15.013 How do I write programs for the Apple Graphics Tablet?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
15.013 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 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 contains the state of the pen (up or down), I don't recall which bit it
is nor if one can distinguish between 'pen up' and 'pen out of reach but key
pressed'
$0281, $0282 contains the absolute X position of the pen
$0283, $0284 contains the absolute Y position of the pen
As far as I can remember the values have about 12 bit resolution (!).
Usually I needed screen coordinates (280 x 192), so I dropped the 4 least
significant bits and did some additional adjustment. Unfortunately, I don't
recall what coordinates will be there in the 'out of reach but key pressed'
case.
By: Oliver Schmidt
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