Re: [CAD_CAM_EDM_DRO] Parallel Port to Power Supply Grounding Differential

Thanks for the clarification... I am also planning to use Mach 1 or similar
app, and will need to interface the limit and other safety circuits, which
makes the necessity to connect to an outside ground a greater possibility.
Bear with me while I go over my setup.....I am using (2) toroid transformer
power supplies with secondaries completely ground isolated from the AC
supply, and currently the DC negatives are chassis connected. At present the
AC chassis ground is not connected to the case (although likely should be
eventually for safety). I took a look at the parallel port grounds, and
they are tied in common with the PC chassis ground, at the backplane metal
connector.
If I were to install a separate permanent cabled ground, say a stranded
#14AWG, between the PC and the DC supply ground, and ensure it was always
connected before powering either device, wouldn't that take care of the
problem? That is, if I ensured I am powering from the same AC socket, can't
I reasonably expect the grounds to be at the same potential, although there
will possibly be some ground current through the ground connection?
Additionally I can use a laptop possibly as a controller, which is fed from
an isolated wall cube of course, so isn't that similarly approached?

If I really need a breakout board I 'll get one, but I want to be sure why I
am doing this and what other corrections can be made to correct the
potential problem. Thoughts?


R. William Kichman, P.E.
Kichman Engineering Associates
103 Old Furnace Road
Cornwall, PA 17016-0643
tel/fax 717/270-0714

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, July 08, 2003 1:06 AM
Subject: Re: [CAD_CAM_EDM_DRO] Parallel Port to Power Supply Grounding
Differential


>
>
> Bill Kichman wrote:
>
> >Just wondering how other folks are ensuring they don't blow out their
parallel ports via voltage differentials between the parallel port's #18-25
ground pins and the chassis ground at the power supply case? I am using
Gecko 320's and a spare PC case housing a transformer fed linear 75V power
supply (minus grounded to case) and (3) servo drives with rotary encoders. I
measured the voltage differential between the power supply grounded case and
the parallel port cable's grounds and measured about 3V. I guess I can
manage to keep these grounds separate, since the Gecko uses opto-isolated
interfaces, but should the (2) get inadvertently connected, I wouldn't want
to "release the smoke genie". I read somewhere that somebody sells
optoisolated breakout boards, but the (2) I could find in my search were not
isolated. What are my options?
> >
> >
> The Gecko drives all use opto-couplers to isolate their inputs from the
> motor power
> supply. The proper wiring is to use the +5 V from the computer (either
> game port
> or a disk drive connector) on Gecko drives set up for a positive common
> terminal
> for the optos. There should be no connection to the grounds of the
> parallel port.
>
> If you have the Gecko drives that can only work with a negative common
> terminal,
> then that common should be connected to the parallel port's ground. You
> will
> likely need pull-up resistors from the step and direction signals to a
> +5 V derived
> from the computer as above.
>
> You should not make any other connections between the parallel port and
> the Gecko
> drives. You can ground the DC - on the Geckos to the power supply case,
> and have
> that ground connected through the safety ground pins of the wall socket.
> This
> single-point ground avoids ground loops as much as possible. Since the
opto
> isolators withstand at least 1000 Volts, the 3 V offset you measured
> should be
> of no concern. If you use a shielded cable between the computer and the
> Gecko
> box, the current will flow ONLY through the CASES of the equipment, not
the
> circuit boards (assuming things are built right). This will not harm
> the equipment
> in general, but could cause problems when cables are connected and
> disconnected.
> It would be best practice to have the computer and the Gecko box plugged
> into the
> same outlet to avoid ground loops in this case.
>
> Jon
>
>
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