I believe the PMDX plug-in supports only one probe and it must be mapped to the input signal Mach4 calls "Probe" (and G31), not any of the "ProbeX" where "X" is a number (and the corresponding G31.X) . See the PMDX plug-in release notes section 9, "Probing Features and Restrictions". I'm pretty sure the probe can be connected to any input pin on whichever SmartBOB you have, but it must be an input on the SmartBOB. It cannot be on any other board. That is because the probing motion happens IN the SmartBOB, not in Mach4, and the SmartBOB can only see signals physically attached to it.