FANUC SRVO-230 / SRVO-231 Chain Abnormal
What SRVO-230 and SRVO-231 mean
On the teach pendant and alarm log the controller displays SRVO-230 CHAIN1 ABNORMAL or SRVO-231 CHAIN2 ABNORMAL, sometimes with an additional numeric suffix identifying the specific input involved. These are safety-chain alarms, not standard servo amplifier or motor faults. FANUC R-30iA, R-30iB, and R-30iB Plus controllers run every safety-critical input – emergency stop, fence or gate, and deadman/enable – through two independent, redundant circuits, chain 1 and chain 2, that must always report the same state.
SRVO-230 is logged when the controller detects a single-channel mismatch on chain 1 (the +24V side), and SRVO-231 when it detects one on chain 2 (the 0V side). Either condition means the controller cannot confirm the input is being monitored correctly, so it holds the alarm and blocks servo power and motion until the mismatch is corrected and cleared through the documented reset procedure.
Common causes of a chain mismatch
The most common cause is an external safety device or its wiring, not the controller itself. Likely causes, roughly in order of frequency, include a defective emergency-stop pushbutton or contact block, a fence/gate interlock switch that is misaligned or wired with mismatched contacts, a deadman/enable switch on the pendant that is worn, sticky, or partially operated, oxidized or welded safety relay contacts, and loose terminals, broken conductors, or damaged connectors in the safety-chain wiring. A failed controller-side E-stop/safety interface board or input circuit is possible but less common than an external device fault.
The controller compares specific redundant signal pairs depending on model and safety-option configuration. Typical pairs on R-30iA/R-30iB systems include:
- EES1/EES11 vs EES2/EES21 – emergency-stop circuits, panel and pendant
- EAS1/EAS11 vs EAS2/EAS21 – fence or gate interlock circuits
- SD4/SD41 vs SD5/SD51 – servo-disconnect inputs
A brief mismatch can also occur if one redundant contact changes state faster than its pair right after an E-stop press or fence opening, without any actual hardware defect.
Troubleshooting steps
Lock out and de-energize the robot and cell power whenever you open enclosures or touch safety wiring; do not bypass or jumper any safety-chain contact to work around the alarm.
- Record the exact alarm text, suffix number, and any companion alarm shown at the same time
- Confirm all E-stops are released, all gates and fences are closed, and the deadman switch is in the required state before doing anything else
- Operate each E-stop, fence switch, and deadman device one at a time, watching both channel indications at the safety interface to see which one lags or fails to change
- With power locked out, inspect the suspect device wiring for loose terminals, broken conductors, damaged connectors, and correct normally open/normally closed wiring
- Check the associated safety relay or contact block for synchronized operation, contact resistance, and mechanical travel
- Compare chain 1 and chain 2 continuity or voltage at the controller safety interface for the specific input named by the alarm suffix
- If external devices and wiring check out, inspect the controller E-stop/safety interface board and its connectors
Clearing and resetting the alarm
Correct the cause of the alarm first; R-30iB alarm-code documentation directs the technician to resolve the fault before attempting a reset. Before touching the pendant, remove personnel from the safeguarded space and verify all E-stops are released, gates and fences are closed, and the deadman device is in the required state.
A documented R-30iB reset path is MENU, ALARM, then the displayed single-channel reset function – F4 RES_CH1 for a chain 1 fault, or RES_CH2/RES_2CH for chain 2 depending on software revision – followed by confirmation (commonly F4 YES) and then RESET on the pendant or operator panel. If both SRVO-230 and SRVO-231 are present, reset the indicated channel and press RESET again to clear any remaining indication once the safety devices are confirmed restored. A second documented path is MENU, NEXT PAGE, SYSTEM, SYSTEM SETTING, the chain-error-reset execution setting, and the pendant reset key shown there. Exact labels and menu numbers vary by controller generation and software; confirm against the applicable manual. A reset that clears the alarm without recurrence for a period is not proof the circuit is sound – if it comes back, treat it as an unresolved intermittent fault.
When it points to a hardware failure
SRVO-230 and SRVO-231 are safety-chain alarms and, by themselves, do not identify a failed axis motor, servo amplifier, pulsecoder, or mechanical reducer. Do not replace drive hardware based on these codes alone unless an independent servo, encoder, or mechanical alarm is also present.
Hardware failure in this context usually means a safety relay with contacts that have oxidized, welded, or lost mechanical synchronization; a contact block or interlock switch with incorrect or degraded contact logic; a harness with a broken conductor, damaged connector, or loose terminal on one channel only; or a controller-side E-stop/safety interface board or input circuit that no longer reads one channel correctly. You can tell the difference from a wiring or device fault by comparing chain 1 and chain 2 readings directly at the controller interface while cycling each device individually – if both channels track together at the board but not at the device, the fault is external; if they disagree even at the board, suspect the interface board itself.
How Robotics Integration helps
Robotics Integration supports maintenance teams working an SRVO-230 or SRVO-231 fault on FANUC R-30iA, R-30iB, and R-30iB Plus controllers, from initial diagnosis through parts-level repair. Over the phone or remotely, a technician can walk through the alarm history, help you identify which signal pair and channel is implicated, and outline the specific checks to run on the suspect E-stop, fence, deadman, or relay circuit before anything is opened up.
Where the fault traces to controller-side hardware, we perform component-level diagnosis and repair on E-stop/safety interface boards, connectors, and related wiring, preserving the original redundant architecture, isolation, and contact design rather than substituting generic parts. On-site service is available for cases that need hands-on wiring inspection, relay and contact-block testing, or full post-repair verification of every E-stop, fence, deadman, and reset function across both channels.
Looking up a different code? See the full FANUC and robot alarm code list, or read about our FANUC robot repair.
Get help with this alarm now
To get a faster read on an SRVO-230 or SRVO-231 fault, have this information ready before you call: the controller model (R-30iA, R-30iB, or R-30iB Plus) and software version, the complete alarm text including any suffix numbers, the full recent alarm history if both chain alarms have appeared, which axis or cell the robot is in, and what the robot was doing – running, in teach mode, or stopped – when the alarm first appeared. Note which E-stop, fence, or deadman device was last operated before the fault.
Call 1-602-449-1556 to talk through the alarm with a technician, get direction on the next diagnostic step, or arrange component repair or on-site support for the safety-chain circuit on your FANUC controller.
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