Opendiag OBD-II Schematics & PCB Layout

oshw-logo-100-pxBack in 2000 I created some open-source hardware: An RS-232 to OBD-II interface. I’m posting the details of that project here so that I can shut down my old website. I’m no longer interested in this project, but it gets a surprising amount of traffic every day. I wanted to make sure it’s still available here in case anyone wants it.

By now there are undoubtedly far better approaches for connecting computers to cars, so please keep in mind that this information hasn’t been updated since 2002.

PCB logo

This page contains plans to build a device to interface RS-232 (a laptop computer) to the ISO9141-2 / ISO14230 / SAE J1962 (OBD-II) diagnostic connector on many Volkswagen, Audi, Seat, Skoda, and Subaru automobiles. It should also work on many pre-OBD-II models. It has been tested on my two vehicles, a 1998 VW GTI 8v and a 1998 VW Passat GLS.

Please note that this page does not contain software. Free software was a goal of the Freediag project. Commercial software is also available which works with this interface.

Please don’t email me questions about this project.

This document is viewed by hundreds of people a day. It has helped thousands build the interface described here. The information presented here is as complete and accurate as necessary for a person of sufficient skill to build their own interface. If you require help, please seek it from a discussion group or your local electronics guru. I lack the time (and often, the ability) to answer questions. Thanks for your understanding.

Here is a parts list for Digi-Key:

Item Qty Part Number Description Application
2 2 2N3904-ND NPN SML SIG G.P. AMP&SWITCH TO92 T1 & T2
4 10 1.0KQBK-ND 1.0K OHM 1/4W 5% CARBON FILM RES R1,R2,R5,R6,R7,R9
5 5 1.5KQBK-ND 1.5K OHM 1/4W 5% CARBON FILM RES R3
7 5 150KQBK-ND 150K OHM 1/4W 5% CARBON FILM RES R8
9 1 A23279-ND 09 MSFL PLUG RA 318 (SL,FM,BL) DB9 MALE
10 1 A23305-ND 09 MSFL RCPT RA 318 (SL,FM,BL) DB9 FEMALE

On 5/21/2002, the cost of these parts was US$8.33, plus a $5 handling fee and shipping charges. You can get other parts, such as perf board, etc., at Fry’s or maybe Radio Shack. I do not sell any parts. You need to get them from an electronics distributor such as Digi-Key.

Frequently Asked Questions & Answers:

Question Answer
Will this device work with my car? If your car supports VAG, ISO9141-2 or ISO14230, it should work. That includes cars sold by Volkswagen, Audi, Seat, Skoda, and Subaru (1996+). Most vehicles sold by Ford, GM, or Chrysler are not electrically compatible with this interface.More information on OBD-II and ISO standards can be found here.Many newer cars (model year 2001+) utilize two K-lines and this device only supports one. If you’d like to contribute a schematic or other information on how to support dual K-lines, please contact me. But don’t ask me — if you don’t see the information here, I don’t have it. Thanks!
Do you sell these things? No. You can buy a complete hardware and software package from Ross-Tech at a very reasonable price.
What kind of capacitor should I use? It isn’t critical. If you use a polarized capacitor, make sure you get the polarity correct. (You should be able to figure that out yourself.)
Does it really work? Yes, hundreds of people have built this thing, and it works. I suggest you replace the 140k resistor with a 150k resistor for the best results, however.
I can’t find the NEC 2501-3. What should I use? There is no such thing as a 2501-3. I’ve simply stuck three 2501-1‘s together and called it a 2501-3.In North America, you can easily find the NTE NTE-3098 (Compatibility confirmed by Dale Kirstein). Fry’s Electronics carries NTE parts.You can also try the Infineon SFH615-A2 (Compatibility confirmed by Nigel Middleton)
My OBD-II (J1962) connector only has electrical contacts in pin positions 4, 5, 7 and 16. Why is pin 15 (L-line) missing? Not every car has an L-line connection. If yours doesn’t, don’t worry. This circuit will still work.
My OBD-II connector has a contact for pin 4 (or pin 5) but not both. I suggest you connect the two ground pins together on the circuit board. Some cars only provide one of the ground connections.
Is free software available? Yes, but it hasn’t been updated since 2003. Check out Freediag for more information.The Opendiag discussion group at Yahoo Groups is another resource.Ross-Tech seems to have discontinued their free version of VAG-COM.
How do I make a circuit board? I have written a basic guide that explains how built the board in the photos. These days I use cheap PCB manufacturers in Malaysia and China. A few can even accept EAGLE files directly.
I’m interested in writing software. Where can I find technical documentation on OBD-II online? You probably want a copy of HS-3000 from the Society of Automotive Engineers. The cost is around US$240 (in 2009).
My computer doesn’t have a serial port. The Keyspan USA-19HS is an excellent USB to Serial (RS-232) adapter for this application. Be aware that most other USB to Serial adapters will give you lots of trouble with this circuit. You have been warned.
I have another question that you haven’t answered here. Please look elsewhere on the internet, or ask your friendly local electronics guru.
Thanks for sharing. How can I express my gratitude? Send me a postcard:Jeff Noxon
10735 Valley Forge Dr
Houston, TX 77042 USA


