This document introduces Signaling Line Circuits (SLCs), which allow communication between fire alarm panels and intelligent, addressable devices. SLCs connect intelligent detectors that identify various threats, addressable modules that interface conventional devices, and addressable pull stations. Devices communicate on the SLC using one of two protocols: Classic Loop Interface Protocol (CLIP) or FlashScan. While CLIP is compatible with legacy systems, FlashScan provides faster communication and supports more devices per loop. The document describes NFPA wire styles for SLCs and discusses isolator modules and bases that can protect critical elements of an SLC from faults.