I have an end of an ethernet cable but I don't know where it goes. I want to see if it is connected to a device at the other end.

Looking at the pinout you have a TX+/TX- and an RX+/RX- pair of wires. From Google I have seen something about auto-negotiation and I believed this was the way to detect it, however when connected to an oscilloscope, I can't see anything.

Is there a specific way in which to detect if the cable is plugged in? (Thinking of using voltage if I could see some.) Is it possible to detect this? Well, it must be, because how do hubs/switches detect if something is plugged in?

Are there any resources which can help?

EDIT: I have tried using a scope connected directly to the pins, using DC blocking capacitors, and a transformerless differential line receiver but I havent had any change in the bogus 200V reading im getting.

Has anyone done this before? Is there a circuit which is known to work?

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    \$\begingroup\$ Doesn't it go into the ether? \$\endgroup\$ – fuzzyhair2 Jul 22 '14 at 13:01
  • \$\begingroup\$ one end goes into hub the other is loose, how can i detect the hub's presence through the disconnected end of the cable? \$\endgroup\$ – NoLiver92 Jul 22 '14 at 13:02
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    \$\begingroup\$ I'd just connect it to a switch and see if one of the lights goes on. \$\endgroup\$ – Simon Richter Jul 22 '14 at 13:02
  • \$\begingroup\$ Simon, a switch would be good BUT this project requires me not to use a switch (theres a bigger project behind it) \$\endgroup\$ – NoLiver92 Jul 22 '14 at 13:03
  • \$\begingroup\$ No, connect it to check for the connection, and then disconnect it later. \$\endgroup\$ – fuzzyhair2 Jul 22 '14 at 13:04

There are several ways to determine whether the other end of the cable is connected to a device.

Assuming this is point to point ethernet with RJ-45 connectors, you can look at the DC resistance between the wires of a pair. This type of ethernet is transformer coupled, so will have a low DC resistance. If nothing is connected to the other end, then the lines will be open and have a infinite resistance.

Or, you can look for link pulses. These are pulses occasionally sent by each end so that the other end knows there is a device out there.

  • \$\begingroup\$ I was looking at the link pulses but I can not pick them up (think im connecting to oscilliscope wrong) \$\endgroup\$ – NoLiver92 Jul 22 '14 at 13:30
  • \$\begingroup\$ @NoLiver92 - add a diagram or at least through explanation of your connection to the question. You might also find this useful: fpga4fun.com/10BASE-T4.html though if I recall some of that plays "try it today pragmatic" games with capacitive rather than transformer coupling. \$\endgroup\$ – Chris Stratton Jul 22 '14 at 15:54

Ethernet over RJ45 is sent using a differential signalling. This means that the wires in each of the two pairs is driven to opposite logic levels. The voltage levels are ±0.85V.

When a device is disconnected, there will be an absence of this voltage. So what you can do is to check to see whether a differential signal exists on the TX pair or RX pair (depending on which end you want to see).


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    \$\begingroup\$ Note that this signal is only AC. You will never see a DC voltage between pairs with something like a voltmeter due to the average always being 0. \$\endgroup\$ – Olin Lathrop Jul 22 '14 at 14:52

Routers and switches have a uni or inni connection. Dcn rl-ime the correct way to test all cables and switches routers Etc is by sending a modulated signal set to modulation of switches etc and see if you get any jitter in packets. Need a test set to put on both ends near and far. If your just connecting a home router get a small fluke tester for basic cat5 cables and follow directions in the box


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