Each smoke sensor is connected to a single relay device as depicted on p. 6, 3rd figure from top (right column) in the document "Smoke sensors".

The lines named "load" in that figure are connected to the rack connector placed on the rear panel of the apenext rack: one to pin 6 and the other to pin 1(or 2,3,4,5 one for each smoke sensor). The model is simple: one can consider the "load" outputs of the relay as a switch which is closed when alarm is setted and open in the other case. In other words the resistance measured between pin 6 and pin 1 (or 2,3,4,5) should be 0 when alarm is on and infinity when alarm is off.

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