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

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

> 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.

This is on the parallel port input pins (11,12,13,15). Unless I'm
mistaken (which is by no means aout of the question 8) the inputs
don't sink current so pullups wouldn't add any current load to the
parallel port. Pulling down is no problem since thats done on the
breakout board through the transistor side of the opto. Even though
the resistors would be in parallel, 10k external pullups would only
add 0.5mA that the breakout board would have to sink.

Sourcing from the input pin pullups wouldn't change at all since
the "secondary" pullups on the breakout board would get their
current from the floppy cable.

However, its also occurred to me that, just like the outputs, some
ports may be just 3.3 volt. 5V pullups could possibly destroy the
inputs. Too risky!

Damn, now I'm right back to my original design with no changes.
Learned a ton in the process though!

Merci to all who participated!