Allen-Bradley Fault 16 — I/O Not Responding
What This Fault Means
I/O Not Responding means one or more I/O modules on the backplane or remote rack have lost communication with the controller. Outputs on the affected module go to their configured fault state.
Common Causes
- Loose or damaged backplane connection
- I/O module hardware failure
- EtherNet/IP network interruption to remote rack
- RPI (Requested Packet Interval) set too aggressively for the network
- Power supply droop causing module brownout
Recommended Fix
- Open I/O tree in Studio 5000 — identify which module shows a red X.
- Reseat the module firmly into the backplane.
- Check the power supply voltage at the backplane (24VDC within ±10%).
- For remote I/O, verify the EtherNet/IP cable and switch port LEDs.
- If a single module repeatedly faults, swap it with a known-good spare.
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