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From: TLC (all posts)
Date: 1999-03-22
Subject: Re: Lightpen

Hi!

> > That is right, but there is no possibility of using this registers for an
> > acceptable resolution. Main processor is far too slow for this.
> > When sign came in you need at least 3 op to execute while read
> > from $FF1C, and as you said, LSB is not present there. (Cannot be
> > used for precise horizontal position.)
>
> Hm. In multi-color mode it's just enough 'cause it doubles the pixels, in
> hi-res mode you would need some more precise positioning but i don't know
> if there would be any.

Rachy was right. Even if the register's resolution matches the resolution of multicolor mode, it still does not matter. The problem is,
you still cannot catch the minute when the pen activates its line.

If you had a triggered input, I mean, some additional electronics which
_captures $ff1e when the input line is active, you'd be able to determinate this position. Else, not. The $ff1e register doesn't even reflect the least significant two bits right (when read by the processor), due to the fact that the reading positions are also determined by the clock division.

> > > Yes. I looked for it and found. $FD0D is the "Program-Reset/Status"
> > > register of the ACIA. (Program reset for writing and status for reading)
> > >
> > > It's layout:
> > That is all nice, but I see no connection then in these two stories...
>
> I just wrote the correct joystick and keyboard handling... :)

Hmmm, setting the ACIA someway, may not be able to catch the needed position?

> > Lavina's picture shows there is nothing but a joy-port connection.
> > The program might be wrong then?
>
> Maybe it's for an other lightpen that uses user port.

Is not it the fact, that a lightpen which is only connected to the joystick port may have designed for the C64 (or if I'm right, also the Vic-20), and the other (userport one) for the Plus/4? I'm just in doubt if reading a lightpen position from the joystick port (without add logic in the lightpen) could or should be impossible.

Levente