Version

0.8.0

Go library for communicating with cars OBD-II system using ELM327 based USB-devices.

To make this library as good as possible - feedback, bug reports and feature requests are very welcome in the GitHub issues of this project.

There are more than 10 different OBD-II signal protocol variations used by the various cars that exist. To avoid having to handle all the details of these protocols the ELM327 exists. The ELM327 acts a facade between the computer and the car. You talk to the ELM327 using a simple text based protocol similar to the Hayes command set and the ELM327 takes care of the communication details of the car.

As shown in the diagram above this library connects to a serial device of the operating system. The library is not concerned with what is connected to that serial device, whether it’s a bluetooth USB-dongle with a ELM327 at the other end or a ELM327 connected directly via an USB-cable.

Communicating with the ELM327 is similar to communicating with a web server. You make a request and wait for a response. However, in this context we are calling a command and waiting for one or more responses.

This library is designed to be used in a way that resembles the way you physically use the device. You have a type called Device that represents a ELM327 device connected to the computer. This Device then has a function called RunCommand that sends a command to the actual device and then waits for a response.

This library aims to be as type safe as possible, which means that you don’t deal with raw text commands, instead you have different command types.

All command types need to implement the OBDCommand interface to be able to be run on the device. Since there are A LOT of OBD commands, you can easily extend this library, by just implementing the OBDCommand interface of your commands.

Let’s start by looking at some example of how you use the library.

Note: these examples are performed on Linux. If you are using another platform there should be minimal changes, but they are not documented yet. Go ahead and put a 👍 on issue #11 if you think this should be prioritized.

First of all, you need to plug in your ELM327 device into your computer and get the path to the device. You can plugin the device and check dmesg, this is what I get on my computer:

$ dmesg | tail
[359720.858480] usb 6-2: Manufacturer: FTDI
[359720.858482] usb 6-2: SerialNumber: A503GJEX
[359720.897717] usbcore: registered new interface driver usbserial
[359720.897733] usbcore: registered new interface driver usbserial_generic
[359720.897748] usbserial: USB Serial support registered for generic
[359720.901755] usbcore: registered new interface driver ftdi_sio
[359720.901767] usbserial: USB Serial support registered for FTDI USB Serial Device
[359720.901839] ftdi_sio 6-2:1.0: FTDI USB Serial Device converter detected
[359720.901913] usb 6-2: Detected FT232RL
[359720.904481] usb 6-2: FTDI USB Serial Device converter now attached to ttyUSB0

Now that I know that the device is available at /dev/ttyUSB0 I can use the library to connect to the device and check the ELM327 version of the device:

example1.go

package main

import (
	"flag"
	"fmt"
	"github.com/rzetterberg/elmobd"
)

func main() {
	serialPath := flag.String(
		"serial",
		"/dev/ttyUSB0",
		"Path to the serial device to use",
	)

	flag.Parse()

	dev, err := elmobd.NewTestDevice(*serialPath, false)

	if err != nil {
		fmt.Println("Failed to create new device", err)
		return
	}

	version, err := dev.GetVersion()

	if err != nil {
		fmt.Println("Failed to get version", err)
		return
	}

	fmt.Println("Device has version", version)
}

Note: These examples uses the function NewTestDevice, which uses a mocked ELM327 device. To use a real ELM327 device, you instead use NewDevice. The reason why a mocked device is used is because the examples should be runnable without using a real device.

$ go run example.go
Device has version OBDII by [email protected]

The next step is to run some OBD commands on the device. For this we need to plug in the ELM327 into our car and turn on the ignition.

Like mentioned before you use the function RunCommand that accepts a OBDCommand to run. A OBDCommand has 3 responsibilities:

• Tell the ELM327 what command to run
• Store the value
• Convert the value to a common format

So you start out by creating a new OBDCommand that does not contain a value. You then take that OBDCommand and call the RunCommand function with it. RunCommand will then return the OBDCommand with the value from the car.

