Parallel Port to Power Supply Grounding Differential

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?
Thanks

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

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