Heidenhain TNC “Guard Doors Are Open” Alarm: Four-Layer Safety-Chain Isolation and the Four Post-Change Tests

– No transition → either a sensor wiring fault or a PLC input card failure (not an NC software fault).

– It transitions but the NC still reports door open → the problem is in the NCK-PLC interface or in an MP that gates the flag.

Step 3 — force / monitor online (layer 3)

• Using the PLC online monitor (PLCdesign or the builder’s programming tool), force-toggle the door input bit and watch the NC behaviour.
• If the NC responds to the force but not to the physical switch → the wiring between switch and PLC input is the fault point.

4. Four root causes and how to handle each

About “hold vs controlled stop vs immediate stop” on door-open: which MP governs it varies by control series and builder PLC project, so this article does not invent a specific parameter number. On site, trace the door-related marker back to its source in the builder’s parameter list / PLC program.

5. Interaction with touch probes (TS / TT): trigger inhibit when the door is open

Heidenhain TS (touch trigger probe) and TT (tool touch probe) families communicate with the control over a dedicated interface. The PLC is responsible for gating the probe trigger signal with the door state — a probe triggered while the operator’s hand is inside the work envelope is a serious hazard.

When a door alarm and a probe-related symptom appear together, confirm in this order:

1. Probe power LED on the receiver unit (TS module) — must be steady, not flashing.
2. Stylus deflection test in MDI with the door closed — should produce a PROBE 1.0 reading or equivalent in the position display.
3. Probe-ready bit PLC marker — should toggle 0 → 1 within one IPO cycle of the probe trigger.
4. Probe enable flag in the NC software — must show active. If it is inactive while the door is closed, the door-open flag is stuck in the NC even though the physical switch is healthy.

Key point: the probe itself is usually not the fault — the gating logic around it is.

6. The four verification tests after any change (don’t just clear the alarm)

After any change to a door / safety-related function, perform a documented re-validation — clearing the alarm is not the same as fixing the problem.

1. Door-open test: confirm the spindle cannot start in any mode (try MDI, Manual and Auto); confirm probe triggering is inhibited.
2. Door-close test: with the door shut and the safety relay healthy, confirm full functionality is restored.
3. Failure-mode test: with the door closed, disconnect one channel of the safety switch at a time — the safety relay must trip and the machine must enter a safe state.
4. Recovery test: restore the channel → close the door → run the documented reset sequence. The machine must not auto-resume motion.

Record the date, technician and result for all four tests and keep them in the machine’s technical file.

7. Troubleshooting checklist

1. Read the alarm text carefully: “door open” vs “safety circuit interrupted” — the former points at the door switch, the latter at the whole chain.
2. Put the control in Manual / Programming mode (never Auto with the door open).
3. Open the PLC diagnostic screen and find the door-status flag bit.
4. Open/close the door repeatedly and check whether the bit transitions in real time — if not, move to the mechanical layer.
5. Force the door input bit online: if the NC responds to the force but not the real switch → the wiring between switch and input is the fault.
6. With E-stop active, measure the door-switch contact: open ≥ 1 MΩ; closed > 5 Ω means replace.
7. Verify the safety relay shows both channels healthy with contacts closed.
8. Diff the parameters against a sister machine; treat any safety-related delta as suspect.
9. After the change, run the four tests and file the records.
10. Never defeat a safety function with parameters.

FAQ

The TNC always says “door open”, but the switch measures closed — what’s going on?

It means the PLC is reading the door-closed condition as false while the physical switch is fine. The usual causes are: a failed door-switch contact (high resistance on an NC contact, typically after 5–7 years in service), a broken wire between the switch and the PLC input card, or a PLC input channel failure. Measure continuity across the switch (target ≥ 1 MΩ open). Then check the PLC input bit, then the NCK-PLC interface, in that order. Do not force the flag “closed” via parameters to clear the alarm — that removes the protection, it does not fix the machine.

After I replace a door switch or touch the safety circuit, what counts as “done”?

Clearing the alarm is not enough. Run at least the four tests and document them: (1) door-open — the spindle must not start in MDI, Manual or Auto, and probe triggering must be inhibited; (2) door-close — full function restored with the safety relay healthy; (3) failure-mode — with the door closed, disconnect one safety-switch channel at a time; the safety relay must trip and refuse to reset until the fault is cleared; (4) recovery — restore, close the door, perform the documented reset sequence; the machine must not auto-resume motion. Record date, technician and result for each.

The door alarm and a probe alarm appear together — which do I chase first?

The door, not the probe. The PLC gates the probe trigger with the door state, so a set door-open flag inhibits probing. Check in this order: probe power LED on the receiver (must be steady, not flashing) → stylus deflection test in MDI with the door closed (should read PROBE 1.0 or equivalent) → probe-ready PLC bit toggling 0 → 1 → probe enable flag active in the NC. If the probe enable flag is inactive while the door is closed, the door-open flag is stuck in the NC even though the physical switch is healthy. The probe itself is usually fine — the gating logic is the problem.

Last updated: 2026-10-28