Attention

IronPLC implements many parts of the IEC 61131-3 standard and is working toward full Structured Text support. Key features still missing include ranges, enum and I/O mapping.

Write PLC Programs with an AI Agent

This guide shows you how to connect an AI coding agent to IronPLC’s MCP server so that the agent can write, validate, and compile IEC 61131-3 programs on your behalf.

MCP (Model Context Protocol) is an open protocol that lets AI agents call external tools. IronPLC ships an MCP server called ironplcmcp that gives any compatible agent access to the IronPLC compiler — the agent can check syntax, run semantic analysis, look up error explanations, and compile programs without you running commands manually.

Note

This guide assumes you have installed the IronPLC Compiler. See Installation if you have not already installed it.

Prerequisites

  • IronPLC installed with ironplcmcp on your PATH. The MCP server is included automatically when you install IronPLC.

  • An MCP-compatible AI agent. See the next section for setup instructions for several popular agents.

Configure the MCP Server

Add IronPLC as an MCP server in your agent’s configuration. Pick the tab that matches your agent.

  1. Open Settings ‣ Developer ‣ Edit Config. This opens claude_desktop_config.json in your editor.

  2. Add the ironplc entry to the mcpServers object:

    {
  "mcpServers": {
    "ironplc": {
      "command": "ironplcmcp"
    }
  }
}

  3. Save the file and restart Claude Desktop.

  4. In a new conversation, look for the tools icon (hammer) in the chat input area. Click it to confirm that IronPLC tools appear in the list.

  1. Open the Cline sidebar in VS Code.

  2. Click MCP Servers, then Configure MCP Servers. This opens cline_mcp_settings.json.

  3. Add the ironplc entry to the mcpServers object:

    {
  "mcpServers": {
    "ironplc": {
      "command": "ironplcmcp"
    }
  }
}

  4. Save the file. Cline reloads automatically.

  5. Confirm that IronPLC tools appear in the MCP Servers list with a green indicator.

  1. Create a file called .mcp.json in your project root:

    {
  "mcpServers": {
    "ironplc": {
      "command": "ironplcmcp"
    }
  }
}

  2. Start Claude Code in that directory. The MCP server connects automatically.

  3. Type /mcp to confirm that the IronPLC server is listed and connected.

Write a Motor Start/Stop Program

Ask your agent to write a motor start/stop program. Copy or adapt the prompt below to match your requirements:

Write an IEC 61131-3 Structured Text program for a motor
start/stop circuit. Requirements:

- A momentary Start pushbutton
- A normally-closed Stop pushbutton
- An overload relay contact
- A motor contactor output
- A running indicator lamp

Include a CONFIGURATION block so I can compile and run it.

The agent will draft the program and validate it automatically.

Hint

Under the hood, the agent calls IronPLC’s check tool to validate the code against the IEC 61131-3 standard. If there are errors, it calls explain_diagnostic to look up the error explanation, fixes the code, and checks again — all without you running any commands.

After one or more rounds of checking, the agent should produce a working program. The result will look similar to:

PROGRAM MotorControl
VAR
    StartButton : BOOL;    (* Momentary start pushbutton *)
    StopButton : BOOL;     (* Normally closed stop pushbutton *)
    OverloadTrip : BOOL;   (* Overload relay contact *)
    MotorContactor : BOOL; (* Motor contactor output *)
    RunningLamp : BOOL;    (* Running indicator lamp *)
END_VAR

    (* Seal-in circuit: Start latches on, Stop or Overload breaks *)
    MotorContactor := (StartButton OR MotorContactor)
                      AND StopButton
                      AND NOT OverloadTrip;

    RunningLamp := MotorContactor;

END_PROGRAM

CONFIGURATION config
    RESOURCE resource1 ON PLC
        TASK MainTask(INTERVAL := T#100ms, PRIORITY := 1);
        PROGRAM prog1 WITH MainTask : MotorControl;
    END_RESOURCE
END_CONFIGURATION

Iterate on the Design

You can refine the program in follow-up messages. For example, ask the agent to add a star-delta starter:

Add a star-delta starter with a 5-second changeover timer
using TON. Add Star, Delta, and Main contactors.

The agent will modify the program, validate the changes with check, and produce an updated version:

PROGRAM MotorControl
VAR
    StartButton : BOOL;
    StopButton : BOOL;
    OverloadTrip : BOOL;
    Running : BOOL;
    StarContactor : BOOL;
    DeltaContactor : BOOL;
    MainContactor : BOOL;
    RunningLamp : BOOL;
    ChangeoverTimer : TON;
END_VAR

    Running := (StartButton OR Running)
               AND StopButton
               AND NOT OverloadTrip;

    ChangeoverTimer(IN := Running, PT := T#5s);

    StarContactor := Running AND NOT ChangeoverTimer.Q;
    DeltaContactor := Running AND ChangeoverTimer.Q;
    MainContactor := Running;

    RunningLamp := Running;

END_PROGRAM

CONFIGURATION config
    RESOURCE resource1 ON PLC
        TASK MainTask(INTERVAL := T#100ms, PRIORITY := 1);
        PROGRAM prog1 WITH MainTask : MotorControl;
    END_RESOURCE
END_CONFIGURATION

Hint

When you are satisfied with the program, ask the agent to compile it. The agent calls the compile tool to produce bytecode that the IronPLC runtime can execute.

Tips for Effective Prompts

  • Specify inputs and outputs explicitly. Name the variables and their types so the agent does not have to guess.

  • Mention timing requirements. Include timer values, scan cycle assumptions, or pulse durations as appropriate.

  • Reference standard library blocks by name. Use TON, TOF, CTU, SR, and other IEC 61131-3 blocks when you know what you need.

  • Ask the agent to validate. Tell the agent to check the code before considering it final. Some agents do this automatically; others benefit from the reminder.

  • Request a specific dialect. If your target runtime needs a particular IEC 61131-3 edition, mention it (for example, iec61131-3-ed3 for third-edition features).

See Also