This wire is used as common alarm signal.  Once one detector is
locally triggered, the others enter a remote trigger state.  Once no
detectors are locally triggered, the others leave their remote trigger
state.

I think you want to know what the signal consists of, and what it is
relative to.  ;-)  Me too.

I have a set here which I've studied behaviourally, but not
electrically.

--=20
James Cameron
http://quozl.linux.org.au/
--=20
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