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I've found several guides online on how to build a passive ethernet hub, the design is always the same:

passive Ethernet hub by Miroslav Adzic

I'm wondering what each of the 2 anti-parallel diodes do? For both positive and negative current there's always one diode conducting, so what's the difference to just bridging the wires directly?

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  • \$\begingroup\$ Modern Ethernet uses four pairs, and five-level signalling, and this trick won't work with that set of protocols. Maybe, if it falls back to 100baseT or earlier, it could work (but termination of the transmission lines will be compromised). \$\endgroup\$
    – Whit3rd
    Commented Oct 28, 2020 at 23:06
  • \$\begingroup\$ @whit3rd modern ethernet uses glass fiber and leds or lasers \$\endgroup\$
    – Passerby
    Commented Oct 29, 2020 at 21:29

4 Answers 4

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Ethernet uses \$\pm0.85\;\mathrm{V}\$ signaling so a signal will only be able to "activate" one diode pair. So any transmission only has a chance to reach the two immediate neighbors (no current will flow more than 1 "hop" round the loop).

Now you can notice the trick which will work only with 3 devices: both of computer A TX lines' have RX lines from computers B and C in immediate neighborhood and it's own RX lines on the far side of the loop. You really don't want to hear what you are sending since it would trigger the collision detection algorithm.

Two interesting points:

  1. The diodes will heavily attenuate the signal so it probably won't work over longer distances.

  2. You cannot use resistors since they would linearly attenuate the signal and after going all the way around the loop it would finally reach your own RX line. It would be attenuated but the receiver circuit is very sensitive so it would still be able to detect is as a collision. You need a nonlinear element (like a diode) that provides a sharp cutoff.

PS. It is a really clever circuit. I cannot imagine inventing something like that on my own. :)

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I think the voltage drop over the diodes between xmt and rcv for each leg is high enough so it can't see itself - but can see the others - probably only works for exactly 3 legs

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  • \$\begingroup\$ One thing I noticed now: ethernet over twisted pair transmits on pin 1,2 and receives on 3,6. So the circuit interconnects send1/recv1 send2/recv2... I'm not really understanding how this works. And: did I get that right that each diode pair is basically only acting as a resistor? \$\endgroup\$
    – user3203
    Commented Mar 2, 2011 at 6:28
  • \$\begingroup\$ I think you could continue expanding the number of ports but the number of diodes will grow exponentially. \$\endgroup\$
    – Kortuk
    Commented Mar 29, 2011 at 12:12
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Each diode have a voltage drop, so if you apply voltage between A1, A3 there will be voltage drop across each diode, and both B and C computers will receive signal = voltage drop on 1 diode.

Why not connect directly? Probably that's about collision detection, no ideas actually...

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I'm clueless too, but one thing the diodes do is create an hysteresis loop which probably suppress noise under 0.7V. I think it's some sort of security device that has more to do with standard electronic communications and less with Ethernet specifically. But it's just a thought, not something that I know for sure.

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