The hardware utilized are RS-485 transceivers wired for open collector operation through the use of a pullup and pulldown of the separate data lines. Characters are transmitted in the common 8N1 format.
Messages start with a Message ID (MID) character and finish with a checksum at the end. 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. 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. The protocol is maintained by SAE International. Common higher layer protocols that operate on top of J1708 are SAE J1587 and SAE J1922. With respect to Open System Interconnection model (OSI), J1708 defines the physical layer.
SAE J1708 is a standard used for serial communications between ECUs on a heavy duty vehicle and also between a computer and the vehicle.
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