Electrical – Detecting disconnected ethernet cable


I want to detect if an ethernet cable has become disconnected using an Arduino. The Arduino will be plugged into a wall, not battery powered.

Using a LM358 seemed like the correct fit. I hooked up orange & orange stripe to one side of the op amp and green & green stripe to the other side. Looking at the output on my scope, it never goes below ~3.5 volts. If I measure the difference with my volt meter, the difference goes to 0 when the cable is disconnected.

Looking around online, it seems as though I would need some kind of feedback loop. But if I do that, I believe that would tie one of the ethernet wires to the ground or power rail of the Arduino.

Assuming the LM358 would work, what would the circuit look like?

I don't need or want network capabilities on the Arduino itself. I'm looking to build a passive device that can detect the connection state between two other devices.

Best Answer

What are you trying to accomplish? If this "Ethernet" is actually plugged into an Ethernet device then what you're looking for is the framing data coming down the line. An Ethernet link is constantly active and detecting the idle signal is just as difficult as detecting data. It's also going to change depending on whether you're looking at 10/100 vs. Gigabit signalling. The "best" way to do this is to tap into the Phy chip and looking for a carrier active pin. Trying to impose a transparent tap is asking for trouble as you're suddenly generating all sorts of signal integrity issues.

If this is simply a a Cat-5/6/6e cable that's not actually carrying Ethernet frames then you're on your own as there's not enough info in your original question to answer that.

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