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I'm muxing Ethernet MDI between two PHYs – one of which is 10/100 (2-pair) and the other of which is 1+Gbps (4-pair). A single set of magnetics (4-pair) serves both PHYs via the mux. Does anybody have any insight as to how to properly terminate the two MDI pairs that are unused when the 10/100 PHY is selected via the mux? My current thought was the approach outlined in this response. enter image description here

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  • \$\begingroup\$ I don't see how that answer you linked to relates in any way, because you are switching the magnetics to two different PHYs and that diagram shows the connections between magnetics and the connector. Can you please add a diagram to make clear what you are switching and where. \$\endgroup\$
    – Justme
    Commented Oct 14, 2023 at 21:12
  • \$\begingroup\$ @Justme Diagram added. I was interpreting the section of the diagram I linked to marked "unused" as a potential termination strategy that would be applicable, here. \$\endgroup\$
    – foxtrot
    Commented Oct 14, 2023 at 21:31

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I don't think it matters much because your path from magnetics to connector must be equal no matter which PHY you use.

If you want you can of course terminate the pins that the PHY does not use in same way than the pairs the PHY does use.

But the next problem is, it depends on the PHYs if they are compatible with the same magnetics connections. It is entirely possible that you not only need to swith 8 MDI lines of four pairs, but also the magnetics center taps, because some PHYs connect the center taps differently. Some with current mode drivers have it connected to driver supply voltage. The PHYs may use different driver voltages. Also some PHYs have voltage mode drivers where the center tap must not be connected to any potential.

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  • \$\begingroup\$ I omitted this detail from my diagram. The 10/100 PHY has a voltage-mode driver and wants center taps referenced to GND with some DC blocking caps. For the 1Gbps PHY, I was considering putting another set of magnetics in front of it. I think I can do this within the insertion loss budget. Any thoughts? \$\endgroup\$
    – foxtrot
    Commented Oct 14, 2023 at 23:26
  • \$\begingroup\$ It also occurred to me that this is basically how midspan PoE injectors work – so I think an additional set of magnetics (with center taps configured appropriately for the corresponding PHY) should be fine, here. I'm still curious to hear if you have any thoughts, though, since you seem to be pretty knowledgeable about these things! \$\endgroup\$
    – foxtrot
    Commented Oct 14, 2023 at 23:47

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