Microsoft Bus/InPort was the fastest PC mouse interface until PS2, and easiest to emulate. Checking mouse position cost one interrupt + 10 in/out port opcodes. PS2 is one interrupt + 4 in port opcodes + out interrupt acknowledge, but there is tons of commands one has to implement. Serial mouse is simple but costs 3 interrupts and uses slow data transfer speed (default 1200bps).
Microsoft didnt totally abandon its older InPort mice, they actually sold InPort to Serial/PS2 adapter (C3K9ILMA) allowing one to reuse old inport mouse in newer computer https://www.ebay.com/itm/375576855239
Good info on Alps 64H101. Long time ago when looking at inPort card I couldnt figure out how it worked with only one cd4020 counter. I guessed incorrectly 64H101 to be some 74H101 version. Now its obvious cd4020 is used for generating interrupts.
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Microsoft didnt totally abandon its older InPort mice, they actually sold InPort to Serial/PS2 adapter (C3K9ILMA) allowing one to reuse old inport mouse in newer computer https://www.ebay.com/itm/375576855239
Good info on Alps 64H101. Long time ago when looking at inPort card I couldnt figure out how it worked with only one cd4020 counter. I guessed incorrectly 64H101 to be some 74H101 version. Now its obvious cd4020 is used for generating interrupts.