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knat53
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Autosensing passive PoE?

Sun Jan 23, 2022 4:57 am

My understanding is that passive PoE has no autosensing capabilities; however, I was surprised to see my hEX PoE automatically power up when I connected it to a CRS112-8P-4S-IN switch.The hEX PoE documentation says it only supports passive PoE for input.

Any idea how the switch detected it should turn power on the ethernet port connected to the hEX PoE? Or does the hEX PoE actually support 802.3af/at for input?

The PoE state on the CRS112-8P-4S-IN ethernet port was set to 'auto on'. I expected that I would need to set this to 'forced on' to power up the hEX PoE but it worked automatically.
 
mkx
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 12:20 pm

Passive PoE still requires client to behave in certain way ... which is that it has to provide certain resistivity between certain pairs of UTP wires (which explains why it fails on long UTP cables which add to resistance). The difference between passive PoE and 802.3 af/at/bt is that 802.3 PDs (powered devices) have to alter resistivity in certain steps to identify particular class of PD ... this forms a kind of a handshake between PSE and PD ... which doesn't happen in case of passive PoE where clients are passively waiting for power to be applied.

BTW, CRS112-8P-4S-IN can only act as 802.3 af/at PSE if one uses (non-standard) 48V power supply with it. Ditto for hEX PoE.
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knat53
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 12:52 pm

Passive PoE still requires client to behave in certain way ... which is that it has to provide certain resistivity between certain pairs of UTP wires (which explains why it fails on long UTP cables which add to resistance). The difference between passive PoE and 802.3 af/at/bt is that 802.3 PDs (powered devices) have to alter resistivity in certain steps to identify particular class of PD ... this forms a kind of a handshake between PSE and PD ... which doesn't happen in case of passive PoE where clients are passively waiting for power to be applied.

BTW, CRS112-8P-4S-IN can only act as 802.3 af/at PSE if one uses (non-standard) 48V power supply with it. Ditto for hEX PoE.
@mkx I'm aware of the 48V requirement, and I am using the 48POW for the CRS112-8P-4S-IN to support 802.3 af/at.

I appreciate the background information on how the handshake is performed but I'm still unclear why the hEX PoE is being powered automatically by the PSE. Are you saying that passive PoE devices such as the hEX PoE also get powered automatically due to the resistance they set?
 
mkx
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 2:16 pm

Are you saying that passive PoE devices such as the hEX PoE also get powered automatically due to the resistance they set?
Yes.
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knat53
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 2:46 pm

Do you have a reference to a standard on how this is implemented? I'd like to do more reading to understand this better.
 
mkx
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 2:54 pm

Wikipedia has a good article on 802.3 af/at/bt PoE standard.

Mikrotik's proprietary passive PoE is described in this article.
BR,
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BarbaraCollins
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Re: Autosensing passive PoE?

Sun Jan 23, 2022 6:29 pm

Hello! Passive PoE still requires client behavior in the form of specified UTP wire pairs with a predetermined resistivity (which explains why it fails on long UTP cables which add to resistance). Passive PoE, in which the clients passively await the application of power, does not need PDs (powered devices) to adjust their resistivity in order to identify a certain class of PD; this forms a sort of handshake between the PSE and the PD.
 
millenium7
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Re: Autosensing passive PoE?

Mon Jan 24, 2022 8:36 am

802.3af/at standard PoE detection works properly every time
passive PoE detection I do not like at all and I really wish it wasn't a thing. I've had several times when i've plugged something in and passive PoE has decided to kick in and supply power. This absolutely can and likely will kill end devices (and potentially source router/switch port)

I've killed a few USB-Eth adapters but those are cheap and easy to replace
I wouldn't push your luck with plugging PC's with integrated NIC's into passive PoE ports. Set it to 'off' manually
 
ConnyMercier
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Re: Autosensing passive PoE?

Mon Jan 24, 2022 10:55 am

I concur with @millenium7,
802.3af/at standard PoE works every time and absolutely no risk of blowing up Network-device !!!

BUT,
I work with a lot of old & new industrial Network-Device that don`t support standard PoE IEEE 802.X.
Mikrotik passive PoE detection is a WAY, WAY better solution, then just using a pasive-POE injector.
 
millenium7
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Re: Autosensing passive PoE?

Mon Jan 24, 2022 11:10 am

Nothing against passive PoE, just passive PoE 'detection'
You don't need to use the 'auto' mode, just manually turn it off/on

In addition passive PoE detection doesn't always work when it is supposed to either. Powering a Cambium radio from a MikroTik for example, most of the time it will not detect that it needs power and you need to manually turn it on
 
ConnyMercier
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Re: Autosensing passive PoE?

Mon Jan 24, 2022 1:33 pm

I like "passive PoE detection" Feature, because it removes the Voltage, if the device gets disconnected
For Exemple: Preventing us during a troubleshooting Session from blowing up our Laptops :D

Until now, we haven`t experienced any of the Problems you mentioned on our Customer-Networks.
Maybe we are just Lucky !
 
tdw
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Re: Autosensing passive PoE?

Mon Jan 24, 2022 2:06 pm

It very much depends on the ethenet interface design in the device being plugged in - if each pair has its own individual terminating resistor and isolating capacitor nothing untoward happens, however to save money often the terminating resistors share a single isolating capacitor so any potential between pairs will cause current to flow through the terminating resistors.

Whilst 24V passive PoE will only result in 160mA flowing through the 2x75ohm terminating resistors this produces 4W - way more than the rating of the resistors causing them to burn out. In most cases the ethernet will still operate satisfactorily but with the pairs no longer correctly terminated issues may arise, especially on long cable runs in electrically noisy environments.

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