Electronic – How to tell which alternative is used in PoE

poereverse-engineering

I need to disable the power over ethernet being output by a device to prevent any damage to routers and switches. After looking more into how PoE works, there appears to be two alternatives to how the power is sent over the wire.

Alternative A – Uses phantom power. This method involves sending power alongside the data signals since they are at such different frequencies.

Alternative B – Uses the unused wires of a 10/100 cable to send power.

I'm not certain as to whether the device is 10/100 or if it supports gigabit connections. If there is a way to discover the gigabit capabilities that would tell me that it cannot be alternative B. I don't have many details on the hardware, as it's part of a larger system.

Best Answer

I think this document helps Introduction Basic principles of PoE Power over Ethernet (Page 4)

Spare-pair power feed

Spare pair power feeds utilize wire pairs that are not otherwise used, the voltage being placed directly on the spare pairs 4/5 (+) and 7/8 (-). This method can be used only on networks with eight-wire cabling. This does not apply to Gigabit Ethernet, because here all eight wires are used for signal transmission and there are therefore no spares.

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Phantom power feed

A phantom power feed has the voltage coupled to wire pairs 1/2 ( ) and 3/6 (+). This method can be used for networks with four-wire or eight-wire cabling.

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So you can check with a voltmeter, if there is a voltage in pins 4/5 - 7/8 then the method used is Spare-pair power feed.

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