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.

Related MCP server: @inscada/mcp-server

Tags

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.