CAN Data Bus Data Telegram

CAN Data Bus Data Telegram

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.





The data telegram for transmitting CAN messages through the CAN data bus consists of seven sequential blocks:

- Start of Frame: Marks the start of a CAN message (Start bit) and synchronizes all stations.

- Arbitration Field: This field consists of the identifier (identifier and) (address) with 11 bits and one control bit request) (remote transmission request bit). This control bit indicates if the message contains a data frame or a remote frame without bytes.

- Control Field: To distinguish (control bits) 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: The data field can contain between 0 and (data) 8 bytes of data. A CAN message with a length of 0 can be employed to synchronize distributed processes.

- CRC Field: The CRC Field (Cyclic-Redundancy- (check field) check field) consists of 16 bits and is used to detect transmission errors.

- ACK Field: The ACK Field (Acknowledgement Field) (receipt contains a confirmation signal from all confirmation) receivers that have received the CAN message without error.

- End of Frame: Identifies the end of the data telegram (frame end)

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