[EricH]

I should probably warn you that in about two weeks, that sort of question will result in a "You should really, really, read the Game Manual". But, seeing as I don't know what the 2013 Manual will have to say, I'll refer to the 2012 Game Manual, Section 4.1.9, which has all the 2012 rules on the control system.

[R52] specifies one programmable NI cRIO to control the robot (along with firmware revision), and that other controllers shall not be used.
The blue box for [R65] explains that per [R52], the robot must be controlled by the cRIO; other devices (custom circuits) must go through the cRIO to originate command signals.