Controller Area Network (CAN)

WHAT´S CAN

The Controller Area Network (CAN) doesn't try to reach heaven like the Tower of Babel. CAN is a serial bus system especially suited for networking "intelligent" devices as well as sensors and actuators within a machine or plant.

CAN is a serial bus system with multi-master capabilities, that is, several CAN nodes can request the bus simultaneously. The serial bus system with real-time capabilities is the subject of the ISO 11898 international standard and chose cover the lowest two layers of the ISO/OSI reference model. In CAN networks there is no addressing of subscribers or station in the conventional sense, but instead, prioritized messages are transmitted. A transmitter sends a message to all CAN nodes. Each decide on the basis of the identifier received whether they should process themessage or not. The identifier also determines the priority that the message enjoys in competition for bus access.

The relative simplicity of the CAN protocol means that very little cost and effort need to expended on personal training; the CAN chips interfaces make applications programming relatively simple. Introductory courses, function libraries, starter kits, host interfaces, I/O modules and tools are available form a variety of vendors permitting low-cost implementation of CAN networks.

Low-cost controller chips implementing the CAN data link layer protocol in silicon and permitting simple connection to microcontrollers have been availalbe since 1989. Today there are more than about 25 CAN protocol controller chips from than 10 manufacturers announced, not included gate array solutions.

The use of CAN in most of European passenger cars and the decision by truck and off-road vehicle manufacturers guarantees the availability of CAN chips for more than 10 years. Other high volume markets, like domestic appliances and industrial control, also will increase the CAN sales figures. Up to spring 1996 there are more than 10 million CAN nodes installed.

One of the outstanding features of the CAN protocol is its high transmission reliability. The CAN controller registers a stations error and evaluates them statistically in oder to take appropriate measures. These may extend to disconnecting the CAN node producing the errors.

CAN networks can be used as an embedded communication system for microcontrollers as well as an open communication system for intelligent devices.

The CAN serial bus system, originally developed for use in automobiles, is increasingly being used in industrial as well as building automation. If the requirements for passenger cars networks are compared with those for industrial field bus systems, the similarities are remarkable. In both cases some of the major requirements are: low cost, the ability to function in a difficult electrical environment, a high degree of real-time capability and ease of use.

Some users, for instand in the field of medical engineering, opted for CAN because they have to meet particulary stringent safety requirements. Similar problems are faced by manufacturers of other equipment with very high safety or reliability requirements (e. g. robots, lifts and transportation systems).

Each CAN message can transmit from 0 to 8 bytes of user information. Of course, you can transmit longer data information by using segmentation. The maximum transmission rate is specified as 1 Mbit/s. This value applies to networks up to 40 m. For longer distances the data rate must be reduced: for distances up to 500 m a speed of 125 kbit/s is possible, and for transmissions up to 1 km a data rate of 50 kbit/s is permitted.