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

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ghidera2000"
<ghidera2000@y...> wrote:

> --- 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 ;)
>

Extra pullup resistors could cause more harm than good. In essence
if they already have pullups then putting another one in puts them in
parallel with each other. While this will make the pull-up more
severe it will also load the pin more when it's trying to go low.
You could exceed the sink capability of the parallel port if it also
has to sink the opto current.

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