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Navigate to Siemens HMI operator panel > Diagnostics > Machine Data > search for $MN_MM_SERVO_FIFO_SIZE

Document the current value (typically 0, 1, or 2). For NCU link systems with active link axes, this MUST be set to 3.

Set $MN_MM_SERVO_FIFO_SIZE = 3 via machine data editor. Verify write permissions and that system is in offline/program mode.

Do not change while NC program is running or during active axis motion

Save changes, perform a warm restart (Reset via NCK menu), and verify alarm clears on next power cycle or system initialization.

Navigate to Diagnostics > Axis Configuration menu and identify which axes have NCU link status active (typically shown as 'Link' or 'Distributed' in axis list).

Cross-reference active link axes against machine data $MN_AXCONF_MACHAX_LINK_xxx entries (where xxx is axis number). Confirm link axes are intentionally configured.

If link axes are not required for this application, disable link configuration by setting corresponding axis entries to non-link type via machine data, then clear alarm.

If link axes are required, ensure $MN_MM_SERVO_FIFO_SIZE = 3 and perform warm restart to re-establish NCU link handshaking protocol.

Check system software version via Diagnostics > System Information. Verify SINUMERIK 840D version supports NCU link feature (840D/840D sl or later).

Review machine data backup or commissioning documentation to determine if $MN_MM_SERVO_FIFO_SIZE was explicitly set during initial setup or left at default (typically 0).

Compare current machine data snapshot against reference backup using Siemens ServiceSoftware or backup tool. Look for $MN_MM_SERVO_FIFO_SIZE discrepancy between link-enabled and baseline configurations.

Apply corrected parameter set with $MN_MM_SERVO_FIFO_SIZE = 3, save to non-volatile memory, and perform cold restart (power cycle NCU) to force full reinitialization of servo and link protocols.

Cold restart may clear temporary diagnostic buffers; capture diagnostic logs beforehand if troubleshooting ongoing NCU link issues