This article is intended to give engineers a brief overview of the SAE J1939 protocol. For details on the specification itself, please visit the Society of Automotive Engineers. Here is a printable version of this J1939 introduction and Bosch CAN specification.

SAE J1939 is a replacement for the SAE J1587/J1708 network which has been used for medium and heavy duty commercial vehicles for the past 20 years. Today SAE J1939 is used as the standard communications bus for commercial vehicle’s diagnostic and control applications. Due to its popularity and success, it has been adopted by the agricultural (ISO 11789) and marine industries (NMEA 2000).

Several different application layers exist and are known as device profiles. For example, CiA 401 defines the application layer (i.e. device profile) for a generic I/O device while CiA 402 defines operation for servo drives, frequency inverters and stepper motors. Altogether there are more than 50 separate and defined device profiles.

J1939 Overview

SAE J1939 is a high level communications protocol, which operates on a Controller Area Network (CAN) bus. SAE J1939 specifies exactly how information (e.g. engine RPM) is exchanged between electronic control units (ECUs) on a vehicle.

It defines the data’s priority, size, scaling, and offset. For example, J1939 specifies engine RPM to have a default priority of 3, to have a size of 16-bits, a resolution of 0.125 rpm/bit, and an offset of 0.

The standard goes on to define many other aspects, including message timeouts, how large messages are fragmented and reassembled, the network speed, the physical layer, and how applications acquire network addresses.