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Power down the machine and open the 840D control cabinet. Physically inspect for the NCU link module (check slot assignments documented in your machine's hardware configuration drawing).

Ensure machine is de-energized and lockout/tagout procedures are followed before accessing cabinet.

Verify the module part number matches the NCU link variant specified in the factory configuration. Cross-reference with the Siemens 840D hardware manual (typically found under NCK I/O module sections).

If module is missing, contact your Siemens service representative to procure and install the correct NCU link module. Do not attempt operation without the hardware.

After installation, power cycle the control and verify MD 30360 and related NCU link parameters are properly configured via Diagnostics > Machine Data > NCU Link menu.

Check all connector cables between the NCU link module and the main NCK board. Reseat all connectors firmly (typically ribbon cables or backplane connections in the 840D cabinet layout).

Power must be OFF during connector inspection and reseating.

Verify the control software and NCU link module firmware versions match compatibility requirements. Check via Diagnostics > System Information > Software Versions and compare against the NCU link module documentation.

If versions are incompatible, perform a firmware update for either the main control or module as recommended by Siemens service bulletin (typically via Service > Software Update in the HMI).

Firmware updates must not be interrupted; ensure UPS or stable power supply is connected.

If hardware detection still fails after reseating and firmware alignment, replace the NCU link module as it likely has internal hardware failure.

Access Diagnostics > Machine Data and navigate to the NCU Link parameter group. Check if MD 30360 (NCU Link Enable) is set to TRUE/1 or if other NCU link parameters contain non-default values.

Review the machine's configuration history: if parameters were imported from a backup or transferred from a different 840D configuration, verify those settings are appropriate for this machine's actual hardware configuration.

If NCU link hardware is not available and not planned for installation, manually reset NCU link MD parameters to factory defaults (typically MD 30360 = FALSE and related communication parameters to NULL/disabled state).

Save the corrected MD via File > Save and perform a control restart (warm restart via HMI or soft power cycle) to clear the alarm and prevent automatic MD reset on future startups.