http://en.wikipedia.org/wiki/On-board_diagnostics#Standard_interfaces

OBD-II diagnostic connector

The OBD-II specification provides for a standardized hardware interface—the female 16-pin (2x8) J1962 connector. Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, the OBD-II connector is required to be within 2 feet (0.61 m) of the steering wheel (unless an exemption is applied for by the manufacturer, in which case it is still somewhere within reach of the driver). SAE J1962 defines the pinout of the connector as:

1	Manufacturer discretion.GM: J2411 GMLAN/SWC/Single-Wire CAN. VW/Audi: Switched +12 to tell a scan tool whether the ignition is on.	9	Manufacturer discretion.GM: 8192 baud ALDL where fitted.
	Bus positive Line of SAE-J1850 PWM and SAE-1850 VPW		Bus negative Line of SAE-J1850 PWM only (not SAE-1850 VPW)
3	Ford DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc. Chrysler CCD Bus(+)	11	Ford DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc. Chrysler CCD Bus(-)
	Chassis ground	12	Not connected
	Signal ground	13	Manufacturer discretion Ford: FEPS - Programming PCM voltage
	CAN high (ISO 15765-4 and SAE-J2284)		CAN low (ISO 15765-4 and SAE-J2284)
	K line of ISO 9141-2 and ISO 14230-4		L line of ISO 9141-2 and ISO 14230-4
8	Manufacturer discretion. Many BMWs: A second K-Line for non OBD-II (Body/Chassis/Infotainment) systems.		Battery voltage

The assignment of unspecified pins is left to the vehicle manufacturer's discretion.

OBD-II 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 V_batt
  • The active/dominant state is driven low with an open-collector driver.
  • Message length is Max 260Bytes. Data field MAX 255.
• 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 used 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).

• 0 – Automatic
• 1 – SAE J1850 PWM (41.6 kbaud)
• 2 – SAE J1850 VPW (10.4 kbaud)
• 3 – ISO 9141-2 (5 baud init, 10.4 kbaud)
• 4 – ISO 14230-4 KWP (5 baud init, 10.4 kbaud)
• 5 – ISO 14230-4 KWP (fast init, 10.4 kbaud)
• 6 – ISO 15765-4 CAN (11 bit ID, 500 kbaud)
• 7 – ISO 15765-4 CAN (29 bit ID, 500 kbaud)
• 8 – ISO 15765-4 CAN (11 bit ID, 250 kbaud)
• 9 – ISO 15765-4 CAN (29 bit ID, 250 kbaud)
• A – SAE J1939 CAN (29 bit ID, 250 kbaud)