The project page contains an in-depth walkthrough of how the PS/2 connection talks to the keyboard hardware along ... or even hook up your custom game controller to spice up the experience.
It will allow you to connect either a PS/2 keyboard or a USB keyboard in fallback mode to the Expert using an ATMega328P and a bespoke DIN-13 to DB-13 cable. As it turns out, there’s a wealth of ...