[[File:Computer Museum of America (02).jpg|thumb|فأرة من تصميم شركة اسيروكس ألتو]]
In the 1970s, the [[Xerox Alto]] mouse, and in the 1980s the Xerox [[فأرة ضوئية]], used a [[
Rotary encoder#Incremental rotary encoder|quadrature-encoded]] X and Y interface. This two-bit encoding per dimension had the property that only one bit of the two would change at a time, like a [[شفرة منعكسة]] or [[Johnson counter]], so that the transitions would not be misinterpreted when asynchronously sampled.<ref>Richard F. Lyon (1981), [http://www.bitsavers.org/pdf/xerox/parc/techReports/VLSI-81-1_The_Optical_Mouse.pdf "The Optical Mouse, and an Architectural Methodology for Smart Digital Sensors"], Xerox PARC report. "The counters needed for X and Y simply count through four states, in either direction (up or down), changing only one bit at a time (i.e. 00, 01, 11, 10). This is a simple case of either a Gray-code counter or a Johnson counter (Moebius counter)."</ref>
The earliest mass-market mice, such as on the [[Apple mouse#Models|original Macintosh]], [[أميغا]], and [[أتاري إس تي]] mice used a [[D-subminiature]] 9-pin connector to send the quadrature-encoded X and Y axis signals directly, plus one pin per mouse button. The mouse was a simple optomechanical device, and the decoding circuitry was all in the main computer.
The [[DE-9 connector]]s were designed to be electrically compatible with the [[
Joystick#Electronic games|joysticks]] popular on numerous 8-bit systems, such as the [[كومودور 64]] and the [[أتاري 2600]]. Although the ports could be used for both purposes, the signals must be interpreted differently. As a result, plugging a mouse into a joystick port causes the "joystick" to continuously move in some direction, even if the mouse stays still, whereas plugging a joystick into a mouse port causes the "mouse" to only be able to move a single pixel in each direction.
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