Specification for the OpenXC JSON message format
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OpenXC Message Format Specification

Version: v0.7.0-dev

This specification is a part of the OpenXC platform.

An OpenXC vehicle interface sends generic vehicle data over one or more output interfaces (e.g. USB or Bluetooth) as JSON or Protocol Buffers (protobuf).


The JSON format is the most flexible and easiest to use. The format is fully specified in the JSON.mkd file in this repository. a more flexible option than binary, but is less compact and therefore takes more bandwidth and processing power.

The JSON format is best for most developers, as it is fairly efficient and very flexible.

Binary (Protocol Buffers)

The binary format is encoded using Google Protocol Buffers. The format is specified in the file openxc.proto. The descriptions of the messages can be foud in the JSON specs - the binary format mirrors this.

The binary messages are published by the VI using the standard length-delimited method (any protobuf library should support this).

The binary format is best if you need to maximize the amount of data that can be sent from the VI, trading off flexibility for efficiency.

Message Pack

MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON, but it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves

For protocol specification visit: https://github.com/msgpack/msgpack/blob/master/spec.md

We are using the following lib: https://github.com/camgunz/cmp

MessagePack provides a binary alternative to ProtoBuf. There are pros & cons to each so you can decide what works best for your project.

Trace File Format

An OpenXC vehicle trace file is a plaintext file that contains JSON objects, separated by newlines (which may be either \r\n or \n, depending on the platform the trace file was recorded).

The first line may be a metadata object, although this is optional:

{"metadata": {
    "version": "v3.0",
    "vehicle_interface_id": "7ABF",
    "vehicle": {
        "make": "Ford",
        "model": "Mustang",
        "trim": "V6 Premium",
        "year": 2013
    "description": "highway drive to work",
    "driver_name": "TJ Giuli",
    "vehicle_id": "17N1039247929"

The following lines are OpenXC messages with a timestamp field added, e.g.:

{"timestamp": 1385133351.285525, "name": "steering_wheel_angle", "value": 45}

The timestamp is in UNIX time (i.e. seconds since the UNIX epoch, 00:00:00 UTC, 1/1/1970).

Official Signals

These signal names are a part of the OpenXC specification, although some manufacturers may support custom message names.

  • steering_wheel_angle
    • numerical, -600 to +600 degrees
    • 10Hz
  • torque_at_transmission
    • numerical, -500 to 1500 Nm
    • 10Hz
  • engine_speed
    • numerical, 0 to 16382 RPM
    • 10Hz
  • vehicle_speed
    • numerical, 0 to 655 km/h (this will be positive even if going in reverse as it's not a velocity, although you can use the gear status to figure out direction)
    • 10Hz
  • accelerator_pedal_position
    • percentage
    • 10Hz
  • parking_brake_status
    • boolean, (true == brake engaged)
    • 1Hz, but sent immediately on change
  • brake_pedal_status
    • boolean (True == pedal pressed)
    • 1Hz, but sent immediately on change
  • transmission_gear_position
    • states: first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, reverse, neutral
    • 1Hz, but sent immediately on change
  • gear_lever_position
    • states: neutral, park, reverse, drive, sport, low, first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth
    • 1Hz, but sent immediately on change
  • odometer
    • Numerical, km 0 to 16777214.000 km, with about .2m resolution
    • 10Hz
  • ignition_status
    • states: off, accessory, run, start
    • 1Hz, but sent immediately on change
  • fuel_level
    • percentage
    • 2Hz
  • fuel_consumed_since_restart
    • numerical, 0 - 4294967295.0 L (this goes to 0 every time the vehicle restarts, like a trip meter)
    • 10Hz
  • door_status
    • Value is State: driver, passenger, rear_left, rear_right.
    • Event is boolean: true == ajar
    • 1Hz, but sent immediately on change
  • headlamp_status
    • boolean, true is on
    • 1Hz, but sent immediately on change
  • high_beam_status
    • boolean, true is on
    • 1Hz, but sent immediately on change
  • windshield_wiper_status
    • boolean, true is on
    • 1Hz, but sent immediately on change
  • latitude
    • numerical, -89.0 to 89.0 degrees with standard GPS accuracy
    • 1Hz
  • longitude
    • numerical, -179.0 to 179.0 degrees with standard GPS accuracy
    • 1Hz

Signals from Diagnostic Messages

This set of signals is often retreived from OBD-II requests. The units can be found in the OBD-II standard.

  • engine_load
  • engine_coolant_temperature
  • barometric_pressure
  • commanded_throttle_position
  • throttle_position
  • fuel_level
  • intake_air_temperature
  • intake_manifold_pressure
  • running_time
  • fuel_pressure
  • mass_airflow
  • accelerator_pedal_position
  • ethanol_fuel_percentage
  • engine_oil_temperature
  • engine_torque


Copyright (c) 2012-2014 Ford Motor Company

Licensed under the BSD license.