Re: light pens on Apple II (and reading >1 light pen on the same computer)

[Linards Ticmanis <ticmanis@gmx.de>]

mad.scientist.jr@gmail.com wrote:
> Someone (thank you merlins ghost) sent me an article on how to build a
> light pen for the commodore 64 and write programs to detect it. The
> c64 has a pin on one of the joystick ports that is set up for light
> pen input. However, only one of the joystick ports has the pin, so it
> is not possible to plug in and simultaneously detect input from 2 or
> more light pens. Since the Apple II seems more hackable than the 64, I
> am wondering if it would be possible to modify the c64 light pen to
> work with the Apple and plug in & detect >1 light pens simultaneously.

Since no dedicated light-pen input exists on the Apple II, getting
proper resolution and being able to do much else in your program except
watching for the stick signal will mean designing a slot card.

The slot card would watch the video scanner do its work (by looking at
the address lines during the first half of every 6502 cycle) and thereby
determine the position of the beam. It would latch that position as soon
as the pen signal arrives, and later, when asked, deliver that
information to the CPU.

-- 
Linards Ticmanis