OBDII signal protocols

There are five signaling protocols that are permitted with the OBD-II interface. Most vehicles implement only one of the protocols. It is often possible to deduce the protocol used based on which pins are present on the J1962 connector:

  • SAE J1850 PWM (pulse-width modulation — 41.6 kB/sec, standard of the Ford Motor Company)
    • pin 2: Bus+
    • pin 10: Bus–
    • High voltage is +5 V
    • Message length is restricted to 12 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 kB/sec, standard of General Motors)
    • pin 2: Bus+
    • Bus idles low
    • High voltage is +7 V
    • Decision point is +3.5 V
    • Message length is restricted to 12 bytes, including CRC
    • Employs CSMA/NDA
  • ISO 9141-2. This protocol has an asynchronous serial data rate of 10.4 kBaud. It is somewhat similar to RS-232; however, the signal levels are different, and communications happens on a single, bidirectional line without additional handshake signals. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
    • pin 7: K-line
    • pin 15: L-line (optional)
    • UART signaling
    • K-line idles high, with a 510 ohm resistor to Vbatt
    • The active/dominant state is driven low with an open-collector driver.
    • Message length is restricted to 12 bytes, including CRC
  • ISO 14230 KWP2000 (Keyword Protocol 2000)
    • pin 7: K-line
    • pin 15: L-line (optional)
    • 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(250 kBit/s or 500 kBit/s). The CAN protocol was developed by Bosch for automotive and industrial control. Unlike other OBD protocols, variants are widely use outside of the automotive industry. While it did not meet the OBD-II requirements for U.S. vehicles prior to 2003, as of 2008 all vehicles sold in the US are required to implement CAN as one of their signaling protocols.
    • pin 6: CAN High
    • pin 14: CAN Low

All OBD-II pinouts use the same connector, but different pins are used with the exception of pin 4 (battery ground) and pin 16 (battery positive).

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