CAN bus

CAN bus

(Controller Area Network bus) A rugged, digital serial bus designed for industrial environments. Introduced by Bosch in the mid-1980s for in-vehicle communications, it is used in myriad applications including factory automation, building automation, aircraft and aerospace as well as in cars, trucks and buses. CAN bus replaced bulky wiring harnesses with a two-wire differential cable (the two wires carry inverted voltages to decrease interference).

CAN provides services at layers 1 and 2 of the OSI model and uses a broadcast method for placing frames on the wire somewhat similar to Ethernet. Bus distance is based on speed, ranging from a maximum of 40 meters at 1 Mbps to a maximum of six kilometers at 10 Kbps. At speeds up to 125 Kbps, CAN provides fault tolerance. If one of the two wires is cut or shorted, the other keeps transmitting.

In a vehicle, both low- and high-speed CAN buses are used. For example, window, lighting and seat control only need low speeds, while engine, cruise control and antilock brakes require high speeds. Two or three CAN buses may be used in a vehicle; for example, a high-speed bus may be dedicated only for safety (air bags, seat belt tensioners, etc.).

The Bus Hampers After-Market Installations
The CAN bus has made it challenging to add certain after-market products to a vehicle. In older cars, there were discrete wires for everything, but CAN bus signals are digital frames that have to be analyzed to determine their purpose. For example, taxi meters and many GPS navigation systems need to monitor vehicle speed. If speed pulse is no longer available as a discrete wire, a CAN bus interface, such as the CANM8-NAV unit from Bridgewater Electronics (www.bridgewater-electronics.co.uk), can convert speed pulse frames to a speed pulse signal.

CANopen and CiA
Introduced in 1995, CANopen is a high-level application layer protocol that provides services for processes, data and network management. The international organization that governs the CANopen protocol is CAN in Automation (CiA). For more information, visit www.can-cia.de.