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1. Introduction to J1939

J1939 is a higher-layer protocol based on Controller Area Network (CAN). It provides  serial data communications between microprocessor systems (also called Electronic Control Units - ECU) in any kind of heavy duty vehicles.

The main advantages of using CAN as a field-bus technology are reduced wiring (CAN requires only two wires between nodes), extremely reliable communication, easy implementation and improved maintenance and service capabilities, which consequently not only produce better vehicle performance, but also help to reduce production costs.

J1939-based protocols are used in:

  • Diesel power-train applications

  • In-Vehicle networks for trucks and buses

  • Agriculture and forestry machinery (ISO 11783)

  • Truck-Trailer connections

  • Military vehicles (MiLCAN)

  • Fleet management systems

  • Recreational vehicles

  • Marine navigation systems (NMEA2000)

The protocol features of J1939 are based on two older SAE (Society of Automotive Engineers) specifications:

1. SAE J1708
SAE J1708 specifies on the physical layer of the communication link. It uses RS485 as an electrical layer operating at 9600 baud. (Note: Unlike RS232/485 there are no message collisions under CAN). Messages under J1708 start with a Message Identification Character, followed by the data information and a checksum. The message length is 21 characters (or less) and each data character is 10 bits long. Each character starts with a start bit of low polarity.

2. SAE J1587
SAE J1587 is a joint SAE/TMS "Recommended Practices for Electronic Data Exchange Between Microcomputer Systems in Heavy-Duty Vehicle Applications". It regulates the communication and standardized data exchange between ECUs based on J1708 networks.

Note: The situation regarding documents/literature on J1708 and J1587 is as dire as with J1939.

The J1939 specification is described by a number of SAE documents, the SAE J1939 Standards Collection:

Document 

Description

J1939

Recommended Practice for a Serial Control and Communications Vehicle Network*

J1939-01

Recommended Practice for Control And Communications Network for On-Highway Equipment
J1939-02 Agricultural and Forestry Off-Road Machinery Control and Communication Network**

J1939-11

Physical Layer - 250k bits/s, Twisted Shielded Pair

J1939-13

Off-Board Diagnostics Connector

J1939-15

Reduced Physical Layer, 250k bits/sec, Un-Shielded Twisted Pair (UTP)

J1939-21

Data Link Layer

J1939-31

Network Layer

J1939-71

Vehicle Application Layer

J1939-73

Application Layer - Diagnostics

J1939-74

Application - Configurable Messaging

J1939-75

Application Layer - Generator Sets and Industrial

J1939-81

Network Management

* This document seems to be obsolete. The hyperlink on the SAE web site produces an error message.

** This document is not listed in the "Core J1939 Standards" list on the SAE web site, but it can be found through their search feature.
Reference: http://www.sae.org/technical/standards/J1939/2_200608