Does This Switch Comply With 802.3 POE?

Here's the deal, look at most any reference pinout for POE 802.3af and you'll find something like this:

Notice how it says positive is on pins 1 and 2 for mode A?

But running this simple setup:

Notice the negative sign on the reading, even though pin 1 is connected to the positive.

Here are the close-ups:

Cables are both straight-thru patch.

Have used other cameras, switches, meters and cables with same result.

Also, when testing mode b, everything works as expected:

easy enough to replicate.

Any ideas if I'm interpreting the diagram wrong or doing the test wrong?

Update: I'm thinking that for some reason the switch is reversing things because of MDI/MDI-X. Not sure why, since the cables are straight....

Scratch auto-MDI/MDI-X, as that only swaps TD and RD but should not affect POE polarity.

Only thing that would do this that I know is a crossover cable, which I'm not using.

Sometimes people confuse electron flow with polarity. This could be the case here?


im just a practical kind of guy who thinks that if the diagram says + and I hook up the + lead to it that the meter should read +.

is that confused?

While I'm not saying it is the case here, but I have seen devices that have stated a terminal was +, but it was referring to electron flow out of that terminal, not the actual polarity, which is -.

I have seen devices that have stated a terminal was +, but it was referring to electron flow out of that terminal.

How did you know it was referring to "electron flow" and not just "backwards"?

Of course by convention, electrical diagrams run from positive to negative, even though in copper we know that the charge carriers are moving the opposite direction.

Even so this causes little practical confusion, since the + is still +.

Also the document that says this is the actual 802.3 spec from the IEEE.

I would like to see what a real Cisco switch would read.

Because I put a meter on it and it was backwards. I asked their support why that was and it was their reply. It was many years ago in a different industry. I never understood it.

Update: Now offering from $0.70 to $1.05* in voting credits for anyone who can provide polarity information for their switch (while powering a device).

No photo required, just list the brand/model and the polarity of pins 1-2 and 3-6.


*Must make small joke to be eligible for funny bonus.

Ok, I'll double that!

Heres a decent "whitepaper" that has different information from a manufacturer that only focuses on power...

That document calls out 1/2 as Negative and 3/6 Positive. Which falls in line with what you are reading.

Other things I read actually stated that there is no "standard polarity" defined in the 802.3 standard, which I dont really believe, but its out there.

I did find this wording about the specification:

"The PSE will expect that the Signature Resistance will be after some form of Auto-polarity Circuit and will compensate for the DC offset in the Signature."

I did not find that in the spec document, but I would be curious if you tried 3-4 different devices, if its possible that you end up reading different polarities depending on the manufacturer of the PoE chip in the PD unit

Thanks Sean, that's an interesting document, and the first I've seen that calls out negative on 1-2.

...its possible that you end up reading different polarities depending on the manufacturer of the PoE chip in the PD unit.

I don't think you would get different polarities by changing the PD, though. Changing PSE's can definitely do it though.

According to the spec, the PD has to have a bridge rectifier on input, which makes it work regardless. This was done in case a crossover cable is in use.

So why do I care? Because, I am working on a network analyzer and thought I could determine whether a crossover cable was in use by the polarity.

But apparently, regardless of the standard, a single polarity cannot be counted on in the field, so now it is nothing more than just a curiosity of why some ended up that way.

For the record, I have tested the following brands of PSE's

  1. Netgear 3-6 +
  2. Dlink 3-6 +
  3. TPlink 3-6 +
  4. Linksys 3-6 +
  5. Everfocus 3-6 +
  1. Cisco 1-2 +
  2. Allied 1-2 +
  3. Silvertel 1-2 +

Thanks again.

1, no upvotes for SP?

I just gave him 4...


I even gave him a funny on the first post because of his sarcastic use of "whitepaper".