Chapter 3
Connector and LED Information
3-3
ni.com
+5 V Power
The +5 V terminals on the I/O connector can be
u
se as either an o
u
tp
u
t or
an inp
u
t. Both terminals are internally connected on the USB-621
x
.
+5 V Power as an Output
Beca
u
se the USB-621
x
devices are b
u
s powered, there is a 50 mA limit on
the
total
c
u
rrent that can be drawn from the +5 V terminals and the digital
o
u
tp
u
ts. The USB-621
x
monitors the total c
u
rrent and drops the voltage
on all of the digital o
u
tp
u
ts and the +5 V terminals if the 50 mA limit is
exceeded.
+5 V Power as an Input
If yo
u
have high c
u
rrent loads for the digital o
u
tp
u
ts to drive, yo
u
can
exceed the 50 mA internal limit by connecting an ex5 V power
so
u
rce to the +5 V terminals. These terminals are protected against
u
ndervoltage and overvoltage, and they have a f
u
se to protect them from
short circ
u
it conditions
1
. If yo
u
r USB-621
x
device has more than one +5 V
terminal, yo
u
can connect the external power s
u
pply to one terminal and
u
se the other as a power so
u
rce.
USB Device Fuse Replacement
(USB-621
x
Mass Termination Devices)
USB-621
x
Mass Termination devices
have a replaceable 0.75A,125V f
u
se (Littelf
u
se part n
u
mber 0453.750) that
protects the device from overc
u
rrent thro
u
gh the +5 V terminal(s).
To replace a broken f
u
se in the USB-621
x
Mass Termination, complete the
following steps while referring to Fig
u
re 3-1.
1.
Remove the USB cable and any I/O signal wires from the device.
2.
Remove the fo
u
r Phillips screws on the bottom of the device to remove
the device top. Yo
u
may have to remove the r
u
bber feet.
1
USB-621
x
Screw Terminal devices have a 350 mA self-resetting f
u
se. USB-621
x
Mass Termination devices have a 750 mA
u
ser-replaceable socketed f
u
se.
