Aucbvax.2461
fa.works
utzoo!decvax!ucbvax!works
Wed Jul 29 17:27:44 1981
Mouse Guts
>From OLSON@DEC-MARLBORO Wed Jul 29 17:19:06 1981
Conglomeration of responses to "How does a mouse work?":

Currently, a mouse is a small box with a fairly large (1-2 cm
diam.)  ball bearing captivated so a fraction of it lies outside
the bottom of the box.  As the box is rolled around, two wheels
positioned perpendicular to one another pick off the rotational
motion of the ball in their plane only (they slip in all other
planes).  Hence we get two rotational motions, one for each
component of the two-dimensional motion of the mouse.

The direction and (over time) speed of the rotation shafts are
measured by disks attached to the shafts encoded with a gray
code, and read either photoelectrically (via led and phototran-
sistor) or mechanically (via brushes).  The grey code output
might look like 00 01 11 10 00 going one way and 00 10 11 01 00
going the other, so we can tell the difference in direction.

Some historical information about mice, according to Bill Barns:

The mouse as originally invented by Doug Engelbart and Bill
English and patented by them (rights assigned to their employer
at the time, Stanford Research Institute (now SRI International))
consists of two high precision potentiometers connected mechan-
ically to metal wheels with rather sharp edges, approximately 2
inches in diameter, and set at right angles to each other as close
as possible without touching.  [This seems a kludgey way to get
this motion, because the pots would "pin" occasionally, even if
they were 20-or-more-turn.]  The pots are mounted on a metal base
plate to which is attached a bracket.  On the bracket there are
(in the original, and still popular configuration) three switches
which are triggered by buttons about 5/16 inch diameter [8 mm]
which rest upon spring metal fingers attached also to the bracket.
The bottom of the metal finger rests on the momentary-contact
actuator of the microswitch.  This arrangement puts some "click"
into the feel.  The switches typically are SPST with a common
ground so that for a three button mouse there are four wires for
the switches and one wire for the non-ground side of each pot - 6
total.  The mouse wheel voltages are fed into an analog->digital
converter of about 10 to 12 bit resolution and at appropriate
times, some logic samples the digital value and does the
appropriate thing.

Engelbart lives on at Tymshare, and English went to Xerox PARC
and gave birth to Alto etc., (not sure if he's still there).
Bill estimates the invention of the mouse between 1962 and 1967,
and *guesses* 1963/4.

By the way, I got several warnings about suits, patents that I
musn't breach, etc, which I condense below:

Patents to SRI and Xerox apply to a number of features of the
design.  The Englebart/English Patent is probably still in
force, and it covers both digital and analog mice.  [I was
warned to check the patents before building my own.  I really
don't think that building your own personal whatever falls
under patent laws (unless possibly if you sell it).]

Thanks to Jerry Farrell (Farrell at PARC-MAXC), Bill Barns
(Barns at OFFICE), Craig Everhart (Craig Everhart at CMU-10A),
and Steven Kirsck (SK at MIT-MC).

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