What's got me wondering here is that the 6845 has a minimal resolution of one
character width, typically 8 bits. It has an upper limit on count rate, and
between that and the dot clock frequency, you get an upper limit on the number
of pixels per line for every frequency of character clock.
Now, I haven't looked at the 6845 spec for a couple of decades, (It was new
then.) but IIRC, the minimal resolution of the value in the light pen register
is the character clock, which is the dot clock DIV pixels-per-character.
You can, of course, draw whatever you want in the video refresh RAM, but I'm not
clear on how this process works in conjunction with the function of the 6845.
Could you tie those two together, plz?
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 17, 2001 5:40 PM
Subject: Re: Light Pens ...
> > > > What do you suppose they did to keep the cursor centered? Did the
integrate
> > the
> > > > waveform from the light pen? How would they handle the vertical
centering?
> > Do
> > >
> > > No, it was genrally a lot simpler than that.
> > >
> > > The 'cursor' might have been a 3*3 array of blocks, which I will number :
> > >
> > > 1 2 3
> > > 4 5 6
> > > 7 8 9
> > >
> > Viewed from the 6845, is the block 1 scan line high? The minimal width is
>
> The block is a 3 by 3 array of the size of object that can be detected by
> the light pen. If the light pen has a resolution of one 'character cell'
> -- say 8 pixels square -- then the cursor is 24 pixels square. If the
> lightpen can resolve individual pixels (and we discussed yesterday
> latching the bit counter to be able to do this) then the cursor is 3
> pixels square. And so on.
>
> Remember that although the cursor is a 3 * 3 array of blocks/pixels, the
> hardware/software moves it about 1 block/pixel at a time, keeping the
> block/pixel numbered 5 under the end of the pen. So the resolution of the
> pen is still 1 block/pixel -- you don't lose a factor of 3 in the
> resolution or anything like that.
>
> > probably 8 bits, or whatever else the character width is. How do you
suppose
> > they got the resolution down to the individual pixels? IIRC, the MOT raster
>
> I think the main problem is making a light pen that will detect
> individual pixels reliably (this is a matter of both optics and
> electronics). It's not hard to latch the bit counter as well as the video
> memory address so as to be able to have a bit address available to the
> software.
>
> > > Quite posslby. A single-chip microcontroller could handle this easily
> > > (the fast stuff -- determining the coordinates of the lightpen -- is
> > > still done in hardware, say by a 6845, all the software has to do is work
> > > out what block in the cursor currently corresponds to that location, and
> > > move the cursor if necessary).
> > >
> > Getting the horizontal location right, as you'd have to in order to do
precise
> > drawing, might be a bit more complicated, don't you think? an 8-pixel
extent of
>
> I'd want to use hardware for that. The 6845 to get the byte address and a
> 3 bit counter + latch to get the bit address in that byte. Apart from
> that, it's no harder to give a pixel address than the address of 8
> adjacent pixels.
>
> The problem is the lightpen itself. It would have to focus a single pixel
> onto the phototransistor. And it would have to have a sufficiently fast
> amplifier to give a nice sharp pulse from a single pixel. And of course
> the CRT would have to have a short persistance.
>
> -tony
>
>
Received on Tue Jul 17 2001 - 21:09:21 BST