Vehicle bus

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A vehicle bus is an electronic communications network that interconnects components inside an automobile, bus, industrial or agrigultural vehicle, ship, or aircraft. Due to the specialised requirements of each type of deployment (including environmental constraints, cost, reliability and realtime characteristics), conventional computer networking technologies (such as ethernet and TCP/IP) are rarely used. Typical Vehicle electronic components that communicate with each other on the vehicle bus are Engine Control Modules (ECM), Transmission Control Modules (TCM), Anti-lock Brake System Modules (ABS) etc.

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 limitations placed on the emission gases for the automobiles, it became impossible to attain this level 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 radio was likely the only electronic device in an automobile, but now almost every component of the vehicle has some electronic feature. As mentioned above some of the typical electronic modules on today's vehicles are the Engine Control Module (ECM), the Timing Control Module (TCM), 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. The vehicle network is the medium of data exchange.

The automotive industry quickly realized the complexity of wiring each module to every other module. See the illustration below for an example of how this wiring scheme would look.

This 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.

The illustration below provides an overview of this design.

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

Low Cost
Immunity from external noise
Ability to operate in harsh environments
Overall robustness and reliability

Although the vehicle network did not place too much emphasis on the data throughput, the demand for more on-board computing is continuing to drive changes to these networks to provide higher-speed communication between modules.

Architecture

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.

Some examples of transmission media use in vehicle networks:

Single wire
Twisted Pair
Optic Fibre
Power-line communication[link]

Common vehicle busses include:

Local Interconnect Network (LIN) - a very low cost in-vehicle sub-network
Controller Area Network (CAN) - an inexpensive low-speed serial bus for interconnecting automotive components
J1939 and ISO11783 - an adaptation of CAN for agricultural and commercial vehicles
FlexRay - a general purpose highspeed protocol with safety-critical features
Media Oriented Systems Transport (MOST) - a highspeed multimedia interface
Keyword Protocol 2000 (KWP2000) - a protocol for automotive diagnostic devices (runs either on a serial line or over CAN)
Vehicle Area Network (VAN)
DC-BUS [link] - Automotive power-line communication multiplexed network
IDB-1394
SMARTwireX

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

External links

http://www.interfacebus.com/Design_Connector_Automotive.html

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