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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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AC Disconnect and DC Disconnect
So far, we have been implicitly assuming what is called DC disconnect. This involves the PSE taking a simply voltage-current measurement to determine if there is a valid PD on the other side or not.
This manner of implementation, which really was just Ohm’s law, unfortunately ran into intellectual property (IP) battles however strange that may sound. So an alternative method came into being, one that involved injecting a low-frequency AC wave on the line. This depends on the fact that when a PD is connected to a port, the AC impedance measured on its input terminals is significantly lower than in the case of an open port (disconnected PD). AC disconnect depends on the PSE being able to “find” the capacitor on ...

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