FANUC SRVO-021 SRDY Off Alarm Recovery

FANUC · intermediate · 30 min

Diagnose and clear the FANUC SRVO-021 "SRDY off" servo alarm — from checking the 3-phase amplifier supply to correcting the FSSB amplifier configuration after a controller reload.

⚠️ Safety first: This procedure exposes you to 200 VAC three-phase power at the servo amplifier. Follow lockout/tagout, use a rated multimeter, and never open the controller cabinet while energized unless you are qualified and the task requires it.

What SRVO-021 means

SRVO-021 "SRDY off (Group:i Axis:j)" is raised when the controller's HRDY signal is ON but the amplifier's SRDY signal stays OFF, with no other servo alarm present. SRDY is the feedback the servo system sends back to confirm the amplifier's MCC (magnetic contactor) has actually closed. If the host commands the MCC on but the amplifier never reports it closed, the controller flags SRVO-021.

In plain terms: the controller asked the drive to power up the motors, and the drive did not confirm it did.

Most common causes

1. Low or missing amplifier supply — one or more phases of the 200 VAC three-phase feed to the servo amplifier is low or lost.
2. Loose or damaged power / brake connectors between the amplifier, the mechanical unit, and the robot connection cable.
3. Wrong FSSB / amplifier configuration — after a controller image reload or a Controlled Start, the FSSB line number, amplifier axis number, or amplifier type no longer matches the hardware. This is the classic "it faulted right after a reload" case.
4. Failed hardware — servo amplifier, servo motor, or a power/brake cable in the mechanical unit.

Step-by-step recovery

1. Read the full alarm. On the teach pendant press MENU → ALARM → Active and note the exact group and axis in the SRVO-021 text — that tells you which amplifier and axis to inspect.
2. Check the amplifier supply. With the cabinet safely de-energized for access, then re-measured live only if qualified, verify each phase-to-phase voltage of the 200 VAC feed at the amplifier input. If any reading is 170 VAC or lower, trace the problem back to the input transformer, breaker, or MCC contactor.
3. Inspect the connectors. Check every power and brake connector on the amplifier, the robot connection cable, and inside the mechanical unit. Look for bent terminals, back-out pins, or heat damage; reseat or replace as needed.
4. If the fault started after a reload, open MENU → SYSTEM → Config / FSSB setup and confirm the FSSB line number, amplifier axis number, and Amplifier Type match your hardware. For a robot whose brake signal is routed through a 6-axis A06B-6400-series amplifier, the Amplifier Type must be set to 1. Correct it, then cold-start the controller.
5. Cycle power and reset. Power the controller off, wait for full discharge, power on, then press RESET on the teach pendant. If SRVO-021 clears and does not return, the fault was configuration or a loose connection.
6. If it persists, the amplifier, motor, or a mechanical-unit cable is the likely failure. Swap the suspected amplifier or cable with a known-good unit, or contact your FANUC service office with the group/axis and the measured supply voltages.

When to escalate

If the supply is healthy, connectors are sound, and the FSSB configuration is correct but SRVO-021 still returns immediately on power-up, treat it as an amplifier or motor hardware failure and involve FANUC service rather than repeatedly resetting.