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.

Is this for input or output? The Gecko's already have opto's on the
input so they will work with direct current sinks or using a non-
inverting buffer chip (i.e. 74LS541). Driving opto's is nothing more
than driving an LED lamp. They are current devices so you have to
pass the rated current (10 - 15ma on most). Since, like you stated,
TTL/NMOS parallel port chips will normally sink more current the opto
is normally given +5 from the computer through a current limit
resistor on the anode and the signal pin is connected to the cathode
so that a low turns it on. Most of the motor drive units either have
TTL inputs referenced to same ground as parallel port (Rutex,
Xylotex) or have their own opto inputs, so building opto isolation on
the step and direction pins is often redundant. If you need to
trigger larger external loads, a SSR (Solid state relay) has it's own
opto input and will work directly off of the parallel port using a
current sinking setup (low = on)

If you need opto inputs to the parallel port then you have a
different configuration where the opto input is driven from the
signal (limit switches, table sensors, etc) with an isolated voltage
source and the opto transistor is used to pull the parallel port line
low through a collector resistor.

IMO, unless you work REAL cheap you would be ahead to buy one of the
available IO breakout boards like the Campbell Designs model that has
all of the buffers, opto's and isolated power supplies. Consider you
already have a full day of design invested and probably at least 2 or
3 more plus that much more to build, troubleshoot, modify, test and
finalize a design you will have worked for about $2.50/hr (less if
you have to buy parts). If you are retired or consider your free
time to be of no value then I guess the tradeoff is justified.

I am an EE with enough experience and spare parts to build any kind
of IO interface I want. I bought a Campbell board.