I have pulled some sentences out of your new serial voltage discussion.
"The Waysmall STUART, HWUART and Tweener boards are currently the only
boards which provide RS-232 level serial interfaces. These can be plugged in
directly to the serial port on the back of your PC, or into a USB-to-serial
I think there could be some confusion with the USB-to-serial adaptor part
since many of these supply logic level signals while others, especially
those that terminate in a DB9, supply RS-232 level signals. Perhaps
identifying it as a USB-to-RS-232 adapter in the last sentence solves the
problem. A Google search on USB to RS-232 seems to lead to the correct
devices if you want to connect the adapter to the RS-232 port on the Gumstix
expansion board. Otherwise, one could connect a logic level USB-to serial
adapter to the appropriate logic level pads on the expansion boards.