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Navigate to Diagnostics > Machine Data > $MN_MM_SERVO_FIFO_SIZE in the NCK menu (or via HMI parameter editor).

Verify the current value. For NCU link systems with distributed axes, this MD must be set to 3 or 4. Value 3 is standard; use 4 only if link cycle latency requires additional buffering.

Value must not be 0, 1, or 2 when link axes are active.

If value is incorrect, modify $MN_MM_SERVO_FIFO_SIZE to 3, save, and perform a warm restart (PLC > Restart).

Changing this MD requires a system restart; plan downtime accordingly.

Verify axis assignment in Diagnostics > Axis Data to confirm axes are correctly configured as link axes (check axis type and link NCU assignment).

Check Machine Data $MN_IPO_CYCLE to confirm the interpolation cycle time (usually 4 ms, 8 ms, or 12 ms depending on control mode).

Verify link communication cycle in Diagnostics > System Data > Link Configuration. Link cycle must be ≥ 2× the IPO cycle. If link cycle is too fast or IPO too slow, the FIFO cannot keep up.

Link cycle and IPO cycle timing must be coherent; consult machine builder specifications.

If IPO cycle is much faster than link cycle, increase $MN_MM_SERVO_FIFO_SIZE from current value to 4 to add an extra buffer slot.

Perform warm restart and monitor for alarm recurrence during axis motion. If alarm persists, review link cable integrity and switch/hub latency on the SINUMERIK link network.

Check Diagnostics > Axis Data and review the link status and mode of all axes. Identify which axis triggered the alarm (from alarm log if available).

Verify that $MN_MM_SERVO_FIFO_SIZE is set to 3 or 4 before enabling any link axes. If axes were switched to link mode dynamically, a full system restart is required.

Link axis mode changes require warm restart; do not attempt online reconfiguration.

Clear the alarm, set $MN_MM_SERVO_FIFO_SIZE to 3, and perform a warm restart (PLC > Restart or power-cycle NCU).

After restart, verify link axis connectivity in Diagnostics > Link Status and confirm all remote NCUs are communicating. Re-enable axes and test smooth motion.