Information Bus: Description and Operation
Bus-System, Function
Bus:
The term bus refers to a system that transports and distributes data.
Audi A4 employs different Bus-systems. These systems operate on different levels. The control module with indicator unit in instrument cluster builds the interface (Gateway) between the different bus-systems.
CAN:
The Controller Area Network (CAN) is a bus system that operated using two wires. They are called bus wires. Data is transferred to the connected control modules serially (one after the other) across the bus wires.
These control modules communicate with each other via the CAN-bus, this means they exchange data via the bus-system.
- The CAN-bus comfort system
- The CAN-bus operates at a speed of 100 K bits/s (100,000 bits per second).
- Operates even if one wire is faulty, but only in emergency mode (e.g. open circuit).
- E.g. the following control modules are connected.
- Control module with indicator unit in instrument cluster -J285- (interface to CAN-bus system)
- A/C Control Head -E87- Steering Column Electronic Systems Control Module -J527-
- Additional control modules connected to the CAN-bus comfort system.
- CAN-bus system
- The CAN-bus operates at a speed of 500 K bits/s (500,000 bits per second).
- Operates only if data can be transferred on both wires (safety mode)
- E.g. the following control modules are connected.
- Control module with indicator unit in instrument cluster -J285- (interface to CAN bus comfort system)
- The Engine Control Module (ECM)
- Additional control modules connected to the CAN-bus system
NOTE:
- The term "CAN-bus" refers to a system that transports and distributes data.
- Different control modules are connected to the CAN-bus comfort system and CAN-bus system, depending on vehicle equipment.
- Data is simultaneously transferred via the data lines (wires) "CAN-Hi" and "CAN-Lo" (Two line CAN-Bus). Depending on Bus-system version, theses wires are twisted together in order to minimize interference from other systems and shielded from other wires.
- The wire connections between the control modules, via which data is transferred, are referred to as data wires.
- Data is transferred to the connected control modules serially (one after the other) via these data wires, this data cannot be checked using workshop equipment.
- Data is sent and excepted from various control modules in regular intervals as telegram (data message). Frequency between telegrams that a control module sends is dependent on the validity of the transferred data and is between 0.01 and 0.2 seconds.
- If a connected control module obtains no information within a specified time period, or if received data signals cannot be evaluated, a malfunction is stored in the DTC memory.
- The reception of the information e.g. "air conditioner" is displayed in measuring value block display groups 125 and 126.
- If a control module cannot supply the required data (or only a specific information) available due to component malfunction, a malfunction notice is sent with data signal, which requires checking the DTC memory of the control module.
Example:
The Outside Air Temperature Sensor -G17- is not connected, the Control module with indicator unit in instrument cluster -J285- therefore cannot sent the measuring value of this sensor in the data message.
The instrument cluster stores this malfunction and sends a malfunction notice instead of the measured value in data signal.
The A/C Control Head -E87- recognizes the malfunction notice when evaluating the telegram and stores "Instrument cluster, please check DTC memory".