So I have never used the parallel port in Mach3 since I have always used a SmoothStepper. My *guess* is that if you use a 1 us wide pulse, they will come out 5x more often that a 5 us wide pulse would. The higher number of pulses would make the system more responsive, but you might miss a pulse easier. Again, I am guessing how they are doing it in the parallel port version since we just ignore those parameters in motor tuning.