DBC Parser and Runtime Decode Module Documentation

Overview

This module provides a minimal but extensible DBC parser and runtime decode foundation for the FrameTap project.

Its purpose is to:

• parse DBC files
• store parsed frame and signal metadata
• build a tree for future UI integration
• build a runtime-ready decode database
• decode live CAN frames
• decode CAN trace records using the same decoder

The implementation follows a simple and practical design:

• parser logic is separated from UI logic
• runtime decode structures are separated from tree structures
• Qt is not required at the parser or decoder level
• the same decode engine can be reused for live traffic and trace replay

This is not a full production-grade DBC implementation yet, but it is a strong architectural base.

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High-Level Architecture

The module is divided into two main paths.

1. Parse and UI path

Used for:

• browsing frames and signals
• later integration with Qt Model/View
• displaying metadata

Pipeline:

DBC file
   ↓
DbcParser
   ↓
DbcDatabase
   ↓
DbcTreeBuilder
   ↓
TreeNode hierarchy
   ↓
future Qt UI

2. Runtime decode path

Used for:

• live CAN decoding
• CAN trace decoding
• fast lookup by CAN ID

Pipeline:

DBC file
   ↓
DbcParser
   ↓
DbcDatabase
   ↓
DbcDecodeBuilder
   ↓
DecodeDatabase
   ↓
DbcDecoder
   ↓
Decoded values

This separation is intentional.

The tree is useful for UI, but it is not the main data structure for runtime decoding.

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Why the Runtime Decode Layer Exists

A tree structure is convenient for browsing, but a runtime decoder needs something different:

• fast lookup by CAN ID
• direct access to signal decode definitions
• minimal overhead during repeated decoding
• the same logic for live frames and trace frames

Because of that, the design uses a dedicated runtime-ready structure:

• DecodeDatabase
• DecodeFrame
• DecodeSignal

This avoids forcing UI-oriented structures into a decode role they were not meant for.

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Module Layout

Parsed DBC structures

Files:

• signal_info.h
• frame_info.h
• dbc_database.h

These store a readable representation of the parsed DBC file.

UI tree structures

Files:

• tree_node.h
• tree_node.cpp
• dbc_tree_builder.h
• dbc_tree_builder.cpp

These convert parsed DBC content into a tree hierarchy suitable for UI and model/view usage later.

Runtime decode structures

Files:

• decode_database.h
• dbc_decode_builder.h
• dbc_decode_builder.cpp

These convert parsed DBC content into a structure optimized for decoding.

Runtime decoder

Files:

• dbc_decoder.h
• dbc_decoder.cpp

These perform actual decoding of raw CAN frames using DecodeDatabase.

Parser

Files:

• dbc_parser.h
• dbc_parser.cpp

These parse the DBC file itself.

Demo

File:

• main.cpp

Used as a small integration example.

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Parsed Data Structures

SignalInfo

Represents one signal as parsed from the DBC file.

Fields:

• name
• startBit
• length
• isLittleEndian
• isSigned
• factor
• offset
• minimum
• maximum
• unit
• receivers
• comment

Notes:

• receivers is a list because a signal may have more than one receiver ECU
• factor and offset define physical conversion
• this structure is close to DBC content and easy to inspect

Physical value rule:

physical = raw * factor + offset

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FrameInfo

Represents one frame as parsed from the DBC file.

Fields:

• name
• canId
• isExtended
• pgn
• hasPgn
• dlc
• transmitter
• comment
• signals

Notes:

• signals is a list of SignalInfo
• isExtended is determined during CAN ID normalization
• pgn is derived using simplified J1939 logic when applicable

---

DbcDatabase

Top-level parsed DBC container.

Conceptually:

DbcDatabase
 └── vector<FrameInfo>

This is the central structure produced by DbcParser.

---

UI Tree Layer

TreeNode

The UI tree contains three node types:

• Root
• Frame
• Signal

Example hierarchy:

dbc
 ├── EngineData
 │    ├── EngineSpeed
 │    ├── OilTemp
 │    └── CoolantTemp
 └── VehicleData
      ├── VehicleSpeed
      └── Odometer

Each node stores either:

• FrameInfo
• SignalInfo

The tree is intended for browsing and later Qt model integration.

