[no subject]

If you're asking about old-time system smoke detectors, they were all
four-wire smokes, as far as I recall.  However, some of the smokes had
internal end of line relays.  In other words, each smoke had four circuit
wires forming what you might call an "A" loop and a "B" loop.  A relay
inside each smoke would place an open on the "A" loop if power to the smoke
was disconnected.  Manufacturers soon realized that a single relay at the
last smoke served the same purpose, and cost less.

- badenov