CAN-Bus System, Function

CAN-Bus System, Function

CAN-Bus:
CAN-Bus is the term used to describe a data transfer and distribution system.

There are various CAN-Bus systems in the Audi A4. These systems operate on different levels. The Instrument Cluster Control Module J285 forms the interface (gateway) between the various CAN-Bus systems.

CAN:
The "Controller Area Network" is a CAN-Bus system employing two wires. These are referred to as CAN-Bus wires. Data are transmitted serially via the CAN-Bus wires to the connected control modules.

These control units communicate with the aid of the CAN-Bus systems, i.e. they use the CAN-Bus systems to exchange information (data). 186 The CAN-Bus
- The CAN-Bus comfort system

- Operates at a transmission rate of 100 kbaud (100000 data units per second)
- Continues to operate even in the event of failure of one of the two wires, however only in emergency operation (e.g. because of an open circuit).
- Examples of connected control modules:
- Instrument Cluster Control Module J285 (interface with drive CAN-Bus system)
- Air conditioner operating and display module, Climatronic Control Module J255
- Steering Column Electronic Systems Control Module J527
- For further control units linked to the CAN-Bus comfort system, refer to

- The drive CAN-Bus system

- Operates at a transmission rate of 500 kbaud (500000 data units per second)
- Only operates if data can be transferred on both wires (safety configuration)
- Examples of connected control modules:
- Instrument Cluster Control Module J285 (interface with CAN-Bus comfort system)
- relevant engine control module
- For further control units linked to the drive CAN-Bus system, refer to

NOTE:
- "CAN-Bus" is the term used to describe a data transfer and distribution system.
- Depending on vehicle equipment, various control modules may be linked to the convenience or drive CAN-Bus system.
- The data are transferred simultaneously in the CAN-Bus systems via the "CAN-Hi" and "CAN-Lo" signal wires (two-wire bus data systems). Depending on the version of the CAN-Bus system, these wires may form twisted pairs to minimize interference from other systems. They may also be screened against the other wires.
- The wires between the control units which are used to transmit data are referred to as signal wires.
- These signal wires are used for serial transmission of data to the connected control modules. The data cannot be checked using workshop equipment.
- The data are transmitted and received by the various control modules at regular intervals. The interval between two data blocks transmitted by a control unit is governed by the priority of the data transferred and is between 0.01 and 0.2 seconds long.
- If a connected control unit does not receive any information within a specified period or if the data received cannot be evaluated, a fault is stored in the DTC memory.
- The reception of the information is displayed for example for the air conditioner in the measuring value block, display groups "125" and "126".
- If a control module cannot provide the data required (or just one item of information) because of a component fault, a DTC message is transmitted together with the data requesting readout of the DTC memory in this control unit.
- Example:
- - The Outside Air Temperature Sensor G17 is not connected and the Instrument Cluster Control Module J285 can therefore not transmit the measured value of this sensor in its data.
- The Instrument Cluster Control Module J285 saves this fault and sets a malfunction in its data instead of the measured value.
- The operating and display module, Climatronic Control Module J255 detects the DTC message on evaluating the data block and saves the information "Instrument Cluster Control Module, read out DTC memory" in the DTC memory.