TraneSpotting

Reverse engineering tools for the Trane ComfortLink II protocol used in residential HVAC systems.

Goal

Enable custom thermostats (Home Assistant, etc.) to communicate with Trane ComfortLink II compatible equipment:

• Trane/American Standard communicating furnaces (SV92, etc.)
• Trane/American Standard communicating heat pumps (XV19, etc.)
• Via BAY24VRPAC52DC relay panel

Protocol Background

ComfortLink II is based on Honeywell EnviraCOM 2.0 protocol.

The XL850/XL950/XL1050 thermostats are manufactured by Honeywell for Trane. This is significant because it means existing EnviraCOM documentation and tools may be applicable.

Two Possible Physical Layers

Your system may use one of two physical layers:

Type	EnviraCOM (Native)	RS-485
Wires	3 (R, C, D)	2-4
Data signal	Modulated 24VAC	Differential ±5V
Baud rate	120 bps	9600-38400 bps
Interface	Custom circuit needed	$10 USB adapter

First step: Determine which physical layer your system uses!

Project Status

Phase 0: Physical Layer Identification - Start Here

Quick Start

Installation

# Create virtual environment
python -m venv venv
source venv/bin/activate

# Install dependencies
pip install -e .

# For development
pip install -e ".[dev]"

# For analysis tools (pandas, matplotlib)
pip install -e ".[analysis]"

# For hardware design tools (circuit schematics)
pip install -e ".[hardware]"

Step 1: Identify Physical Layer

Before capturing data, determine your bus type:

Option A: Visual Inspection

• Count wires between thermostat and relay panel
• 3 wires (R, C, D): Likely EnviraCOM
• 4+ wires with labeled A/B: Likely RS-485

Option B: Oscilloscope Measurement

1. Probe the data line
2. EnviraCOM: 24VAC signal with slow (8ms) transitions
3. RS-485: Fast differential signal (microsecond transitions)

Option C: Use Signal Analyzer (if you have logic analyzer captures)

python -m tools.signal_analyze capture.csv

Step 2a: If RS-485

# Get USB to RS-485 adapter (FTDI chipset recommended)

# Auto-detect baud rate
python -m tools.baudrate_detect -p /dev/ttyUSB0

# Start capturing
python -m tools.capture -p /dev/ttyUSB0 -b <detected_baud>

Step 2b: If EnviraCOM (Native)

You'll need custom hardware. Options:

1. Find a W8735A Serial Adapter (discontinued 2013, check eBay)
2. Build the DIY interface circuit - See docs/hardware/enviracom_interface.md
   • Complete schematic with BOM (~$15 in parts)
   • Uses H11AA1 + 4N35 optocouplers for isolation
   • ESP32/Arduino compatible
3. Generate circuit diagrams:

   pip install -e ".[hardware]"
   python -m tools.hardware.enviracom_schematic --format png

4. Generate PCB manufacturing files (Gerber, BOM, etc.):

   # Install cuflow dependencies
   pip install -e ".[pcb]"
   
   # Clone cuflow (not available on PyPI)
   git clone https://github.com/jamesbowman/cuflow.git /tmp/cuflow
   export PYTHONPATH="/tmp/cuflow:$PYTHONPATH"
   
   # Generate PCB files (MCU header version)
   python -m tools.hardware.enviracom_pcb
   
   # Generate USB version (direct computer connection)
   python -m tools.hardware.enviracom_pcb --variant usb

Step 3: Analyze Captures

# Analyze a capture file
python -m tools.analyze captures/raw/capture_*.bin

# Export to readable format
python -m tools.analyze capture.bin --export packets.txt --format parsed

# View specific packet
python -m tools.analyze capture.bin --packet 42

Tools

Tool	Description
tools/capture.py	Capture RS-485 bus traffic
tools/analyze.py	Analyze captured packets
tools/replay.py	Replay packets (use with caution)
tools/baudrate_detect.py	Auto-detect RS-485 baud rate
tools/signal_analyze.py	Analyze oscilloscope/logic analyzer captures
tools/packet.py	Packet parser library
tools/hardware/enviracom_schematic.py	Generate EnviraCOM interface circuit diagrams
tools/hardware/enviracom_interface.py	EnviraCOM interface BOM and pinout
tools/hardware/enviracom_pcb.py	Generate PCB layout and Gerber files (via cuflow)

Project Structure

trane_protocol/
├── captures/
│   ├── raw/           # Binary capture files
│   └── parsed/        # Analyzed/exported data
├── docs/
│   ├── hardware/      # Wiring and setup guides
│   └── protocol/      # Protocol documentation
│       ├── README.md           # Protocol overview
│       └── enviracom_physical.md  # Physical layer details
├── tools/
│   ├── capture.py     # RS-485 bus capture
│   ├── analyze.py     # Packet analysis
│   ├── replay.py      # Packet replay
│   ├── packet.py      # Packet parser
│   ├── baudrate_detect.py    # Baud rate detection
│   └── signal_analyze.py     # Waveform analysis
├── tests/
├── config.example.yaml
├── pyproject.toml
└── README.md

Related Projects

• EnviraCOM (GitHub) - Honeywell EnviraCOM interface
• Net485 - Multi-brand HVAC RS-485 protocol project
• Infinitive - Carrier Infinity integration
• gofinity - Go library for Carrier/Bryant

Key References

• Bloominglabs EnviraCOM Info - Physical layer details
• US Patent 6373376B1 - EnviraCOM hardware implementation
• CocoonTech Forums - Community discussion

Safety Warnings

• 24VAC is dangerous - Use proper isolation when building interfaces
• Only connect to communication bus lines, NEVER directly to 24VAC power
• Keep a working thermostat available as backup
• Don't experiment during extreme weather
• Start with passive monitoring before sending any packets
• Understand packets before replaying them

Contributing

Contributions welcome! Especially:

• Confirmation of physical layer type for specific equipment
• Packet captures from different equipment configurations
• Protocol documentation updates
• Checksum algorithm identification
• EnviraCOM interface circuit designs
• Message type mappings

License

MIT License - See LICENSE file