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It would be useful to measure electrical parameters (such as I,U,R and perhaps L,C or other) at the end of the cable, when the other end is disconnected or connected, and analyze the physical layer of the Ethernet computer network. For example when you occur problems on a network adapter, packet losses, loops or get an electric shock when touch the metal parts of equipment.

Some clarifying questions:

  1. Is it possible to do this using one or several multimeters?
  2. If it's possible then what and where exactly measure (between the wires or pairs of the cable or relative to ground)?
  3. Could this give some additional and more detailed diagnostic information about the cable or NIC?
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    \$\begingroup\$ You can, somewhat tediously, test for shorts, connectivity, and most cross-wiring errors. \$\endgroup\$ – Hot Licks Jan 19 '15 at 19:18
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The only thing you might be able to test is the continuity of the cable. If you are attempting to test to see if data can traverse the cable than this is an okay way to test. If you are looking to test data rates, etc. this is not the method you should use.

I have also experienced Layer 1 (Physical Layer) issues where the cable was fine but the female port (The jack on your Ethernet card or motherboard) had bent risers that were not making good contact with the Ethernet connector.

Other than continuity (Does point "A" have an electrical connection to point "B") the electrical test you would be able to perform with a multimeter would have nothing to do with data rates.

PoE testing is best observed at the device, switch, or router because part of the IEEE standard for PoE requires an end-point to negotiate with the end supplying power and no current will be present if you just connect a test lead.

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To add:

  1. Yes, it's possible, but unless the jacks/terminations are in bulk uninstalled state or very close to one another (like same room) you're going to need test leads going between one of your multimeter conductors and and the other end of the cable. Many cheapo "cable testers" do exactly this but they have two seperate ends which don't need to be physically connected to show continuity like a typical multimeter does.

  2. Just measure continuity on each conductor, one at a time. Remember that the pins are not straight through: http://www.creativeitresources.com/wp-content/uploads/2014/11/RJ45T568AandT568BPatchCable.jpg

  3. No. This will be a very basic, layer 1 troubleshooting tool only.

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  • \$\begingroup\$ You don't really need to run a test lead to the other end of the cable, since you have seven other wires in the cable you can use to do that. You do need someone at the other end to move jumpers around, or you will get a lot of exercise running back and forth. \$\endgroup\$ – Hot Licks Jan 19 '15 at 19:19
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  1. Possible. If the cable in question is structured cabling you may want someone else team up with you to move jumpers at the other end (a pair of good sharp tweezers are good enough as a jumper) and for patch cables multimeters with really thin sharp probes can work.
  2. Continuity of the cable. Check if cable is correctly wired as a straight through, and no cable is shorted over. If you have advanced tools like function generator and spectrum analyzer you can try do some high frequency crosstalk tests.
  3. If all you have is a bunch of multimeters all you will get is whether the cable works or not. If you have the advanced gear like function generators and spectrum analyzers you can also test the maximum speed the cable in question can carry (e.g. 10BASE-T, 100BASE-TX, 1000BASE-T, 10GBASE-T or 40GBASE-T, at speeds 10, 100Mbps, 1, 10 and 40Gbps)
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Yes, it is possible to test Ethernet twisted pair with multimeter, but with the multimeter it is very difficult because you need to match your cable end or socket with the multimeter pin. Also it is difficult to maintain proper pairing of the wire.

There are some best twisted pair testing tools which are used by professionals such as CableIQ Qualification Tester which quickly identifies the cabling problem and also verify wire pairing and detect installation defects like “split pairs”. With this device you can also determine length of a cable or distance to a break or problem area.

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