Vasilis Stergiopoulos has built a RJ45 LAN cable tester. The circuit was originally designed to use a 555 and 4017 IC, but Vasilis has published the schematic and source code in assembly for using the Attiny2313 instead.
The Attiny2313 produces successive outputs at port D, where the LAN-cable is connected. If a straight cable is connected between connectors A and B, then the LEDs will glow in successive order. If a cross cable is connected, then the glow order will be as noted on the board, i.e. 1,2,7,4,5,8,3,6. If any other glow order occurs, it means that the cable is not wired correctly. If one or more LEDs don’t glow, then we have a corresponding open circuit. A remote unit with only the B connector (B2) can be used if both cable ends are not accessible in the same location.