CAN data bus data telegram - GF54.00-P-0005-01A

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:

• Standard format
• Expanded format

Only the standard format is currently in use at DaimlerChrysler.

Fig 1: CAN Data Bus Data Telegram

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 the identifier (address) with 11 bits and one 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 expanded format the control field (6 bit) contains a reserved bit for future expansions and the number of bytes contained in the data field in the last 4 bits.
• Data Field: (data) The data field can contain between 0 and 8 bytes of data. 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) consists of 16 bits and is used to detect transmission errors.
• ACK Field: (receipt confirmation) The ACK Field (Acknowledgment Field) contains a confirmation signal from all receivers that have received the CAN message without error.
• End of Frame: (frame end) Identifies the end of the data telegram

• Intermission:

  (Intermission) 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: (idle)

  If no control unit transmits, the CAN data bus remains idle until the next message is sent.