6 Apr
1998
6 Apr
'98
8:11 p.m.
> > Can I make one,
> > get one REALLY cheap, or operate the thing without one? Also, it
>
> Don't know. I bet it emits RF, and the tablet probably has a circuit
board with
> traces running horizontally on one side an
vertically on the other.
My Dauphin
detects the
pen in this manner.
The older sumagraphics tables like the Bit Pad 1, the ID series and the
Apple graphics tablet worked by sending a magnetic pulse along some made of special alloy. I believe this pulse travelled
at approximately
the speed of sound in the wires - there was a shock wave that travelled
along them caused by magnetostriction.
Magnetostriction?
The puck was a simple sense coil. I had to rewind one
of mine once, and
I
seem to remember it was something like 11 turns of
30swg wire. That
would
be a start anyway.
Also I seem to remember that the buttons on the puck had 100k resistors
in series with them. The other side of the button was grounded. This is
also critical - otherwise noise breaks through into the sense coil
amplifier.
/ 100k
Gnd----/ o---\/\/\---- Button input
Do you know the pinout for the plug?
Certainly the Bit Pad 1 came in a serial version (and also a GPIB
version and a parallel version).
There's another type of tablet that consists of an XY matrix of PCB
tracks - plain copper PCB tracks. They are individually driven by a set
of decoder/driver chips which are driven by a simple microcontroller.
Again the puck is a simple sense coil.
Thing is, it has a much better resolution than the spacing between the
tracks. And there's no extra hardware, like high-speed-ish counters.
I've
never figured out how that one works - any clues?
Well, I have this second one. My guess is that it uses capacitance,
like touch-to-turn on lamps.
-tony
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