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OBDII On-board Diagnostics Resources

Some of the datasheet links have not been revised recently.  They can all be found on the manufacturer's web sites.  The broken links will be repaired soon.  Sorry for the inconvenience.

Information on OBDII and other in-vehicle networks

There are many types different in-vehicle networks.  Light vehicles and heavy vehicles use different styles of networks.  Different manufacturers have different network types.  A number of vehicle network standards have been introduced which are being adopted by the vehicle manufacturers. However, the great thing about standards is that there are enough of them that everybody can choose their own.

The state of California introduced some uniformity to in-vehicle networks when they defined their OBDII requirements (On-Board Diagnostics).  They defined a data protocol and a standard vehicle connector which enables a device (called a scantool) to query the vehicle's computer and ask it if the vehicle is polluting. The idea is that the vehicle monitors itself and it only need be queried once in a while. There is now a OBD3 proposal which will have the cars reporting pollution violations themselves via radio.  Europe is also formulating their own version of OBDII called EOBD.

While the OBDII standard defined a single data protocol and physical connector, it allowed three different electrical interfaces to be used; J1850 PWM, J1850 VPW, and ISO-9141-2.  More electrical interfaces are being approved for OBDII purposes such as KWP2000 and CAN.  The result is that there are a number of different flavours of OBDII.  The following definitions will help explain the differences between the OBDII vehicle network flavours and non-OBDII networks.

Here is a summary of some of the vehicle networks standards... 

J1850 VPW (Variable Pulse Width) – this is the 10.4kbps protocol used mainly on GM vehicles.  Single wire.

J1850 PWM (Pulse Width Modulated) – this is the 41.6kbps protocol used mainly on Ford vehicles.  Dual wire.

ISO9141-2 – This is the 10.4kbps asynchronous protocol used mainly on Chrysler, European, and Asian vehicles.  The protocol messages are the J1850 messages but the physical interface is different.   The protocol bit format is the same as an asynchronous PC serial port but the unique bit rate and voltages require a converter to be used for PC to vehicle communications.  A slow 5bps handshake sequence is require to initialize the ISO interface to start communications.  Most vehicles will terminate the communications session if a command is not received for 5 seconds.  Single wire.

ISO/DIS14230-4 (also called Keyword Protocol 2000 or KWP 2000) – This is a newer version of the ISO9141 protocol.  It has a provision for a fast initialization which may be used instead of the slow 5bps handshake sequence.

CAN (Controller Area Network) - CAN is becoming the most widely used bus for in-vehicle communications.  It is a high speed/high performance protocol.  Dual wire.

LIN (Local Interconnect Network) -  LIN is a low cost, low speed, in-vehicle network intended to complement the higher performance CAN network.  Vehicle systems with modest network requirements are placed on a LIN sub-network to reduce costs. The LIN sub-networks connect to the vehicle's CAN network via gateways.  LIN uses a single-wire enhanced ISO9141 implementation (non-OBDII protocol).

Ford's UBP (UART Based Protocol) - Proprietary, non-OBDII diagnostic protocol.

Chrysler's CCD - Proprietary, non-OBDII diagnostic protocol.

GM's 8192 UART (ALDL Assembly Line Diagnostic Link) - Proprietary, non-OBDII diagnostic protocol.

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
              (this 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

OBD2 Vehicle Network Information


J1850 Standards

The best source of information on the J1850 protocol itself is the book of SAE standards called HS-3000, “SAE On-Board Diagnostics for Light and Medium Duty Vehicles - Standards Manual 1999”.  It is available from B & B electronics bookstore.  The book is also available directly from the SAE bookstore.

The HS-3000 manual was written by a committee and it isn’t an easy read.  You will find the signal levels and bit timings for the protocols.  You will find the message formats for many of the messages although you will be flipping pages since the information is distributed through a number of different standards documents.  You will not find vehicle or manufacturer specific message format information.

The IC manufacturers datasheets provide a good summary of the standards documents and they are much easier to read.  The explanation of the J1850 VPW format was not very clear in the SAE standards.  Intersil and Motorola make some J1850 VPW parts and their datasheets have good explanations of VPW.  The Oki datasheets have a good explanation of the J1850 PWM protocol.

