PLANET FSD-808P - Manual (Page 17)

17
How is Power Transferred Through the Cable
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these are
used for 10BASE-T and 100BASE-T. The specification allows two options for using
these cables for power as shown in Figure 2 and Figure 3:
The spare pairs are used. Figure 2 shows the pair on pins 4 and 5 connected
together and forming the positive / negative supply, and the pair on pins 7 and 8
connected and forming the positive / negative supply. (In fact, a late change to the
spec allows either polarity to be used)
POWER SOURCING
EQUIPMENT (PSE) POWERED DEVICE
(PD)
SPARE PAIR
SPARE PAIR
SIGNAL PAIR
SIGNAL PAIR
Converter
DC / DC
RX
TX
TX
RX
48V
4
5
1
2
3
6
7
8
4
5
1
2
3
6
7
8
Converter
+ / -
+ / -
Figure 1 - Power Supplied over the Spare Pins
The data pairs are used. Since Ethernet pairs are transformers coupled at each
end, it is possible to apply DC power to the center tap of the isolation transformer
without upsetting the data transfer. In this mode of operation the pair on pins 3
and 6 and the pair on pins 1 and 2 can be of either polarity.
POWER SOURCING
EQUIPMENT (PSE) POWERED DEVICE
(PD)
SPARE PAIR
SPARE PAIR
SIGNAL PAIR
SIGNAL PAIR
DC / DC
RX
TX
TX
RX
48V
4
5
1
2
3
6
7
8
4
5
1
2
3
6
7
8
+ / -
+ / - Converter
Figure 2 - Power Supplied over the Data Pins