Re: ParPort output low - hi-Z or ground? opto to parallel

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <info@t...> wrote:

> For maximum noise immunity I would either run the led side
> from the same voltage level as the parallel port or buffer the pin
> output and shift the voltage level up to the +5 using open

collector

> buffers.

You know, I'm now thinking my original design was actually
(accidently) more correct.

I used the parallel port outputs as output high and fed into ULN2803
Darlington arrays to buffer the signal and the '2803 drove the opto
leds. At 2.0V input the '2803 can sink >200mA which is way more than
enough for the optos.

About the only thing missing would be the pullups for the parallel
port input pins. Personally, even though my parallel port has a
perfect +5V on the input pins, I'd still go with pullups. In my job
as a mechanic (ships then buildings) for >20 years - I've learned
that Murphy spares no one and doesn't know what "common sense" is ;)

The schematic is in the Electronics_101 files area in the Stepper
Drivers folder.