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Check the diagnostic buffer for additional alarms related to PLC communication or bus errors that occurred simultaneously.

In PLC diagnostics, verify that the PLC cycle time is not exceeding limits and that DB communication areas (DBx for NC-PLC interface) are accessible and not corrupted.

Power cycle the complete system (NCU and PLC) to reinitialize the communication interface between NC and PLC.

Ensure machine is in safe state and all active programs are saved before power cycle.

Check PROFIBUS/PROFINET connection status and cable integrity between NCU and PLC if using external PLC configuration.

Verify that the PLC program and NC configuration versions are compatible and match the commissioning documentation.

Compare the PLC interface data block structure (DB10, DB11, etc.) with the NC configuration in the machine data to ensure consistency in size and format.

Reload the PLC program and NC configuration from a known good backup, ensuring proper download sequence (typically PLC first, then NC data).

Document current machine data settings before restoring backup. System downtime will occur during reload.

If no backup available, perform a complete NC-PLC interface synchronization using Startup Tool or STEP7/TIA Portal to rebuild interface connections.

Boot the NCU and observe LED indicators during startup for any hardware fault indications (red LEDs or error patterns on NCU front panel).

Run NCU diagnostics from the HMI Service screen to check system memory integrity and hardware component status.

Check CompactFlash card for errors: remove, inspect for physical damage, test in card reader, and consider replacement if errors detected.

Only remove CompactFlash card when NCU is completely powered down. Use only Siemens-approved replacement cards.

If hardware fault is confirmed, contact Siemens service for NCU replacement or module repair, documenting all alarm history and diagnostic results.