What did the author actually build?

A single HTML file, with no libraries, that simulates an FX5UC PLC driving an MR-J5-40G servo over CC-Link IE Field Basic in the browser. Almost all of the code was written by Claude Code.

What AI mistakes were found?

Four cases include ladder logic that did not change after switching modes, JOG+ and JOG− buttons placed opposite to the machine diagram, "-0 r/min" shown while stopped, and an override switch that stayed slow after being turned OFF.

How can people with real hardware help?

The GitHub repo has an Issue form labeled "real-machine check" for reporting where the simulator differs from actual equipment. Code writing is not required.

Large Language ModelsAI Coding AssistantsZenn AI/ML1h agoOct 24, 2026, 01:00 JST

Non-coder builds PLC servo simulator with Claude Code in a day, 16 PRs

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3 Key Points

What happened
Starting from a GitHub repo, the author — who knows PLCs and servos but does not write code — had Claude Code build a browser simulator of an FX5UC PLC driving an MR-J5-40G servo over CC-Link IE Field Basic, reaching 16 pull requests in one day.
Why it matters
The author reports four AI mistakes caught by touching the screen and checking behavior, such as ladder logic not updating after switching modes and a stopped axis showing "-0"; the AI's own browser checks also caught some errors, but whether the PLC and servo specs match real hardware remains unverified.

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Context & Analysis

The author's workflow was deliberately simple: look at the simulator screen, send a screenshot with a one-line Japanese request, let Claude Code research, fix on a branch, verify in the browser and open a pull request, then merge after reading the PR description. This loop produced 16 pull requests in a single day from the moment the repository was created.

What makes the account concrete is the error list. Mode switching left the running ladder unchanged because saved samples were treated as user-authored after a separate PR changed the sample content. The JOG+ and JOG− buttons were arranged opposite to the machine diagram, while the motion itself was correct. A stopped axis displayed "-0 r/min" and "-0 %" because a tiny negative calculation remained after rounding. The override stayed slow after being turned OFF because the previous value stayed in the device. The author notes that touching the screen catches many mistakes, especially visual ones, and that Claude Code's own browser checks also helped — but neither the author nor the AI could verify whether the PLC and servo specifications match real equipment, leaving that to people with hardware knowledge.

FAQ

What did the author actually build?: A single HTML file, with no libraries, that simulates an FX5UC PLC driving an MR-J5-40G servo over CC-Link IE Field Basic in the browser. Almost all of the code was written by Claude Code.
What AI mistakes were found?: Four cases include ladder logic that did not change after switching modes, JOG+ and JOG− buttons placed opposite to the machine diagram, "-0 r/min" shown while stopped, and an override switch that stayed slow after being turned OFF.
How can people with real hardware help?: The GitHub repo has an Issue form labeled "real-machine check" for reporting where the simulator differs from actual equipment. Code writing is not required.

Zenn AI/MLRead Original Article

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