RE: Re: Parallel Port Interface

Excellent suggestion Doug, thanks.

I think I'm going to abandon the opto idea in favour of this one.

My drives are L297/L298 based on the suggested design from SGS for the chips
and there is no optoisolation on the boards. I think the NAND gate will do
nicely, though.

Thanks again,
Gary

> Date: Sat, 03 Nov 2001 11:41:27 -0700
> From: Doug Fortune <pentam@...>
> Subject: Re: Re: Parallel Port Interface
>
> currinh@... wrote:
> >
> > For what it is worth, here's what I've done.
> >
> > I designed and built the opto-isolation circuit (quite
> > an accomplishment for me) and this worked well. However,
> > it inverted the signals and EMC did not like this.
> >
>
> I think I suggested this idea before (using single ended signals,
> buffering them to higher strength, and using a Schmitt trigger
> type device (with built in hysteresis):
>
> Using a 74LS132 quad 2-input positive NAND Schmitt trigger TTL
> chip (under a buck), you can use it this way:
>
> As it is a NAND, if you hook one input up to GND and the other
> input to your signal (and I am presuming this chip is right next
> to the encoder, for buffering the signal down to your driver/PS
> box), then you get:
> - the buffering
> - Schmitt trigger hysteresis (good for noise reduction)
> - the signal is NOT INVERTED.
>
> However, if you do want the signal inverted (to fix the problem
> above where EMC, or some other program, doesn't like it some
> particular way) hook one input up to +5, and the other input to
> your signal, then you get:
> - the buffering
> - Schmitt trigger functionality
> - the signal is INVERTED.
>
> In fact, since the chip has 4 NAND circuits, you can hook a
> circuit up both ways when you originally solder it together,
> and select what you want (Inverted or Not Inverted) to suit
> the circumstances.
>
> Doug Fortune
> http://www.cncKITS.com