Now I design the fire alarm panel i use ATMEGA16 for the controler , i wonder how i can detect loop cable exspecialy for cable that i connect to contact relay (i use relay to activated solenoid) because some time that cable got voltage 24vdc..., for input sensor i use adc input ATMEGA16 to detect the fire sensor and to detect loop cable n its work well...
Why don't you try using a window comparator circuit. Fire alarm circuits work as a "short" for alarm and an "open" is circuit trouble. Using a LM339 Quad comparator would allow you to build 2 complete comparator circuits. If the micro you are using has comparator inputs then they should be able to be setup as a window comparator.
Check out this site for setting up a window comparator: **broken link removed**