It is OK to connect your PMDX-424 to the computer and run Mach4 without your motor drivers or stepper motors attached.
Step pulses will still be generated and your DROs will react as if the motors were present because this is an "open loop" system.
You should leave the red jumper clip installed on the E-Stop input or replace it with a normally closed switch because the sample profiles expect the switch to be connected.
