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There are two conductors in the SLC that carry data to and from the panel and the modules and detectors, and it also carries a small amount of power.
The Signaling Line Circuit (SLC) uses the same two conductors to carry data to the modules from the panel, it carries data to the panel from the modules, and it carries power to run the computers inside the modules from the panel.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer


The Signaling Line Circuit (SLC) is a Data/Power Bus

By Douglas Krantz

The SLC isn't an RS232 communication circuit. The SLC isn't an RS485 communication circuit. Proprietary to each manufacturer, the SLC is its own type of communication-line/power-supply circuit.

The Signaling Line Circuit

Carrying signals back and forth inside the fire alarm control panel's circuit board (or for that matter, the internal circuits for any other computer system), is a data bus. This data bus is between the Central Processing Unit (CPU) and the memory, the displays, the controls, and the input and output ports.

Carrying signals back and forth outside the control panel is another data bus. This data bus is between the fire alarm control panel and all the fire alarm detectors and modules. It carries signals throughout the building. In the case of the fire alarm system, this outside-the-panel data bus (Signaling Line Circuit or SLC) also carries power for the attached detectors and modules.

Data Bus Makes the Fire Alarm System Addressable

Because this is a data bus, it carries more information than a simple alarm/no-alarm condition of the devices. Specifying which device should be receiving information from the fire alarm panel, and which device is sending information to the fire alarm panel, this data bus carries addressing information. The addressing information is the way the panel can specify which module it's talking to, and which module is sending data to the panel.

The heart of the addressable fire alarm system is the addressing data being sent along the SLC.

Internal and External Compatibility

Between the fire alarm panel's internal data bus and the fire alarm system's external data bus (SLC) is a data translator. So the two different kinds of data buses can talk to each other, it converts or translates the data formats.

The external port connections for this translator are the SLC screw terminals on the fire alarm panel.

It's Still a Conventional Fire Alarm System

Even though the fire alarm system is addressable, all inputs and outputs connected to the addressable fire alarm system are conventional.
  • The manual pull stations are conventional pull stations with internal or external addressable modules
  • The waterflow switches are conventional waterflow switches with addressable modules
  • The gatevalve tamper switches are conventional tamper switches with addressable modules
  • The smoke detectors are conventional smoke detectors with internal addressable modules
  • The heat detectors are conventional heat detectors (even if they are electronic sensor types) with internal addressable modules
  • The control relays are physical relays with internal addressable modules
  • The Notification Appliance Circuit (NAC) modules are relays, switching between a supervisory circuit and external 24 volt power, with internal addressable modules

Modules and Detectors on the SLC

To interpret the data for the conventional fire alarm inputs and outputs, and attached to the Signaling Line Circuit (SLC), data translating modules are required.

The modules convert the conventional input and output information to and from data on the SLC so the Signaling Line Circuit can:
  • Carry alarm and supervisory data to the fire alarm panel from the module input ports (detectors and input modules)
  • Carry control data to the output modules from the fire alarm panel (riser control modules and control relay modules)
  • Carry supervisory data to the fire alarm panel from the supervised output modules (Notification Appliance Circuit {NAC} risers)
  • Carry trouble data from all the modules to the fire alarm panel
  • Carry addressable polling data (equivalent to an "I'm OK" check) between the panel and the modules

Power

In addition to carrying data, the SLC also carries power from the fire alarm panel to operate the modules, external supervisory circuity, internal relays, and any internal detectors.

Data/Power Bus

Even though the Signaling Line Circuit (SLC) is made up of only two conducting wires, and even though the wires and terminations are all custom installed, between the fire alarm control panel and all the input and output modules, the SLC is a data and power bus.


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