Classic Macintosh Mouse Adapter
As I speculated before, the USB adapter I built last year works just fine with the 9-pin DB9 mouse from the original Mac and Mac Plus:
The Arduino source is available here. Here’s how I wired up the Mac mouse:
|Mouse port pin||Arduino pin|
|6 (NC)||(not connected)|
Note that I added pull-up resistors on the mouse pins as I should have done from the start.