Does anyone know how interconnect signaling for fire alarms works?

[ryanc@infosec.exchange]

Does anyone know how interconnect signaling for fire alarms works?

Mine have one signal wire plus mains (neutral and live), and I assume it's probably one of:

(normally floating, active x volts DC)
(normally neutral, active x volts DC)
(normally floating, active neutral)

[ra6bit@infosec.exchange]

@ryanc In the US I believe it is white (neutral), black (hot), red (signal) with Red to black normally floating, alarm signal is notionally+9VDC red to black but 7 to 20VDC in practice. White to black is 120VAC.

[ryanc@infosec.exchange]

@ra6bit makes sense. I think I'm going to need to use a meter and then set them off.