Vehicle bus

Background

The main driving forces for the development of vehicle network technology have been the advances made in the electronics industry in general and government regulations imposed, especially in the United States, in order to make the automobiles environmentally friendly.

With stringent emission standards for automobiles, it became impossible to attain the required degree of control without the help of on-board computing devices. On-board electronic devices have also contributed substantially to vehicle performance, occupant comfort, ease of manufacture and cost effectiveness.

At one time, a car radio was likely the only electronic device in an automobile, but now almost every component of the vehicle has some electronic feature. Typical electronic modules on today's vehicles include the Engine Control Unit (ECU), the Transmission Control Unit (TCU), the Anti-lock Braking System (ABS) and body control modules (BCM).

An electronic control module typically gets its input from sensors (speed, temperature, pressure, etc.) that it uses in its computation. Various actuators are used to enforce the actions determined by the module (turn the cooling fan on, change gear, etc.). The modules need to exchange data among themselves during the normal operation of the vehicle. For example, the engine needs to tell the transmission what the engine speed is, and the transmission needs to tell other modules when a gear shift occurs. This need to exchange data quickly and reliably led to the development of the vehicle network, as the medium of data exchange.

The automotive industry quickly realized the complexity of wiring each module to every other module. Such a wiring design would not only be complex, it would have to be altered depending on which modules were included in the specific vehicle. For example, a car without the anti-lock brake module would have to be wired differently than one that included anti-lock brakes.

The industry's answer to this problem was to create a central network in the vehicle. Modules could be 'plugged' into the network and would be able to communicate with any other module that was installed on the network. This design was easier to manufacture, easier to maintain and provided the flexibility to add and remove options without affecting the entire vehicle's wiring architecture. Each module, a node on the vehicle network, controls specific components related to its function and communicates with the other modules as necessary, using a standard protocol, over the vehicle network.

Networks were not new, but their application to the vehicle was. The networks for the vehicles called for:

Although the vehicle network made modest demands on data throughput, the demand for more on-board computing is continuing to drive changes to these networks to provide higher-speed communication between modules. The control area network include the receiver and transmitter for the host to controller transmission and interlinking between the computers

Protocols, physical media and connectors

There are several network types and protocols used in vehicles by various manufactures. Many companies are encouraging a standard communication protocol, but one has not been settled on.

Protocols

Common vehicle buses protocols include:

Physical transmission media

Some examples of physical transmission media use in vehicle networks:

Connectors

Additionally, many major car manufacturers use their own proprietary vehicle bus standards, or overlay proprietary messages over open protocols such as CAN.

Early history of buses on GM vehicles

The earliest known GM data communication link from one electronic module to another was introduced in the 1980 model year Cadillacs. The link was one wire point to point, linking the engine control module to the fuel data panel. In model year 1981 all GM passenger cars for the US market used a similar data link to a test connector for assembly line diagnostics. The value of this data in diagnosing emission systems after customer delivery was quickly identified. Scanner tools were made to view and interpret the data stream. The data line was the same line that turned on the "Check Engine" lamp in the instrument panel. Data was sent at 80 bits per sec. 9 bits per frame, 21 bytes (frames), 2.4 sec./transmission.

In 1981 model year Cadillacs, two 80 bit point to point links were present, with the second link going from the engine control module to the climate control module.

In the 1982 model year, fuel injection engine control modules had the data port moved to a dedicated full-time connection (Corvette and 2.5 L engines).

In the 1983 model year, the data rate on the fuel injection engine control modules was doubled to 160 bit/s.

In the 1984 model year, Corvette Instrument panel used data from the fuel injection engine control module's data link.

In the 1985 model year, Cadillac FWD "C" vehicle had an electronic system that had point to point data links between 5 electronic modules and a dedicated assembly line diagnostic connector.

On the 1988 and 1989 model year Buick Reatta and 1986–1989 model year Buick Riviera touchscreen CRT equipped vehicles, an 8,192 bit/s data bus was implemented between the body computer module and the assembly line connector, climate control module, and CRT controller. This was GM's first multi-drop data bus. All GM vehicles adopted this bus as new engine control modules were developed. This bus remained the standard GM vehicle bus until 1996 when the J1850 VPW bus replaced it.