It may have occurred to you by now that even a modest-sized property could require plenty of door and window sensors; thus, if we used one input for each sensor, we'd soon run out unless we put more and more port expanders onto the system. The same is true for commercially available security systems.
So, the way this is dealt with is by creating zones, with each zone containing a group of sensors. A bedroom, for example, may be defined as one zone with a window sensor, a door sensor, and movement detector forming that zone. In this scenario, each sensor is connected to the next in a series (or daisy-chained); if one of them triggers, it will alert the monitoring system that there was a trigger in the zone. Obviously, though, it may not necessarily be the actual detector, which in most applications isn't really an issue.
However, this can introduce some challenges when we're considering mixing normally open and normally closed type sensors within a zone, but this is something...
