brucemack / hello-obd2

Car diagnostic port interface (ISO-9141) for a microcontroller

Overview

This project is a simple demonstration of how to communicate with a car via the OBD-II (on board diagnostic) connector. This connector allows you to read diagnostic codes from the car's engine control unit (ECU) and to get some real-time performance data.

This project supports the ISO-9141-2 K-Line interface. The more modern CAN bus is not supported yet - that's a different project.

This has been tested on a 2005 Toyota Corolla and a 1999 Honda Civic (D16Y7 engine).

The design has been used to read DTC codes out of the Civic.

The code is here. This is a C program developed using the Arduino platform.

To make things easier to test, a companion ECU simulator was also constructed. This simulator allows testing without needing to connect to an actual vehicle.

Hardware

Custom hardware was built to provide physical connectivity to the car. Here's what the development platform looks like:

The purple PCB was designed and manufactured specifically for this application. Please see this related project for more information about the hardware implementation.

This interface uses the Teensy 3.2 microcontroller. The Teensy controller is helpful here because it supports more than one hardware serial port. A test has also been done using an Arduino Nano (clone) using a software serial port. See notes below.

The K-Line interface is challenging because it sends and receives on the same wire. This is a 12V interface pulled up by a 560 ohm resistor and pulled down by a 2N3904 NPN transistor. An extra transistor stage is used as an inverter. Special arrangements need to be made in software to ignore the "echo" of the transmitted data on the single wire.

One other challenge is that the ISO-9141 specification defines a slow handshake at the start of the connection: the so-called "5 Baud" interface. Because the Teensy 3.2 (or most any controller) is not capable of generating such a slow transmission rate, we use "bit banging" to generate that 2 second handshake. A separate I/O pin is used for this purpose to avoid conflicts with the UART. Please see the "CTL0" input in the schematic below.

Normal transmission happens at 10,400 baud (8/none) as defined in the ISO spec.

Here's the schematic, as simulated in LT-Spice:

Here's the initial prototype:

A standard SAE J1962 OBD2 connector is used (male). Pins used are:

• 5 - Signal ground
• 7 - ISO-9141 K-Line
• 12 - +12V

The interface board also has red +12V power LED, a green indicator LED (means that the ECU is connected), and a +5V regulator to power a microcontroller from the vehicle's power supply.

Software Notes

The ISO document that describes the physical interface is here

Please see this page for a list of the PIDs used.

Notes on Integration with an Arduino Nano

It would be helpful to demonstrate support for the Arduino Nano, given how ubiquitous this part is. There are two challenges that need to be studied:

• The Nano only has a single hardware UART which is connected to the USB connector on the board. We want to leave that available for user interaction (console) so it can't be used for communicating with a vehicle. An alternate UART is needed.
• There are Arduino libraries that support "software serial" capability on arbitrary pins. However, the mainstream library distributed by Arduino doesn't support simultaneous bidirectional communications. Note that we don't necessarily need bidirectional communications since the K-Line is a one wire protocol, but the original code was developed using hardware UARTs that supports bidirectional and it will be a lot easier if all implementations work this way.

Paul Stoffrogen (of Teensy fame) has created this library that supports bi-directional software serial ports. In my experience, Paul knows what he is doing!

The latest version (2.09) has been tested using this configuration and it worked on the first try.

The Nano used for this test was not an authentic Arduino board. I used an ELEGOO Nano clone which contained a CH340 USB interface which is a cheap substitute for the FT232. The only tricky thing with these clone Nano boards can be getting the right USB driver for the CH340 chip.

Hookup notes:

• Pin D9 on the Nano goes to pin 1 (TX, green) on the ISO-9141 board.
• Pin D8 on the Nano goes to pin 2 (RX, yellow) on the ISO-9141 board
• Pin D2 on the Nano goes to pin 3 (CTL, white) on the ISO-9141 board
• Pin D10 on the Nano goes to pin 4 (LED, blue) on the ISO-9141 board
• Pin 5V on the Nano goes to pin 5 (5V, red) on the ISO-9141 board. WARNING: Don't make this connection if you are already powering the Nano over the USB connection. We don't want to have two 5V supplies connected at the same time. This can result in damage to the USB host.
• Pin GND on the Nano goes to pin 6 (GND, black) on the ISO-9141 board

Set your serial console to 9600 baud.

Demo Pictures (Nano)

Here's the complete setup, attached to my ECU Simulator:

Here's what the console output looks like. On connection, the demo asks the vehicle for the list of supported PIDs, any current diagnostic trouble codes (DTCs), and then polls for the engine RPM every 15 seconds.

The ECU simulation reports two trouble codes: P0122 (throttle position sensor) and P1509 (IACV sensor failure).

Standards

• Electrical: https://cdn.standards.iteh.ai/samples/16737/e6b719fd44c345a792656f6d19e6cee4/ISO-9141-1989.pdf (note: probably an unauthorized copy)
• Protocol: http://forum.amadeus-project.com/index.php?act=attach&type=post&id=9491 (note: probably an unauthorized copy)
• Good detail: https://www.irjet.net/archives/V4/i7/IRJET-V4I7181.pdf

Example

Here's the initial output as seen on a 2005 Toyota Corolla:

Analysis (using this: https://en.wikipedia.org/wiki/OBD-II_PIDs):

• Status:
  • No check-engine light (MIL)
  • No DTCs saved
  • Tests available for Components, Fuel System, Misfire
  • Tests available for Oxygen sensor, Evaporative system, at Catalyst
  • No tests are incomplete
• PIDs supported
  • 01 - Status
  • 02 - Freeze DTC
  • 03 - Fuel system status
  • 04 - Engine load
  • 09 - Long term fuel trim (2)
  • 0A - Fuel pressure
  • 0B - Intake manifold pressure
  • 0C - Engine speed
  • 21 - Distance traveled with MIL indicator
  • 22 - Fuel rail pressure
  • 23 - Fuel rail gauge pressure
  • 24 - Oxygen sensor 1

Copyright

Copyright (C) 2024 - Bruce MacKinnon

This work is covered under the terms of the GNU Public License (V3). Please consult the LICENSE file for more information.

This work is being made available for non-commercial use. Redistribution, commercial use or sale of any part is prohibited.