After hacking the plastic off a PS/2 mouse extension cord, he wired everything up according to the pinout. Programming the PIC to understand PS/2 commands was a little strange. [David] is used to ...

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/2 connector fitted, it may well have come with an adapter for a 9-pin RS232 serial port.