Re: ParPort output low - hi-Z or ground?

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

> Spent a day drawing up an optoisolation scheme for the parallel
> port. I know the inputs go between +5V and 0V but, when you set an
> output to low, is it connected to ground or does it go hi-Z?
>
> Silly me, I was thinking in terms of sourcing current from the port
> (OUTput...) and getting fancy with transistors. Just as I finished
> the drawing (As usual!) I realized that its TTL and TTL is much
> better at sinking. If it goes to ground then a couple resistors and
> 1N4148's are all I need to drive the opto-leds.

Hi,
I'm also working on a a3977 board, after a week of work I've decided
that having a 6n136( 4n25 won't do it,it need a lot of current going
thru it on the input to have a good CTR) opto conected to the
parallel port thru a 1k resistor,(led side of the coupler) and a 7404
i.c on the output side of the 6n137 to bring the signal to be in fase
with the input signal is the best way(I think).
there is no electric connection in bettwen one side of the opto and
the other, so if anything goes wrong with the drive it will have no
efect on the parallel port.
I also have added a current reduction circuit based on the 555 i.c
If you're using a 4n25 opto type you need to supply 5v to it on the
input side.
the 6n136 has a very good transfer ratio so as litlle as 1.2UA is
enough to get it going on the other side.
tested the board today, and it works very well, up to the computer
limits(40Khz)step pulse.

Good luck
Fogassa