I think irving2008 has hit the nail on the head. My guess is some sellers are not differentiating betwixt modules for PWM of DC motors and what is required for steppers. Bang goes any warranty, I guess I'll be snipping pins 1 & 15. Close inspection shows some modules of this type have the pins 1 & 15 brought out to Berg links. If anyone else buys one of these I'd advise them to check that little bit more carefully than I did.

Ho hum!

I like Jonathon's DAC idea.