Did it have a little adapter in the box? There was a time when if you ... He’s created a PS/2 to RS232 mouse converter, and it takes the form of a little PCB with an AT90S2313P microcontroller ...

A simple converter can be sourced online, but the only thing he could find locally that was even close was a USB to RS-232 converter. He used that component to craft his own USB to RS-485 bridge.