Conventional Notification has been around for as long as fire alarm systems themselves have been around. The concept here is very simple, apply power to a device, and it makes a noise, flashes a strobe, or both. This is the current standard for how most alarm systems are made today.
In the early days of fire alarm, these devices used unsupervised, high voltage to sound. Some systems just acted as a big light switch. If an initiating device was activated, power would be applied to the horn or bell circuit until the pull station was reset. Some other systems would only sound off the devices a few times and then automatically stop, as the initiating device did not need to be reset manually. Most of these systems are well behind us now.
In modern conventional notification, we have 24 VDC circuits that are supervised by a resistor at the end of the line. That way, when a device is taken down, a break in the wire, or a short, the system can go into trouble with out the system needing to be in alarm to fix it. This supervision voltage is generally very low. The concept of sounding off the devices, however, remains the same. When the system is in alarm, the NAC outputs 24 VDC which sounds off the horn/strobes.
Circuitry is also very basic in this regard. In order for the circuit to be properly supervised, it must be impossible to take down a device with out causing an open circuit. To do this, modern manufactures require devices to be wired as shown below.
In modern Notification Devices, strobe synchronization is very important. Devices such as the ones shown above allow for synchronization, and many brands have offered sync functionality for many years. Most conventional devices will do their own thing with out a synchronization code being sent down the wire, but some may not properly function with out the sync code. For more about that, check out Synchronization Code.