Data Bus System Function
Data Bus System Function
Data bus:
Data bus refers to a system for transporting and distributing data.
Audi A4 uses various data bus systems. These systems function on different levels. The instrument cluster control module (J285) forms the interface (gateway) between the various data bus systems.
CAN:
A "controller area network" is a data bus system that uses two wires. These are referred to as data bus wires. The data is transmitted serially, i.e., in series, via the data bus wires to the connected control modules.
These control modules communicate via the data bus systems, i.e., they exchange information (data) via the data bus systems. Refer to 186 The CAN data bus
• The comfort data bus system
- Functions with a transmission rate of 100 kbaud (100,000 information units per second)
- Functions if one of the two line connections fails in emergency mode (e.g. due to an open circuit).
- The following control modules are connected for example
- Instrument cluster control module (J285) (interface to the powertrain data bus system)
- The Climatronic control module (J255) A/C control head of the A/C system
- Auxiliary heater control module (J364) (only in vehicles with an original Audi parking/auxiliary heater)
- Steering column electronic systems control module (J527)
- Additional control modules connected to the comfort data bus system.
• The powertrain data bus system
- Functions with a transmission rate of 500 kbaud (500,000 information units per second)
- Only functions when data can be transmitted on both wire connections (protective circuit).
- The following control modules are connected for example:
- Instrument cluster control module (J285) (interface to the comfort data bus system)
- The respective engine control module
- Additional control modules connected to the powertrain data bus system.
• "CAN bus" refers to a system for transporting and distributing data.
• Depending on the vehicle equipment, various control modules are connected to the comfort data bus system or to the powertrain data bus system.
• The data is transmitted in the data bus systems simultaneously via the "CAN-Hi" and "CAN-Low" data lines (two-wire data bus systems). Depending on the data bus system version, these wires are twisted together to minimize interference from other systems. They can also be shielded with respect to the other wires.
• The wire connections between the control modules used to transmit data are referred to as data lines.
• The data is transmitted serially, i.e., in series, to the connected control modules via the data lines. This data cannot be checked with workshop equipment.
• The data is transmitted and expected by the various control modules in regular intervals as telegrams. The time interval between two telegrams transmitted by a control module depends on the importance of the transmitted data and takes between 0.01 and 0.2 seconds.
• If a connected control module does not receive any information within a defined time period or if the received data telegrams cannot be evaluated, a fault is stored in the DTC memory.
• Information receipt is displayed for example for the A/C system in the measured value block of display groups "125" and " 126". Refer to => [ Display Group 125 ] Measured Value Blocks From Display Group 020.
• If a control module cannot provide the requested data (or only certain information) on the basis of a component fault, a fault message requesting read out of the DTC memory in this control module is transmitted with the data telegram.
• Example:
• The outside air temperature sensor (G17) is not connected, and the instrument cluster control module ( J285) cannot transmit the measured value of this sensor in the data telegram.
• The instrument cluster control module (J285) saves this fault and includes a fault message instead of the measured value in the data telegram.
• When evaluating the telegram, the Climatronic control module (J255) A/C control head recognizes the fault message and saves the information "read DTC memory of the instrument cluster control module" and in the DTC memory.