Signaling Line Circuit (SLC)
The Signaling Line Circuit or SLC in a fire alarm system is a power and computer style data bus. It is used to provide power to the computers inside each module and detector (each device), carry information from the panel to the devices, and carry information from each device to the panel.
Because this is a life-safety system, all power and communication has to be extremely reliable. In essence, the power for the devices and each signal sent take turns on the wires so there is no electronic confusion between the signals.
Addressing the Devices
The panel has no idea where in the building the devices are located, and the devices don't know where they are either. The panel only calls out to the devices using the number programmed into each device (its address), and the devices respond when their number is called out by the panel.
It's an "I'm OK" system. The panel sends data out on the Signaling Line Circuit (SLC) and basically says "Number 27, are you there?" Module or Detector 27 had better answer back data saying "I'm here and I'm OK" or the panel assumes that device number 27 has a problem.
When device 27 doesn't answer, device 27 says there is some sort of trouble, or device 27 says there is an alarm, the panel looks up the location information on line 27 in a look-up table. At the time the fire alarm system was installed in the building, the look-up table in the fire alarm system's database was provided by the fire alarm installer.
The look-up table has lines in it that match the numbers assigned to the modules and detectors. Each line in the look-up table also has a sting of numbers and letters (characters).
Keep in mind, though, the panel isn't very smart; the panel doesn't know what this string of characters means.
The panel then displays the string of characters on its screen. It's up to the original installer to make sure the string of characters uses the words of the location.
Power and Computer Style Data Bus
- The panel provides power to the devices
- The panel uses data to call out a number for one of the devices attached to the SLC and asks "Are You OK?"
- The device with that number programmed into it responds to the panel using data
- If everything is not OK, the panel displays numbers and letters (characters) from its look-up table