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

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<davemucha@j...> wrote:

>
> Assuming a seperate external power supply and running the opto at

10-

> 15mA, the LED section of the opto will drive the parallel port

side.

>
> The parallel port side would either sink or source depending on

which

> way you have it connected.
>
> So, assuming you are only referring to the 5 inputs, parallel port
> pins 10,11,12, 13 and 15 see
> http://www.doc.ic.ac.uk/~ih/doc/par/
>
> I too would be interested in how you decide to connect them.
>
> Dave

Assumimg that the parallel port pins will source enough current to
overcome the leakage current of the opto transistor all you need do
is connect the collector of the opto NPN to the port pin and the
emitter to parallel port common ground (several PP pins). With the
signal driving the input LED with an isolated power supply the
collector will go low on activation of the input. If you wanted to
use a normally closed set of contacts to actiavte from you could
force the opto led to turn off on an interruption and causing the
transistor to turn off and the pin to go high.

An opto goes not have current gain so you will not get any buffering
from using one. If signal levels or drive are an issue, buffering
on one side or the other (or both) is required. You can get level
translation, for example where the input is 3.3V like some of the
new parallel ports. If the source will supply the required current
then the output can be referenced to any voltage within the specs of
the opto transistor. Since optos are current devices and require a
relative large amount of current with less than unity gain, they
exhibit a level of noise immunity that a high impedence input does
not.

In most of the buffered breakout boards the outputs are normally
buffered and conditioned while the inputs are opto isolated to break
the common ground between the machine sensors and the PC.