Years ago I invented an in-circuit network cable tester. It has been on my web site for about 10 years, but no one makes anything like it. So I thought I would re-post it to my blog as the search engines seem to find things here much faster.
The scanning tester is also my own design but several other people make very similar devices. It just uses a 555 timer and a 4017 to sweep through all 8 wires to verify that they are connected and that they are in the right order. This is a great device for testing home made network cables or ones that may have been damaged while they were in use. It also tests and detects network crossover cables.