Vector Imagery

[Tony Duell]

> 
> I've already cracked open the meter and tweaked the internal load
> resistor.  The simple 8-resistor ladder I built _works_, with the
> exception of a noticable dip in voltage between 2^5-1 and 2^5, and
> 2^6-1 and 2^6, and 2^7-1 and 2^7.

What resistor vavlues are you using, and what tolerance?

> 
> > Second, The output port is TTL? CMOS?  IF TTL the output swing is
> > not to 5V to with even light loads.  IF CMOS output voltage will sag
> > if the load is too great.
> 
> To be honest, I have no idea - it's an integrated port on a modern
> machine.  If I had to guess, I'd say it's a CMOS port, but I don't
> know that for a fact.
>

Write 0xFF to the port, and measure the voltage on the data lines with a 
(fairly high impedance) voltmeter.  A CMOS driver will give you almost 
5V, a TTL one will be quite a bit lower (it might only be about 2.5V).

Then connect up your 'DAC', write FF again, and measure the voltages
again. If they're significantly lower, your DAC is loading the port too
much, use higher value resistors. You might also try writing 0 to a
particular port line, measuring the voltage there with the others at all
0's and all 1's. Then do the same thing with the line you're monitoring
set to 1. Repeat those tests for all 8 port lines. If you see the voltage 
on the line being monitored change when you change other bits, again I 
think the resistors are too low.

-tony