CAN data bus data telegram - GF54.00-P-0005-01A
WARNING: This page is about a different car, the 2009 Mercedes-Benz CLK350 and 2008 Mercedes-Benz CLK350. However, it is still accessible from the selected car via links, so may be relevant.
During normal operation, messages consist of the following transfer frames:
- Data frame for transmitting CAN messages (such as coolant temperature, etc.).
- Remote frame for requesting CAN data from a different control unit.
- Error frame informs all connected control units that an error has occurred and that the most recent CAN message is invalid.
The CAN protocol supports two different data frame formats, with the only difference residing in the length of the identifier:
- the standard format
- the expanded format
Only the standard format is currently in use at DaimlerChrysler.
The data telegram for transmitting CAN messages through the CAN data bus consists of seven sequential blocks:
- Start of Frame: (Start bit) marks the start of a CAN message and synchronizes all stations.
- Arbitration Field: (identifier and Request) This field consists of an identifier (address) with 11 bits and a control bit (Remote Transmission Request-Bit). This control bit indicates if the message contains a data frame or a remote frame without bytes.
- Control Field: (control bits) To distinguish between the standard and extended format the Control Field (6 Bit) contains the IDE-bit (Identifier Extension Bit), a reserved bit for future expansions and in the last 4 bits the number of data bytes contained in the data field.
- Data Field: (data) The data field can contain between 0 and 8 Bytes of data contained. A CAN message with a length of 0 can be employed to synchronize distributed processes.
- CRC Field: (check field) The CRC Field (Cyclic-Redundancy Check Field) comprises 16 Bit and serves as security for detecting transmission errors.
- ACK Field: (confirmation of receipt) The ACK Field (Acknowledgment Field) contains a confirmation signal from all recipients which have received the CAN message free from faults.
- End of Frame: (frame end) Identifies the end of the data telegram
- Intermission: (interval) Interval between two data telegrams The interval must contain at least 3 bits. The next message can then be sent by any control unit.
- IDLE: (bus idle) If no control unit transmits, the CAN data bus in idle condition until the next data telegram is sent.