CAN Data Bus Data Telegram

The following transmission frames are used for data telegrams in normal operation:

- Data Frame
for sending messages (e.g engine coolant temperature).

- Remote Frame
for requesting data from another control module.

- Error Frame
All control modules connected are notified that a fault has occurred and the last message is invalid.

The CAN protocol supports two different formats of data frames which only differ exclusively in the length of the identifier: the standard format and the extended format. Only the standard format is currently used at Mercedes-Benz.







The data telegram for sending messages via the CAN data bus consists of seven sequential fields:

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

- Arbitration field: (Identifier and request)
This field consists of the identifier and (address) with 11 bits and a remote transmission request bit. This remote transmission request bit identifies whether it is a telegram with data frame or remote frame without data bytes.

- Control field: (Control bits)
The control field (6 bits) contains the IDE bit (Identifier Extension Bit) to distinguish between the standard and extended format, a reserve bit for future extensions and in the last 4 bits the number of data bytes included in the data field.

- Data field:
The data field can contain between 0 and 8 bytes of data. A message of 0 length can be used for synchronization of distributed processes.

- CRC Field:
The CRC field (Cyclic Redundancy Check Field) comprises 16 bits and serves as a safeguard for recognizing transmission faults.

- ACK Field:
The ACK Field (Acknowledgement Field) contains a confirmation signal of all receivers which have received the message error-free.

- End of Frame:
Identifies the end of the data telegram.

- Intermission:
Intermission between two data telegrams. It must be at least 3 bits. The next data telegram can then be transmitted by any control module.

- IDLE:
If no control module is transmitting, the CAN data bus is idle until the next data telegram is transmitted.