I have an ethernet cable tester, it takes a 9v battery, has some 4000 series logic and some LEDs and blinks them based on the integrity of the cable. This new tester takes some nearly useless old technology and makes a fantastically simple device that simply reports the state of the ethernet link to actual running network equipment. I will state flat out that I got this idea from this instructables article and that I added no content to it whatsoever. I am posting about it because I want more people to know about how easily this hack can be done, and how useful it is. To start off with we have an 8p8c to DA15 network adapter. Yes, I am going to be pedantic about connector naming, see here for the D-subminiature wiki article, and here for the RJ45 vs 8p8c debacle. This is commonly known as an AAUI (or AUI) to RJ45 adapter, meant for connecting two ethernet connections that are not electrically identical. The hack is truly simple, just hook up ground and power (supposed to be 12v, we’re running it at 9v) and away we go. There is also a momentary push button so power is normally off, but that’s optional.
The two ethernet converters I modified had different indicator lights, the Link light on each light up when a valid device is at the other end, and one even had an Rx light that glows when the network sends broadcast messages. I have no idea how such little hardware can determine a valid link, but I assume there is a hardware level detection done independent of any actual data transfer over the cables. The whole gallery is here.