Specifications

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Appendix B Frequently Asked Questions
The following sections address general questions about how data bus systems work. See “Testing
Bus Systems” on page 44 for bus information specific to BMW vehicles.
B.1 What is a data bus?
The data bus is a like a telephone line hooked between control units. It links the individual control
units to form one large system. The more information a control unit has of the overall system, the
better it can control specific functions that may affect the health of the whole system. Automotive
computer networks are radically transforming how automotive systems and components function.
Late model BMW vehicles have a major increase in vehicle electronic systems which need to
exchange large volumes of information efficiently without taking up too much space. If a separate
wire was required for each new item of information exchanged, the increase in wires and pins
would be prohibitive. Buses reduce the number of wires and pins and at the same time increase
available information. Buses make more space available through smaller control units and
connector plugs.
Buses are used for the following reasons:
• Fast communication speed—Buses provide high-speed data transfer between control units.
• High level of reliability—The number of moving parts and connectors are reduced. Buses
have a low error rate because they have continuous verification of the transmitted information.
• Reduction in harness size—The number of wires between control units is greatly reduced.
• Multiple utilization of sensors—One sensor’s information can be used by multiple modules,
eliminating the need for duplicate sensors.
• Adding new bus functions—More information can be transferred, which means it is easier
to add new bus functions without first requiring additional data input.
• Easy to add more controllers (expanding buses using software)—Other controllers can
be installed in the future without major changes to the harness or existing systems. When a
new ECM with more information is added to the bus, often software updates are the only
modifications necessary.
NOTE:
i The CAN data bus conforms to international standards—control units from different
manufacturers can be installed on a bus and interchange data, even though they were built by
different manufacturers.