joshvp / opendbc

democratize access to car decoder rings

opendbc

The project to democratize access to the decoder ring of your car.

DBC file basics

A DBC file encodes, in a humanly readable way, the information needed to understand a vehicle's CAN bus traffic. A vehicle might have multiple CAN buses and every CAN bus is represented by its own dbc file. Wondering what's the DBC file format? Here a good overview.

How to start reverse engineering cars

opendbc is integrated with cabana.

Use panda to connect your car to a computer.

Good practices for contributing to opendbc

• Comments: the best way to store comments is to add them directly to the DBC files. For example:

  CM_ SG_ 490 LONG_ACCEL "wheel speed derivative, noisy and zero snapping";
  

  is a comment that refers to signal LONG_ACCEL in message 490. Using comments is highly recommended, especially for doubts and uncertainties. cabana can easily display/add/edit comments to signals and messages.

• Units: when applicable, it's recommended to convert signals into physical units, by using a proper signal factor. Using a SI unit is preferred, unless a non-SI unit rounds the signal factor much better. For example:

  SG_ VEHICLE_SPEED : 7|15@0+ (0.00278,0) [0|70] "m/s" PCM
  

  is better than:

  SG_ VEHICLE_SPEED : 7|15@0+ (0.00620,0) [0|115] "mph" PCM
  

  However, the cleanest option is really:

  SG_ VEHICLE_SPEED : 7|15@0+ (0.01,0) [0|250] "kph" PCM
  

• Signal's size: always use the smallest amount of bits possible. For example, let's say I'm reverse engineering the gas pedal position and I've determined that it's in a 3 bytes message. For 0% pedal position I read a message value of 0x00 0x00 0x00, while for 100% of pedal position I read 0x64 0x00 0x00: clearly, the gas pedal position is within the first byte of the message and I might be tempted to define the signal GAS_POS as:

  SG_ GAS_POS : 7|8@0+ (1,0) [0|100] "%" PCM
  

  However, I can't be sure that the very first bit of the message is referred to the pedal position: I haven't seen it changing! Therefore, a safer way of defining the signal is:

  SG_ GAS_POS : 6|7@0+ (1,0) [0|100] "%" PCM
  

  which leaves the first bit unallocated. This prevents from very erroneous reading of the gas pedal position, in case the first bit is indeed used for something else.