-------------------big snip--------------

I can't believe how complicated this is getting.
All you need is a couple of resistors at the remote site on the relay 
contacts, to make things easy 10K normal, 1K alarm.
Stuff some DC down the line through a current limiting resistor and use 
a basic circuit to measure the current on the line.
You then have:
No current = open circuit
Max current = short circuit
10K current = normal
1K current = alarm
This is how we do things in the Fire Alarm industry

Regards,

Dave

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