Douglas Krantz - Technical Writer - Describing How It Works
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What Happens to the SLC when a Device is Disconnected?

The Signaling Line Circuit (SLC) is a single pair of electrical conductors that connects all the devices and the control panel together. If the SLC doesn't continue past a device, the devices no longer connected to the panel don't communicate.

What Happens to the SLC when a Device is Disconnected?


Greetings Douglas,

In an addressable fire system, if I disconnect the wires for SLC (Signaling Line Circuit) loop from the in or terminals of a detector, will the other devices get power? How do the other devices communicate?

Thank you, J N

Purpose of the Signaling Line Circuit

A Signaling Line Circuit (SLC) has two purposes:
  1. An SLC is used to send signals between the control panel and the devices on the loop
  2. An SLC is used to provide power from the control panel to the devices on the loop

To understand what is going on electrically, though, we have to look at an SLC from an electrical point of view. From an electrical standpoint, an SLC is two pieces of copper conductors:
  • The positive wire of an SLC is electrically one conductor - even though there are splices at each device, the positive wire of the SLC is still one conductor
  • The negative wire of an SLC is electrically one conductor - even though there are splices at each device, the negative wire of the SLC is still one conductor

Sending and Receiving Signals

Often, we think of the signals on the SLC as being sent from the panel to a specific device on the SLC, and the specific device sends its signal to the panel. That is only partially true.

Panel to Device - The panel sends a signal on the Signaling Line Circuit (SLC). The SLC, being a single pair of conductors, carries that single signal to all of the devices attached to the circuit. Only the device that has the address included at the beginning of the signal pays attention to the signal. All the other devices ignore the signal.

Device to Panel - A single device sends a signal on the SLC. The SLC, being a single pair of conductors, carries the signal to all the devices and the control panel. The control panel is the only device that has the address included at the beginning of the signal, so the control panel is the only device that pays attention to the signal.

If part of the Signaling Line Circuit is disconnected from the panel, the devices on the part of the SLC that is disconnected don't communicate.

Power

The addressable detectors and devices and the addressable input and output devices all have electronics inside them. They need power to operate.

The control panel is the power supply. The panel provides power to the SLC, and the devices receive their power to operate from the SLC.

If part of the Signaling Line Circuit is disconnected from the panel, the devices on the part of the SLC that is disconnected don't receive the power they need to operate.

Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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