> Anyone got any bright ideas on a device which can be plugged into the end
of a single wire (say one core in a power cable) and the device will return
a length from the end of where the cable ends.

A TDR


> anyone know how to build such a beastie? Anyone ever built one?

Tektronix :)

> I envisage something along the lines of a pulse with a delay which is then
translated to length, but the detection of the echo, and processing speed
may make it harder. Would a PIC be able to keep up?

Doubtful indeed, unless the cable is very long, and you don't need any
accuracy.
 Most 50 ohm coaxial cable has a V factor of .66, and the pulse goes out and
comes back, so figure .33C as a starting point.

At 300 MHz, a wavelength in free space is 1M, so to get accuracy to the
nearest meter, you'd have to count at 600*.33 or 200 MHz

Even scenix can't play that game.

Other cables are different, basically the higher the Z, the higher the C.

A pic (or any other micro) could control it, but couldn't directly measure
it.