J1708 or by it’s complete name SAE J1708, is a standard used for serial communications between ECUs on a heavy duty vehicle and also between a computer and the vehicle.

If you’re familiar with data communication or networking, the SAE J1708 is categorized in the Physical layer of the Open System Interconnection model (OSI).

Common higher layer protocols that operate on top of J1708 are SAE J1587 and SAE J1922.

This post is under ‘gadget’ since I was looking for a way to interconnect my GPS device to my car a few months back.

The standard defines a 2-wire 18 gauge wire cable that can run up to 130 feet (40 m) and operates at 9600 bit/s. A message is composed of up to 21 characters, unless the engine is stopped and the vehicle is not moving in which case transmitters are allowed to exceed the 21 byte max message length. Messages start with a Message ID (MID) character and finish with a checksum at the end. Characters are transmitted in the common 8N1 format.