Information provided by: Andrew Gale
[ Uncs Note: As far as I'm aware, no pen actually ever got released
for the Sam ]
Very simple really - the SPEN pin on the lightpen/audio port is
usually pulled high internally with a resistor. This means that ports
248 and 504 are constantly updated with the x and y coordinates
respectively of the raster beam. If SPEN is held low then those ports
'stick' at their current value. So, basically, your lightpen circuit
should hold SPEN low and blip it high very briefly when the raster
beam passes in front of the photodetector (you could use a photodiode,
phototransistor, photodarlington - preferrably a fast one) - this
shouldn't take much more than one i.c.! Note that the x-resolution is
not too great, and that line interrupts (often used for palette
changes) will not operate. Sorry.