Planet FSP-804P User Manual page 20

Powered Device
A powered device (PD) is a device powered by a PSE and thus consumes energy.
Examples include wireless access points, IP phones, and IP cameras. Many PDs
have an auxiliary power connector for an optional, external power supply.
Depending on the PD design, some, none, or all power can be supplied from the
auxiliary port, with the auxiliary port sometimes acting as backup power in case of
PoE supplied power failure.
How Power is Transferred through Cable
A standard Cat5 Ethernet cable has four twisted pairs, but only two of these are
used for 10BASE-T and 100BASE-TX. The specification allows two options for using
these cables for power, shown in Figure 1 and Figure 2:
The spare pairs are used. Figure 1 shows the pair on pins 4 and 5 connected
together and forming the positive supply, and the pair on pins 7 and 8 connected
and forming the negative supply. (In fact, a late change to the spec allows either
polarity to be used).
POWER SOURCING
EQUIPMENT (PSE)
+
-
48V
20
4
5
SPARE PAIR
1
TX
2
SIGNAL PAIR
3
RX
6
SIGNAL PAIR
7
8
SPARE PAIR
Figure 1: Power Supplied over Spare Pins
POWERED DEVICE
(PD)
4
5
1
RX
2
DC / DC
3
Converter
Converter
TX
6
7
8