Comments by "LoneTech" (@0LoneTech) on "How the Computer Mouse Works - Computerphile" video.

  1. They would detect electrically which sort of port they were connected to; PS/2 ports pull both data and clock lines up with a resistor, while USB ports pull their D- and D+ lines down (and detect low or full speed by which line is pulled up first by a device). So the microcontroller in the mouse would switch to PS/2 mode if it saw a high level. If it didn't, it would try pulling one line up to start the USB device connection sequence. There also exist rare active converters, which contain their own microcontroller (or even ASIC, but less likely) to talk both protocols.
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  2. There were more iterations of these technologies; I have an optical mouse that uses stripes in two directions on the mouse pad, each with their own sensor. This type only works when straddling the two zones. Some higher end workstation mice also used a rather complicated metal mouse pad where I think the directions were optically separated; they looked a bit like CDs to the eye.
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