The Signal Protocols of OBD-II

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OBD-II Currently has five signaling protocols.

It is often possible to make an educated guess as to which protocol is being used on a specific car based on the pins present in the connector. Manufactures mostly adopt and use only one throughout their range. For instance, Jaguar uses ISO 9141-2.

SAE J1850 PWM

  • (41.6 kbaud, standard of the Ford Motor Company).
  • pin 2: Bus-
  • pin 10: Bus+
  • Characteristics
  • High voltage is +5V
  • Message length is restricted to 11 bytes, including CRC
  • Employs a multi-master arbitration scheme called 'Carrier Sense Multiple Access with Non-Destructive Arbitration' (CSMA/NDA)
SAE J1850 VPW (Variable Pulse Width) (10.4/41.6 kbaud, standard of General Motors).
  • pin 2: Bus+
  • Characteristics
  • Bus idles low
  • High voltage is +7V
  • Decision point is +3.5V
  • Message length is restricted to 11 bytes, including CRC
  • Employs CSMA/NDA (see above)

ISO 9141-2. This protocol has a data rate of 10.4 kbaud, and is similar to RS-232. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.

  • pin 7: K-line
  • pin 15: L-line (optional)
  • Characteristics
  • UART signaling (though not RS-232 voltage levels)
  • K-line idles high
  • High voltage is Vbatt
  • Message length is restricted to 11 bytes, including CRC

ISO 14230 KWP2000 (Keyword Protocol 2000).

  • pin 7: K-line
  • pin 15: L-line (optional)
  • Characteristics
  • Physical layer identical to ISO 9141-2
  • Data rate 1.2 to 10.4 kbaud
  • Message may contain up to 255 bytes in the data field

ISO 15765 CAN (250kbit/sec or 500kbit/sec). The CAN protocol has become a popular standard outside of the USA car industry and has made significant impact in the OBD-II Share of the market. From 2008, all vehicles sold in the USA will be required to use the CAN bus, this will remove the need for the five distinctly different protocols.

  • pin 6: CAN High
  • pin 14: CAN Low

Note that pins 4 (battery ground) and 16 (battery positive) are present in all configurations. Also, ISO 9141 and ISO 14230 use the same pin out, thus you cannot distinguish between the two simply by examining the connector.