I wanted this thing to be totally self-contained. No power packs, no external cords, etc... The only power I have available inside the machine at the interconnection point is the +5vdc logic supply, and the parallel port can't provide any power of it's own. (Why didn't they just include one 5V pin???) Since the power from inside the machine is already DC, the suggestion above won't work.
I really don't need much current at all. Just enough so that the PC sees a logic high.
Some of the DC-DC converters are isolated. Looks like they chop up the input DC, effectively turning it into AC, and run it across a transformer, and filter it back to DC again
I'm just looking for a completely isolated connection. Input and output. Would i be safe with just a direct connection? Probably, but i'd rather be safe, and account for every possible mode of operation and failure, than have a fried pc.