When a fire alarm system is first set up, there are two types of devices that are programmed: one type of device is the Control Panel and the other type of device are all the smoked detectors, heat detectors, pull stations, flow switches, relay outputs, etc. in the Building.
When it is first installed, the control panel is programmed so it can know every device that is connected on the loop. The designer of the building's fire alarm system has a list of devices that will be connected, and assigns an address number to each device. On this list is the address number, the device type (smoke detector input, flow switch input, relay output, etc.), and what to do when each input device goes into alarm.
Assigned with each address number is a short description of the device to be displayed on the screen of the control panel. The words for device 27 could be something like "Smoke Detector 2nd Floor by Room 210". When the smoke detector at address 27 sends in an alarm, the panel turns on red alarm light and displays on its screen, "Alarm Device 27" and "Smoke Detector 2nd Floor by Room 210".
At the time the panel is first programmed, it is also given commands of what to do when Device 27 goes into alarm, commands like "Turn On NAC1, NAC 2, NAC3 (Notification Appliance Circuits or Horns and Strobes)", and "Turn On 52 (Relay Output)".
Devices in the Building
Each device in the building (each smoke detector, each flow switch, each relay, etc.) is assigned the same number that the control panel has assigned to it. The devices don't know about the descriptive words in the control panel, the devices only know what kind of device they are, and what address has been dialed in on the addressing thumbwheels on the back of the device.
The manufacturer of the fire alarm system might use the addressing thumbwheels, or a set of little DIP switches, or the address number may have been assigned inside each device using programming of the device by the panel, but each device has its own address.
The control panel doesn't know if the devices are connected to the loop like they're supposed to be connected. To find out, every few seconds the panel checks each device, one at a time, to make sure each device is still connected.
The polling is a little like a school teacher taking attendance. The teacher calls out a person's name, everybody hears the name but only the person with that name says "Here"; the fire alarm control panel calls out an address, all the devices on the loop receive the address but only the device programmed with that address responds with the equivalent of "Here".
After the control panel polls device 27, if there's no response from the device saying "Here", the control panel assumes there's trouble with device 27, turns on the yellow trouble light, and displays "Trouble Device 27" and "Smoke Detector 2nd Floor by Room 210".
When they're polled, the devices actually send back to the panel more information than just "Here"; when they're polled, the devices usually send back "Here, and OK". The devices can also send back information like "Here, and Dirty", or "Here, and In Alarm", or "Here, and In Fault".
When the panel receives "Here, and OK", the panel does nothing. When the panel receives anything else, the panel turns on the correct light, displays the correct message, and if commands have been programmed in, performs whatever command has be programmed.