connecting stepper driver to PC

[BillTodd]

You don't connect the ground together, that's the whole point of opto-isolation.
(you should still connect the earths to the PC and lathe at their plugs)

The current flows out of the 5v from the PC's USB (or keyboard connector ) through the LED in the opto-isolator then back into the PC via the printer port (the printer port driver device or breakout board is happy to 'sink' the 10-20mA required to light the LED).

[berkut]

Thank you very much for your reply Bill. I think that I now understand how it all works.

What did you mean by "you should still connect the earths to the PC and lathe at their plugs"?

I was concerned about whether I could control such a driver properly since I think that my parallel port is only 3.3V but, in this case, it doesn't matter because the parallel port is just acting as a switch to ground.