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Power down the machine completely and lockout. Inspect all FSSB cables (typically fiber optic cables with orange or black jackets) from the CNC to each amplifier for physical damage, tight bends, or pinch points.

Always power down before disconnecting FSSB cables to prevent amplifier damage.

Disconnect and reconnect each FSSB connector at both the CNC side and amplifier side, ensuring positive latching clicks. If fiber optic, clean connectors with appropriate fiber optic cleaning tools and inspect for cracks.

Never look directly into fiber optic cable ends - can cause eye damage.

Check the FSSB cable routing to ensure cables are not running parallel to power cables, near welders, or other high EMI sources. Maintain minimum 6-inch separation from power lines.

If available, swap suspected FSSB cable with a known good cable or swap cable positions between amplifiers to see if the error follows the cable. Replace cable if fault moves.

Access the CNC diagnostic screens (Maintenance > Servo > FSSB or similar path depending on control series). Identify which axis amplifier number is reporting the error in the alarm detail or sub-code.

Check all LED indicators on the suspect amplifier. Record the LED status pattern and compare to amplifier documentation. Red LEDs or no LEDs typically indicate amplifier fault conditions.

Verify the amplifier has proper input power (200-230VAC single phase or 3-phase depending on model). Check fuses and circuit breakers dedicated to that amplifier in the cabinet.

Measure voltage with appropriate safety equipment and PPE - lethal voltages present.

If possible, swap the FSSB position of the suspect amplifier with another axis amplifier (both cables and parameter assignment) to determine if the fault follows the amplifier hardware or remains on the bus position. Replace amplifier if fault follows the unit.

This requires parameter changes - backup parameters before making modifications.

Verify that the last amplifier in the FSSB chain has the termination resistor properly installed (typically a connector with built-in resistor or termination plug on the OUT port of the last amp).

Check CNC parameters for correct FSSB configuration. Verify parameter 8100-8103 series (control series dependent) matches the actual number of amplifiers and their axis assignments on the bus.

Review any recent service work or amplifier replacements. Ensure the FSSB daisy-chain sequence is correct and no amplifiers were bypassed incorrectly leaving an unterminated segment.

If termination resistor is suspect, measure resistance value if accessible (typically 110-120 ohms for FSSB) or replace with known good termination connector from Fanuc spare parts.