A DBC file describes a car’s network traffic, what modules are on it, and their messages and signals.

Example

BO_ 342 STEERING_SENSORS: 6 EPS
SG_ STEER_ANGLE : 7|16@0- (-0.1,0) [-500|500] “deg” EON
SG_ STEER_ANGLE_RATE : 23|16@0- (1,0) [-3000|3000] “deg/s” EON
SG_ COUNTER : 45|2@0+ (1,0) [0|3] “” EON
SG_ CHECKSUM : 43|4@0+ (1,0) [0|3] “” EON

Here is what it means:

BO_ 342 STEERING_SENSORS: 6 EPS

STEERING SENSORS Describes the group of signals. All of the signals for one ID come from a single ECU so all messages are typically related in some way.

6 Amount of bytes the message contains. There is a maximum of 8.

EPS This is the an abbreviation of the module that is sending the message. In this case, it is the electrical power steering controller.

SG_ STEER_ANGLE : 7|16@0- (-0.1,0) [-500|500] “deg” EON

  • 7|16: This signal starts at bit position 7, and the size is 16 bits. 16 bits is common for a steering angle. Here is what this looks like in cabana’s bit view:
16 bit value, starting at position 7 — 7|16

Refer to numbering below to understand positioning:

Big Endian
Little Endian
  • @0-: 0 indicates that the signal is big-endian (it would be 1 for little-endian), and signed (+ indicates an unsigned value). Honda values all have big endianness.
Example of big endian bit position numbering, the highlighted value would be 3|4@0+

(-0.1,0): A number to scale by. (Multiply by this value)

  • [-500|500]: Min and Max, 0|0 is unset.
  • “deg”: Units, in this case, it is degrees.

The example above describes the steering sensor message, an example of one frame is shown below:

Hex

Binary

Displayed in cabana

Cabana makes it easy to visualize a DBC file

On it own, it is not obvious that this value contains the steering value, but using comma.ai cabana’s graphing tool, we can see:

Viewing car’s steering angle with DBC + Cabana

Once you track down one signal, others are easier to find, like the left blinker, note the bit value changing to 1 under the cursor:

Viewing steering angle and blinker with Cabana

The tool shown in the gifs is called cabana. You can try a demo drive here.

You can easily create your own using a panda.

engineer, hacker, early-adopter https://github.com/energee https://energ.ee

engineer, hacker, early-adopter https://github.com/energee https://energ.ee