During renovation I've asked the electrician to use two long patch cables originally running from my router to create RJ45 sockets with the intent to later connect the router to the socket using a standard patch cable. So there is a patch cable with one plug cut of and replaced by a socket.

Now, the patch cable connection doesn't work: a device connected to the other side of the cable (either of the two) doesn't detect signal. I've checked one of the sockets which has replaced the original plug and it seems to be wired correctly (read: "as specified on the connector") as 568B.

Since I have problem wrapping my head around what pin is supposed to be connected where on the various cable connections (my confusion is such, that reading How to wire up two RJ45 sockets? didn't help), my questions are:

  1. Does it matter whether the original patch cable was wired as 568B or 568A, straight or cross?
  2. Should I be using a cross patch cable or even a less standard wiring to connect the router to the socket? Or is it likely that the connector is either wired incorrectly (missing contact somewhere) or that the cable has been damaged?
  3. Can you please suggest how to proceed to locate the problem (part of the cable has plastered into the wall, so I would prefer not having to cut it out/replace it)?

Update: devices see temporarily a sign of signal, yet don't pick it up for serious communication, Linux kernel log:

[x0.041818] e1000e eth0: NIC Link is Up 100 Mbps Full Duplex, Flow Control: Rx
[x0.041822] e1000e eth0: 10/100 speed: disabling TSO
[x0.884898] e1000e eth0: NIC Link is Down
  1. Could this be a sign of bad contact in the new socket connector?


the problem was actually in the socket connector - it has metal casing and the individual cords were slightly sticking out touching the casing thus short circuiting some five of the lines all together.

  • \$\begingroup\$ The answer to 1) depends on what you've got plugged into both ends of it. Lots of things can auto-negotiate which pair is which, so it doesn't matter if you use a crossover cable or not for them. But some things can't. \$\endgroup\$
    – Hearth
    Commented Nov 18, 2021 at 1:02
  • \$\begingroup\$ should have used F-F rj45 inserts insteead of cutting cable, \$\endgroup\$ Commented Nov 18, 2021 at 3:42
  • \$\begingroup\$ Obviously you should take a photo how the cable is wired on each end now. There's no way to tell what could be the issue. \$\endgroup\$
    – Justme
    Commented Nov 18, 2021 at 5:49

2 Answers 2

  1. It does not really matter whether your house wired 568B or 568A, as long as termination is the same on both ends. Electrically they are compatible.

  2. Usually router connected to device with straight cable. So, connect it to the socket with straight cable. Again, does not matter which one. Same on the other side - connect device to the socket using straight cable.

  3. If you have multimeter the easiest way to check electrical continuity of the wiring in the wall is to buy 6" patch cable, cut it in half, strip the ends and insert into both sockets. Then:

  • twist all wires on one side together and check continuity between one wire and all the others on the other side. They should all ring.
  • twist 4 pairs of wires (e.g. green to white-green) on one side and check continuity of each pair and between the pairs. Each pair should ring, between pairs there should be no connection.

You can also use a battery and a lamp instead of multimeter, if you don't have one.

If all wires in a wall check out properly then the problem must be in your patch cables. Luckily they are dime a dozen nowadays.

The only other potential problem that I can think of is bad wiring job, for example too long untwisted ends or sockets not rated for the speed. This usually affects gigabit connection. 10 and 100 megabit should be fine.

  1. Bad connection most certainly can be a reason for communication problems. I just don't know if what you have is caused by one. I do recall struggling a lot with extension I made using two patch cables and $1 F-F coupler from Monoprice. $5 one did not fare any better. Only when I bought (awfully overpriced, IMHO) $15 coupler my troubles went away.
  • \$\begingroup\$ Thanks! Nice hint with checking with another patch cable, I have a long one at hand, wil try. Would you please also kindly check new question nr. 4? \$\endgroup\$
    – peterph
    Commented Nov 18, 2021 at 1:25
  • 1
    \$\begingroup\$ I updated the answer. Also, I did not recommend cutting a nice long patch cable, I suggested buying tiny 6 inch for the test. These should be available for under a dollar, the category does not matter for continuity test. Or you can use the cut off plugs from old cable, if there is enough wire left on them to strip and make connection. \$\endgroup\$
    – Maple
    Commented Nov 18, 2021 at 4:50
  • \$\begingroup\$ Thanks. I actually have a lot of cable and some spare connectors left, so the part that goes into the socket I can check easily. The other side (plug) will be a bit bigger challenge :) \$\endgroup\$
    – peterph
    Commented Nov 18, 2021 at 23:36

Go straight through. It doesn't matter which wiring convention you use as long as it's the same on both ends (in fact, even if you swap them, it's still probably fine with modern equipment, as all Gigabit Ethernet equipment is required to be built to not care whether the cable is "straight" or "crossover").

As for detecting cable problems... well, it's almost certain that you have one. Some network cards have time domain reflectometry hardware, accessible through a BIOS menu or windows device manager. It tells you the "cable length" for a given pair. If one pair reports a different length than the others, you have a break in the cable. If you plug it into a remote NIC and one or more pairs still say "open" then you have a connector problem.


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