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Access Diagnostics > System > Communication in the 840D control interface. Monitor the FastPlcCom cycle counter and error counter for discontinuities or rapid increases, indicating dropped handshakes.

Check Machine Data MD 11000 (PLC communication cycle time) and MD 11010 (PLC communication timeout). Verify these values are appropriate for your PLC scan time. Typical timeout should be 2-3x the PLC cycle time.

Do not set MD 11010 below 100ms on standard systems; values too low will cause nuisance alarms.

In the PLC program, verify the OB1 (main cycle) execution time does not exceed the FastPlcCom cycle time. Use the PLC cycle time display in the PLC diagnostics menu to confirm synchronization.

Power cycle the control system (full hard reset) to reinitialize the NCK-PLC communication stack and clear any stale handshake states.

In the NCK menu, navigate to Diagnostics > Memory > Shared Memory and verify the PLCIO buffer status shows 'OK'. If status is 'ERROR' or 'UNINITIALIZED', the buffer requires reinitializing.

Check the PLC startup sequence: Access PLC > Startup in the control menu and ensure MD 37100 (Enable PLCIO) is set to '1' and that the PLC cold start (warm restart if applicable) completes before NCK requests data.

A PLC warm start may not fully reinitialize shared memory; perform a cold start (full PLC download) if alarm persists.

Verify PLCIO buffer size in MD 11020 (PLCIO input area size) and MD 11021 (PLCIO output area size) match the actual PLC data declarations. Use the PLC editor to confirm the total I/O structure size.

If the alarm recurs, perform a PLC full download and NCK system reset (Control Panel > Maintenance > Reset System) to rebuild shared memory structures.

Check the PLC status in the Diagnostics menu: navigate to PLC > Status and verify the PLC is in RUN state and OB1 is executing cyclically. If PLC is STOP, error, or in HALT state, this is the root cause.

Review the PLC error blocks: In PLC Diagnostics > Error Info, check if OB121 (task priority error), OB122 (cycle time exceeded), or OB80-OB87 (runtime errors) were triggered. These indicate a PLC program problem, not hardware.

Examine OB1 cycle time: Diagnostics > PLC > Cycle Monitoring. If OB1 consistently exceeds the FastPlcCom cycle time (MD 11000), reduce the PLC user program load or increase MD 11000 to allow headroom (typically 10-20% margin).

Reload the PLC program from a known-good backup and perform a full restart. If the alarm clears, the PLC source code contained a latent bug; if it persists, the issue is hardware or configuration.