connecting stepper driver to PC

Hello everyone,

I have bought two 2.5A stepper motor drivers to power the slides on my CNC lathe but it is not clear to me how to connect them to my PC.

The drivers need connecting to two output pins on the the parllel port for step and direction and a 5v connection to power the opto-isolator. This sounds fine except that the driver has no connection on the control side for the ground. I understand that the input and output shouldn't use the same ground pin as this would make the opto-isolator useless.

I plan on getting 5v from one of the PC's USB ports (or directly from the power supply). Where should I connect the ground of this, and where should I connect the parallel port's ground? Should I connect them together or do they have a common ground inside the PC?

I would greatly appreciate any help with this as I can't progress until I have it clear in my head what I need to do. I also don't want to risk damaging the drivers and/or motors or the PC.

I have looked at other stepper drivers since I bought the ones I have and every one I have seen has a connection for the ground on the input side.

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).

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

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.