🚀 go-pugleaf

>>I am interested in a cheap way to monitor some slowly changing
>>voltages (read: DC) in a piece of test equipment.  8-bit resolution
>>is fine.
...
>>Does anyone know if a Soundblaster is a reasonable way to do this?

There's something else...I'm not 100% sure about the original Soundblaster,
but a lot of the clones have an AGC on the input...you'll get a meaningless
number if you try to read it statically, unless you somehow disable the
AGC.  There is no software switch on my Thunderboard to do this, and there's
no schematic in the manual, so I haven't tried to modify it.

Also, programming information on some of these boards is pretty sketchy.
I can't figure out how to statically read my TB, either - I sort of know
how to do DMA sampling, but I haven't gotten it to actually work yet.  I
have the Unix driver sb.c (on internet), but would love some DOS "C" code,
if anybody's got it.

>Couldn't you use a game(joystick) port?

Not likely.  The PC joystick isn't an A/D converter, it's an op-amp
functioning as a comparator.  Think of it this way: if you have an
R-C time delay,and you charge up the capacitor all the way, then remove
the charging voltage and put the resistor across the capacitor.  You can
figure out how long it will take the voltage across the resistor to drop
with the time constant formula, R*C.  If C stays the same, and you
put a comparator looking for, perhaps, 0.63 times the charging voltage,
you can measure the "R" value with a timer and a one-bit input.

That's how a PC joystick input works...when you call the "READ JOYSTICK"
routine in the BIOS, the routine charges (or discharges, I'm not sure which
way it works) the cap, lets it discharge through the 'R' (the pot on the
joystick), and measures how long it takes to charge/discharge to a certain
voltage.  It's not very accurate, but it's certainly good enough for
game playing.  Putting a voltage on it wouldn't work very well, though.

Incidentally, the fact that the computer is doing the timing with
tight loops is why joysticks don't work too well in Windows.  Windows
mucks up the timing.  My 486-33 doesn't like the joystick at all.  I
have an AMI BIOS, I have heard rumors that other configurations support
the joystick just fine.


Chris

-- 
Christopher E. Piggott, WZ2B                    cep4478@ultb.isc.rit.edu
President                                       wz2b.ampr [44.69.0.1]
Rochester Institute of Technology               wz2b @ WB2PSI.#WNY.NY.USA.NA
Amateur Radio Club K2GXT                        CEP4478@RITVAXA.BITNET