@billgoats ADB
On a related note, I wrote my own ADB host firmware years ago for an ADB -> USB adapter because all the open-source adapter projects at the time only supported keyboards. I wanted to use an ADB trackball (Kensington Turbo Mouse). However, the ADB standard only supports 2 button mice without driver extensions, and I was never able to figure out what special command the Kensington driver sent to the trackball to get it to send button presses for buttons 3 and 4. I eventually gave up and took a much less rare serial version and just wired up the buttons and optical encoders to the microcontroller to have it act as a USB mouse.