We haven't been checking the Mach4-related forums...
Can you please let us know which Mach4 plugin version you have been having problems with probing? We fix some early bugs a while ago and are eager to know whether anybody is still having issues with it.
* early PoKeys57CNC boards (v1.0 and v1.1) required an external pull-up resistor to be added to the probing input due to a hardware bug.
* pin 17 should be selected as probing input with invert option enabled. This should also work with simple contact-based probes.
There was a typing error - pin 19 should be used for probing
, as described in the manual and pinouts.