Problem reading Quad Encoder, possible Cortex issue

I have a Cortex that is not reading quadrature shaft encoders. The other day a student was complaining that the quad encoder would work, then suddenly stop working. Power cycling the Cortex would fix it temporarily, then it would fail again. Eventually (same day) it stopped working altogether. We tried different ports without success. I attached a second quad encoder to the same shaft and it worked. The next day it worked for a while, then started failing just as the first one did. Eventually (same day), that one too stopped working. I’ve tried different codes (known working code) and different ports, and neither quad works. I’ve moved the quads to another robot and verified that they work fine. It definitely seems as though the first Cortex just doesn’t want to read a quad. I’ve updated the firmware with no change. To be clear, every other sensor installed seems to be working fine.

The other symptoms include the area of the Cortex closest to the LED’s is very hot. Much hotter than other Cortexes and much hotter than I am comfortable seeing it run. Also, rather than flashing two green lights, it flashes yellow.

What is your suggestion?


Sorry to hear about your issue with the Quadrature Encoders. Thanks for the accurate description. There is probably a short in one of your Analog or Digital port cables. Remove all of the Analog and Digital connections. Turn on the VEX ARM® Cortex®-based Microcontroller. It will probably stay cool now. If it still gets real warm underneath the status LEDs, contact tech support ( If it does not get warm, connect a few of your cables at a time, then see if it gets warm. You will probably find one of the cables has been cut and has the red and black wires touching.

Thank you. I was able to determine that a potentiometer went bad. The Cortex is no longer heating up and it is, at least for now, reading all sensors including the quad. I’ll return here and/or notify tech support if that returns to a fail condition. Thank you for your help.