I am not sure what you mean by "all my Power supply GND terminals".
Power supplies typically have both a negative output terminal and a
safety ground. The safety ground is often called Earth and it may be
a bare screw that seats into the enclosure, but sometimes it is on
a terminal strip.
The negative terminal of the power supply that powers you stepper
drivers is often connected to the chassis and safety ground from your
mains power cord. This is workable because most stepper drivers have
opto-isolated inputs. If the drivers do not have isolated inputs, you
may find that things work better if you do not connect the negative
power input to safety ground.
The GND terminals of the PMDX-422 are not a safety ground and
should not normally be connected to the safety ground, including
cases where the stepper drivers do not have opto-isolated inputs.
This is why we do not recommend that you connect the negative
side of the stepper motor power supply to the chassis or safety
ground when the driver does not have isolated inputs.
If you use an external 5 volt power supply for the PMDX-422 you
should connect this power supply's safety ground to your chassis
and mains safety ground, but do not connect the negative terminal
to the chassis or safety ground. The negative terminal of the power
supply should connect to the GND terminal of J9 on the PMDX-422.
With all this being said, an external power supply is not normally
needed. When you do use an external power supply, resetting the
PMDX-422 will require removing the USB connection AND turning
off this power supply.
The primary concern is avoiding ground loops. Desktop computers
and some laptops internally connect the negative side of their
power system to the mains safety ground. This means that the
ground in the USB cable is connected to the mains safety ground
and there is little you can do about it. A ground loop can happen
when the USB ground and therefore the GND of the PMDX-422
get connected to the safety ground at the machine. This will
result in currents flowing through the USB cable's GND connection
and shield. These currents contain a lot of noise and can result
in communications problems for devices like the PMDX-422 that
do not have isolation built into their USB ports.
In summary, unless you have a good reason to use an external
power supply, do not use one, and make sure that the GND of
the PMDX-422 is not connected to the safety ground at your
machine.
