PSE/PD Interface Specifications

For the 10BASE-T and 100BASE-TX IEEE802.3af networks, IEEE802.3af defines Power Interfaces (PIs), which are interfaces between PSE/PD and network cables. Currently, it has defined two power supply modes, Alternative A (1, 2, 3, 6 signal wire pairs) and Alternative B (idle wire pairs 4, 5, 7, and 8). The following is a description of the two power supply modes:

Power supply through signal wire pairs (Alternative A)

As shown in the following figure, a PSE can supply power to a PD through signal wire pairs. Because DC and data frequency does not interfere with each other, electric current and data can be transmitted through the same wire pair. For electric cables, this is a kind of "multiplexing". Wires 1 and 2 are connected to form a positive (or negative) polarity, and wires 3 and 6 are connected to form a negative (or positive) polarity.

Figure 12-1 Alternative A Power Supply Mode with 10BASE-T and 100BASE-TX

Power supply through idle wire pairs (Alternative B)

As shown in the following figure, a PSE can supply power to a PD through idle wire pairs. Wires 4 and 5 are connected to form a positive polarity, and wires 7 and 8 are connected to form a negative polarity.

Figure 12-2 Alternative B Power Supply Mode with 10BASE-T and 100BASE-TX

According to IEEE802.3af, standard PDs must support both power supply through signal wire pairs and power supply through idle wire pairs, while PSEs need only support either of the two modes.