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.

[Ladyada] and [pt] had an old keyboard from NeXT, but since it used a custom protocol it wasn’t usable with modern hardware. So they built a custom device to convert the NeXT protocol to USB.… ...