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