4 Jul
2006
4 Jul
'06
3:17 p.m.
Don Y wrote:
Josh Dersch wrote:
s/buffer/holding latch/
Brad Parker wrote:
Always load X (for example) before Y. Then, insert an extra
buffer between X's parallel port and the X D/AC. Clock BOTH I would guess that a simple parallel port adc on
a PC could do very
well displaying a reasonable number of dots on a scope with intensity
control... not very impressive given what fpga's do now shading
polygons,
but fun none the less.
A couple of years ago I tried this -- I built two very (very) simple
parallel-port DACs (one for X, the other for Y) and I had them
connected to linux machine with two parallel ports. I wrote a small
test app to rapidly write data to the output pins on the parallel
ports. Plotting points on the oscilloscope worked OK (aside from the
poor precision of my DACs), but drawing arbitrary vectors seemed to be
impossible because I couldn't figure out a way to change the data sent
to the parallel ports for X and Y at /exactly/ the same time. Since X
would be changed slightly before Y (or vice versa) I'd get a stairstep
effect instead of a single line from point A to B.
Maybe a faster machine would be able to reduce this effect (I was
using a Pentium Pro 200), or clever use of DMA... D/AC's off of the Y "load" pulse. So,
write to X doesn't
alter the X D/AC *but* saves the output in X's "holding latch".
Then, updating Y causes X's held value to fall into the X D/AC
at the same time that Y's value is falling into the Y D/AC.