Re: [CAD_CAM_EDM_DRO] Re: Opto / relay recommendation

In a message dated 31-May-02 17:27:16 Central Daylight Time, rab@...
writes:

> So if I'm wiring up one of these anyway, is it worth putting all of the
> parallel port I/O pins that I'm using through a ULN2003 ?

Would > it be ok handling step/direction signals ? Is there a practical

> limit to the frequency of step/direction signals it can handle ?

> Would it help to protect the parallel port on the computer ?

Rab: Forgive my intrustion into this question you submit to Steve, but I
have used dozens of ULN2003 and ULN2004, and I see a misunderstanding you
seem to have, and I THINK I can help:

First question: The ULN2003 is a "driver" for switching loads heavier than
"logic gates". Such as an electromechanical relay that requires 12 VDC at
100 mA. It is a "current SINK Darlington" (seven of 'em in each 16-pin DIP),
so it acts as an INVERTER. That is, the "polarity" of the ULN's output is
the OPPOSITE of the PC's port connected to it. Unless you had added another
inverter (74HC04, etc.) before the ULN...

Second question: The Gecko drives have opto-isolators as inputs. That is,
each input-pin "lights up" a LED in a little device inside that photo-couples
that to a "photo-transistor" that then signals the Gecko circuitry. This
"hassle" electrically-isolates the inputs from EMI noise from the nasty ol'
outside world. It is WONDERFUL in doing that! BUT, for every advantage,
there is a DISadvantage: These inputs measure 16 mA load with 5 VDC "common"
(as specified). That is a bit much for the printer-port. OH, it'd probably
"work", but only barely. Better: Use something like the ULN2003 to "buffer"
the signal to the Gecko inputs. (or, probably better, just a "dedicated"
74HC gate would do fine, so long as that gate would not be expected to drive
some other "logic input", as the opto-isolator load "pulls it down" a bit).
BUT, the ULN2003 alone INVERTS the logic-sense of the parallel-port output,
so you WOULD need an inverting buffer, before the ULN... I have used
74HC540 (EIGHT inverters in ONE package!) when needing an "inverted"
printer-port, or the 74HC541 when I just want "isolation", as this is a
NON-inverting buffer. It separates the parallel-port a bit from the outside
world.

Further to this second question, the ULN will "pull DOWN" only. That is, it
will NOT "pull UP", so it is NOT a good candidate for routing the "step" and
"direction" outputs to an ordinary logic-gate type of input. HOWEVER, the
Gecko inputs, as said, are just LED's, and if you "jumper" those per the
instructions for -ve going inputs, and use a +5VDC input-common, this will
work. And, though not a "high-speed device", I would think the ULN2003 could
be expected to function OK on up into the HUNDREDS of kilohertz. MORE than
fast enough for "stepper stuff".

Finally, your third question: Yes, but as said, it is not exactly ideal for
this. Do what I said two ¶'s above: Use either the '540 or '541 octal
buffers, and the ULN2003 after those if you need to switch a relay-coil or
light some LED-array, etc.

Hope I have not been TOO confusing! Lemme know! Jan Rowland


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