FANUC SRVO-037 IMSTP Input Alarm: Causes & Fix
What the SRVO-037 IMSTP alarm means
The controller displays the alarm as SRVO-037 IMSTP input (Group: %d), with the group number identifying which motion group triggered it. The alarm text tells the technician that the *IMSTP signal for a peripheral-device interface is OFF. On FANUC R-30iA, R-30iB, and R-30iB Plus controllers, *IMSTP is the UOP immediate-stop input, normally mapped to UI[1]. UI[1] is active-low: it must stay ON for the robot to run, and going OFF asserts an immediate stop processed through software, similar in effect to an emergency stop.
When SRVO-037 fires, the robot stops motion immediately, the running program halts, an alarm is logged, and servo power is removed from the drives. The robot cannot resume automatic operation until UI[1] is restored and the alarm is cleared.
Common causes of SRVO-037
Because *IMSTP is a UOP safety input, the most common causes are external to the robot controller itself. Check these first, in order of likelihood:
- An open emergency-stop circuit or pushbutton contact somewhere in the cell
- An open or tripped safety relay contact feeding the UOP interface
- A dropped or de-energized PLC output that drives *IMSTP
- A broken conductor, loose terminal, or damaged connector in the UOP wiring
- Loss of the 24-V control power supplying the UOP I/O interface
- A PLC handshake fault, where the PLC fails to hold the permissive state during startup, fault recovery, or a communications loss
Companion UOP alarms often appear alongside or instead of SRVO-037 and help isolate the fault:
- UI[2] *HOLD off stops program execution but is a different signal than *IMSTP; do not confuse the two
- UI[3] *SFSPD off points to a safety-speed circuit problem, separate from immediate stop
- UI[8] ENBL off means the enable input is missing, which can block motion even after UI[1] is restored
Any of these signals can accompany a broader safety-chain problem rather than an isolated *IMSTP failure.
Step-by-step troubleshooting
Work through this sequence before replacing any parts:
- Place the cell in a safe state and follow lockout/tagout procedures before touching wiring inside the controller cabinet or safety circuit.
- Open the UOP input status display on the controller and locate UI[1] (*IMSTP).
- Have someone operate the PLC, safety relay, and field emergency-stop devices while you watch UI[1] change state.
- Compare the signal at the PLC output, the field terminal, and the FANUC input to find where the circuit opens.
- Flex cables and touch connectors, terminal blocks, and cabinet doors while watching UI[1]; a state change under movement points to intermittent wiring, not a robot configuration problem.
- Check companion signals, UI[2], UI[3], and UI[8], to see whether the fault is isolated to *IMSTP or part of a wider safety-chain issue.
- Verify the UOP wiring against the electrical drawings for the specific installation, since signal allocation and interface hardware vary by controller and option package.
Clearing the alarm and verifying the fix
Once the external *IMSTP circuit is confirmed closed and UI[1] shows ON at the controller, press RESET. SRVO-037 typically clears at that point; if it does not, recheck the input status display before assuming a controller fault. After reset, verify that servo power establishes normally and that no new safety alarm appears when you attempt to move the robot.
A practical recovery sequence: put the cell in a safe state, identify the device or PLC output that controls *IMSTP, restore the emergency-stop and safety-relay conditions, confirm 24-V control power and wiring continuity, verify UI[1] is ON, press RESET, then recheck servo-ready operation.
If the installation uses the Enable UI Signals setting ($OPWORK.$UOP_DISABLE on R-30iB documentation), confirm it matches the actual cell: enabled whenever UOP safety inputs are wired in, disabled only if the robot genuinely has no peripheral UOP interface. Forcing UI[1] ON or disabling UOP inputs to hide a bad safety device is not an acceptable fix for an integrated cell.
When SRVO-037 is a hardware failure
SRVO-037 is a UOP input condition, not an axis amplifier overcurrent or motor feedback fault, so pulsecoder, servomotor, and amplifier replacement is rarely the answer. The hardware that actually fails in *IMSTP circuits is on the interface side: PLC output cards, safety relays, interface relays, fuses, terminal blocks, connector pins, and the controller’s UOP I/O board or module itself.
Suspect the interface hardware when UI[1] stays OFF after every external emergency-stop device, safety relay, and PLC output has been verified good, or when the PLC reports its output ON but the FANUC UOP monitor still shows UI[1] OFF. In that case the fault lies between the field terminal and the controller input: a failed relay, a blown fuse, a damaged connector pin, or a bad input circuit on the I/O board. Board-level testing includes checking power rails, input circuits, and connectors for damaged traces or solder joints before condemning the board.
How Robotics Integration helps
Robotics Integration works SRVO-037 IMSTP faults on FANUC R-30iA, R-30iB, and R-30iB Plus controllers by phone and remote session first: reviewing the alarm history, UOP input states, and PLC handshake logic with your technician to isolate whether the fault is in field wiring, the PLC program, or controller-side interface hardware.
When the fault points to a failed UOP I/O board, interface relay, or connector, our repair process includes connector and terminal inspection, fuse and relay replacement where applicable, board-level power-rail and input-circuit testing, and repair of damaged traces or solder joints. Repaired boards are tested under real input voltage and load, cycling UI[1] and related UOP signals repeatedly to catch intermittent faults before the unit goes back into service.
When the problem needs hands on the machine, we send a technician on-site to trace wiring, test the safety chain, and verify the fix with the robot running under normal PLC control.
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 diagnosis, have this information ready before you call: the controller model (R-30iA, R-30iB, or R-30iB Plus), the full alarm text exactly as displayed including the group number, the alarm history showing when SRVO-037 first appeared and how often it repeats, which axis or motion group was active, and what the robot was doing at the moment the alarm triggered (running a program, jogging, or idle).
If you can, also note the current state of UI[1] on the UOP input display and whether the alarm follows a specific PLC state, e-stop button, or door interlock. Call Robotics Integration at 1-602-449-1556 to walk through the alarm with a technician, arrange controller I/O board repair, or schedule an on-site visit.
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