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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Vendors Cheating on Y-caps—to Our Advantage
We will now explain how some magjack vendors cheat a little here. We know, or at least assume, that each port should have one Y-cap. But in configurations like 2 × 4, some vendors take the top and bottom ports of this stacked configuration and have all their eight twisted pairs share a single Y-cap. What is worse, their datasheet makes no mention of this.
You will need to cut into the housing to catch this. Or do an LCR measurement as explained previously and you will get half the net capacitance you were expecting. This sharing can cause unwanted interactions between top and bottom RJ-45s and is not considered a very good idea for that reason. But because the net capacitance is significantly reduced, ...

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