I took a Simplex smoke detector base wiring diagram and simplified it to show the basic wiring. In this case there is one terminal for the positive side of the zone and two terminals for the negative side of the zone. The separate terminals for one side of the IDC is for detector head supervision. Whether to use the positive side or the negative side to accomplish the detector head supervision is up to the manufacturer.
In the drawing terminals 1 and 2 have parts that form a contact that closes the connection between them when the detector head is installed in the base. If the detector head is removed that contact opens which breaks the connection to the End Of Line Resistor so a trouble will show on the panel. Terminal 4 is the positive side of the circuit. The two wires that go into terminal 4 must be separate wires, not a loop with the insulation removed. That is to provide supervision if that connection is broken somehow. Terminal 3 is used for the external LED connection if needed.
