How do I convert a 12v-0V square wave signal to 5V-0V to feed to USB port on computer?

I have a project  that involves a 12V to 0V square wave signal that I need to feed to a tablet type of computer via the tablets USB port.  Where the signal goes after the tablet gets it is the software programmers problem. This system was orignally made to feed the signal to pins 10 and 12 of the LPT port on a desktop PC. Signal return was made to pin 25 of same LPT port.  The infrastructure used on that system is no longer available.  Any help greatly appreciated.  

Use a USB-LPT adapter, or a USB-serial adapter.

Connect the 12V-0V signal via a 4k7 resistor to a 5.1V zener diode's cathode, connect the diode anode to ground. For extra security, put a standard, 1N4148 diode across the zener, but anode - cathode and cathode to anode.