National Instruments Data Acquisition Device NI USB-621x - +5 V Power; +5 V Power as an Output; +5 V Power as an Input; USB Device Fuse Replacement

Chapter 3 Connector and LED Information
NI USB-621x User Manual 3-3 ni.com
+5 V Power
The +5 V terminals on the I/O connector can be use as either an output or
an input. Both terminals are internally connected on the USB-621x.
+5 V Power as an Output
Because the USB-621x devices are bus powered, there is a 50 mA limit on
the total current that can be drawn from the +5 V terminals and the digital
outputs. The USB-621x monitors the total current and drops the voltage
on all of the digital outputs and the +5 V terminals if the 50 mA limit is
exceeded.
+5 V Power as an Input
If you have high current loads for the digital outputs to drive, you can
exceed the 50 mA internal limit by connecting an external +5 V power
source to the +5 V terminals. These terminals are protected against
undervoltage and overvoltage, and they have a fuse to protect them from
short circuit conditions
1
. If your USB-621x device has more than one +5 V
terminal, you can connect the external power supply to one terminal and
use the other as a power source.
USB Device Fuse Replacement
(USB-621x Mass Termination Devices) USB-621x Mass Termination devices
have a replaceable 0.75A,125V fuse (Littelfuse part number 0453.750) that
protects the device from overcurrent through the +5 V terminal(s).
To replace a broken fuse in the USB-621x Mass Termination, complete the
following steps while referring to Figure 3-1.
1. Remove the USB cable and any I/O signal wires from the device.
2. Remove the four Phillips screws on the bottom of the device to remove
the device top. You may have to remove the rubber feet.
1
USB-621x Screw Terminal devices have a 350 mA self-resetting fuse. USB-621x Mass Termination devices have a 750 mA
user-replaceable socketed fuse.