Just to add to Mark's post, we both know that a USB to serial cable works, but we're looking for a simplified modification than the current solution.

The original mod was designed to work with the parallel port, but later someone adapted this to serial using a three transistor / diodes and a handful of 10K resistors circuit. From what we can deduce it uses the DTR and RTS lines as level shifters, and the transistors as a simple switch so that it makes / breaks connections on the modified commercial PCB.

Hope that helps

Malcolm