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Just What Is a Signaling Line Circuit (SLC)?

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A Signaling Line Circuit carries data to and from the devices in the field of a fire alarm system, and carries power to operate the devices.
A Signaling Line Circuit (SLC) carries information in the form of data to and from the field devices for the fire alarm system, and also carries power from the control panel to the devices.
By Douglas Krantz

The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss. Designed to be connected to the SLC, the only devices used on the SLC are the Main Computer (Fire Alarm Control Panel or FACP) and Sub-Computers (Modules and Detectors).

Terminology

Just to keep things straight, the NFPA uses the words Signaling Line Circuit (SLC) because that's what the circuit does --- the SLC allows devices to signal each other.

I often like to use the words Data and Power Circuit because that's what the circuit uses to accomplish the Signaling.


SLC Signaling Using Data

The Signaling Line Circuit is used by the main computer (Fire Alarm Control Panel or FACP) to communicate with all the sub-computers (Detectors and Modules).

The data sent from the FACP to the detectors and modules carries information like address and polling, and sometimes programming. The data sent from the detectors and modules to the FACP carry address, acknowledgement of the polling (equivalent of "I'm OK"), alarm, supervisory, and trouble signals.

SLC Power

The power itself is provided by the FACP. While the provided power is seemingly weak, it is enough to power all the sub-computers on the SLC and also the LEDs, the Detectors, and the Supervision of the conventional circuits controlled by the SLC modules.

Time Shared Signaling and Power

The power and signaling is accomplished on a time-share basis: much of the time the FACP is providing full power to the devices; sometimes the FACP is sending data to one or more detectors or modules; sometimes a detector or module is sending data to the FACP.

Devices and Modules are Interpreters

Between the SLC and Conventional circuits, the sub-computers on the SLC are interpreters.
  • Output Modules --- They interpret the data sent from the FACP into supervised riser outputs (Class A or Class B circuits for audible and visual devices) and relay contact outputs.
  • Input Modules --- They interpret from Class A or Class B circuits into data sent to the FACP.
  • Detectors --- Even the detectors, inside the same container or box as the associated sub-computers, can be thought of as conventional fire alarm smoke or heat detectors connected to input modules.

Signaling Line Circuit --- Data and Power Circuit

On its data-buss, just like the internal communications inside any computer, the signaling on the SLC between the devices and the FACP is done with data.

The power to run the Devices (modules and detectors) is included on the SLC.

The data from the devices to the FACP, the data From the FACP to devices, and the power for the devices is all provided for on a time share basis on the SLC.



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