Re: ParPort output low - hi-Z or ground? opto to parallel

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <info@t...> wrote:

>
> >
> > I am looking at an external power supply thru a resostor on the
> > isolated side of the opto. I can drive the LED with 10-15mA and
> > attain isolation from the opto as the power supply and resistors
> and
> > LED side of the optos would be on a totally seperate circuit.
> >
> > On the parallel port side, It would appear that a non-buffered
> > connection would have the port pins (10-13) connected to the opto
> and
> > the other side to ground.
> >
> > A state change of the inputs would result in a state change of

the

> > outputs.
> >
> > Would a pull-up or pull-down on the parallel port side offer any
> > benifits ? and which would be best for the parallel port side of
> > things ?
> >
> > Dave
>
> I am assuming your reference to the isolated side is to the
> transistor side? If you are using this as an isolated input to the
> parallel port then the transistor side is connected to the parallel
> port pins. The emitter (led) side needs to be driven by a separate
> isolated power supply which can be any voltage as long as the

series

> current limit resistor is sized to set current to the correct
> amount.

I see the opto as a base side that has power and an LED and is driven
like any LED. Typically 10-15 volts.


The transistor side has a collector and an emitter.

Using a seperate power supply to drive the base, then all that is
left is the transistor side.

> On the transistor side you would connect the emitter to PC ground

and

> the collector to the parallel port pin input pin. A pullup

resistor

> of 4.7K to +5 (+3.3) would make sure that the pin goes high when

the

> transistor is off but may not be needed if there are already

pullups

> in place. Hook it up without the pullup and monitor the voltage on
> on the collector with no signal on the opto led. If it has a solid
> +4 or greater then you don't need a pullup.

That is the question, or should I say answer I was looking for. Test
for the need, and use if needed.

thanks !


Dave

> I would not trust a parallel port pin to SOURCE 10 - 15ma. The
> parallel port pins on a lot of PC mother boards are NMOS or CMOS

and

> are designed to be signal lines into a printer (high impedence). I
> learned the lesson the hard way when I switched to a newer computer
> on my engraving machine and it started doing strange and wonderful
> things. In the PC world parallel port cables are supposed to be

kept

> to under 8' (I know, you can get more but its risky).
>
> While I agree with Jon that running the led side with +5 on the

anode

> and +3.3 on the cathode side will work, that leaves about a

milliamp

> or so of "bias" current flowing when the opto is supposed to be

off.

> That's below the turnon threshold but it takes less noise to cause

a

> problem. For maximum noise immunity I would either run the led

side

> from the same voltage level as the parallel port or buffer the pin
> output and shift the voltage level up to the +5 using open

collector

> buffers.