Here are some pictures of my first assembled prototype.

This is an early prototype. The jumper wires on the bottom are not necessary with the current board layout.

Top of prototypeBottom of prototype


This is the artwork used in the latest revision of the board. It is a single-layer design. If you plan to etch your own board, use the Postscript or PDF files in the files section for greater accuracy.

Low-res of the PCB artwork

This picture shows component values and placement. Note that the 2501-3 is actually three NEC 2501-1 optoisolators. The Digi-Key part number is PS2501-1-ND.

Diagram showing component placement on the top of the PCB

Part Substitutions

The following parts are suitable replacements:

  • NEC 2501 Optoisolator: Infineon SFH615-A2 (Thanks: Nigel Middleton) or NTE NTE-3098 (Thanks: Dale Kirstein)
  • 2N3904 Transistor: NTE NTE-123AP (Thanks: Dale Kirstein)

These substitutions have been used by others who have built this device.


The files linked below and the images on this page (also referred to as “software”) are Copyright © 2000 Jeff Noxon, and are distributed under the terms of the GNU Public License. By downloading these files you agree to the terms of the license.

The GPL license has been chosen for two reasons. First, the documentation necessary to make this project possible was released to the public under open terms. Second, I have spent a great deal of time on this project. Any commercial vendor who sells this device (or one derived from these files) is obligated under the license to provide the schematics and/or board layout to their customers.


Files / Downloads

COPYING.TXT – License agreement for all files – IMPORTANT
ISO_B1.PS – Board layout, revision B1, Postscript
ISO_B1_LJET.PCL – Board layout, revision B1, PCL, 300dpi, HP LaserJet
ISO_B1_LJET4.PCL – Board layout, revision B1, PCL, 600dpi, HP LaserJet 4+
ISO_B1.PDF – Board layout, revision B1, PDF (for Adobe Acrobat Reader)
SCHEMATIC_B1.PDF – Schematic (PDF Format)
ISO_B1.SCH – Schematic (EAGLE Format)
ISO_B1.BRD – Board Layout (EAGLE Format)

OpenDiag – Aims to develop GPL’d software to use with this device. OpenDiag also provided the basis for this schematic.

B&B Electronics – Sells a rather expensive, but nice, SAE J1962 to DB9F cable compatible with this project. (Broken link, old SKU OBDIIJ1962 seems to be unavailable.)

Multiplex Engineering – Sells an alternative, much cheaper SAE J1962 to DB9F cable. See this e-mail for more information. I have not tried this cable. If you buy one and it works, please let me know!

Ross-Tech – Sells VAG-COM diagnostic software, which is compatible with this interface.

Cadsoft – Sells the Windows & Linux CAD software (EAGLE) used to develop the board.


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