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.
If you are the original author of this post and wish to have it removed, please contact me through the Private Messages at midibox.org/forum
DrBunsen.
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.”