I have a dev board with a RJ45 connector which has magnetics integrated. I need put it into a case and mount another RJ45 on the panel, sadly I have only another connector which also integrate the magnetics. If I can use a cable simply connect the RX-, RX+, TX-, TX+, of my board connector to the RD-, RD+, TD-, TD+ pins of the connector on the front panel, but left the CT pins open?

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I've tried that, and it works, at least. But more explanations are welcome here. And is it a good or bad design?

  • \$\begingroup\$ Perhaps remove the connector and wire to the panel connector straight from the board? Or just try it, you may get away with another set of transformers in line, if the cables are kept short. \$\endgroup\$
    – tomnexus
    Dec 22, 2015 at 8:27
  • 1
    \$\begingroup\$ I actually think there's a good chance it will work. The transformers are low loss 1:1 so another one shouldn't hurt. Don't connect the CTs to anything. Ground the chassis GND pin separately to your board ground. LEDs you need to make a plan. \$\endgroup\$
    – tomnexus
    Dec 22, 2015 at 9:10
  • \$\begingroup\$ @tomnexus: Thanks. The LEDs is not important, I can omit them. I'm thinking now if I can float P10 (CHS GND pin) too. \$\endgroup\$
    – diverger
    Dec 22, 2015 at 10:18

1 Answer 1


Connecting three magnetics (both ends and the middle connector) in series is bad design, it may work but Ethernet specifications allow/require one magnetics on each end. I assume, with this configuration you can't reach the maximum distance of a 100m between two Ethernet devices.

  • \$\begingroup\$ Thanks for your answer. Can you give more information about how the "middle" magnetics affect the maximum distance? \$\endgroup\$
    – diverger
    Jan 10, 2016 at 4:54
  • \$\begingroup\$ It simply consume energy so you have less energy to drive the line. \$\endgroup\$
    – WalterH
    Jan 10, 2016 at 15:17
  • \$\begingroup\$ @diverger each set of magnetics will cause some signal loss, so if your system is already marginal for signal integrity (because you are running at the limit of the range) then the extra set of magnetics may push things over the edge. \$\endgroup\$ Feb 26, 2016 at 1:03

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