Is it possible to detect rx+ rx- tx+ tx- wires of an ethernet cable having only a part of cable with multimeter or somehow else?

I mean the situation when you have just one side of the ethernet cable. The other side is connected to a working switch or hub far away. And the guy that did the other side of the cable didn't use standard wiring (green and orange pairs).

I am talking about 100mbit cables, not 1000mbit.

  • 1
    Please mind that it is NOT enough to use same wiring on both connectors. You MUST respect the pairings for it to work properly, specially with long cables (IE: pin 1 must be paired with 2, and 3 with 6). – LatinSuD Jun 20 '14 at 9:22

Call someone at the far away place and have them read out the color order to you. Or walk over there. The max length of the cable is 100m...not that much exercise.

Or better yet, cut that jack off and wire it properly.

If the pairs are in the wrong order the cable can still work, but will confuse the next guy. If they were not wired with pairs in the right places, you wont have "twisted pairs". That is essential for proper transmission, especially on longer runs.

A multimeter wont do you much good. A cable tester will. My fluke cable tester will try to establish a connection to the device on the other end on different combinations of pairs and report which ones worked.

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    +1 "Or better yet, cut that jack off and wire it properly" – Chris S May 20 '14 at 0:25
  • or if its short, just buy a pre-built one and save yourself the hassle. – Sirex May 20 '14 at 2:05
  • Will a cable tester try to establish a connection with a real device like switch? – POMATu May 20 '14 at 10:36
  • @roman yes. I believe the one I have is a fluke nettool. They aren't cheap, but they save a lot of time and effort diagnosing bad cables. Seriously though - just cut the end off and wire it up properly. – Grant May 20 '14 at 11:34
  • This is more like a theoretical question not a real problem that should be solved.But thanks a lot for info about fluke cable tester, that is the solution that I was looking for. – POMATu May 20 '14 at 11:41

You can test the cable either with multimeter or a cable tester, although the multimeter reading is not accurate at all. Anyway...

Testing a cable with cable tester is pretty straight forward.

If you don't have a cable tester, you can still check if the cable pairs are okay. What you need to do is check the resistance between each pair (between 1 - 2 and 3 - 6 pins on the RJ45 connector).

Keep in mind that the resistance depends on the cable length, conditions, etc...

For example: You should get a reading of about 2-2,5 Ohms for a 5m FTP Cat5.

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