I am building ethernet connectors and a switch into a product. There is a requirement to include PoE which I haven't done before. This has required the use of a new magnetics chip to expose the two centre taps.

The new magnetics chip has a different pin ordering to the one used previously, so as in the below image, there is a crossover introduced in the routing:

PCB layout issue

I believe that the transformers in the magnetics are symmetric. I'm therefore hoping that I can simply swap the connections to the + / - (p/n) pins of the magnetics:

enter image description here

(This would apply to both the TX and the RX pairs)

This is what the datasheet for the magnetics (Bel Fuse S558-5500-25-F) shows:

enter image description here

The centre tap is - I believe - genuinely in the centre and to my understanding the dots indicate that the coils are the same polarity.

Does anyone have any reason not to make the swap I intend to improve my routing?

The alternative would be to put the magnetics on the underside of the PCB, but this introduces unwanted layer changes, and potentially an interference issue. That or I find different magnetics.

  • \$\begingroup\$ I don't feel qualified enough to give a full-fledged answer to this, but if you're swapping BOTH the local and line sides of a given pair, as you draw in the diagram, I can't see how this could cause problems. They should be symmetric as you say. \$\endgroup\$ – Glenn Willen Sep 17 '19 at 13:28
  • \$\begingroup\$ I'm in exactly the same boat! I can't see it making a difference, but can't say conclusively... \$\endgroup\$ – Chuck990 Sep 17 '19 at 13:33

As long as the RJ45 connector line pinout adheres to the standard, you shouldn't have any problems. The magnetics are a passive device, they don't care about RX + or RX-.

You can even get away with RX/TX crossing on the connector side, as most modern NICs and switches have auto MDI/MDI-X capability. Some PHYs allow you to set the transmission polarity in their configuration, but I wouldn't rely on that capability.

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    \$\begingroup\$ Gigabit transceivers are definitely not sensitive to polarity (they are required to be able to auto-switch both polarity and RX/TX). But for 10/100 I can't find a clear answer. \$\endgroup\$ – Glenn Willen Sep 17 '19 at 14:48
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    \$\begingroup\$ Is that ALL Gigabit transceivers or just one in particular? Do you have a source for that please? \$\endgroup\$ – Chuck990 Sep 17 '19 at 14:58
  • \$\begingroup\$ Check the one you plan on using, you don't need all of them. From your layout, you can wire the connector either way. \$\endgroup\$ – Lior Bilia Sep 17 '19 at 17:34
  • \$\begingroup\$ I have selected this as the correct answer, and when creating the prototype PCB, it was shown that this works with these devices at least. \$\endgroup\$ – Chuck990 Dec 5 '19 at 11:52

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