Douglas Krantz - Technical Writer - Describing How It Works
Get the Book Make It Work - Convetional Fire Alarms
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Why are Modules Needed in a Fire Alarm System?

The modules attached to the Signaling Line Circuit, or SLC, are the inputs and outputs of the fire alarm system. Rather than have all the devices, with all their wires, connect directly to the control panel, the devices connect to modules and the modules connect to a single SLC.

The modules on a signaling line circuit, or SLC, are inputs and outputs for a fire alarm system

Greetings Douglas,

I really enjoyed your website. It was very informative. However, there are a few things for which I am trying to get more clarity on. It is the role of modules in the SLC! Why exactly are they needed, and how do they work. If you could help or maybe even guide on where I could find these answers, I will be very grateful.

Thank you, T

To understand why there are modules connected to the Signaling Line Circuit (SLC), step back and think in terms of "Building Wide Fire Alarm System". The idea of a building wide fire alarm system can be thought of as a Fire Alarm System (FAS), or more accurately Fire Detection and Alarm System (FDAS).

Fire Detection and Alarm System

A FDAS (Fire Detection and Alarm System) is an electrical means of detecting fire and warning people of fire. The words "conventional" and "addressable" don't even fit into the description of a Fire Detection and Alarm System.

Fire Detection Device

A switch is an electrical device used for the detection of fire:
  • Water flowing throws an electrical switch - flowing water has detected fire
  • A person having seen a fire, runs to the manual pull station or manual call point and throws an electrical switch - the person detected fire
  • A heat detector detects heat or smoke detector detects smoke and throws an internal switch - the smoke detector or heat detector has detected fire

Fire Alarm (Notification) Device

A Notification Appliance (horn, strobe, speaker, etc.) warns people of a fire:
  • A fire horn makes noise - it has warned people of fire
  • A fire strobe flashes its light - it has warned people of fire
  • A speaker on the wall makes noise and uses a voice - it has warned people of fire

Fire Control Device

A controlled device, while not really detecting fire or directly warning people of fire, is used to protect people from fire or smoke:
  • A door holder stops holding a door - it has prevented fire and smoke from progressing
  • A damper in a ventilation system closes - it has prevented fire and smoke from progressing
  • An air handling system is shut down - it has prevented smoke from progressing
  • An elevator is stopped to keep it away from a fire floor - it has protected people from fire and smoke

Building Wide FDAS vs Control-Box-on-the-Wall

When considering a fire alarm system to be a detect and warn system, the control panel takes on a secondary role: it doesn't detect fire, it doesn't warn people of fire.

On the other hand, fires can occur anywhere in the building, and people who need to be warned of fire can be anywhere in the building. The fire alarm system has to be all over the building.

Not all Fire Detection and Alarm Systems have a control-box-on-the-wall. Stand-alone smoke alarms for residences are inside a single box on the ceiling; they have a smoke detector and a sounder inside the box. Many of them don't have control systems at all. The smoke alarms, though, detect fire and warn people of fire.

Waterflow switches use an outside fire horn to tell the firefighters that water is flowing in the fire suppression system, but the switch-to-outside-horn circuitry usually doesn't use a control-box-on-the-wall.

Fire alarm systems, therefore, are the detectors, switches, sounders, strobes, and contacts either connected to the control panel, or just connected to each other. In other words, the control panel is only that, the controls for the Fire Detection and Alarm System (FDAS).

Conventional Control Panel

The control panel has screw terminals for the inputs (Initiating Device Circuit or IDC), screw terminals for control relay outputs for smoke control or other functions, and screw terminals for outputs to power the horns and strobes (Notification Appliance Circuit or NAC).

For each set of screw terminals, there is input or output circuitry connected to the terminals. If there are 10 IDC inputs, there are 10 sets of IDC input circuitry; if there are 4 NAC outputs, there are 4 sets of NAC output circuitry, if there are 3 relay control outputs, there are 3 control relays.

Between the sets of IDC input circuitry, the sets of NAC output circuitry, and the control relay, there is a single, two-way, communication line to the Central Processing Unit (CPU) of the fire alarm control panel. This is called a computer bus, and it is inside the printed circuit board of the Fire Alarm Control Panel (FACP).


The Central Processing Unit (CPU) of the control panel is the guts of the FACP (Fire Alarm Control Panel).

The CPU (Central Processing Unit) has been programmed to know what each individual input is and whether it's an alarm or supervisory input, and whether the input latches or can reset on its own. The CPU has also been programmed to know what each NAC output is and whether it's going to be continuous sounding or temporal.

For its internal rules of operation, the CPU has even been programmed to know which output NAC and/or relay will activate with an alarm comes in on an individual IDC input.

Internal Bus Communication Line

The CPU uses the Internal Bus Communication Line to receive the alarms from the IDC inputs, and to tell which NAC or relay output will activate when an IDC input goes into alarm. This is a single communication line that has everything connected to it. All the devices take turns talking, so they don't interfere with each other.

Input, Supervised Output, and Control Modules

At this point, use an imaginary scissors. In your mind cut the circuit board for the FACP (Fire Alarm Control Panel) so that each IDC input (both the terminals and the input circuitry), each NAC output (both the terminals and the output circuitry), and each relay output (both the terminals and the actual control relay) is on a separate printed circuit board.

Once the cutting is done, each IDC input, each NAC output, and each Control Relay output is on a stand-alone printed circuit board. Even the CPU is now on a stand-alone printed circuit board.

Now, use imaginary wires to make an External Bus Communication Line connecting all the stand-alone circuit boards together, including the CPU. This external bus will replace the internal bus so all the inputs, outputs, and the CPU can communicate with each other again.

Re-Named External Bus

At this point, rename the external bus to a Signaling Line Circuit or SLC. You have now converted a conventional fire alarm system to an addressable fire alarm system.

What About Addressable Detectors?

I have seen where one fire alarm manufacturer has taken another manufacturer's off-the-shelf conventional smoke detector, glued an addressable input module to the back of the detector, and called the combined assembly an Addressable Smoke Detector.

There's another manufacturer that uses its own conventional pull station, mounts an addressable input module to the back of it, and calls the assembly an addressable pull station.

In essence, all addressable detectors and pull stations are regular detectors and pull stations, with an added addressable input module.

Addressable fire horns, addressable fire strobes, and addressable fire speakers are the same way. They are normal fire horns, fire strobes, and fire speakers that have addressable NAC output modules inside them.

Even addressable control relays are just relays, with addressable control modules inside them.

Role of the Modules on the Signaling Line Circuit (SLC)

  • Addressable Input or Monitor Modules, and Addressable Input Devices use the SLC to send alarm signals to the CPU (Control-Box-On-The-Wall)
  • Addressable Supervised Output Modules (NAC) use the SLC to receive turn-on signals from the CPU (Control-Box-On-The-Wall)
  • Addressable Control Modules use the SLC to receive turn-on signals from the CPU (Control-Box-On-The-Wall)

The Signaling Line Circuit carries the signals that are used to communicate between the detection devices, the alarm output devices, the control output devices, and the CPU (Control-Box-On-The-Wall).

Douglas Krantz
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