Vehicle bus

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 arer Engine Control Modules (ECM), Transmission Control Modules (TCM), Anti-lock Brake System Modules (ABS) etc.

Background

The main driving force (pun intended) 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 bacame impossible to attain this level of control without the help of on-board computing devices. On-board electronic devices also have contributed substantially to vehicle performance, occupant comfort, ease of manufacture and cost effectiveness. Where as at one time a radio probably was the only electronic device in an automobile, nowadays practically everything in the automobile has some electronic feature in them. As mentioned above some of the typical electronic modules on today's vehicles are ECM, TCM, 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. (The engine needs to tell the transmission what the engine speed is, the transmission needs to tell everybody when the gear shift occurs, ABS needs to tell the engine and transmission that the wheels are locking). That is how vehicle networks came about. The vehicle network is the medium of data exchange.

Let us say that every module needs to exchange data with every other module. Let us say Engine Control Module needs to tell all other modules what the engine speed is, Transmission control module needs to tell all modules what the current gear is etc. Wiring the vehicle to connect all the modules together to achieve this communication is a nightmare let alone being highly error prone.

Now imagine the scenario where some vehicles on the assembly line don't have the ABS option, some don't have power locks/windows option etc. requiring different wiring harnesses. Stocking up on wiring harnesses for all possible combination of options and installing these in the vehicles will become a Herculean task. As a result cost of the vehicle goes up. Also because of the complicated wiring diagnosing and fixing problems becomes a challenge.

The industry's answer to this nightmare was the use of networks in vehicles. This allowed for ease of manufacture, better maintenance capability and flexibility in being able to add and remove options without affecting the entire vehicle's wiring architecture. Each module, a node on the vehicle network, is an island of control; controlling specific components related to their function and communicating 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:

- 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 (remember, amount of data that can be transferred in a second) it is changing quickly as more and more computing is done on-board requiring faster networks.

There are several network types and protocols used in the vehicles by various manufactures. Even though there is a push to standardize one network protocol for all manufacturers that dream is still a long way away.

Some examples of transmission media use in vehicle networks:

- Single wire

- Twisted Pair

- Optic Fibre

Common vehicle busses include: