Re: Parallel Port Interface / correction!

Thanks Mariss,

One mystery down, many more to go...

Gary

--- In CAD_CAM_EDM_DRO@y..., mariss92705@y... wrote:
> Hi,
>
> A schmidt trigger device (an inverter for instace) might have a
> logic "1" trip level at 4V and a logic "0" trip at 1V. This is
> entirly diffrent from "normal" logic that may have a threshold of
> 2.5V.
>
> Think of it this way. If you have a signal with +/-.5V noise riding
> on it and you barely get above 2.5V, then the noise could take you
> thru the thresold several times, giving false steps. For the same
> reason it "squares up" slow rising or falling inputs.
>
> On a schmidt triggered device noise is less of an issue since it
> would have to drop the signal from 4V to 1V before it would be
> noticed.
>
> By analogy a schmidt trigerring is kind of like a microswitch. It
> takes a certain pressure to trip the switch and a lot less pressure
> to release it. That is to say, you can't "tease" the switch. Either
> it is closed or it is open.
>
> On opto-isolators: Never drive an opto with an opto. Optos have
very
> leisurely rise and fall times, meaning any noise on the line would
> have ample time to take the signal thru the threshold several times
> before signal would finally settle at a "1" or a "0".
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., Gary.Rose@c... wrote:
> > Doug,
> >
> > Can you explain a little why the SCHMITT trigger is significant
> (like
> > what is it)?
> >
> > TIA
> > Gary
> >
> > --- In CAD_CAM_EDM_DRO@y..., Doug Fortune <pentam@h...> wrote:
> > > Correction! Not a NAND (lost my mind apparently),
> > > we need an EXCLUSIVE-OR gate as Alan suggests.
> > >
> > > But the '136 is not a SCHMITT trigger either..
> > >
> > > So here is an alternative (I can't remember the
> > > chip I used last time, can't find my book...),
> > > so the suggestion instead is a:
> > >
> > > Non-Inverted:
> > > - 74'341 or 74'344 octal buffer/line driver with Schmitt
> > > trigger
> > >
> > > Inverting:
> > > - 74'340 octal buffer/line driver with Schmitt trigger
> > >
> > > Both have the same pinouts, so if you use a socket, you
> > > can just switch chips to change between inverting and non-
> > inverting.
> > > All three devices are 3 state, but we will just enable them
> > > permanently (using pin1 and pin19).
> > >
> > > Bonus points to Alan for keeping me honest....