In testing my parallel port opto-islolation board, I discovered that it only drops to around 2.6v for a 0. I invert each signal to energize the reverse phase of each stepper winding but that signal is a solid 0 and 5 volts, after coming through the not gate.
I'm thinking of using a second hex inverter to double invert the first one's signal back to original to get a crisply delieneated digital output, for both phases.
How well will this method protect my computer's parallel port?

 

You need to post your schematic, indicating where the problem area is.