21700

In Program Execution, pause the NC program at the 21700 alarm block. Use Diagnostics > Probe Signals menu to verify current probe input status (typically digital input DI assigned in MD 36910).

Do not resume program; probe is already triggered.

Execute a manual clearance move: jog the axis away from the workpiece/stop surface to release mechanical contact. Monitor the probe signal indicator in the NCK Diagnostics display to confirm the signal returns to inactive/quiescent state.

Ensure adequate clearance distance to prevent re-triggering during the move.

Insert a G0 (rapid positioning) clearance block or dwell (G04 P__) in the NC program before the failing MEAS/MEAW block to guarantee probe signal stabilization between measurements.

Verify MD 36910 probe input assignment and MD 36920 (Probe debounce time) are correctly configured for your probe hardware. Increase debounce time if signal bouncing is suspected.

Power-cycle the machine control: perform an Emergency Stop (EStop), then restart the SINUMERIK 840D. This resets the NCK digital input state and clears any latched signal conditioning faults.

This is a temporary workaround; hardware faults will recur.

Using the offline diagnostics interface (Siemens STARTER or Service Panel), navigate to Hardware > Digital Inputs and monitor the probe input bit (assigned in MD 36910) over 10+ seconds while the probe is mechanically undeflected. If the bit remains logic 1 or toggles erratically, the probe or input module has failed.

Inspect the physical probe connector (usually sub-D or M12) for corrosion, bent pins, or poor contact. Test continuity of signal lines with a multimeter if applicable. Check probe stylus for mechanical binding or deformation.

Disconnect power before testing electrical connections.

If probe input remains stuck, replace the probe head assembly or the probe signal conditioner module. Update probe offset data (MD 36921 for X/Y/Z offset) after hardware replacement.

Access NCK Parameter menu: Control Panel > Diagnostics > Parameters. Verify MD 36910 points to the correct DI channel assigned to your physical probe connector on the digital input module (typically I0.0 to I7.7 notation).

Check MD 36900 (Probing Enable) = 1 and MD 36911 (Probe active level, typically 0=low-active, 1=high-active) matches your probe hardware specification. Consult the probe manufacturer's documentation for signal polarity.

If parameters were incorrect, correct them, save the machine data, and perform a warm restart (Control Panel > Restart > Warm). Re-run the measurement block.

Incorrect polarity will cause the opposite alarm: 'probe not deflected when expected'.

After correction, manually test the probe by touching the stylus to a reference surface and confirm the Diagnostics > Probe Signals indicator toggles on/off correctly.