The need for serial communication in vehicles
Many vehicles already have a large number of electronic control systems. The growth of automotive electronics is the result partly of the customer’s wish for better safety and greater comfort and partly of the government’s requirements for improved emission control and reduced fuel consumption. Control devices that meet these requirements have been in use for some time in the area of engine timing, gearbox and carburettor throttle control and in anti-block systems (ABS) and acceleration skid control (ASC).
The complexity of the functions implemented in these systems necessitates an exchange of data between them. With conventional systems, data is exchanged by means of dedicated signal lines, but this is becoming increasingly difficult and expensive as control functions become even more complex. In the case of complex control systems (such as Motronic) in particular, the number of connections cannot be increased much further.
Acceleration Skid Control
Moreover, a number of systems are being developed which implement functions covering more than one control device. For instance, ASC requires the interplay of engine timing and carburettor control in order to reduce torque when drive wheel slippage occurs. Another example of functions spanning more than one control unit is electronic gearbox control, where ease of gear-changing can be improved by a brief adjustment to ignition timing.
If we also consider future developments aimed at overall vehicle optimization, it becomes necessary to overcome the limitations of conventional control device linkage. This can only be done by networking the system components using a serial data bus system. It was for this reason that Bosch developed the “Controller Area Network” (CAN), which has been internationally standardized (ISO 11898) and has been “implemented in silicon” by several semiconductor manufacturers.
Using CAN, peer stations (controllers, sensors and actuators) are connected via a serial bus. The bus itself is a symmetric or asymmetric two wire circuit, which can be either screened or unscreened. The electrical parameters of the physical transmission are also specified in ISO 11898. Suitable bus driver chips are available from a number of manufacturers.
The CAN protocol, which corresponds to the data link layer in the ISO/OSI reference model, meets the real-time requirements of automotive applications. Unlike cable trees, the network protocol detects and corrects transmission errors caused by electromagnetic interference. Additional advantages of such a network are the easy configurability of the overall system and the possibility of central diagnosis. The purpose of using CAN in vehicles is to enable any station to communicate with any other without putting too much load on the controller computer.
Use of the CAN network in vehicles
There are four main applications for serial communication in vehicles, each having different requirements and objectives.
Industrial applications of the CAN network
A comparison of the requirements for vehicle bus systems and industrial field bus systems shows amazing similarities:
- low cost
- operability in a harsh electrical environment
- high real-time capabilities and
- ease of use
are equally desirable in both sectors. The standard use of CAN in Mercedes-Benz’s “S” Class and the adoption of CAN by US commercial vehicle manufacturers for fast transmissions (up to 1 MBit/s) has made industrial users prick up their ears. Not only manufacturers of mobile and stationary agricultural and nautical machinery and equipment have chosen to use CAN, is has also been the choice of manufacturers of medical apparatus, textile machines, special-purpose machinery and elevator controls. The serial bus system is particularly well suited to networking “intelligent” I/O devices as well as sensors and actuators within a machine or plant.
The textile machinery industry is one of the pioneers of CAN. One manufacturer equipped his looms with modular control systems communicating in real time via CAN networks as early as 1990. In the meantime several textile machinery manufacturers have joined together to form the “CAN Textile Users Group”, which in turn is a member of the international users and manufacturers group “CAN in Automation”. Similar requirements to those of the textile machinery are to be found in packaging machinery and machinery for paper manufacture and processing.
In the USA a number of enterprises are using CAN in production lines and machine tools as an internal bus system for networking sensors and actuators within the line or machine.
Some users, for instance in the medical engineering sector, decided in favour of CAN because they had particularly stringent safety requirements. Similar problems are faced by other manufacturers of machinery and equipment with particular requirements with respect to safety (e.g. robots and transport systems).
Apart from the high transmission reliability, the low connection costs per station are a further decisive argument for CAN. In applications where price is critical it is of essential importance that CAN chips be available from a variety of manufacturers. The compactness of the controller chips is also an important argument, for instance in the field of low-voltage switchgear.