Summagraphics tablet?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed May 5 18:46:56 2004

> Well I suppose they run as a SBC, taking input from the pad and
> converting to the necessary protocol before spitting it down the serial
> line.

The 'pad' has no intellegence of its own, and does nothing without the
microcontroller.

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.

The pulse is detected by a coil in the pen/puck, and fed back to the
control electronics. The microcontroller, aided by some external
counters, measures the time delay, and thus works out where the pen/puck
is on the tablet. The process is then repeated for the other axis of the
tablet using a coil and wires at right angles to the first one.

If you want to know how I discovered all this, well, <mumble> years ago I
obtained an Apple Graphics Tablet. This is a Summagraphics tablet without
the electronics, but with the counters, etc, on an Apple ][ plug-in card.
Not having an Apple ][ at the time, I made an interface, loosely based on
the Apple ][ card (the schemaitc is in the manual) to plug into my CoCo.
After sorting out noise problems due to common ground leads (to tie into
another thread!), I got it working fairly well..

I have the Bit Pad 1 on my PERQ, and a somewhat older one, based on the
same principles, on the I2S systems. I have all the manuals and
schematics, and they all work in much the same way.

>
> My tablet's got both a pen and a puck; maybe that's a pen connector?

On the BP1, the pen and puck plug into the same 7 pin DIN connector
(allowing for 4 buttons on the puck).

-tony
Received on Wed May 05 2004 - 18:46:56 BST