The standard VEX motors operate on roughly 5V DC, so in order for the Cortex or PIC to control a larger motor, a second power supply must be used. However, the VEX Cortex and PIC both regulate the voltage they provide the motors, so simply hooking up a larger power supply won’t work – and doing so would risk damaging the actual microcontroller.
To solve this problem, we can introduce an intermediary component – a VICTOR 884 Speed Controller. The VICTOR sits between the 12V power supply, VEX microcontroller, and 12V DC motor, isolating the microcontroller from the high voltage. Check out instructions for hooking up the VICTOR, along with a video here: http://www.robotc.net/blog/?p=745