mcp-plc-bridge

mcp-plc-bridge

An open-source MCP (Model Context Protocol) server that lets an LLM read sensors and control actuators on a PLC over Modbus TCP. Built against OpenPLC as the PLC runtime, but the bridge itself only depends on standard Modbus TCP, so it works with any Modbus TCP-capable PLC or simulator.

Architecture

LLM (e.g. Claude) <--MCP--> mcp_plc_server.py <--Modbus TCP--> PLC runtime
                                    ^
                                    |
                       plc_client.py (Modbus helper)
                                    ^
                                    |
                         thermal_sim.py (optional digital twin
                         for analog tags, see below)

• plc_client.py — thin Modbus TCP client (coils and 32-bit REAL values).
• mcp_plc_server.py — MCP server exposing list_tags, get_sensor_reading, and set_actuator tools, backed by a small tag map.
• thermal_sim.py — standalone digital-twin process for a simulated analog control loop (a setpoint, a valve position derived from it, and a temperature that drifts toward the setpoint over time). This is pure Python, not PLC ladder logic — a stand-in for a real PID loop so the demo has a live analog signal to show off. Skip it if you only care about the boolean tags.
• mock_plc_server.py — a plain Python Modbus TCP server you can run instead of a real PLC runtime, useful for trying the bridge without installing OpenPLC at all.

Tags

Tag	Type	Writable	Notes
motor	bool	yes	Digital output
input1	bool	yes	Digital input
input2	bool	yes	Digital input
TempsetPoint	float	yes	Temperature setpoint
Valve_position	float	no	Computed from TempsetPoint
Actual_Temp	float	no	Drifts toward TempsetPoint over time

Setup

1. Install Java (required by OpenPLC) and Python 3.10+.

2. Install OpenPLC Editor and set up a PLC runtime (native install or Docker) — see OpenPLC's own docs for this part, since it depends on your OS and OpenPLC version.

3. Create a venv and install dependencies:

   python -m venv venv
   venv/Scripts/activate   # or `source venv/bin/activate` on Linux/macOS
   pip install -r requirements.txt
   

4. In OpenPLC Editor, build a program with the tags above (or edit the tag map in mcp_plc_server.py to match your own I/O layout), deploy it, and start the PLC.

Running

# Optional: only if you're using the analog tags
python thermal_sim.py

# The MCP server itself (this is what a Claude client connects to)
python mcp_plc_server.py

To try the bridge without any PLC at all:

python mock_plc_server.py   # in one terminal
python plc_client.py        # in another, to sanity-check the connection

Connecting to Claude

Point your MCP client at:

• Command: path to your venv's python (or python.exe on Windows)
• Args: path to mcp_plc_server.py

Where exactly to configure this depends on which Claude client you're using — check its settings for an "Extensions," "Connectors," or "MCP servers" section, or its claude_desktop_config.json if it uses the classic mcpServers format.

Once connected, you can ask things like "What's the current temperature reading?" or "Turn on the motor."

License

MIT — see LICENSE.