With respect to Open System Interconnection model (OSI), J1708 defines the physical layer. Common higher layer protocols that operate on top of J1708 are SAE J1587 and SAE J1922.
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.
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. Transmission is accomplished by controlling the driver enable pin of the transceiver. This method allows multiple devices to share the bus without the need for a single master node. Collisions are avoided by monitoring the bus while transmitting the MID to ensure that another node has not simultaneously transmitted a MID with a higher priority.
|B||KL15||constant hot (Bat+) or ignition (10A fuse)|
|E||J1939 GND||CAN ground|
H,J are not connected. A and B are twisted in cable. C and D are twisted in cable. F and G are twisted in cable.
Typical SAE J1708/J1587 MIDs:
Instrument Panel: 140,234
Vehicle Management System: 142
Fuel System: 143
Cab Climate Control: 146, 200
Suspension: 150, 151, 152, 153
Vehicle Navigation: 162, 191
Vehicle Security: 163
Tires related: 166, 167, 168, 169
Particulate Trap System: 177
Refrigerant Management systems: 190
Tractor/Trailer Bridge: 217, 218
Collision Avoidance Radar: 219
Driveline Retarder: 222
Vehicle Sensors to data converter: 178
Safety Restraint System: 232
Forward Road Image Processor: 248