All Fire Alarm Systems
A fire alarm system is a Detect Fire and Warm People about the Fire system. It has automatic detection devices like smoke detectors, heat detectors, and waterflow switches; it has manually operated input devices like pull stations and MCPs (Manual Call Points).
The detection devices send their alarms to a Fire Alarm Control Panel or FACP. Automatically deciding what to do when there are alarms, the control the control panel has circuitry to sound the alarm. Providing electrical power to the detection devices and notification appliances, the control box also has a power supply.
NAC Provides Power to Sound the Alarm
When the control panel (FACP) sounds the alarms, it provides power to the horns and strobes (Notification Appliances).
When the FACP isn't sounding the alarm, it is providing backwards voltage on the Notification Appliance Circuit (NAC). When it's backward voltage, the wires in the NAC circuit are being checked; the backwards voltage can't be used by the horns or strobes. When the FACP is sounding the alarm, it is providing forward voltage on the NAC; because the voltage is now forward, the horns and strobes can use the power and sound the alarm to warn people.
Unless it has addressable horns and strobes, that process is common to almost all fire alarm systems.
Fire Alarm Control Panel
No manufacturer will ever tell you or tell me how their Fire Alarm Control Panel works. The most they will ever you or me is that the panel works.
Conventional and Addressable
Most building wide fire alarm systems have conventional devices, and the addressable systems, besides having conventional devices, also have addressable devices. The addressable devices are most often connected to the panel using a SLC circuit (Signaling Line Circuit).
The conventional devices are often connected directly to the panel on IDC circuits (Initiating Device Circuits) and on NAC circuits (Notification Appliance Circuits). Often, using conventional circuitry, the conventional devices are connected to addressable modules, and then the addressable modules are connected to the panel using the SLC.