AVR-Chat

In the alarm industry, this is called a supervised circuit.

A resistor at the far end sets the loop current.  At the panel, a window 
comparator checks that against high and low limits.  shorted loop alarms 
high, open loop alarms low, short to ground or panel power also 
alarms.  Design for ANY conductor to withstand short to panel power if it's 
in the same cable.