It is a bit early to be asking most of these questions.
They are realistic questions for a mature high end product.
The PMDX-422 is not a high end product, and it was released only a week ago.
Currently our expectations are:
1) Probing will be supported. It is not ready yet, but will be a free firmware
upgrade when is becomes available. Upgrades are downloadable from our
web site and installed using a dialog box on our plug-in configuration page.
2) Not quite sure what you mean by protected mode probing. The PMDX-422
will stop motion when the probe is triggered. It does not depend on Mach4
to read the signal and then request a stop.
3) Spindle encoder sync will be possible in the future. This will come for lathe
threading first, then we can consider rigid tapping. The Mach4 G-code manual
for milling lists a G84.2 for rigid tapping, but I am not sure if Mach4 actually
addresses this yet.
4) The PMDX-422 will not home to an encoder index signal. There simply is not
enough I/O available on the PMDX-422 to provide for all the signals needed. Our
forthcoming high end product, the PMDX-426, will have industrial features including
encoder assisted homing. Price and schedule for the PMDX-426 are still to be determined.
Try reading the Quick Start Guide for the PMDX-422 here:
http://www.pmdx.com/Doc/PMDX-422_Quick_Start_Rev0_2.pdfWhile it is still preliminary, it will answer a lot of questions about the board.