A Signaling Line Circuit (SLC) is more of a concept than a specific wiring method / power source / signal protocol. Each manufacturer uses different methods for their circuits. Some manufacturers have used three types of SLC, or more. No two types of SLC are the same.
Most SLCs are two-wire, carrying power from the panel to the detectors, and signals between the panel and the addressable devices in the rest of the building. Some SLCs are four-wire, two wires for power, one wire for the panel to send signals to the devices, one wire to send signals from the devices to the panel.
The SLC power supplies can be 16 volts, 20 volts, 22 volts, 24 volts, 30 volts. The SLC power supply could even be 12 volts (nominal).
Some SLCs can be analog, using pulse-width modulation for some parts of the signals; some SLCs can be totally digital, using only computer style data signals.
The protocol, though, is strictly propriety; no manufacturer gives out their protocol.
Currently, world-wide, there are many dozens of SLC types in use. You will have to contact the manufacturer of the system you're working with to get more detailed information.