Information on Controller Area Network (CAN), CANopen, DeviceNet, J1939

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2. Overview - Controller Area Network and J1939*

* This chapter is an excerpt from "A Comprehensible Guide to Controller Area Network" by Wilfried Voss

The standard CAN message frame uses an 11-bit message identifier (CAN 2.0A), which is sufficient for the use in regular automobiles and any industrial application, however, not necessarily for off-road vehicles.

The Society of Automotive Engineers (SAE) Truck and Bus Control and Communications Subcommittee had developed a family of standards concerning the design and use of devices that transmit electronic signals and control information among vehicle components. As a result, the higher layer protocol SAE J1939, based on CAN, was born, which was required to provide some backward-compatible functionality to older RS485-based communication protocols (J1708/J1587).

In order to serve these demands, the CAN standard needed to be enhanced to support a 29 bit message identifier. The ISO 11898 amendment for an extended frame format (CAN 2.0B) was introduced in 1995.

The 29 bit message identifier consists of the regular 11 bit base identifier and an 18 bit identifier extension. The distinction between CAN base frame format and CAN extended frame format is accomplished by using the IDE bit inside the Control Field. A low (dominant) IDE bit indicates an 11 bit message identifier, a high (recessive) IDE bit indicates a 29 bit identifier.

An 11 bit identifier (standard format) allows a total of 2¹¹ (= 2048) different messages. A 29 bit identifier (extended format) allows a total of 2²⁹ (= 536+ million) messages.

Both formats, Standard (11 bit message ID) and Extended (29 bit message ID), may co-exist on the same CAN bus. During bus arbitration the standard 11 bit message ID frame will always have higher priority than the extended 29 bit message ID frame with identical 11 bit base identifier and thus gain bus access.

The Extended Format has some trade-offs: The bus latency time is longer (minimum 20 bit-times), messages in extended format require more bandwidth (about 20 %), and the error detection performance is reduced (because the chosen polynomial for the 15-bit checksum is optimized for frame length up to 112 bits).