Automotive On-Board Diagnostics II Bus, ODBII Description

OBDII Bus

On-Board Diagnostics II Bus

OBDII Bus Description

OBD2 [On-Board Diagnostics II] defines a communications protocol and a standard connector to acquire
data from passenger cars. It 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. OBDII will light a 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 OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to
interface with the vehicle.

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. Developed in 1994, J1850 may be phased out for new designs. The ISO 9141-2
single-wire asynchronous interface operates at 10.4kbps

Implementing J1850, pdf,

Intel

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Automotive On-Board Diagnostics II Bus, ODBII Description

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

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Engineering Design Key words for this page: Automotive Buses, Auto Bus types, Motor vehicle Bus Standard,
Passenger cars, Interface Standard Data Bus, Specification, Spec, Interface, IC, Physical Interface, On-Board
Diagnostics II Bus, Topology, Description, Physical Interface, X-by-wire, Automotive, Auto, Car, carmaker,
OBDII, OBDI, OBD2, ASRB, Cable Buses, Diagnostics.

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