SRVO-056

Identify the affected axis

On the teach pendant, press MENU > ALARM > F1 [TYPE]. Review the alarm history to identify which axis number is associated with SRVO-056. The axis number will be indicated in the alarm detail (e.g., SRVO-056 Axis 1). Record this information as it determines which FSSB cable to inspect.

Check if the alarm occurs on the same axis repeatedly or rotates between axes. A rotating pattern suggests controller-side FSSB issues, while a fixed axis points to that specific cable or amplifier.

Power down and inspect FSSB cables

Perform a controlled shutdown (MENU > SYSTEM > Master Off, then main disconnect). Trace the FSSB fiber cables from the axis control board in the controller cabinet to the servo amplifier for the affected axis. Check for tight bend radius (must be >30mm), cable pinching in cable tracks, and any visible damage to the orange fiber jacket. Disconnect both ends of the cable by pressing the release tab and pulling straight out.

Never look directly into a fiber optic cable end or connector port while power is applied. Invisible laser light can cause permanent eye damage.

Clean fiber optic connectors properly

Use a Fanuc-approved fiber optic cleaning kit (or lint-free optical wipes with 99% isopropyl alcohol). Clean both the cable ferrule ends and the connector ports on the axis control board and amplifier. Wipe in one direction only using a twisting motion. Inspect with a fiber optic microscope if available - the core center must be completely clear with no residue. Allow 30 seconds for alcohol to evaporate completely before reconnection.

The FSSB uses 650nm red laser light. If you have a fiber optic test light or visual fault locator, you can verify continuity and check for cracks that cause light leakage along the cable length.

Reconnect and verify FSSB signal strength

Reconnect the cleaned cables ensuring they click firmly into place. Power up the system. Navigate to MENU > SYSTEM > Variables > $SCR.$FSSB_DIAG[axis]. Check the signal strength values (varies by controller generation, but typically should be >-15dB for good signal). Values between -15 and -20dB indicate marginal signal quality that can cause SRVO-056.

On R-30iB controllers, you can also access MENU > MAINTENANCE > FSSB Monitor to view real-time communication statistics including error counters and retransmission rates for each axis.

Replace suspect cable if cleaning fails

If alarm persists after cleaning or signal strength is poor, replace the FSSB cable with a genuine Fanuc part (typically A660-20xx-Tyyy series depending on length). After replacement, perform a full power cycle and verify the system boots without alarms. Test the affected axis through its full range of motion under low speed (25% override) and monitor for alarm recurrence.

Keep a known-good FSSB test cable (2-meter length) in your tool kit for quick swap diagnostics. This can immediately confirm whether the issue is cable-related or in the electronics.

Check for environmental EMI sources

Survey the robot cell for high-EMI equipment operating near the controller or amplifier cabinet. Common culprits include resistance welders, induction heaters, large motor starters, VFDs, and plasma cutters. Note whether SRVO-056 correlates with operation of specific equipment. Check that FSSB fiber cables maintain separation (minimum 100mm) from power cables, especially servo motor power cables.

Use an oscilloscope to monitor the 24VDC control power supply at the servo amplifier during operation. Voltage spikes exceeding ±10% during alarm occurrence indicate power supply noise coupling into the FSSB circuits.

Verify system grounding integrity

Measure ground resistance between the controller cabinet ground bar and the servo amplifier cabinet ground point using a multimeter. Resistance should be <0.1 ohms. Check that the main ground cable is at least 8mm² and has secure bolted connections (not sheet metal screws). Verify the amplifier mounting surface is clean, bare metal with proper star washers for ground continuity.

Use lockout/tagout procedures before measuring ground connections. Even with power off, capacitors in servo amplifiers can retain lethal voltage for several minutes.

Install or check ferrite suppressors

Inspect both ends of the FSSB cables where they connect to equipment for snap-on ferrite cores. Add ferrite cores (Fanuc P/N A660-8019-T505 or equivalent) within 50mm of each connector if not present. Also verify that servo motor power cables have ferrite cores installed near the amplifier end. Check that existing ferrites are not cracked or degraded.

For severe EMI environments, consider routing FSSB cables through conductive flexible conduit that is grounded at both ends. This provides additional shielding for the connector ends where EMI pickup is most likely.

Review and adjust noise filter parameters

Access MENU > SYSTEM > Variables and check the settings for EMI suppression parameters. Set $EMG_AFETR[axis] = 50-100ms to add filtering to emergency stop signals. Verify that parameter 2011 (Emergency Stop Filter Time) is set to 50ms or higher if nuisance trips occur. For servo-specific filtering, check parameter 2157 (Position Error Detection Filter) is enabled.

These filters add slight delays to fault detection but prevent EMI-induced nuisance alarms. Document original values before changes. If SRVO-056 disappears with filtering, you've confirmed EMI as root cause and should pursue better shielding/grounding.