DrD, I have 2 separate boards I am trying to interface with. The one is no problem accessing the Arduino rx/tx pins. The other is a custom board on a "3018 CNC". It uses the 328p chip it just doesn't have an easy way to connect to the rx/tx pins. ** I run into the same issues with lots of these CNC boards (ex. Ramps 1.4 for Mega, CNC Shield for UNO) They don't allow access for a "rigid" connection. So what I was looking to do is just connect to the USB port. I have everything working by sending serial commands through RX/TX pins from the software side just want a "easy" way to connect to any board.
You mentioned those other communications channels. Is there some way to use them through USB?