---

name: can-dbc-matrix#

description: Parse and analyze CAN DBC matrix JSON files exported by canmatrix. Use when working with CAN bus databases, DBC files, CAN messages, signals, ECUs, or when the user provides a canmatrix-exported JSON file for analysis, decoding, or signal lookup.

CAN DBC Matrix JSON Parser#

Parse CAN database JSON files exported by the Python canmatrix library. The input is always a JSON file produced by canconvert with --jsonExportAll (full export mode).

Quick Reference: JSON Top-Level Keys#

KeyTypeContentmessagesarrayCAN frames with nested signalsecusobjectECU name → comment mappingattributesobjectGlobal DB attributesvalue_tablesobjectNamed value enumerationssignal_definesarraySignal attribute definitionsframe_definesarrayFrame attribute definitionsglobal_definesarrayGlobal attribute definitionsbaudratenumberCAN bus baudratefd_baudratenumberCAN FD baudrate

Core Workflow#

1. Load the JSON with json.load()
2. Navigate using the structure documented in json-schema.md
3. Process messages and signals as needed

Common Tasks#

List All Messages#

python

import json

with open("can_db.json", encoding="utf-8") as f:
    db = json.load(f)

for msg in db["messages"]:
    print(f"0x{msg['id']:03X} {msg['name']}  DLC={msg['length']}  "
          f"cycle={msg.get('cycle_time', 'N/A')}ms  TX={msg.get('transmitters', [])}")

Find a Signal by Name#

python

def find_signal(db, signal_name):
    for msg in db["messages"]:
        for sig in msg["signals"]:
            if sig["name"] == signal_name:
                return msg, sig
    return None, None

msg, sig = find_signal(db, "VehicleSpeed")

Decode a Raw CAN Frame#

python

def decode_frame(db, arb_id, data_bytes):
    """Decode raw CAN data bytes into physical signal values."""
    msg = next((m for m in db["messages"] if m["id"] == arb_id), None)
    if not msg:
        return None

    results = {}
    for sig in msg["signals"]:
        raw = extract_raw_value(data_bytes, sig)
        physical = raw * float(sig["factor"]) + float(sig["offset"])

        # Value table lookup
        val_str = sig.get("values", {}).get(str(int(raw)))
        results[sig["name"]] = {
            "raw": raw,
            "physical": physical,
            "unit": sig.get("unit", ""),
            "enum": val_str,
        }
    return results

def extract_raw_value(data_bytes, sig):
    """Extract raw integer from CAN data bytes for one signal."""
    start_bit = sig["start_bit"]
    bit_length = sig["bit_length"]
    is_big_endian = sig.get("is_big_endian", False)
    is_signed = sig.get("is_signed", False)

    bits = ''.join(f'{b:08b}' for b in data_bytes)

    if is_big_endian:
        byte_pos = start_bit // 8
        bit_in_byte = start_bit % 8
        collected = []
        pos = byte_pos * 8 + (7 - bit_in_byte)
        for _ in range(bit_length):
            if 0 <= pos < len(bits):
                collected.append(bits[pos])
            curr_byte = pos // 8
            curr_bit = pos % 8
            if curr_bit == 0:
                pos = (curr_byte + 1) * 8 + 7
            else:
                pos -= 1
        raw = int(''.join(collected), 2)
    else:
        raw = 0
        for i in range(bit_length):
            byte_num = (start_bit + i) // 8
            bit_num = (start_bit + i) % 8
            if byte_num < len(data_bytes):
                if data_bytes[byte_num] & (1 << bit_num):
                    raw |= (1 << i)

    if is_signed and raw >= (1 << (bit_length - 1)):
        raw -= (1 << bit_length)
    return raw

Physical Value Formula#

text

physical = raw_value * factor + offset
raw_value = (physical - offset) / factor

Signal range: [min, max] in physical units.

Generate Message/Signal Summary Table#

python

def summarize(db):
    rows = []
    for msg in db["messages"]:
        for sig in msg["signals"]:
            rows.append({
                "Message": f"0x{msg['id']:03X} {msg['name']}",
                "Signal": sig["name"],
                "Start": sig["start_bit"],
                "Len": sig["bit_length"],
                "Factor": sig["factor"],
                "Offset": sig["offset"],
                "Unit": sig.get("unit", ""),
                "Min": sig.get("min", ""),
                "Max": sig.get("max", ""),
                "Endian": "BE" if sig.get("is_big_endian") else "LE",
            })
    return rows

Filter by ECU (Transmitter/Receiver)#

python

def messages_by_tx(db, ecu_name):
    return [m for m in db["messages"] if ecu_name in m.get("transmitters", [])]

def signals_received_by(db, ecu_name):
    results = []
    for msg in db["messages"]:
        for sig in msg["signals"]:
            if ecu_name in sig.get("receivers", []):
                results.append((msg, sig))
    return results

Important Notes#

• Message id is a decimal integer. Convert with hex(id) for display.
• is_extended_frame: True = 29-bit CAN ID, False = 11-bit standard ID.
• is_fd: True = CAN FD frame (up to 64 bytes), False = Classic CAN (up to 8 bytes).
• factor/ offset may be strings or floats depending on export settings. Always cast to float().
• values dict maps raw integer (as string key) → enum label string.
• Motorola (big-endian) bit numbering: canmatrix exports in "lsb" format by default.
• cycle_time is in milliseconds. 0 or absent means event-triggered.

DBC to JSON Conversion#

Use canmatrix's built-in CLI tool:

bash

pip install canmatrix
canconvert input.dbc output.json --jsonExportAll

Other supported input formats: .arxml, .kcd, .dbf, .xls(x), .sym, .ldf.

See json-schema.md for the complete JSON structure reference.