What causes FANUC alarm 5311?

Incorrect parameter settings for the FSSB configuration. Faulty or damaged FSSB cables or connectors. Incompatible amplifiers or servo motors connected to the FSSB. Incorrect wiring or connection order of the FSSB components.

How do I fix FANUC alarm 5311?

1. Check the FSSB parameter settings in the Fanuc control to ensure they match the actual hardware configuration. 2. Inspect the FSSB cables and connectors for any damage or loose connections, and replace if necessary. 3. Verify that all the amplifiers and servo motors connected to the FSSB are compatible with the Fanuc control and the parameter settings. 4. Ensure that the FSSB components are connected in the correct order as specified by the Fanuc documentation. 5. If the issue persists, perform a full power cycle of the Fanuc control and the connected FSSB components to reset the system. 6. As a last resort, consult the Fanuc service manual or contact Fanuc technical support for further assistance.

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FANUC Controller

Servo/Axis · Advanced · 30-60 minutes

Alarm 5311

Fanuc Alarm 5311 - FSSB: ILLEGAL CONNECTION

Alarm 5311 FSSB: ILLEGAL CONNECTION occurs when the physical FSSB (Fiber Optic Servo Serial Bus) amplifier configuration does not match the axis/spindle configuration defined in the CNC parameters. This mismatch is detected during power-up initialization when the system scans the FSSB network and compares detected amplifiers against parameter settings for axis assignment, amplifier types, and connection order.

FANUCServo/AxisAdvanced30-60 minutes

5311

Fanuc Alarm 5311 - FSSB: ILLEGAL CONNECTION

FANUC ALARM 5311

Root Cause Summary

Alarm 5311 FSSB: ILLEGAL CONNECTION occurs when the physical FSSB (Fiber Optic Servo Serial Bus) amplifier configuration does not match the axis/spindle configuration defined in the CNC parameters. This mismatch is detected during power-up initialization when the system scans the FSSB network and compares detected amplifiers against parameter settings for axis assignment, amplifier types, and connection order.

Causes and Fixes

1
Navigate to SYSTEM > PARAM and verify parameter 8200-8239 (FSSB axis/spindle assignment). Note which axis is assigned to each FSSB position (e.g., P8201=1 for X-axis on position 1, P8202=2 for Y-axis on position 2).
2
Physically trace the FSSB fiber optic cable connections from the CNC control unit through each amplifier. Verify the daisy-chain order matches parameter assignment. FSSB OUT on control connects to AMP1 IN, AMP1 OUT to AMP2 IN, etc.
Power down the machine before disconnecting or reconnecting FSSB fiber optic cables to prevent damage to optical transceivers.
3
Check FSSB diagnostic screen at DIAGNOSIS > PARAM > SERVO > FSSB. Verify the detected amplifier count and types match expected configuration. Compare detected amp IDs with parameter 8131-8138 (amplifier type settings).
4
If amplifier order is incorrect, either reconfigure parameters to match physical layout or rewire FSSB connections to match parameters. After correction, cycle power and verify alarm clears during initialization.

DIY Feasiblehigh confidence

Call a technician if:

·Parameter changes required beyond basic configuration
·Hardware replacement needed
·Multiple related alarms present

Prevention

Always backup parameters before hardware changes
Document amplifier positions and models
Use proper FSSB cable management
Verify parameter settings after any servo system modifications

Common Mistakes

Assuming amplifier order doesn't matter
Not updating parameters after amplifier replacement
Using non-Fanuc FSSB cables
Restoring parameters from different machine configurations

Related Alarms

5310 5312 5320 401 410

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