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Can I connect Ethernet with PoE ?  

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Gilly Barr
(@gilly-barr)
Can I connect Ethernet with PoE ?

I bought an ethernet switch that has an output port with PoE (power over ethernet), and I was wondering if it's safe to connect this to the MINI??

From what I understand, it's crucial to know if the MINI supports this, since it's ~48v and if it doesn't know how to handle PoE, it can burn the board...

Thanks in advance! 

Posted : 26/01/2021 7:33 am
Fayoh
(@fayoh)
Eminent Member
RE: Can I connect Ethernet with PoE ?

If the switch supports 802.3af/at there will be a detection/negotiation phase before any dangerous voltages are present and it will be perfectly safe.

For a passive injector (ubiquity I think do this on some devices?) I'm not sure. There are ethernet line transformers that are PoE classed, but probably the mini does not have it.

Posted : 26/01/2021 8:09 am
Gilly Barr
(@gilly-barr)
Topic starter answered:
RE: Can I connect Ethernet with PoE ?

@daniel-16

Thanks for the fast response!

Yes, the switch I have is from ubiquity.. 🤦‍♀️

Posted : 26/01/2021 8:37 am
cwbullet
(@cwbullet)
RE: Can I connect Ethernet with PoE ?

I have a Pi Server and it still connects over ethernet despite using POE.  

--------------------
Chuck H
3D Printer Review Blog

Posted : 26/01/2021 11:09 am
Crawlerin
(@crawlerin)
Prominent Member
RE: Can I connect Ethernet with PoE ?

802.3af compliant switch or power injector will not start delivering power until it detects signature from client that it is PoE capable (I think it feeds low voltage and looks for specific resistance measured in kOhms). Then there's the handshake where they negotiate their class and capabilities and only after that switch starts delivering power. It's not like there's live 48 volts on wires 😀 

Here's a good explanation:

Signature detection is performed by injecting a very small voltage on the link to detect a characteristic signature of IEEE-compatible PDs. This is presented with a 25k ohm resistance. Once detected, the PSE knows that higher voltages can be safely applied. A device not supporting this standard will not present this signature and voltage will not be applied. This means that on a remote switch with multiple PoE PSE ports, the connection of various types of Ethernet equipment can be safely done with compliant and non-compliant PoE PDs. It is important to note that Passive PoE injectors do not perform this check and will always apply power on the cable. These devices have visual warnings on the ports to not connect non-PoE complaint devices, however, that does not guarantee that staff at remote sites will not inadvertently connect a device to the wrong port.

And some good more technical video: https://training.ti.com/poe-handshaking  

But if the power injector is passive and has non-standard implementation of for example 12V or 24V (like Ubiquiti passive inline injectors!), it can possibly damage equipment.

Posted : 28/01/2021 12:59 am
Gilly Barr
(@gilly-barr)
Topic starter answered:
RE: Can I connect Ethernet with PoE ?

@crawlerin

Thanks! 
I understand I need to be really careful with passive PoE.. :/

Posted : 28/01/2021 11:20 am
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