J1850 VPW (GM style) Part Datasheets and App Notes

Intersil HIP7010 “J1850 Byte Level Interface Circuit” datasheet.
Intersil HIP7020 “J1850 Bus Transceiver For Multiplex Wiring Systems”
Intersil AN9739 “Protecting The J1850 Bus For Module Loss Of Ground Using The HIP7020”
Intersil HIP7030 “J1850 8-Bit 68HC05 Microcontroller”
Motorola AN1731 "VPW J1850 Multiplexing and Motorola’s Byte Data Link Controller (BDLC) Module"
Motorola AN1212 “J1850 Multiplex Bus Comms Using the MC68HC705C8 & SC371016 Comms IF”
Motorola MC33390D  J1850 Serial Link Transceiver

Search the Motorola site for J1850 for more information on J1850 enabled MCUs.

J1850 PWM (Ford style) Part Datasheets and App Notes

OKI Semiconductor MSM6636B - SAE J1850 PWM Automotive Bus Controller, parallel multiplexed bus
OKI Semiconductor MSM6636 - SAE J1850 PWM Automotive Bus Controller, serial interface
OKI Semiconductor Users' Manual MSM6636
OKI Semiconductor Users' Guide MSM6636

ISO-9141 (Chrysler, European, Asian, International) Part Datasheets

Motorola “MC33199DW  ISO9141 serial link driver (has K& L lines)” PDF Missing.

Motorola MC33290D/P  ISO9141 serial link interface (K line only) PDF file.

Vishay  Si9241A (K line) and  Si9243A (K and L lines) bus transceivers. PDF Files.
Vishay AN602 Driver ICs for ... ISO9141 PDF File (3 pages)

OBD2 White Papers and Articles

Advanced Vehicle Technologies, Inc., "In-Vehicle Networking Briefing Slides" (93 KB).
AVT, Inc., "In-Vehicle Networking Briefing Slides with Presenter's Notes" (105 KB).

These articles summarize the EPA legislation that requires cars to have on-board diagnostic capabilities. The communications network that accesses the diagnostics is discussed in detail.

Evaluation Engineering Computer Networks Take to the Road: Part 1
Evaluation Engineering Computer Networks Take to the Road, Part 2

High-Tech Performance magazine. Critique of On Board Diagnostics by Ray T. Bohacz.

Intel Corp.,  D. John Oliver, "Implementing the J1850 Protocol"

OBD2 Interface Devices

Multiplex Engineering Inc. makes low cost ($72+ US) OBD interfaces for use with your own software.
 AutoTap - OBDII Diagnostic Scanner from B&B Electronics ($290+ US with software, various versions).
 Opendiag Schematics & PCB Layout open source ISO-9141 to RS-232 converter schematics.

Miscellaneous Links

OBD-II - On-Board Diagnostic System home page hosted by B&B Electronics.
B&B Electronics Autotap Cable Pinout PDF file (1 page).

OBD2 Connector Sources

 Multiplex Engineering sells SAE J1962 connector to DB9F cables and SAE J1962 connectors themselves.
 Pioneer Packard is the distributor for Delphi Packard which manufacturers OBD2 connectors.  Their part numbers for the SAE J1962 male connector are: 12110252 shell, 12047581 pins, 12110254 locking clip.
 Test Products Inc : OBDII Diagnostic Connector industrial strength connector for manufacturers.

LIN (Local Interconnect Network)

LIN Organization Homepage

LIN Application Notes and White Papers

LIN Press Conference Press Release 6 Mar 2000  PDF file (34 pages, 2.9MB).
Microchip AN729 LIN Protocol Implementation Using PICmicro PDF file.
Microchip AN729 LIN Protocol Implementation Using PICmicro Source code.

Heavy Vehicle Network Information

SAE J1708 Application Notes and White Papers

National Semiconductor AN-915 Automotive Physical Layer SAE J1708 and the DS36277 PDF file.

Miscellaneous Vehicle Information

Articles, White Papers, and Books

SAE A Simple Way to Prevent Blindzone Accidents PDF file.

Bob Pease's homepage for his book How to Drive Into ACCIDENTS ...and How NOT To.



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