Question on PLC Inputs

Good day, I have a question that no one has been able to adequately explain yet so I'm hoping to get some advice here.
At my workplace, we use an inductive proximity sensor to make sure a gear is rotating. This prox is fed into an Allen-Bradley Flex I/O module which then interfaces with a 503 processor. I believe the prox is in a PNP-Sourcing configuration.
A few weeks ago we were getting intermittent gear not rotating errors, sometimes back to back and sometimes it would run 1/2 hour without a problem. I was watching the input LED on the Flex I/O module but could not detect any flickering so I went online with a laptop and found the input in the program. The input ran to a counter which divided the counts by 2. This counter then reset a watchdog timer set at 300 ms. Most of the time the timer would stay around 8 - 20 ms but at times it would reach 190 - 230 ms. On a whim, I replaced the prox which fixed the problem.
My question is, is it possible for the input LED on the card to stay on even if the prox is intermittent. I can understand not seeing the LED flickering during normal operation since this 4" gear is rotating at around 900 rpm but I would expect to see the LED turn off for 3 seconds or more to allow the watchdog timer to time out. The LED did turn off when I stopped the gear so it was not stuck on.
Thank You, Nate
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Hi Nate,
My only possible explanation would be that the pulses were too fast for the scan to see them. You might even have aliasing between the gear tooth rate and the scan rate. If the LED was hard wired it wouldn't be subject to aliasing because it isn't sampled.
You may want to check the prox speed rating. It doesn't take that many teeth on a gear at 900 RPM to get a relatively high frequency. You may also want to look at a counter module instead of a general purpose I/O module. The counter modules can take a much higher input frequency.
Matthew
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