GF54.00-P-0005A | CAN data bus, function | 18.10.11 |
MODEL 124, 129, 140, 163, 168, 170, 202, 208, 209, 210, 211, 215, 220, 240,
463 up to 31.5.12
MODEL 203, 230 up to Model Year 2008 /AEJ 07
The CAN data bus (Controller Area Network) is a data bus system specially designed for use in the motor vehicle.
The CAN data bus is a bi-directional bus, that is, each connected control unit is able to send and receive information.
The CAN data bus consists of a special twisted two-core cable. The users (control units) are connected to this cable. Transmission of data takes place redundantly via both cables, whereby the data bus levels are mirrored (that is, if the level on one cable is 0, the other cable transmits level 1 and vice versa). Two-line operation is used for two reasons: for fault detection and as a safety concept.
If a voltage peak occurs on just one line (e.g. due to EMC problems), the receivers can identify this as a fault and ignore the voltage peak.
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If a short circuit or a discontinuity occurs on one of the two CAN lines, then due to a safety concept coupled to the software/hardware, the system can change over to single-line operation. The defective data line is shut down.
A specific data protocol controls how and when the participants can send and receive.
The CAN data bus is significantly different to other data bus systems that are based on the participant addressing principle, in that it uses a message-related addressing system.
, which identifies the CAN message content (e.g. coolant temperature). The CAN log permits up to 2048 different CAN messages, the addresses of 2033 to 2048 being permanently assigned. Data capacity per CAN message is 8 bytes.
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A recipient only evaluates those CAN messages (data messages) which are stored in its list of CAN messages to be accepted (acceptance filtering).
Data messages can only be sent when the CAN data bus is free (i.e. once a 3 bit interval has elapsed after the last data message, and no control unit starts to send). The data bus level is then in recessive state (logical 1).
If several control units simultaneously start to send, a procedure is applied whereby the CAN message
with the highest priority takes precedence, without loss of time or bits (attributing).
Each control unit which looses the attributing becomes a receiver automatically and repeats its transmission attempt as soon as the CAN data bus is free again.
Besides data messages, there is also a request message used for calling a particular CAN message.
The control unit that can provide the requested data message then reacts to the request.