Designing IEEE 802.3af Powered DevicesSponsored by: NATIONAL SEMICONDUCTOR

Power ever Ethernet (PoE), or IEEE Standard 802.3af, defines a method for powering Ethernetconnected "powered devices" (PDs) over the same cable that's used for data. The actual circuitry that takes the power off the cable and delivers it to the power converter for servicing the load is called the power interface (PI). When the standard was conceived in 1999, the most common PD envisioned was an enterprise telephone set based on Voice over Internet Protocol (VoIP). Since then, and particularly after the standard's approval in June 2003, a host of novel PD applications has been developed to take advantage of the marriage between data and power.

Picking Off The Power The RJ-45 connector is the business end of an Ethernet cable in the photo on the right. PoE uses the data pairs or the spare pairs in the cable to carry -48 V dc from an endpoint switch or midspan hub to the PD. On the data pairs, pins 3 and 6 supply one side of the dc, and pins 1 and 2 supply the other. On the spare pairs, pins 4 and 5 are paralleled for one side of the dc supply, and pins 7 and 8 are paralleled for the other side.Spare Pairs And Data Pairs The figure to the right shows the normal (top) and crossover (bottom) wire-color codes in an Ethernet cable. Note that a crossover does not affect which pins in the RJ-45 connector are active, but it flips the sense of the dc voltage. The PoE standard specifies a diode bridge in the PD to normalize either cable configuration. The photo of the connector above shows the end of a crossover cable that has the bottom configuration. (Tracer colors on the white wires in each pair in the photo are less conspicuously barber-poled than they are in the drawing.)

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