In order to draw +5 volt power from J9, the jumper at JP1 must
be in the "ON" position. This is not the best way to power an
accessory. This terminal is mainly intended for supplying power
TO the PMDX-422 from an external regulated +5V power source.
The alternate, and preferred, method of supplying +5 volt power
to an accessory from the USB cable is to use the +5S terminal
on J10.
If you must use the +5V power from J9 because switched power is
not acceptable, we recommend no more than 50 mA be drawn from
the +5V terminal of J9. As noted in the Quick Start document, the
actual voltage at this terminal may be as low as 4.5 volts, depending
on your USB host port. The PMDX-422 does not actively limit the current
that is available, but the host port may and this is why we recommend
not more than 50 mA of current. The USB specification allows no more
that 100 mA of unmanaged power total and the PMDX-422 itself needs
about 50 mA to start running.
Our firmware now negotiates with the USB host to allow more current
but there is no guarantee that the host will grant the request. Even if
the host grants the request, the extra current must not be drawn before
the host grants the request. Doing so may cause the port on the host
to reset.
The PMDX-422 offers +5 volts on another terminal, the +5S terminal
on J10 that offers "managed" power that is not switched on until the
host grants the request for extra power. This terminal will usually
have a voltage closer to +5 volts because the management switch
has a lower voltage drop than the protection diode. Up to 350 mA
may be drawn from +5S on J10 if the host USB port allows it.
Harming the protection diode that feeds the +5 power to J9 is very
unlikely because it is rated at 1 ampere. If you measure less than
4.5 volts at the +5V terminal of J9 when JP1 is in the "ON" position,
then there may be an issue with the PMDX-422 or the USB port of
your host computer.
Connecting the PMDX-422 to the host through a passive (non-powered)
hub is not recommended because all devices on the hub must share
the power available from the host USB port.
