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My colleague told me today he saw some sort of RJ45 dongle that intended to simulate a "link up" state for the connected interface (for testing purposes).

Can this be achieved by doing a specific wiring? (i have RJ45 crimping tools)

For instance:

enter image description here

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2 Answers 2

Reset to default
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I never have tried make that kind of connector manually, but theoretically is possible. You only have to connect RX to TX and TX to RX to create a loop in the RJ45. I know that kind connector usually are use to test E1 cards to auto test it.

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@Roid's theory did the trick. Here's the proof of concept:

Requirements

  • Short ethernet cable (~2 inch)
  • RJ45 connector
  • RJ45 Crimper

enter image description here

Peel the cable's coating

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Wire RX to TX

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Crimp it!

enter image description here

Result

enter image description here

enter image description here

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    This will work for 10BASE-T and 100BASE-TX because they use one pair to send and one pair to receive, but not for 1000BASE-T because it uses all four pairs to both send and receive, and it will fall back to 100BASE-TX if it can't communicate bidirectionally on all four pairs.
    – Ron Maupin ♦
    Commented Jul 26, 2016 at 17:20
  • I've seen line and wiring techs with these in their bag. One of them called it his "Sanity Check Plug."
    – Nanban Jim
    Commented Jul 26, 2016 at 21:00
  • Could you please post which pins need to be connected - from the pic it could be 1-8 downwards or 8-1 downwards.
    – user1016274
    Commented Jul 29, 2016 at 10:49

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