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photogate overdetections in some ports

We are using 14 Bpods (r1), controlled with python (PyBpod).

In each Bpod, we use 3 ports, the left and the right supply water and the central one simply detects the position of the animal with the photogate (it does not use the valve to deliver water).

We have a problem that happens randomly on 3 of the Bpods. Occasionally there are bursts of detections on the central port photogate (thousands of in and out detections per second) for a few seconds and then everything goes back to normal.

We changed the position of the problematic ports (we are only using 3 of the 8 possible behavioral ports on each bpod, so we had 5 free) and then restarted the Bpods.
The issue was resolved for a few days, but now it seems to be happening again on one of the Bpods.

What could be the reason for these overdetections? We thought it would be a problem in the port's photodetector, but we were surprised that it stopped happening by changing their position and restarting the Bpods.
Hi Rafa,

I'm also surprised that changing the position of the ports helped.

In the software, there is a panel to enable or disable behavior ports. I'm assuming that you only have the ports that are plugged into port interface boards enabled.

Sources of infrared light (e.g. IR illuminators for imaging, sunlight, IR webcams) can cause events if the light reaches the port's phototransistor. 

Faulty Ethernet cables can also be the culprit.

I'd consider the experiment of swapping state machines between a working rig and the problematic one, to see whether the problem migrates with the state machine, or remains with the ports.

Please let me know what you find!

Thanks Josh for your suggestion.
We changed the bpod that controlled the problematic box for another and the problem persisted, so we can say that the problem is not with the bpod but with the photogates or cables.
We use infrared cameras in all the boxes, perhaps the problem is due to the reflections of infrared light.
It happens much more often when we train a particular animal.

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