SRVO-055

Check alarm detail for affected axis

On the teach pendant, press MENU → ALARM → F1 [TYPE] → select SERVO. Note which axis number is listed with the SRVO-055 alarm. This identifies which amplifier has lost communication. The alarm detail line will show 'Axis: X' where X is the axis number (1-9 for standard robots).

If multiple axes show SRVO-055 simultaneously, suspect the controller FSSB board or main FSSB cable from controller rather than individual amplifier connections.

Power down and inspect FSSB connectors

Perform controlled shutdown (MENU → SYSTEM → Master/Cal → [F1] CONTROLLED START). Disconnect both fiber optic cables (IN and OUT) from the affected axis amplifier. Inspect the ferrule end-face using a fiber scope or bright light for contamination, cracks, or scratches. Check the connector is fully seated and locking mechanism engages with an audible click.

Never look directly into a fiber optic cable end while system is powered. Although low power, it can damage eyesight. Always power down before disconnecting FSSB cables.

Clean fiber optic connectors

Use only approved fiber optic cleaning materials: lint-free wipes with 99% isopropyl alcohol or fiber optic cleaning cassettes (Fanuc part number A660-8012-T398 or equivalent). Clean both the cable ferrule and the amplifier receptacle. Allow 30 seconds drying time before reconnection. Clean in one direction only, never circular motions.

Keep a fiber optic inspection scope in your toolkit. A $200 USB fiber scope can save hours of troubleshooting by immediately identifying contaminated or damaged fiber ends.

Verify FSSB cable routing and condition

Trace the FSSB cables from controller to each amplifier following the daisy-chain topology. Check for kinks, pinch points, or bends tighter than 25mm radius. Verify cables are secured and not hanging loose where robot motion could stress them. Inspect the orange outer jacket for any cracks or damage.

FSSB cables should never be zip-tied tightly in bundles with power cables. Keep minimum 50mm separation from AC servo motor power lines and 100mm from welding cables to prevent electromagnetic interference affecting the optical transceivers.

Replace suspect FSSB cable and test

If contamination cannot be cleaned or damage is visible, replace the FSSB cable segment. Use only genuine Fanuc FSSB fiber optic cables with correct length. After cable replacement, power up and monitor MENU → SYSTEM → FSSB for 'Link Active' status on all axes. Test robot motion through full range to verify alarm does not return under movement.

Keep a 3-meter FSSB cable as a diagnostic spare. You can temporarily bypass a suspect cable segment to quickly isolate whether the cable or amplifier is the root cause.

Verify amplifier power and LED status

With robot powered on (but alarm present), visually inspect the affected axis servo amplifier. Check that the green POWER LED is illuminated steady. Verify the amber FSSB LINK LED (labeled 'OPT' or 'LINK' depending on amplifier model) is ON solid, not blinking or OFF. If FSSB LED is off, the amplifier is not communicating on the bus.

The FSSB LINK LED should be steady amber/orange when communication is good. Blinking indicates the amplifier is receiving data but not transmitting properly. OFF means no optical signal received.

Check DC bus voltage at amplifier

Measure DC bus voltage at the amplifier power terminals. For alpha/beta servo amplifiers, DC bus should be approximately 280-310 VDC with 200-230 VAC input. Low DC bus voltage (below 260 VDC) can prevent proper FSSB operation. Check incoming 3-phase AC voltage at the cabinet is within 200-230 VAC ±10% and all three phases are present and balanced within 2%.

HIGH VOLTAGE PRESENT. Only qualified personnel should measure DC bus voltage. Use proper PPE and insulated test equipment rated for 600V minimum. Ensure proper lockout/tagout procedures.

Review amplifier alarm history

On teach pendant, check MENU → ALARM → ALARM HISTORY for any amplifier-specific alarms preceding the SRVO-055. Look for SRVO-075 (overload), SRVO-015 (overvoltage), SRVO-037 (overcurrent), or temperature alarms (SRVO-065). These indicate the amplifier dropped off FSSB due to protective shutdown.

Press F3 [DETAIL] on alarm history entries to see time-stamps. If multiple alarms occurred within 1-2 seconds, the first alarm is usually the root cause and subsequent alarms are cascade effects.

Check amplifier cooling and temperature

Verify amplifier cooling fans are operating and air flow is not obstructed. Check cabinet ambient temperature is below 45°C. Inspect amplifier heat sink for dust buildup. Access MENU → SYSTEM → Variables → $MTRTEMP[x] where x is the motor number to check motor temperature (should be below 90°C). Overheated amplifiers will drop off FSSB.

Use a non-contact infrared thermometer to quickly scan all amplifiers. A single amplifier running 20°C+ hotter than others indicates cooling problems or internal faults in that specific amplifier.