I'm at a loss as to why this is happening.

With the Mach3 Reset button pressed (solid green) i.e. not in reset or emergency mode, the steppers are silent unless I command them to move, everything normal and as it should be.

However if i press the Mach3 Reset button (flashing green/red), the steppers intermittently shudder, when I say shudder, they don't actually move, however I can hear the coils being de-energized/energized momentarily, and I hear a slight ping from the motors. If I place my hand on the motor I can feel it, perhaps every second or two.

I scoped the direction and step signals at the parallel port and I don't see anything that looks like it could trigger the drive. Even with the scope set to single trigger mode and a threshold of about 1 volt. When in run mode with the steppers moving, nice clean 5V square waves, so the scope and triggering are working.

Unless there is an actual E-Stop (not a press of the reset button on the Mach screen), the enable signal to the drives is active.

Anyone experience something like this?

I figured it out. The drive enable signal works opposite to the step and direction signals, and not as the drive documentation indicates, you have to actually apply voltage to the enable opto to disable it. I guess they prefer the fail-unsafe mode. Who designs equipment like this?

Well after determining the drive enable documentation is incorrect. I started to wonder why is my enable signal is dropping out intermittently, while in reset mode.

One of the input parameters to my enable signal is the mach3 "charge pump" signal. I use this to drive a monostable (one-shot). So I scoped my enable signal on one channel, and the Mach3 "charge pump" signal on the other channel, and sure enough every once in a while the charge pump skips a beat, instead of being 12.5kHz, it drops to 6.25kHz for one cycle, that's enough to trigger my enable circuit (my threshold is set to 9kHz).

This problem seems to be related to the Pokeys 57E that I'm using for additional I/O, because if I disable the 57E, Mach seems to be able to keep up and not skip a beat while in reset........... I'm going to have to dig a bit deeper.