Re: Optocoulped Parallel port

stratton@... wrote:

> From: stratton@...
>
> > You don't use the pullup resistor to power the optocoupler. You use
>
> > either the PC
> > power supply or an external +5V power supply, through a resistor,
> > through the
> > LED or optocoupler, and finally to the port output, which will light
> the
> > LED when
> > the output is at logic ZERO, or grounded. More current is available
>
> > that way.
> > This all applies to output from the PC to external devices.
>
> I would agree, however if you are trying to plug into the parallel
> port and the parallel port only, and not have an external power
> supply, then you have to use the port as a current source rather than
> sink. Which is marginal, but tends to work.

Really, without putting a booster transistor on it, or using a logic
gate, I wouldn't expect
it to work on a lot of systems, especially on the laptops. I have
several devices that
I designed that attach to parallel ports, and I had to use pullup
resistors powered
by an external power source to get them to work on all computers
reliably.

On the CNC setup, I happened to have a Servo-to-Go card that provides a
protected +5V output for this and other purposes, so i used that. With
the
+5V from the STG card to the plus side of the opto-22 compatible relay
modules, and the minus side connected to the parallel port, it works
fine.
(I actually have the +5V for the opto relays go through a transistor
that
shuts off the +5 if the E-stop is activated. This is a totally hardware
safety
that shuts off the servo amps, spindle drive and coolant if there is an
E-stop condition.)

Jon