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How to Fix FANUC FSSB Alarm – Causes and Solutions
Quick Fix for FANUC FSSB Alarm
If the FANUC FSSB alarm appears, try these immediate actions:
- Power cycle the controller and all servo amplifiers
- Check FSSB fiber optic cables for loose or damaged connections
- Inspect amplifier status LEDs for communication faults
- Reseat FSSB connectors and verify proper routing
These steps often restore communication if the issue is intermittent.
What is FANUC FSSB Alarm?
A FANUC FSSB Alarm indicates a failure in the FANUC Serial Servo Bus (FSSB) communication system.
FSSB is the high-speed fiber optic network that connects:
- Robot controller (CPU)
- Servo amplifiers
- Robot axes (motors & encoders)
When this communication is interrupted, the controller cannot send motion commands or receive feedback, causing the robot to stop.
What Does FSSB Alarm Mean?
This alarm means the controller has lost or corrupted communication with one or more servo amplifiers.
Typical impact:
- Robot axes become unresponsive
- Servo system shuts down for protection
- Multiple servo-related alarms may appear simultaneously
Common Causes of FANUC FSSB Alarm
Damaged or Loose Fiber Optic Cable
FSSB relies on fiber optic communication, which is sensitive to physical damage.
Common issues:
1. Bent or broken fiber cable
2. Loose connectors at amplifier or controller
3. Dirty or scratched fiber ends
This is the most common cause of FSSB alarms.
Faulty Servo Amplifier
If one amplifier fails, it can interrupt the entire FSSB loop.
- Internal amplifier failure
- Overheating or power fault
- Communication board damage
FSSB Interface Board Failure
The controller’s FSSB interface board may fail.
- No signal output to amplifiers
- Intermittent communication loss
- Hardware degradation
Power Supply Instability
FSSB communication depends on stable system power.
- Voltage drops or spikes
- Faulty power supply unit
- Poor grounding
Incorrect Cable Routing or Installation
Improper installation can degrade signal quality.
- Excessive cable bending radius
- Cable routed near high-voltage lines
- Poor shielding practices
How to Troubleshoot FANUC FSSB Alarm
Step 1: Power Cycle the System
- Turn off controller and amplifiers completely
- Wait 1–2 minutes
- Restart system and check alarm status
Step 2: Inspect FSSB Fiber Optic Cables
- Check for bends, cracks, or damage
- Clean fiber connectors carefully
- Reseat all connections firmly
Step 3: Check Amplifier Status
- Observe LED indicators on each servo amplifier
- Identify which axis/amplifier is not communicating
- Replace faulty amplifier if needed
Step 4: Verify FSSB Loop Integrity
- Follow the fiber loop from controller → amplifiers → back
- Ensure the loop is complete
- Check for any break in the chain
Step 5: Inspect Controller FSSB Board
- Reseat the board inside the controller
- Check for signs of overheating or damage
- Replace if communication remains unstable
Step 6: Test and Monitor
- Restart the system
- Jog robot at low speed
- Monitor for recurring communication faults
Pro Diagnostic Tip
Swap Fiber Optic Cables Between Amplifiers
- Move cables from one amplifier to another and observe alarm behavior.
- If the alarm follows the cable → Likely a fiber optic cable issue (bends, damage, or dirty connectors).
- If the alarm stays on the same axis → Likely a servo amplifier or FSSB interface board fault.
Check Loop Continuity and Node Behavior
- FSSB uses a closed-loop topology; a single faulty node can block communication to downstream axes.
- Inspect LED indicators on each amplifier:
- No light / red alarm → Hardware failure in amplifier or interface board
- Blinking communication LED → Signal instability, often cable-related
Interpretation for Field Isolation
- Cable-related issues: intermittent alarms, blinking LEDs, or alarm moves with cable swaps
- Hardware issues: consistent alarm on one axis, no LED activity, or amplifier fault patterns
- Test system at low speed after isolating suspect components to confirm resolution
This method allows rapid differentiation between signal transmission failures and amplifier/interface hardware faults, minimizing downtime.
Preventing FSSB Alarms
- Avoid tight bends in fiber optic cables
- Keep connectors clean and dust-free
- Ensure proper grounding and cable routing
- Regularly inspect amplifiers and connections
- Maintain stable power supply
Related FANUC Alarm Codes
- SRVO-065 – Servo Communication Error
- SRVO-131 – Encoder Communication Error
- SRVO-133 – Encoder Data Error
- SV041 – Servo Alarm
These alarms often appear together when FSSB communication fails.
Conclusion
- FANUC FSSB Alarm indicates a critical communication failure between the controller and servo system.
- Most issues are caused by fiber optic cables or amplifier faults, and can be resolved by systematic inspection and isolation.
- Quick identification of the faulty component minimizes downtime and prevents further system errors.
FAQ
Q1: What is FSSB in FANUC robots?
FSSB is the fiber optic communication bus connecting the controller and servo amplifiers.
Q2: What causes FANUC FSSB alarm?
Common causes include damaged fiber cables, faulty amplifiers, interface board failure, or unstable power supply.
Q3: Is FSSB alarm serious?
Yes, it stops all servo communication and halts robot motion.
Q4: How do I fix FSSB alarm quickly?
Check fiber optic cables first, then inspect amplifiers and controller interface boards.
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