Hmm, weird. It looks like a faulty Intelligent Motion Systems driver, it's a model IM805. The direction signal acts by dropping the line low, pulling current through an optoisolator onboard.
The series limiting resistors on each line step and direct lines, on the driver's pcb, should be identical, but that one for the step measures 384 ohms, while the other for direction measures 2136 ohms. That resistor is likely partially open, which would make sense. Not sure if the optoisolator is also bad, will know soon enough.