PLANET GSD-804P - Manual (Page 17)

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Figure 1 - Power Supplied over the Spare Pins
The data pairs are used. Since Ethernet pairs are transformer 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.
Figure 2 - Power Supplied over the Data Pins