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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

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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

enter image description here

Wire RX to TX

enter image description here

Crimp it!

enter image description here

Result

enter image description here

enter image description here

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  • 2
    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
    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
    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. Jul 29, 2016 at 10:49

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