It is not the primary runtime decode structure.

---

Runtime Decode Layer

Purpose

The decode layer exists so that decoding can be fast and independent from UI concerns.

Instead of searching a tree, the decoder uses a prepared database with direct lookup.

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ByteOrder

Runtime byte order enum:

• Intel
• Motorola

This is better for decode code than passing around raw DBC characters.

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ValueType

Numeric type enum:

• Unsigned
• Signed

This is clearer than combining multiple boolean flags during runtime logic.

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DecodeSignal

Represents one runtime-ready signal definition.

Fields:

• name
• startBit
• length
• byteOrder
• valueType
• factor
• offset
• minimum
• maximum
• unit
• receivers
• comment

This structure contains all information required for extracting and converting a signal value from raw frame data.

---

DecodeFrame

Represents one runtime-ready frame definition.

Fields:

• name
• canId
• isExtended
• dlc
• pgn
• hasPgn
• transmitter
• comment
• signals

This structure is used directly by the decoder.

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FrameKey

Fast lookup key for runtime frame matching.

Fields:

• canId
• isExtended

This matters because the same numeric identifier must not be confused between standard and extended frames.

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DecodeDatabase

Top-level runtime decode container.

Fields:

• frames
• frameIndexByKey

Conceptually:

DecodeDatabase
 ├── vector<DecodeFrame>
 └── unordered_map<FrameKey, index>

This gives the decoder fast access to a frame definition using CAN ID and frame type.

---

Decoder Layer

RawCanFrame

Represents a raw CAN frame to decode.

Fields:

• canId
• isExtended
• data

This same structure can be used for:

• live CAN bus input
• replayed trace records
• unit tests

---

DecodedSignalValue

Represents one decoded signal result.

Fields:

• definition
• rawValue
• physicalValue
• valid

---

DecodedFrameValue

Represents one decoded frame result.

Fields:

• definition
• signals
• valid

This is the decoder output for one raw frame.

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DbcDecoder

Main runtime decoder class.

Responsibilities:

• find a frame definition by CAN ID
• decode all signals in a frame
• extract raw values
• sign-extend signed values
• convert raw values into physical values

Main methods:

• FindFrame(...)
• Decode(...)

---

Parser Support

The current parser supports the following DBC elements:

• BO_
• SG_
• CM_ BO_
• CM_ SG_

---

Supported DBC Syntax

Frame definition

Example:

BO_ 256 EngineData: 8 EEC1

Parsed fields:

• frame CAN ID
• frame name
• DLC
• transmitter ECU

---

Signal definition

Example:

SG_ EngineSpeed : 0|16@1+ (0.125,0) [0|8000] "rpm" ECU1,ECU2

Parsed fields:

• signal name
• start bit
• signal length
• byte order
• signedness
• factor
• offset
• minimum
• maximum
• unit
• receivers

---

Comments

Frame comment example:

CM_ BO_ 256 "Engine data frame";

Signal comment example:

CM_ SG_ 256 EngineSpeed "Actual engine speed";

Stored in:

• FrameInfo::comment
• SignalInfo::comment

---

CAN ID Normalization

The parser normalizes frame identifiers.

Common DBC behavior:

• extended identifiers may be stored with bit 31 set
• the actual 29-bit identifier must be extracted from that value

The parser therefore stores:

• normalized canId
• separate isExtended flag

This is important both for correct lookup and for future interoperability with live CAN APIs.

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PGN Extraction

PGN is derived only when the frame is treated as extended.

The current logic is simplified J1939 extraction:

• pf
• ps
• dp

This is enough for a practical start but should not be treated as full J1939 validation.

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Decode Flow

Typical runtime decode flow:

RawCanFrame
   ↓
Find frame in DecodeDatabase
   ↓
For each signal:
    extract raw bits
    apply sign extension if needed
    convert to physical value
   ↓
DecodedFrameValue

---

Intel and Motorola Extraction

The decoder currently has separate extraction paths:

• ExtractIntel(...)
• ExtractMotorola(...)

This is important because byte order is not just metadata once decoding starts.

Intel and Motorola require different bit extraction logic.

This is one of the main reasons why the runtime decode layer should be explicit and prepared in advance.

