I began work on this project again today after having set it aside for about a year.

My first attempt to drive a stepper motor wasn't successful. After checking the wiring to make sure the signals from the Propeller were reaching the drive electronics, I then monitored the signal while the program was running but without connecting the Propeller to the drive electronics. While disconnected, the step signals from the Propeller were a nice 0V to 3.3V pulse. When the signal was connected to the drive electronics I found the signal line no longer produced a nice consistent pulse. I'm pretty sure the optocouplers can't be driven with a 3.3V signal.

I'll need to use some sort of level shifter to boost the 3.3V signal from the Propeller to a 5V output the drive electronics can use.