On Fri, Mar 26, 2010 at 5:21 PM, Slavko Kocjancic <[email protected]> wrote:
> Alexey Starikovskiy pravi:
> > As a matter of fact, I already see a big stopper in your board
> usefulness.
> > You put optical isolation between logic and parallel port, I see 2
> > drawbacks of it:
> > 1. parallel ports are quite weak these days (3.3v/~5mA), so driving LED
> > from it directly is not bullet proof.
> >
> All EPP ports are 5V tolerant and I didn't see the EPP port diferrent as
> open colector with pullups. LED are arranged to be driven low with
> external 5V supply. And optocoupler is happy with anithing from 5mA up
> to 20mA. So I see no problem.
>
If the parallel port is indeed that week, your optocouples might not recieve
the 5mA to switch. Especially, if your pre-opto-resistors are calculated for
5V.
But honestly, I would expect an parallel port to being able to drive an
optocoupler.
Other option would be some mosfet gate that then switches the couplers, but
then you have might want to avoid touching the connector pins with bare
hands.
Otherwise, nice project, personally I would have tried to go for a much
smaller board.
> > 2. on both sides you have logic levels _always_.
> >
> ???
> > 3. machine equipment on the other side of your logic has very different
> > set of voltages,
> > 12, 24 volts are common, and sometimes you could find 48, your board
> > does not allow to
> > connect this equipment without yet another layer of optical isolation.
> >
> I'm setup machines without problems on that. The board need 12V
> unregulated for relays only If you have no relay then anithing betwen 8V
> to 30V is good.
> > 4. now you have common ground between all your logic and all 48 inputs
> > and outputs
> > (and your schematics can be stretched to 160)... I doubt that you could
> > keep such ground
> > clear of the machine noise.
> >
> The bad design has big ground loop noise. If somewhere is that problem
> then just put normal optocoupler just here.
> > In general, your design seem to be very close to UBOB board here:
> > http://www.candcnc.com/UBOB.htm
> >
> Seems similar. But my board rely on EPP interface to not stall computer.
> One cycle LPT port take near 1uS! And I need only 1 per byte. UBOB needs
> at least 3. And price is way overpriced. All thing for my plate is
> under 25Eur. (with homemade pcb)
> > The problem with the above seems to be it's price, for about same money
> > its possible to
> > get much more capable board from MESA -- 7i43.
> >
> Yes MESA is good ... .. but without isolation at all!
> Any time I have problem with ground loop problem the relation PC <-->
> Machine is the worst one.
>
> Anyway I don't want to argue is it good or is it bad. Just want simple
> solution for simple benchtop machine.
>
> Slavko.
>
>
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