J1708

J1708

SAE J1708 is a standard used for serial communications between ECUs on a heavy duty vehicle and also between a computer and the vehicle. J1708 makes up the data link and physical layers. Common higher layer protocols that operate on top of J1708 are J1587 and J1922.

The standard defines a 2-wire 18 gauge wire cable that can run up to convert|130|ft|m|0 and operates at 9600 bit/s. A message is comprised 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.

SAE J1708, although still widely used, is replaced by SAE J1939 which is a CAN protocol.

The protocol is maintained by the SAE.

External links

* [http://www.dgtech.com/product/dpa/dpa.php Dearborn Group RP1210 J1708 Protocol Adapter]
* [http://www.dgtech.com/product/dpa/example/example.php TMC RP1210 Example Source Code]
* [http://www.simmasoftware.com/j1708.html SAE J1708 Source Code]
* [http://www.sae.org SAE International Website]


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