Acronym for: controller area network. CAN is a digital communications network, originally designed for use on vehicles but now more widely used. International Standard ISO 11898 (parts 1 to 4) lays down the basis for CAN specifications.

These CAN specifications only define the data link layer of the OSI 7 layer model – see below. This means that “CANbus” is not a complete definition of a network; more detail needs to be included for a complete description. There are a multitude of different standards related to CANbus applications & a few are listed below. Other CAN related standards extend the range of specification to include further layers of the 7 layer model; for example; CANopen, MilCAN, SAE J1939.

The CAN bus protocol itself only lays down the message frame structure to be sent over the CANbus, together with hardware error handling & collision arbitration.

CAN specifications do not define the physical layer, which means that CAN could be operated over a number of different media, although a 120 ohm terminated, 5 volt open collector drive to a Hi & Lo differential twisted pair is usual.

The Society of Automotive Engineers (SAE) has defined network categories for vehicle use:

• Class A: Low speed (~10kb/s); for entertainment systems & the like
• Class B: Medium speed (~125kb/s); for control of body functions, such as lights, heater, wipers
• Class C: High speed (~250kb/s); for real time control such as braking, steering, traction

CAN related specifications (just a few as examples):