4 Jun
2009
4 Jun
'09
9:24 a.m.
Gordon JC Pearce wrote:
Tony is closest ;-)
The USB serial adaptor only seems to put out a really tiny current.
More than a few microamps (!) into a load makes it put out a train of
spikes at about 20kHz more like a sawtooth wave, than nice neat pulses.
I've had problems with USB serial devices feeding various things, but
mostly things that draw too much current. Most USB devices seem to only
have +/-5v swings and if you draw too much current the sag quickly.
Older PC's seem to have much better +/-12v swings and can supply more
current. I have some digital thermostats which draw from DTR and will
only work with a pc - they won't work with the +/-5v of a USB device
because the voltage sags too much.
With these same "problem" devices I've also not had much luck with
simple max232 style outputs; they give a nice +/-10v swing but also sag
quickly. I think the drivers have wimpy NP caps (like .1uf). I could
try beefing those up and seeing if that helps.
-brad