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What magnetics do I need connecting a SBC Ephy to local switch IC?

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Skavin:
I would like to design a carrier board for a bunch of SBC's (NanoPi Duo2) and have them connect to a switch IC on the carrier board and have it present a single RJ45 on that so I can connect them all to my lab LAN at once.
I am not hard set on a switch IC but but something like a RTL8305NB IC with 5 10/100 PHY ports.

What magnetics do I need on the board between the NanoPi and the switch IC's inputs?

If they were totally separate device's connected by cat5e  there would be:
Ephy <-> Transformer <-> common mode choke <-> auto transformer <-> RJ45 <-> cat5e <-> RJ45 <-> auto transformer  <-> common mode choke <-> Transformer <-> Switch IC

I assume that I can remove the cat5e and RJ45 connectors but how far can I go?
Can they just be connected together as the grounds and so voltages should all be the same?

Thank you in advance for any suggestions or pointers to more reading.

ajb:

--- Quote from: Skavin on June 08, 2023, 06:54:12 am ---If they were totally separate device's connected by cat5e  there would be:
Ephy <-> Transformer <-> common mode choke <-> auto transformer <-> RJ45 <-> cat5e <-> RJ45 <-> auto transformer  <-> common mode choke <-> Transformer <-> Switch IC
--- End quote ---

Well, you'd really only have one transformer module with built-in chokes, generally, or a magjack, not three separate magnetic components for each PHY. 

It's possible to capacitively couple two PHYs instead of magnetically couple them.  Note that this is not generally a great idea for ports that connect to the outside world.  There are several appnotes on the idea, here's one: http://ww1.microchip.com/downloads/en/AppNotes/Capacitive%20Coupling%20Ethernet%20Transceivers%20without%20Using%20Transformers.pdf

Skavin:

--- Quote ---Well, you'd really only have one transformer module with built-in chokes, generally, or a magjack, not three separate magnetic components for each PHY.
--- End quote ---

Yes the transformer module has all 3 parts. My thought process was that 2 transformer modules back to back would be silly so what is inside them that is not needed and find surface mount devices that would do the job of what is needed. this path was bolstered by looking at the "HORACO 4 Port Gigabit POE Extender" from Aliexpress that I AssUMe works for some definition of works. It seems to just use a discreet center tapped transformer per pair for its PoE. attached is a board pic stolen from the the listings comments.


--- Quote ---There are several appnotes on the idea, here's one: http://ww1.microchip.com/downloads/en/AppNotes/Capacitive%20Coupling%20Ethernet%20Transceivers%20without%20Using%20Transformers.pdf
--- End quote ---

Thank you this is the information I was after. Documentation of how it should be done is much better than taking direction from a random piece of hardware from the internet :) . It has pointed me down a whole new line of investigation and reading. Who knows this old coder might make some hardware that works in his spare time :)

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