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Example Usage

Parse DBC

DbcParser parser;
DbcDatabase database = parser.ParseFile("example.dbc");

Build UI tree

DbcTreeBuilder treeBuilder;
std::unique_ptr<TreeNode> root = treeBuilder.Build(database);

Build runtime decode database

DbcDecodeBuilder decodeBuilder;
DecodeDatabase decodeDatabase = decodeBuilder.Build(database);

Decode a raw frame

RawCanFrame rawFrame;
rawFrame.canId = 0x123;
rawFrame.isExtended = false;
rawFrame.data = {0x01, 0x02, 0x03, 0x04};

DbcDecoder decoder;
DecodedFrameValue decoded = decoder.Decode(decodeDatabase, rawFrame);

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Unified Decode Strategy

A key design goal is that the same decoder should work for both:

• live CAN frames
• trace replay frames

That means this architecture supports:

live path

live CAN input
   ↓
RawCanFrame
   ↓
DbcDecoder
   ↓
decoded signal values

trace path

trace reader
   ↓
RawCanFrame
   ↓
DbcDecoder
   ↓
decoded signal values

This avoids duplicating decode logic in two separate parts of the application.

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Intended Use in FrameTap

This module is meant to support at least the following FrameTap workflows:

• load a DBC file
• browse frames and signals
• search signals
• drag a signal into a plot later
• decode live CAN traffic
• decode recorded traces
• convert raw values into physical values
• show metadata like units, comments, transmitter, receivers, and PGN

Example combined workflow:

Load DBC
   ↓
Parse into DbcDatabase
   ↓
Build UI tree
   ↓
Build DecodeDatabase
   ↓
Use same decode engine for:
    - live frames
    - trace replay

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Why the Tree Is Not Enough

The tree exists for browsing.

However, runtime decode should not rely on tree traversal because that would introduce unnecessary coupling and inefficiency.

A runtime decoder needs:

• fast key-based access
• minimal interpretation at decode time
• direct signal definitions already prepared

That is why DecodeDatabase is a separate layer.

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Why No Abstract Factory Is Used

At the current stage, abstract factory is intentionally avoided.

The current design is already clean:

parser → parsed database → decode database
                      ↘ tree builder → UI tree

Introducing factory layers now would increase complexity without solving an immediate problem.

If later the project requires multiple output representations or multiple build strategies, that can be added then.

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Current Limitations

This is still a minimal implementation.

Not supported yet:

• multiplexed signals
• VAL_ tables
• BA_ attributes
• BA_DEF_ definitions
• advanced comment handling
• full DBC grammar coverage
• full J1939 validation
• extensive edge-case handling for unusual DBC formatting

Motorola extraction is implemented, but it should still be verified carefully against real-world DBC files and expected values.

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Recommended Next Steps

A practical development order would be:

Stage 1 - already implemented

• BO_
• SG_
• CM_
• normalized CAN ID
• isExtended
• transmitter
• receivers
• comments
• tree representation
• runtime decode database
• runtime decoder

Stage 2

Recommended additions:

• parent pointer in TreeNode
• Qt model adapter
• VAL_ support for enum-style signals
• better display strings for UI
• selective decoding of only chosen signals

Stage 3

Recommended additions:

• multiplexing support
• attribute parsing
• richer J1939 support
• CSV export of decoded traces
• optimized filtering and signal selection

Stage 4

Advanced functionality:

• live plot integration
• signal subscriptions
• per-signal trace decode pipelines
• decoder-assisted export formats

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Build Integration

The module does not depend on any specific build system.

It can be integrated with:

• CMake
• qmake
• Makefile

Just add the source files to the project.

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Summary

This module is now split into two intentionally separate layers:

Parsed representation

Used for:

• storing parsed DBC content
• browsing
• UI tree generation

Runtime decode representation

Used for:

• fast frame lookup
• live CAN decode
• trace decode
• physical value conversion

That separation is the main architectural improvement.

In short, the system now looks like this:

DBC parser → DbcDatabase → DecodeDatabase → DbcDecoder
                         ↘ TreeNode → future UI

This gives FrameTap a much better foundation for real use, because both browsing and decoding are supported without forcing one representation to do the other's job.