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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Predetection/Open-Circuit Detection/Initialization
In Fig. 3.3, Region A is something we have avoided discussing so far. We now explain this a bit more.
For added safety, we want to make sure that there is something like a signature resistor on the other side of the cable, before even attempting to measure it accurately. In particular, we want to guard against short circuits or open circuits. The former could be anything less than 200 Ω (representing the two 75-Ω series-termination resistors of a NIC). The latter would be by IEEE definition anything above 500 k. The simplest way to check for the former is to put an exploratory current source out on the cable and make sure the voltage exceeds a certain low threshold after a certain settling ...

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