[RoboMaster]

You can use an Arduino for other, non prohibited things if you want though. FIRST gives us a lot of free reign for "custom circuits and sensors." So if there's something you know how to do on Arduino but can't find out how to do it on the FIRST control system, you could have the Arduino do the work and the cRIO would just receive or send commands. I guess you could hook Digital I/O pins together and have the cRIO and Arduino send and listen for dedicated On/Off signals along one or more pairs of pins.

Cons are that you can't use the Arduino to control motors, as mentioned, and that having two systems could easily be more complicated than just using the main FIRST control system. It would depend a lot on what you wanted to do with the Arduino.

I've always wanted to use a custom circuit though; it would be pretty cool.