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Is it possible to use Active PoE splitter that does comply to 802.3af, with passive PoE injector?

  • 802.3af does autonegotiate
  • Does negotiation take passive PoE into account?

Edit:
Acording to: This answer @ networkengineering

Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a diode bridge.

That would mean, unless the negotiation does not allow fallback to spare pairs, or let's the DC pass through (dangerous?), or just does not operate at all (would be useless), it should work.

Note: The device also has to power itself before it can do anything, which means it has to by default tap into both possible powersources internally. Even if that is a low current path.

According to: Fundamentals of Power Over Ethernet, page 5, flowchart.

  • Negotiation is managed/initated by injector
  • The splitter does provide a signature

Which does imply, given the standard (previous quote), it should automatically inteligently use the power on the spare lines.

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  • 1
    that depends on your splitter. Does it care if it's attached to a passive source?
    – Ricky
    Feb 19, 2016 at 23:35
  • That is something I am not aware of. It is atypical splitter I am considering purchase of. ...
    – user23063
    Feb 19, 2016 at 23:43
  • (Will answer this question myself if I decide to buy the adapter)
    – user23063
    Feb 20, 2016 at 18:01
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 13, 2017 at 17:42

2 Answers 2

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Generally, no.

There are some splitters that are compatible with pre-standard, proprietary and passive power delivery variants but unless you make a lucky match it won't work. Furthermore, running a passive injector with an incompatible PD on the other end might well cause some damage.

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Passive PoE is entirely non-standard, and can damage devices that are not expecting power or are expecting power at a lower voltage than the injector delivers. Passive PoE systems could run at various voltages, 24V seems to have been the most common, but 12V and 48V passive systems certainly exist.

The initial version of IEEE standard PoE delivered power at 48V and used a fairly crude "negotiation" process. The first phase was the "detection" phase, the power sourcing equipment would supply a low voltage, and look for a signature resistance. If a powered-device was successfully detected it would move to the "classification" phase, the voltage is raised to about 18V and the powered device draws a specific current to indicate it's power requirements. If the power sourcing equipment is able to supply the required power it turns on the 48V supply. Later versions have added more complexity, but we don't need to think about that here.

So what is likely to happen if you connect an IEEE powered device to a passive PoE source? that depends on the voltage.

If the supply is 12V or 24V then most likely nothing happens. The voltage never reaches the level required to turn on the main converter in the powered device. On the other hand if the passive system is 48V there is a good chance that the powered device will turn on it's main converter and work.

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