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Power off the CNC completely. Inspect all fiber optic cables between the CNC FSSB connector (typically on CPU board or interface board) through each servo amplifier and back to CNC. Check for disconnected cables, damaged connectors, or kinked/crushed fiber sections.

Do not look directly into fiber optic cable ends during inspection as laser light can damage eyes. Handle fiber connectors only by the ferrule housing, never pull on cables.

Remove each fiber connector, inspect the ferrule end for contamination or scratches using a fiber scope if available. Clean all fiber connectors using lint-free fiber cleaning swabs and isopropyl alcohol, or use a fiber optic cleaning pen. Verify correct seating with audible click when reconnecting.

Verify FSSB loop continuity: fiber should run from CNC FSSB OUT connector to first amplifier FSSB IN, then amplifier FSSB OUT to next amplifier FSSB IN (daisy chain), with final amplifier FSSB OUT returning to CNC FSSB IN connector, forming complete loop.

Power on CNC and observe servo amplifier LEDs during boot. All amplifiers should show normal FSSB communication LEDs (typically green solid or blinking). Check DIAGNOSIS > SERVO > FSSB STATUS screens to verify which axis amplifiers are detected on the FSSB network.

Identify the FSSB daisy-chain sequence of amplifiers. Disconnect the fiber OUT cable from the first amplifier and connect it directly from CNC FSSB OUT back to CNC FSSB IN, bypassing all amplifiers. Power on to test if alarm clears.

If alarm clears with no amplifiers, reconnect only the first amplifier in the loop (CNC OUT to Amp1 IN, Amp1 OUT to CNC IN). Power on and check for alarm. If no alarm, progressively add amplifiers one at a time to identify which amplifier breaks FSSB communication.

On the faulty amplifier, check LED indicators during power-up: FSSB-related LEDs should indicate communication status. Refer to amplifier model-specific manual (α-series, β-series) for LED diagnostic codes. Common failure is red FSSB LED or no FSSB activity indication.

If a specific amplifier is identified as faulty, verify parameter settings for that axis in SYSTEM > PARAM > SERVO parameters (parameters 2020-2032 range for axis assignment). If parameters are correct and hardware fault is confirmed, replace the servo amplifier interface board or entire amplifier unit.

Before replacing amplifier, document all servo tuning parameters for that axis from SYSTEM > PARAM screens as replacement may require parameter restoration.

With power off, verify the FSSB interface board (if separate from main CPU) is properly seated in the CNC backplane. Check for visible damage, burned components, or corrosion on the board. Reseat the board firmly and ensure all securing screws are tight.

Use proper ESD (electrostatic discharge) precautions when handling CNC circuit boards. Discharge static electricity before touching boards.

Check for any parameter corruption by attempting to load a verified backup parameter set. Navigate to SYSTEM > PARAM and check parameters 8131-8139 (FSSB configuration parameters) and parameter 8140 (FSSB channel settings) to ensure proper FSSB initialization settings are configured.