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As I understand IEEE PoE standards, the PSE detects PoE capability of the connected device (PD) by measuring resistance on certain pairs and then negotiates its PoE requirements.

Questions:

  1. Do passive PoE PSE perform the measurement before outputting power?
  2. Is there any PoE injector or midspan PSE that does so?
  3. Is there a risk of damaging equipment when connecting a non-PoE device down the line of passive PoE injector? Do Ethernet controllers include any protection against such damage? (thinking laptop, basic switch...)
Overnout
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1 Answers1

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The definition of "passive" PoE is that the power supply doesn't try to negotiate or determine anything. That is what differentiates it from "active" - i.e. standard - PoE.

So, to answer your numbered questions:

  1. No measurement is performed. Passive power injection is turned on or off in the settings of the device doing the injection.
  2. By definition, no. If it was doing that, it would be active, not passive.
  3. Yes, you can damage equipment that is not compatible. The manuals for most passive PoE equipment include warnings about this.
Moshe Katz
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