SRVO-021

Check alarm history for FSSB communication errors

Press MENU > ALARM > F1 [TYPE] > select 'Alarm Log'. Look for FSSB-xxx alarms (FSSB-001, FSSB-002, etc.) or SRVO-020 (FSSB communication error) that occurred immediately before the SRVO-021. These indicate which axis and FSSB line has the issue.

SRVO-020 and SRVO-021 occurring together almost always confirms FSSB as the root cause. Note the axis number to identify the specific amplifier chain.

Inspect FSSB fiber optic cables and connections

Power off the controller. Open the cabinet and locate the FSSB cables (orange fiber optic cables). Check all connectors from the CPU board through each axis amplifier in the chain. Look for: bent/damaged fiber tips, loose connections, dust/contamination in connectors, and cable damage from pinching or excessive bending radius (<50mm).

Never look directly into fiber optic cables or ports. Laser light can damage your eyes. Always power off before inspecting connections.

Verify FSSB signal strength

Go to MENU > SYSTEM > Variables. Toggle $DIAG_FO.$FSSB_DIAG to 1 to enable diagnostics. Check variables $DIAG_FO.$FSSB_PWR[line][axis] for receive power levels. Values should be between -12.0 and -3.0 dBm. Values below -15.0 dBm indicate fiber degradation or dirty connections.

On R-30iB Plus controllers, you can view this graphically at MENU > SYSTEM > FSSB > Signal Strength. Replace any fiber cable showing weak signal even if currently functional.

Verify FSSB terminator installation

Ensure the FSSB terminator (small orange plug) is properly installed on the last amplifier's output port in the chain. Check parameter 11020 (FSSB line configuration) to confirm which amplifier should be last. Missing or loose terminators cause signal reflections and intermittent SRDY loss.

Keep spare FSSB terminators in your toolkit. They're inexpensive and frequently lost during maintenance. Part number A660-8020-T589.

Test with known good FSSB cable

If signal strength is borderline or the robot has high-flex applications, temporarily replace the FSSB cable to the affected axis with a known good cable. If SRDY returns, the original cable has internal fiber damage not visible externally. This is common on axes 4-6 in high-duty-cycle applications.

FSSB cables in track motion or through the J3 arm can fail internally while looking perfect externally. Always suspect the cable on J4-J6 of high-speed robots.

Check for additional servo alarms on the affected axis

Press MENU > ALARM > F1 [TYPE] > Alarm Log. Look for SRVO-xxx alarms on the same axis number immediately before SRVO-021. Common associated alarms: SRVO-037 (over-current), SRVO-044 (overheat), SRVO-050 (overload), SRVO-062 (IPM error). These indicate why the amplifier dropped SRDY.

If you see SRVO-065 (excessive position error) before SRVO-021, the problem may be mechanical binding or brake failure, not the amplifier itself.

Monitor DC bus voltage and amplifier status

Go to MENU > SYSTEM > Variables. Check $DIAG_SV.$SRV_AMP_VOLT[axis] for DC bus voltage (should be 280-310V for standard systems, 530-570V for high-voltage). Check $DIAG_SV.$SRV_AMP_TEMP[axis] for amplifier temperature (should be <75°C). Values outside range indicate power supply or thermal issues.

Low DC bus voltage (<260V) can cause intermittent SRDY loss during motion. Check the transformer taps and input voltage to the power supply (parameter 19001 for voltage setting).

Inspect amplifier LED diagnostic codes

With the cabinet open and servo power off, observe the LED indicators on the servo amplifier PCB for the affected axis. On α-series amplifiers: steady green = ready, flashing red = fault detected, alternating red/green = communication error. Note the flash pattern and consult the amplifier hardware manual for specific fault codes.

High voltage (300V+ DC) is present inside servo amplifiers even when servo power is off. Only qualified personnel should work inside the cabinet with proper safety procedures.

Check cabinet cooling and amplifier thermal condition

Verify all cabinet cooling fans are operating (check $DIAG_SV.$SRV_FAN_ALARM). Clean any dust buildup on amplifier heatsinks and heat exchangers. Ensure ambient temperature is <40°C. Thermal issues cause amplifiers to enter protective mode and drop SRDY, especially on high-payload axes 2-3.

If SRVO-021 occurs only after extended operation, use a temperature gun to check amplifier heatsink temps. Over 80°C indicates inadequate cooling or amplifier degradation.

Test amplifier by axis swapping (advanced)

If possible, swap the FSSB and motor connections between the suspect axis amplifier and a known-good axis (swap both motor power cables and FSSB cables, update parameters accordingly). If the alarm follows the amplifier to the new axis position, the amplifier is faulty. If it stays with the original axis, the issue is external (cable, motor, or configuration).

This requires careful documentation of original connections and parameter changes (FSSB line 11020-11023, amplifier type 11306). Take photos before making changes. Only for experienced technicians.