Subscriber Discussion

Does This Switch Comply With 802.3 POE?

U
Undisclosed #1
Feb 25, 2016
IPVMU Certified

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?

U
Undisclosed #1
Feb 25, 2016
IPVMU Certified

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....

U
Undisclosed #1
Feb 29, 2016
IPVMU Certified

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.

Avatar
Jon Dillabaugh
Feb 27, 2016
Pro Focus LLC

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

U
Undisclosed #1
Feb 27, 2016
IPVMU Certified

dunno.

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?

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Avatar
Jon Dillabaugh
Feb 27, 2016
Pro Focus LLC

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 -.

U
Undisclosed #1
Feb 27, 2016
IPVMU Certified

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.

Avatar
Jon Dillabaugh
Feb 27, 2016
Pro Focus LLC

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.

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U
Undisclosed #1
Mar 01, 2016
IPVMU Certified

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.

Thanks!

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

JH
John Honovich
Mar 01, 2016
IPVM

Ok, I'll double that!

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SP
Sean Patton
Mar 02, 2016

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

https://portal.chippc.com/support/downloads/files/PoE_and_IEEE802_3af.pdf

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.

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SP
Sean Patton
Mar 02, 2016

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

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U
Undisclosed #1
Mar 02, 2016
IPVMU Certified

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.

JH
John Honovich
Mar 02, 2016
IPVM

1, no upvotes for SP?

I just gave him 4...

U
Undisclosed #1
Mar 02, 2016
IPVMU Certified

Rectified.

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

SP
Sean Patton
Mar 03, 2016

Sarcasm???

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U
Undisclosed #1
Mar 03, 2016
IPVMU Certified
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U
Undisclosed
Sep 12, 2019

I was taught that PSE's POE chips do Special Things to these various signals and so I find it curious that you're hanging gear directly on those wires and looking for coherent results. I would have thought that violated some rule. I gather this generally works for you. Scary...

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U
Undisclosed #1
Sep 12, 2019
IPVMU Certified

I was taught that PSE's POE chips do Special Things to these various signals and so I find it curious that you're hanging gear directly on those wires and looking for coherent results.

thanks for your concern, but I am no stranger to proper methods and procedures regarding such inquiries.

in this case, even before the first lead was connected, and as common sense would dictate, the entire workspace was ritualistically cleansed by a Wiccan priestess; then I prepared the space further by reciting the entire text from the Latin translation of IEEE RFC 1042 backwards, and followed that with the blood sacrifice of a Certified Cisco Engineer who was suspended by a twisted pair of branches from a spanning tree at my workbench altar.

also, i made sure to use a static discharge wrist band.

better safe than sorry, you know.

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U
Undisclosed
Sep 12, 2019

Keep a fire extinguisher nearby in case you teach yourself how to make the PoE switch catch fire. Legend has it they do that if the power negotiation logic fails.

JS
Jonathan Strauss
Sep 13, 2019

I'm pretty sure most patch cables I see are 568B but could those be 568A?, I'll admit I don't know how to use a multi meter but could this be the case? According to that chart In Mode B the pairs used aren't the ones that get swapped

U
Undisclosed #1
Sep 13, 2019
IPVMU Certified

I'm pretty sure most patch cables I see are 568B but could those be 568A?

they could be.

but what does it matter what the color of the pairs are inside the unopened cable?

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