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||b0||bits of a byte|
|P||1||0||0||0||S||0||0||P is parity. S is on if switch is down|
|P||0||x5||x4||x3||x2||x1||x0||low 6 bits of x data|
|P||0||x11||x10||x9||x8||x7||b6||high 6 bits of x data|
|P||0||y5||y4||y3||y2||y1||y0||low 6 bits y data|
|P||0||y11||y10||y9||y8||y7||y6||high 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…@lapis.berkeley.edu – build something that will last at
Uucp: ucbvax!ucblapis!oster – least until we've finished building it.”