Re: Opto Isolators & Parallel Ports

Tim Goldstein wrote:

> >> Do you know where I might find information on how to opto isolate the
> interface between the drivers and the parallel port? Am now using 4n28's
> but these are not working well. <<

The 4N28 has a current transfer ratio of 10% ! That is very low, and probably
your problem. I use the 4N36 for general use, as it has a 100% CTR. Stay away
from the Darlington transistor versions (4N32, 4N33) as their saturation voltage
may be too high. The 4N35, 4N36, 4N37 series are pin compatible with your
4N28, and vary in isolation voltage, only. Should be a drop-in replacement that
will fix your problem. If you still want to use the 4N28, you will need to
make whatever it is driving demand very little current from the optoisolator,
like a 74HCxx CMOS chip, and use a very high resistance pullup resistor,
at least 10K, probably more like 20-30K.

What specifically, is the problem? Is it that when the PC sends current through
the LED, that the phototransistor is not conducting strongly enough to pull
the output voltage down to a logic low? That is what I'd expect, due to the
low CTR.

Jon