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Below is an edited extract from an email I found on Google's Usenet Archive. I think there is enough information in there to allow us to use some older serial port graphics tablets as X-Y controllers under MIOS.

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Date: Sat, 26 Jul 86 09:12:59 pdt
From: oster%ucbla…@BERKELEY.EDU (David Phillip Oster)
Subject: Re: Digitizing point coordinates (MacTablet)

This letter discusses using Mac compatible digitizing tablets to get high resolution data … since you are not afraid to program, you might try the following:

I assume the MacTablet connects to the Mac using a serial port…

Listen to the tablet using terminal software (Versaterm, naacterm …)(try 9600 baud as an initial guess at the baud rate.)

The tablet probably sends a 5 byte data packet of the following form:

b7 b6 b5 b4 b3 b2 b1 b0bits of a byte
P 1 0 0 0 S 0 0P is parity. S is on if switch is down
P 0 x5 x4 x3 x2 x1 x0low 6 bits of x data
P 0 x11 x10 x9 x8 x7 b6high 6 bits of x data
P 0 y5 y4 y3 y2 y1 y0low 6 bits y data
P 0 y11 y10 y9 y8 y7 y6high 6 bits y data

b6 is 1 only at the start of a 5 byte data packet.

This data is drawn from a Summagraphics data sheet for their Bit Pad 1 digitizer.

Unlike the Mac, point (0,0) is probably at the LOWER-left corner.

If you get data that is consistent with this, then you need only write a basic program to read data at the serial port. Tablets generate so much data that you're program had better check the fullness of the serial buffer and throw away 5 byte data packets periodically to keep the buffer from overflowing. Alternatively, you could write in a language, like C or compiled Pascal, that was fast enough to keep up with the data rate.

Low-resolution Kurta tablets and other-than-MacTablet Summagraphic tablets use this data format when they are not transmitting data as strings of ASCII, base-10 integers.

Kurta's PenMouse, a cordless digitizer considerably cheaper than MacTablet, sends 2400 baud, their other models generally run at 4800 baud. Kurta also makes a MacXL compatible version.

Disclaimer: My opinions are solely my own, but, in evaluating them, you should know that I am currently re-writing Kurta's software.

— David Phillip Oster – “The goal of Computer Science is to
Arpa: o… – build something that will last at
Uucp: ucbvax!ucblapis!oster – least until we've finished building it.”

serial_graphics_tablet.txt · Last modified: 2007/07/26 16:32 (external edit)