Let’s try this out by checking the RPM of the engine. There is a OBDCommand for that defined in the library already, called EngineRPM. We start by creating a new EngineRPM that we call RunCommand with:

example2.go

package main

import (
	"flag"
	"fmt"
	"github.com/rzetterberg/elmobd"
)

func main() {
	serialPath := flag.String(
		"serial",
		"/dev/ttyUSB0",
		"Path to the serial device to use",
	)

	flag.Parse()

	dev, err := elmobd.NewTestDevice(*serialPath, false)

	if err != nil {
		fmt.Println("Failed to create new device", err)
		return
	}

	rpm, err := dev.RunOBDCommand(elmobd.NewEngineRPM())

	if err != nil {
		fmt.Println("Failed to get rpm", err)
		return
	}

	fmt.Printf("Engine spins at %s RPMs\n", rpm.ValueAsLit())
}

There are more than 180 different OBD commands, and cars have different support for these commands. So to avoid sending OBD commands to the car that it does not support we can check what commands the car support:

example3.go

package main

import (
	"flag"
	"fmt"
	"github.com/rzetterberg/elmobd"
)

func main() {
	serialPath := flag.String(
		"serial",
		"/dev/ttyUSB0",
		"Path to the serial device to use",
	)

	flag.Parse()

	dev, err := elmobd.NewTestDevice(*serialPath, false)

	if err != nil {
		fmt.Println("Failed to create new device", err)
		return
	}

	supported, err := dev.CheckSupportedCommands()

	if err != nil {
		fmt.Println("Failed to check supported commands", err)
		return
	}

	rpm := elmobd.NewEngineRPM()

	if supported.IsSupported(rpm) {
		fmt.Println("The car supports checking RPM")
	} else {
		fmt.Println("The car does NOT supports checking RPM")
	}
}

The supported here is a SupportedCommands which is a special type that stores the raw lookup table and exposes two helper functions that reads this table:

IsSupported

Check if given command is supported

FilterSupported

Filters out supported commands from given list

For simplicity there’s a function called GetSensorCommands which gives you a list of all the commands defined in the library. You can use this list of commands and filter out what commands are supported on by car:

example4.go

package main

import (
	"flag"
	"fmt"
	"github.com/rzetterberg/elmobd"
)

func main() {
	serialPath := flag.String(
		"serial",
		"/dev/ttyUSB0",
		"Path to the serial device to use",
	)

	flag.Parse()

	dev, err := elmobd.NewTestDevice(*serialPath, false)

	if err != nil {
		fmt.Println("Failed to create new device", err)
		return
	}

	supported, err := dev.CheckSupportedCommands()

	if err != nil {
		fmt.Println("Failed to check supported commands", err)
		return
	}

	allCommands := elmobd.GetSensorCommands()
	carCommands := supported.FilterSupported(allCommands)

	fmt.Printf("%d of %d commands supported:\n", len(carCommands), len(allCommands))

	for _, cmd := range carCommands {
		fmt.Printf("- %s supported\n", cmd.Key())
	}
}

Besides checking sensor values, you can also check whether the MIL is on and if there are any DTCs:

example5.go

package main

import (
	"flag"
	"fmt"
	"github.com/rzetterberg/elmobd"
)

func main() {
	serialPath := flag.String(
		"serial",
		"/dev/ttyUSB0",
		"Path to the serial device to use",
	)

	flag.Parse()

	dev, err := elmobd.NewTestDevice(*serialPath, false)

	if err != nil {
		fmt.Println("Failed to create new device", err)
		return
	}

	cmd, err := dev.RunOBDCommand(elmobd.NewMonitorStatus())

	if err != nil {
		fmt.Println("Failed to get monitor status", err)
		return
	}

        status := cmd.(*elmobd.MonitorStatus)

	fmt.Printf("MIL is on: %t, DTCamount: %d\n", status.MilActive, status.DtcAmount)
}

Please see the godocs for a more detailed explanation of the library and it’s structure.

• [X] Reading sensor data
• [ ] Reading trouble codes
• [ ] Resetting Check Engine Light
• [ ] Reading freezed sensor data

The project uses quarterly milestones to plan upcoming changes. The current quarter will focus on implementing new features. To see the details of what will be done see the milestone 2018 Q3.

Changes of the library are tracked in the CHANGELOG.

The library has been built and tested on the following platforms:

The library has been used successfully on the following cars: