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Power down the machine completely and lock out power per safety procedures.

Always follow LOTO procedures. Verify zero voltage before touching connections.

Inspect all FSSB cables (typically orange fiber optic cables) for physical damage, kinks, or excessive bending. Check both ends for cracks in connectors.

Do not look directly into fiber optic cable ends. Laser light can damage eyes.

Disconnect and reconnect all FSSB cable connections at the CNC control unit and each servo amplifier, ensuring they click firmly into place.

Power up the system and observe if the alarm clears. If alarm persists, swap FSSB cables one at a time with known good spares to isolate a faulty cable.

Check the system diagnostic screen or FSSB connection display to identify which axis or amplifier number is reporting the communication error.

Inspect the identified servo amplifier for LED status indicators. Compare LED patterns against the amplifier's documentation to identify faults.

Verify the amplifier has proper DC bus voltage (typically 280-310VDC) by checking the voltage display or measuring at the test points.

High voltage present. Use insulated test equipment rated for at least 600V.

If a specific amplifier is identified as faulty and cable swapping did not resolve the issue, replace the servo amplifier module and reconfigure axis parameters.

Record all parameter settings before replacement. New amplifiers require parameter loading and possibly serial number registration.

Access the maintenance/diagnostic menu and navigate to the FSSB setup screen. Verify the correct number of axes and amplifier addresses are configured in parameters 8000 series.

Perform an FSSB axis detection or auto-setup function from the diagnostic menu to allow the CNC to re-scan and identify all connected amplifiers.

This may reset axis assignments. Document current configuration before proceeding.

Check for additional alarms or error history that may indicate a CNC control board fault. Inspect the CPU board and JD1A/JD1B connectors for proper seating.

Static sensitive components. Use proper ESD precautions when handling control boards.

If software corruption is suspected, reload CNC system software and parameters from a known good backup. Contact Fanuc technical support if control board replacement is necessary.

System reload will erase all parameters and programs. Ensure you have complete verified backups before proceeding.