Well, as soon as you tie 2 commons from different supplies together, they are no longer isolated. Period. But whether that is a good thing, bad thing, or not a thing at all depends.
If you have a PMDX-126 and a PMDX-107, here are the sets of isolated connections:
- PMDX-126 parallel ports, step/dir signals on J1 to J4 and output signals on J5 and J6. These all share the same ground connection (we call it "PCgnd" for "PC side ground) that is isolated from the other groups listed below.
- PMDX-126 30Amp relay contacts on J9, isolated from everything else
- PMDX-126 20 Amp relay contacts on J10, isolated from everything else
- PMDX-126 Input signals on J11 and J12 and the Faullt and EStop inputs on J13. These all share the same ground that is isolated from the "PCgnd".
- PMDX-126 "RST", "COM" "ERR+" and "ERR-" on J13 are isolated from all other signals.
- PMDX-107 "Aref", "Aout" and "Agnd". These are isolated from all PMDX-126 signals and also isolated from the "Rev/Dir", "Fwd/Run" and "COM" terminals.
- PMDX-107 "Rev/Dir", "Fwd/Run" and "COM" terminals are isolated from all PMDX-126 signals and also from the PMDX-107's "Aref", "Aout" and "Agnd" terminals.
The starting point for isolation is to keep all motor and spindle power supplies, signals and grounds away from the "PCgnd" and related signals on the PMDX-126.
For those who are running a VFD on their spindle (I know you are not), keep the VFD's common or ground that is used with the 10V speed control input only connected to the PMDX-107's "Agnd" terminal. Often, that common terminal in VFDs is ties directly to one of the AC lines. That could (would?) cause nasty things to happen if it got connected to PCgnd or some other signal.
Motor power should go to the motor drivers. And these drivers should (or usually do) have isolation on the step/dir inputs, which keeps the motor power system isolated from the PMDX-126.
Beyond that, I can't think of a hard & fast rule for what to connect and what to isolate. Generally isolation does two things: keeps"dangerous" voltages out of parts of the system and helps keep noise from high-current supplies (motor supplies and spindle supplies) from adversely.
Bob
