in the box was also an adapter to allow it to be used with the older PS/2 interface. And if you were to go back a few more years into the past, you’d have found when you bought a mouse with a PS ...
[danjovic] has you covered with the open-source EXPS/2 keyboard adapter. It will allow you to connect either a PS/2 keyboard or a USB keyboard in fallback mode to the Expert using an ATMega328P ...
TheRasteri also highlights a few even smaller HIDman variations made by others in the community, including one as small as an on-slot PS/2-to-USB adapter. Even if you can use custom drivers and ...