Douglas Krantz - Technical Writer - Describing How It Works
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How Does Multiplexing of the Signaling Line Circuit Work?

A multiplexed circuit is a circuit that carries more than one signal. In the case of a SLC (Fire Alarm Signaling Line Circuit), the signals take turns so only one signal at a time in being carried by the SLC.

The Signaling Line Circuit has 3 different types of signals multiplexed onto one 2 wire circuit - Data from the fire alarm control panel, Data from the field devices, Power from the control panel.
The 3 different kinds of signal all take turns traveling on the same pair of wires on the Signaling Line Circuit (SLC).

By Douglas Krantz

Depending on what one needs to understand about a circuit, the circuit can be thought about in different ways. Take for instance a Signaling Line Circuit (SLC). A Signaling Line Circuit is a circuit that starts from the SLC output terminals on the fire alarm panel and goes to the addressable devices in the building.

From one point of view, the SLC is a pair of copper conductors connecting all the building's field devices to the fire alarm panel. From another point of view, the SLC is a multiplexed distribution system (power and multiple signal transmission system).

This multiplexed system carries power from the fire alarm panel to the devices in the field, data from the fire alarm panel to the devices in the field, and data from the devices in the field to the panel.

In the communication world, there are several types of multiplexing. A Plain Old Telephone System (POTS) is one type, and a Fire Alarm Signaling Line Circuit (SLC) is another type.

(When reading the following descriptions, keep in mind that the mechanics and electronics of the SLC are not described, only the results of the mechanics and electronics.)

Multiplexed Plain Old Telephone System (POTS)

A telephone circuit, like the land line going from one home to another, has constant power provided by the central office, and continuous two way communication; a person in one home can be talking at the same time as the person is talking in the other home. In other words, the telephone circuit carries power and both signals at the same time; there's no interference.

Multiplexed Fire Alarm Signaling Line Circuit

A Fire Alarm Signaling Line Circuit (SLC) is different; power and each signal are time-shared. A Signaling Line Circuit carries power, or the SLC carries the data from the panel, or the SLC carries data from a field device like a detector or module.

The SLC does not carry more than one at a time; power would interfere with the signals, the signals would interfere with power, and the signals would interfere with each other. It's all time-shared.

Time Shared Power

Power takes turns with data. Other than during the intervals of time that data is sent one direction or the other on the Signaling Line Circuit, full voltage power is constantly sent out from the panel to the modules on the SLC. Power is even sent during the time between bits of data.

The power from the fire alarm panel is used several ways by the devices in the field.

Time Shared Signals from the Input Modules

To send data to the panel, the modules interrupt the power on the SLC.

Input devices to the fire alarm system (detectors and input modules) receive signals of alarms, supervisories, and trouble, and the computers inside the device translate this information to data. Using the SLC data bus, once the information has been translated to data, the input modules send the information to the fire alarm panel.

To be clear, because input modules are included inside each addressable detector's housing, the addressable smoke detectors and heat detectors only look like they are single devices. However, using internal connections, they are really conventional detectors networked to addressable input modules.

Because the input module is inside the same housing as the detector, other signals like sensitivity or dirtiness are sent between the detector and module, and then sent back to the fire alarm panel.

Time Shared Signals to the Output Modules

To send data to the modules, the panel interrupts power on the SLC. Supervised riser control modules and control relay modules, just like screw terminals on the fire alarm control panel, are output ports for the fire alarm system.

Information sent along the SLC data bus is received by the output modules, and according to the information received the output modules take action.

In the case of a supervised riser control module, the output port (using an internal relay) switches between internal supervisory circuitry (to send supervised trouble signals to the fire alarm panel) and a separate power riser input to the module (to switch on the power, audio, or telephone circuit).

The Fire Alarm Panel Polls the Modules

Because this is a time-shared system, the panel acts as the gate keeper. At appropriate times, the panel uses data to address a module, and when addressed, the module sends data back to the panel.

This addressing of a specific module is also a polling request, or an "I'm OK" check. From the panel, polling requests are sent out to the modules one at a time. The addressed module returns with a message.

Besides just saying "I'm OK", information about alarm, supervision, and trouble signals is sent from the modules back to the panel.

One Signal at a Time

Outside of the proprietary differences in protocol (how the data is arranged), and the different baud rates of the signals (how fast the signals are transmitted), between one manufacture's Signaling Line Circuit and the next manufacturer's SLC, there isn't much difference.

The fire alarm system is a Life Safety System

To prevent interference between the signals and power, and to make sure the Signaling Line Circuit will always be fully functional, the signals are sent one direction at a time. During the intervals of time that there is no data being sent either direction, power is sent out from the panel.

Whereas the SLC is a pair of copper wires, from a multiplexed point of view, the SLC is a time-shared power and multiple signal distribution system.
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