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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Types of Powered Devices
In Fig. 2.1, we had shown a very simplified PD. We now expand on that. At the top of Fig. 2.12, we have the most common implementation of a PD interface. We learned that to support Alt-A MDI and Alt-A MDI-X, a bridge rectifier is needed on the data pairs. However, to support Alt-B we actually only need a single diode on the spare pairs, since pins 4 and 5 are always of positive polarity with respect to 7 and 8, as per IEEE requirements. Nevertheless, to keep diode drops and reverse leakages the same on both the data and spare pairs, and to thereby avoid strange incompatibility/interoperability issues, most commercial PDs have identical diode bridges on both sets of pairs, as shown in Fig. 2.12.
FIGURE 2.12   Typical ...

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