010- How do I write programs for the Apple Graphics Tablet?
Description
This article is from the Apple II Programming FAQ, by Jeff Hurlburt with numerous contributions by others.
010- How do I write programs for the Apple Graphics Tablet?
The Apple Graphics Tablet I know of is rather large and heavy;
its pen is attached to it with a (too short) cable; it makes
funny sounds that change when the pen is moved in/out of the
reach of of the tablet.
A long time ago I patched a few programs to make use of this
tablet. To make it clear in the first place - I don't have these
patched versions available anymore :-(. But I found a listing
from which I can tell you this:
To detect the interface card, I looped over all slots to check in
its firmware for
$B0 at location $Cx01 and
$20 at location $Cx09
x being the slot number. I never found this 'signature' in any other firmware.
To check for the pen position one has to poll the tablet (again x
being the slot number):
LDA $CFFF ; switch off all extension ROMS
LDA $Cx00 ; switch on the extension ROM of the tablet
LDA #$Cx
STA $07F8 ; initialize some hidden text screen data area for the tablet firmware
JSR $CBB9 ; call well known location ;-) in tablet firmware
When the pen is in reach of the tablet (up or down) this routine
will return immediately. Else it will block - fortunately, the
routine checks the keyboard strobe ($C010) too and will also
return if a key is pressed, even if the pen remains out of
tablet's reach.
After the routine has returned one can get the information:
$0280 pen state:
bit 0 = 0 Pen down, bit 0 = 1 Pen up
bit 1 = Previous pen state
bit 4 = 0 Pen has been localized, its state and positions are valid.
bit 4 = 1 Pen out of reach but key pressed and then X=Y=0
$0281 Low byte of X pen position
$0282 High byte of X pen position
$0283 Low byte of Y pen position
$0284 High byte of Y pen position
X and Y have 13 bits resolution from 0 to 8191. Using my Apple
Tablet, I get values from 300 to 6350 and the bounds are not
reached.
--Oliver Schmidt, Guillaume Tello, Shaun Olson
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