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I would like a way of testing whether a network is live (connected to a switch) or dead (physical copper leading to nothing but an empty socket at the other end.

I often have to work out which sockets in a room are live or not, and the only current way is to lug around an archaic laptop, plug it in, then wait for it to realise something has happened then check whether it's connected. This is time consuming and wasteful.

I have moderate experience with soldering, and would happily knock up something involving voltage comparators and 555 timers, but I don't know what I can test for. Is it safe to draw any current from the data lines of Ethernet? I also don't know which pins are guaranteed to provide a voltage. What little info I can find online (amongst a plethora of pages explaining what a crossover cable is) seems to contradict what my voltmeter says.

I know this is a little off topic, but I can't think of anywhere else to post it.

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You could use a small standalone Ethernet tester such as this, but the only advantage over a laptop would be size. –  Bob Aug 24 '12 at 8:53
    
It's related to computer hardware, even if loosely, so it's on-topic on Super User. Though it's probably an issue which is encountered more by sysadmins, so Server Fault could be a good place to ask it as well. –  Gnoupi Aug 24 '12 at 8:58
    
Another thought; some laptops have LAN ports with indicator lights, which would provide a faster response based on the presence of a signal rather than the verification of a functioning device as an OS would check for. If you really want to build something yourself, Electrical Engineering may be able to help. You also should be able to find a schematic online. –  Bob Aug 24 '12 at 9:05

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You can buy relatively inexpensive network testers designed for exactly this purpose.

port tester enter image description here

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