Thanks for the reply.

The 2 boards do share a common ground. The LED module has 4 pins: +12, GND, Signal 1 and Signal 2. I share the +12 and GND between both boards (+5 is generated through a voltage regulator on each board).

The specs for the LED board say that you must tie Signal 1 or Signal 2 to ground to control it. So if I connect Signal 1 to GND, it will start flashing one way, and Signal 2 to GND will flash another way. If I do both it flashes a 3rd way, and no pins to GND it will just stay off.

If the Signal pins needed to be set High to control them, I would just connect them to the PIC outputs on my main board. How can I make it so that I can control this external LED board from the PIC on my main board? ie. How do I tie the signal pins to ground?