ADC on a pogoplugV2

[g8jvm]

Hi
has anyone fitted an ADC either as a direct hardware modification, or if one exists an ADC USB, probably a USB might work if the bandwidth started at DC
I only need to trigger an event from a line toggling low to high.
Maybe use the RXD line on the serial port ?????
I'd prefer not to reinvent the wheel if someone has already done it.
Thanks