Obd Ii Iso 9141 Interface

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Andy Whittaker's OBD-II ISO9141 interface; (website down) Silicon Engines ISO 9141 interface: (website down) It is supplied in a robust box with lots of lights. This interface was used to develop the first bits of the freediag project. It is functionally the same as the above two interfaces (with the exception of all the lights and the box). All cars and light trucks built and sold in the United States after January 1, 1996 were required to be OBD II equipped. In general, this means all 1996 model year cars and light trucks are compliant, even if built in late 1995.

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).


On-Board Diagnostics II Bus

OBDII Bus Description

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OBD2 [On-Board Diagnostics II] defines a communications protocol and a standard connector to acquire data from passenger cars. OBD was required by U.S. EPA on all gasoline powered cars and light duty trucks manufactured for the U.S. after 1996 to help monitor/inspect vehicle emissions [as required by the Clean Air Act Amendments of 1990].

OBDII is the second generation of the OBD specification. The first generation OBD monitored fewer emission related components, was not calibrated to a specific level of emission performance, and was not as fell defined. Any passenger cars and truck produced after 1996 uses the OBD II standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.


OBDII will light a warning lamp called a MIL (malfunction indicator lamp), also known as the 'check engine' light on the dash. A scan-tool may also be used to probe the OBDII connector OBDII data as defined by the SAE J1979 standard. The warning light may come on for any number of reasons and manufacturers recommend having the vehicle serviced as soon as possible. However; the Check Engine light could also come on for such simple reasons as filling the tank while the vehicle is running, or leaving the gas cap off. It may take up to three days for the light to go back off, after coming on for a missing gas cap.
The OBD II interface is located in the cab, and must be located with in a certain area within the cab as defined by the standard.

J1962 - describes the standardized 16-pin trapezoidal connector.
The male plug is use to connect to your car computer and the female is for making an extension cable or some other device

The J1850 VPW single wire protocol, used by GM may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 16, with no contact in pin 10. The J1850 PWM two wire protocol, used by Ford may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 10, with no contact in pin 16. The ISO 9141-2 single wire protocol, used by Chrysler may be found on an OBDII bus, if so the connector will have contacts in pins 4, 5, 7, 15 and 16. The protocol and command set is fixed by SAE J1979, so they are the same for all three protocols, only the electrical layers are changed. The CAN Bus may also be found on the OBDII bus

J1850 bus Description

The J1850 bus is used for diagnostics and data sharing applications in vehicles. The J1850 bus takes two forms; A 41.6Kbps Pulse Width Modulated (PWM) two wire differential approach, or a 10.4Kbps Variable Pulse Width (VPW) single wire approach. The single wire approach may have a bus length up to 35 meters (with 32 nodes). A high resides between 4.25 volts and 20 volts, a low is any thing below 3.5 volts. High and low values are sent as bit symbols (not single bits). Symbols times are 64uS and 128uS for the single wire approach. The ISO 9141-2 single-wire asynchronous interface operates at 10.4kbps

OBDII Related Standards

J1962 - SAE (Society of Automotive Engineers) standard defining the physical connector used for the OBDII interface.
J1850 - SAE standard for the Class B Communications Network Interface (standard defines the actual J1850 signaling and timings)
J1939, ISO 11898
J1978 - SAE standard for OBD II scan tools
J1979 - SAE standard for diagnostic test modes
J2012 - SAE standard for EPA emission test report format.
J2178-1 - SAE standard for Class B Communications Network Message: Detailed Header Formats and Physical Address Assignments
J2178-2 - SAE standard for Class B Communications Network Message: Data Parameter Definitions
J2178-3 - SAE standard for Class B Communications Network Message: Frame IDs for Single Byte Forms of Headers
J2178-4 - SAE standard for Class B Communications Network Message: Message Definitions for Three Byte Headers
Code of Federal Regulations [CFR]: 40 CFR; 86.094-17h

OBDII Cable Assemblies

Cable Graphics for a number of different types.
OBD connector to the format indicated.

OBDII to DB15 pin Cable
Female OBDII to 2x2 pin Cable
Right Angle Male OBDII to DB-9pin Cable
Male OBDII to DB9-pin Cable right angle
Male OBDII to Male DB9-pin Cable
Male OBDII to Male DB15-pin Cable
Male OBDII to Female OBDII Cable
BMW to Female OBDII Cable

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OBDII pinout, Signal Assignments

OBD Acronyms:
DLC: Data link connector
DTC: Diagnostic trouble code
ECM: Engine control module
EEC: Electronic Engine Control
MIL: Malfunction indicator light
OBD: On-board diagnostics
OBD-II: Second generation on-board diagnostics
PCM: Power-train control module



Modified 6/13/15
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