Editing Bus mouse (section)

== Electronics ==
Buttons are often simply shorted to ground when pressed, with pull-up resistors on the host side.
Button input often needs to be [[debouncing|debounced]], which because buttons are few and wired direct (there is no [[keyboard matrix|button matrix]])
is typically done through a low-pass filter for each input on the host side.

An [[Opto-mechanical sensor|opto-mechanical mouse sensor]] contains a mouse ball and an optomechanical [[rotary encoder]] on each axis (X and Y). Each encoder produces a "quadrature code" (also called 2-bit Gray code, or "pulse trains") on a pair of lines, which is filtered to logic levels, often by a pair of schmitt triggers in the mouse.
In the pinouts below, the lines have been named (XA,XB) and (YA,YB) so that when moving (moving right/down) the sequence for a pair of inputs is: (0,0), (1,0), (1,1), (0,1). The quadrature code sequence for decreasing value (moving left/up) is in the opposite order.
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Only Commodore Amiga and Archimedes docs have described which pin in a pair is leading.
For PC-98, I found a good third-party doc, albeit in Japanese.
Amiga docs specifies that increasing Y is down, while Acorn's that increasing Y is up.
Apple, Atari and Commodore signals have been corroborated by diffing versions of Retronic Design's source code.

The other interfaces' A/B order are direct from documentation without any interpretation.
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In several implementations (Archimedes, Atari ST, NeXT non-ADB), the mouse is connected to the keyboard and quadrature code is interpreted by the keyboard's [[Keyboard controller|microcontroller]], which in turn presents mouse input intermixed with keyboard input to the host over a type of serial protocol.
On some machines, the ports were also used for game controllers (Apple IIc, Atari ST, Commodore Amiga).