🔓 A deep dive into automotive OEM communication files! From the principles to applications of DBC files, get all the essential information in one go!

1 CAN Description File DBC

The CAN DBC file (CAN Database) is a text file that contains information for decoding raw CAN bus data into “physical values”.

1.1 How to Obtain

The process of encoding sensor data into CAN bus frames is entirely determined by each Original Equipment Manufacturer (OEM). Therefore, DBC^[1] files are proprietary, known only to the OEM manufacturers. Engineers at OEMs typically have access to the complete DBC files for the equipment they produce.

1. DBC File Standardization

DBC files have different encoding/decoding rules for different protocols:

• J1939: Most heavy-duty vehicles use the J1939 protocol
• NMEA 2000: Most vessels use the NMEA 2000 protocol
• ISOBUS DBC: Most agricultural vehicles use the ISOBUS/J1939 protocol
• OBD2 DBC: Most passenger cars support OBD2 connections via the CAN bus (for OBD2 protocol-related content, see 👉CAN Bus | In-depth Understanding of the UDS Diagnostic Protocol)

2 DBC Syntax

In 👉CAN Bus | In-depth Understanding of the CAN Protocol, we have initially understood the meanings of some key terms in DBC files. Below is an excerpt from a DBC file.

Image fromcsselectronics

Message Section:

• Message keyword: starts with BO_ and the DBC ID must remainunique (29 (CAN ID bits) + 3 (extra bits as “extended ID” flag) bits, represented in decimal)
• Message name: must remainunique (1-32 characters, including uppercase and lowercase letters, numbers, and underscores)
• Message length: an integer between 0 and 1785
• Message sender: the name of the transmitting node; defaults to Vector__XXX if none is specified

Signal Section:

• Signal keyword: each message contains one or more signals (starting with SG_)
• Signal name: must remainunique (1-32 characters, including uppercase and lowercase letters, numbers, and underscores)
• Signal start bit and length: separated by “|”, indicating the starting position of the signal, with length indicating the signal length
• Signal storage method:@1 specifies byte order as little-endian (Intel),@0 as big-endian (Motorola)
• Signal formula calculation: (scale, offset) used for linear calculations
• Signal metadata: optional (e.g., [0|0] “”)
• Signal receiver: the name of the receiving node; defaults to Vector__XXX if none is specified

3 DBC Software/Library Applications

Similar to ldf, dbc can also be viewed and edited using a text editor. The following software is available for processing and editing:

• Vector CANDB++: The Vector CANalyzer application includes a handy CANDB++ (Vector DBC editor). It offers comprehensive features, including quick “consistency checks” for DBC files.
• canmatrix^[2]: This open-source Python DBC library can load, edit DBC files and export them to other formats.

Below is an example using canmatrix.

3.1 Loading DBC Files

import canmatrix

# Load DBC file
dbc = canmatrix.formats.loadp("dbc.dbc")
dbc_matrix = dbc.get("")

canmatrix can not only load dbc files but also load ldf files, just by importing the file path.

ldf = canmatrix.formats.loadp("ldf.ldf")

Note: If the loaded ldf variable is None, check if your Python environment has the ldfparser package installed, as canmatrix parses ldf files by calling ldfparser at a lower level. Opening the source code of canmatrix.formats reveals that canmatrix supports many types of file parsing, including:

moduleList = ["arxml", "csv", "dbc", "dbf", "json", "ldf", "kcd", "fibex", "sym", "xls", "xlsx", "yaml", "scapy", "wireshark", "odx", "eds"]

3.2 Getting ECU

import inspect

for ecu in dbc_matrix.ecus:
    print(type(ecu), inspect.getmembers(ecu))

3.3 Getting Frames

for msg in dbc_matrix.frames:
    print(type(msg), msg.__dict__)

3.4 Getting Signals

Analyzing the data structure of dbc_matrix, it is not difficult to find that it has a <code>signals attribute. However, when we directly loop and print, the output is empty. This is because dbc files generally contain many signals, and loading them all into memory at once is unnecessary and slows down performance. Therefore, a lazy loading approach is adopted, where signals are retrieved only after specifying the frame message.

#1 Lazy loading
for signal in dbc_matrix.signals:
    print(type(signal), inspect.getmembers(signal))

#2 Load signals after specifying the frame
for msg in dbc_matrix.frames:
        print(type(msg), msg.__dict__)
        for signal in msg.signals:
              print(type(signal), inspect.getmembers(signal))

3.5 Getting Signal Groups

for msg in dbc_matrix.frames:
        for signalGroup in msg.signalGroups:
              print(type(signalGroup), signalGroup.__dict__)

canmatrix can automate most of the work; you can practice with publicly available DBC files.

Reference Links

[1] DBC: https://www.csselectronics.com/pages/can-dbc-file-database-intro[2] canmatrix: https://github.com/ebroecker/canmatrix

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Unlocking the Core of the CAN Protocol: A Comprehensive Analysis of DBC Files