Summagraphics tablet?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu May 6 17:57:27 2004

>
> > Basically, the microcontroller triggers a thyristor (SCR) on the
> > tablet driver PCB (actually iside the tablet, at one corner) which
> > discharges a capacitor through a 1-turn coil around one side of the
> > tablet. This causes a magnetic pulse to propagage down metal wires
> > across the tablet. IIRC, it sets up magnetostrictive strains in the
> > wires, and thus propagates essentially at the speed of sound.
>
> I'm not sure how accurate this is for the pad I have.
>
> The pad is a two-layer PCB, with the working area covered by a grid of
> printed-circuit "wires", in one direction on one side of the PCB and
> the other direction on the other side. They are surprisingly far

Oh, it's a Kriz tablet (after Stan Kriz at 3 rivers).

I have never fully understood how they work either. The 'wires' in the
PCB carry electrical currents, and are sequenced in some way by the
microcontroller. The puck/pen contains a pickup coil, the output of which
is amplified and fed back to the microcontroller.

There;s no ADC On the pen input, the signal is conveted to a 2-level one.
And the resolution is _much_ higher than the spacing between the 'wires'.

I have the schemaitcs of the PERQ 2 tablet (which uses this principle),
and they don't give much away. One day I'll disassemble the
microcontroller ROM and make sense of it all...

> apart, on the order of =BE". I haven't traced the circuitry enough to
> know how accurate your description is, though I didn't see anything
> like the one-turn coil you describe - everything electrical, besides
> the serial connector, power, and the puck, is on the PCB. Two parallel
> etch runs could form a one-turn coil; I don't recall seeing them, but I

I was assuming this was a magnetostrictive tablet, like the older
Summagraphics ones. Obviously not.

-tony
Received on Thu May 06 2004 - 17:57:27 BST