishes test bits from the CAN messages that are to be sent in the data telegram "CRC Field". The receiver calculates these test bits from the received CAN message and compares them to the received test bits.

Frame check:
This mechanism checks the structure of the transferred frame by comparing the bit fields with the specified fixed format and frame length.
Errors detected using the frame check are referred to as format errors.

Mechanisms at the bit level

Monitoring:
During transmission every station observes the CAN data bus gauge and recognizes the difference between sent and received bits. Both global and local bit errors which occur at the sending unit can therefore be detected.

Bit stuffing:
In each message, a maximum of 5 successive bits may have the same polarity between the "start of frame" and the end of the "CRC field". After 5 consecutive identical bits, the sending unit inserts a bit of the opposite polarity into the bit stream. The receiver removes these bits again after the CAN message reception.

Error handling
If any station on the CAN data bus detects an error, it interrupts the current transmission by sending an error message. The error message consists of 6 dominant bits. When an error message appears a local error is distributed to all the connected control units so that they ignore the transmitted CAN message.

After a short interruption the other control units can transmit CAN messages again according to their priority.

The control unit whose CAN message caused the error also begins to send its CAN message again (Automatic Repeat Request).