Hi James Buwalda,
Friday, 16 August 1996 10:23:13, James Buwalda wrote to Denis MOREAUX
DM> [description of ZX Spectrum Lightpen and of how it could work]
JB> [Repeatition of the Strobe Frame trick]
 JB> If the light level it
 JB> recieves is half the required voltage, it is assumed it is still on
 JB> screen. If at full voltage, it is at a strobe area. If at 0 or less
 JB> that a given amount, it is assumed to be off screen. If the fire
 JB> key is pressed at one of these times, it is either reloaded, flash
 JB> the screen to signify a miss, or kill the person. 
Huh... The Tape In of the spectrum is a binary input : 0 or 1, nothing other.
So, the Spectrum pens and guns have to work somewhere else !!!
 JB> The light pen
 JB> does something similar. The lightpen transmits and recieves at the
 JB> same time. Then at black on a bar code it transmits 0, if at a
 JB> light bar it reflects light and xmits a 1. Simple huh? 
No, it's a light pen to point on screen !!! It allows you to draw directly on
your screen. That's why I said it's the same thing that a Light Gun... It 
don't
sends anything, it only receives the signal from the TV screen.
One thing that make it's detection easier on the spectrum is that the only
interrupt available is lanched at the start of the screen update (just after
the vertical retrace. On the spectrum, the screen update begins with a few
lines of the same color (the border) so there is time available to save the
registers...) I think that IRQ9 should have a similar use on the PC but that
it's not always the case. (old Trident have a jumper to enable/disable IRQ9)
That's all for now!
                       Denis MOREAUX
.!. The future is like the present, only longer.
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