Sorry - been away from the computer for a while. For starters, I would simply wire the SON to whatever its "let the drive run" state (either open or shorted to COM), and leave the servo ready signal unconnected. Once you get the system working, you can see if the servo ready can be tied into the PMDX-126's ERR input. And/or look at having the PMDX-126's "charge pump OK" signal drive the SON input to enable the servos.