IMHO this doesn't refer to X window system but to a so called cross connection, like a null modem cable. Means: you have to wire Tx -- Rx Rx -- Tx GND -- GND If your board gets ground from a power supply or so you might even save the GND connection, but not if it's battery powered. Here the X stands for cross. If my guesses are all wrong, let me know. Dex
