12-61 in fo @ mo xa .c o m www.mo xa .c o m Device Connectivity Multiport Serial Boards > Introduction to CAN 12 Introduction to CAN The CAN serial bus, which was developed for the automotive industry, was introduced in 1986 as the “Automotive Serial Controller Area Network.” It was soon discovered that CAN worked extremely well in other embedded systems applications, and consequently its popularity increased. The list of applications that use CAN includes weaving machines, elevator systems in large buildings, all kinds of ships, trains, aircraft, x-ray machines and other medical equipment, logging equipment, tractors and combines, coffee makers, and major appliances. The CAN serial protocol covers applications that range from high-speed networks to low-cost multiplex wiring. Automotive electronics, engine control units, sensors, and anti-skid-systems, for example, are connected using CAN with bitrates up to 1 Mbps. CAN signals are typically transmitted differentially through a pair of wires, since doing so greatly improves the reliability of signal transmissions even when the network is subject to low signal levels or common mode errors. The two wires are called CAN_H and CAN_L and use 120-ohm termination resistors. Many CAN systems also use twisted pair wires to reduce the effects of electromagnetic interference. CAN systems are popular since they use an inexpensive bus topology, make it easy to connect additional nodes, and are less prone to network failures. The Controller Area Network (CAN) is a serial protocol that allows multiple processors in a system to communicate with each other in an efficient manner. CAN is now the standard for high-speed, mission- critical, real-time control networks for different types of machines, due to the fact that the networks are reliable, relatively simple, and inexpensive. CAN systems are quite versatile, and mechanics and technicians find it easy to repair or replace computer hardware in a CAN system, without affecting the rest of the network in any way. In addition, design engineers can easily modify existing CAN systems for other uses by adding or removing network nodes. CAN Node A CAN Node C CAN Node B 120 ohm CAN_H CAN_L 120 ohm Layered Structure of a CAN Node Application Layer Object Layer -Message Filtering -Message and Status Handling Transfer Layer -Fault Confinement -Error Detection and Signaling -Message Validation -Acknowledgement -Arbitration -Message Framing -Transfer Rate and Timing Physical Layer -Signal Level and Bit Representation -Transmission Medium The object layer and the transfer layer comprise all services and functions of the data link layer defined by the ISO/OSI model The physical layer specifies the physical properties for transferring bits between different nodes and must be the same for all nodes belonging to the same network. The physical layer defines how signals are actually transmitted, but it is not defined to allow transmission medium and signal level implementations to be optimized for their applications. The specifications are designed to achieve compatibility between any two CAN implementations, where compatibility can refer to either electrical features or how transmitted data is interpreted. CAN is subdivided into different layers, as indicated in the accompanying table. The CAN Physical Layer