CAN Data Bus, Function

CAN Data Bus, Function




Shown on example of Controller Area Network bus class C (engine compartment) (CAN-C) in model 220 with engine 112, 113
A1 Instrument cluster
AN3/10 ME-SFI [ME] control unit
AN15/3 ETC control unit
AN15/5 Electronic selector lever module control unit
AN22 AAC [KLA] control and operating unit
AN47 Traction systems control unit
AN51 AIRmatic with ADS control unit
AN63/1 DTR control unit (with code (219a) Distronic (DTR))
AN73 EIS control unit
AN80 Steering column module

Task
- Data interchange between the individual control units
- Readying sensor signals for several systems
- Reducing the number of wires
- Improving the electromagnetic compatibility

Design
The data bus consists of a twisted two-strand data line which connects all the participants to each other in a parallel circuit. The data line lines must not be mixed up (low or high level). With respect to bus adaptation the ME control unit (N3/10), DAS [FBS] control unit (N73/1) (model 163), DAS [FBS] radio frequency/infrared control unit (N54/4) (model 129) and DAS [FBS] radio frequency control unit (N54/3) (model 170) apply as a terminal . In each case a terminating resistor of 120 ohms is installed for this (exception ME control unit (N3/10) on engine 120: 240 ohms each).

Mote: For the resistance measurement of the data line (circuit 15 OFF) the various terminating resistors in the terminals are connected in parallel.

Function
The data are transmitted digitally at various intervals on the data bus. The individual data blocks are defined in a protocol and this also states which data are received or transmitted by a control unit. Each connected control unit is able to transmit or receive data.

The total of the data blocks, the short pause between two transmission intervals and further properties of the data bus are checked constantly. Any faults detected are stored.

Vehicles with EIS [EZS] control unit (N73)
In the vehicle a distinction is made between a fast Controller Area Network bus class C (engine compartment) (CAN-C) and Controller Area Network bus class B (interior) (CAN-B) which is 6 times slower. The EIS [EZS] control unit (N73) forms the interface (Gateway) for this. This does not apply to vehicles with central gateway control unit (N93) (except model 203).

Vehicles with central gateway control unit (N93)
The central gateway control unit (N93) forms the interface (gateway) between the two data busses (except model 203). On all model series the central gateway control unit (N93) serves as an interface for STAR DIAGNOSIS.

Note: The new control unit must be recoded when a control unit is exchanged. Control unit coding can be carried out via a menu in the handheld tester (up to 07/99) or via STAR DIAGNOSIS