General, Data Transmission, Structure

General
The CAN bus (Controller Area Network) is a bus system designed for use in motor vehicles.

The CAN data bus is a bidirectional bus, i.e. each control module connected is able to transmit and receive.

A defined data protocol controls how and when the participants transmit and receive.

As a significant feature which distinguishes it from other bus systems, which are based on the principle of participant addressing, the CAN data bus has message-specific addressing. This means that every message is assigned a fixed address (identifier) which identifies the contents of the message (e.g. ECT sensor). The CAN protocol permits up to 2048 different messages, the addresses between 2033 and 2048 being permanently assigned. The data capacity per message is 8 bytes.

A receiver only uses the messages (data telegrams) which are stored in its list of messages to be accepted (acceptance testing).

Data transmission
Data telegrams can only be sent when the CAN bus is free (i.e. when 3 bit intermission has elapsed after the last data telegram and no control module starts to transmit). The bus level is recessive during this (logic 1).

If several control modules start to transmit at the same time, a procedure applies whereby the message with the highest priority goes first, without loss of time or bits (arbitration).

Each control module which loses the arbitration automatically becomes a receiver and repeats its attempt to transmit as soon as the CAN data bus is free again.

Apart from data telegrams there is also a request telegram for particular data.

The control module, which can provide the required data telegram, then responds to the request.

Structure
The CAN data bus consists of a special twisted two-core cable. The participants control modules are attached to this cable. Data is transferred redundantly via the two lines, the bus level being mirrored, i.e. if the level is 0 on one line, the other line transmits level 1 and vice versa).

Two-line operation is used for two reasons; for fault recognition and as a reliability concept.

If a voltage peak occurs on one line only (e.g. due to EMC problems) the receivers can recognize this as a fault and ignore the voltage peak.

If a short-circuit or open-circuit occurs on one of the two CAN lines, the reliability concept software/hardware triggers switchover to single line operation. The defective transmission line is shut down.