Re: breakout board and diode question

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

> Greetings,
>
> is it possible to protect the parallel port of a p.c. by
> building a breakout board that utilizes diodes instead of
> opto-couplers. i am interested in building a breakout board but
> i dont have the electrical knowledge to design an opto-coupler
> board. any suggestions or instructions on which components
> to use sure would make my day.
>
> thanks
> jeff

If you put the diodes in series with the pins you will loose .6 volts
and the port will not sink current (it won't turn on a load that
requires an active low signal). If you put a diode from gnd to each
pin (anode to ground) it will work to supress negative going spikes
coming from the outside world, however most parallel ports have that
protection built in. Nothing you can do with simple diodes will
protect you from overvoltage or excess current draw (if you want the
port to drive the load). A 5.6V Zener on each pin to ground will
clamp any incoming voltages over the limit to ground and suppress
negative spikes as well. Most of the problems we see with PP pins is
lack of drive or noise related issues. Your best bet is to use an
active device like a 74LS541 octal driver that will isolate the port.
and "sacrafice" itself in the case of gross over voltage. The opto
couplers do more than just protect your parallel port since it
separates the grounds of the two systems and reduces noise
transmition on the ground wires. If you want the best solution Optos
are the answer.