RE: ISA Vector card already done...

> I didn't worry about the max number of vectors, refresh rate, etc. I
> looked at
> it from the point of view of the monitor. I can't remember the slew
> rate off
> the top of my head, but I took the slew rate in in/sec, used
> displayable screen
> size in the horizontal direction (15 inches I think), divided by 1024,
> etc. and
> ended up with 6.xx mhz. I then looked at the clock going into the
> DACs on an
> asteroids and saw a 6 mhz clock.
>
I follow you, but is that the number we really want to be figuring out?
The worst-case vector slew rates would be caused by an instantaneous
re-positioning of the beam cause by changing the DAC outputs in large
steps (like from 0V to +fullscale output). We're only moving a few bits
at a time and changing the output slowly in comparison.

I looked at Asteroids on the scope once and it looked like it was using
a 16ms "frame" speed. The "draw" time was a fraction of that amount. I
want to say something like 6-8ms when things were actually getting
drawn.

Vector display signals "seem" pretty slow. (You can hear the neck
chatter, look at them on a slow scope w/out problems, etc.)

Something just feels kinda "fishy" about needing a super-fast system to
make it work. (Well, we know that Atari's last vector design was DSP
based, so something must be possible...)

> This is the same problem I had with using software. I calculated that
> I would
> be able to run around 20 instruction per update in the yet to be
> released 50mhz
> PIC clone and it looked like I could write a line draw routine that
> would fit...
>
Still should work on the DSP then 'cause loops are free and I've got a
33MHz clock rate internally. I think we're figuring something wrong
though in the necessary speed. I can't explain where the error is, but
it doesn't "feel" quite right. If nothing else, the AD7247A is
basically just a dual-parallel version of the serial 12-bit DAC.
Spendy-- $28 in singles, but that plus the DSP, plus a little glue would
make an entire deflection system.

-Clay
Received on Wed Mar 25 13:27:55 1998