Control Units, Circuits, and Wiring Methods
What the control unit does
The control unit receives initiating signals, applies the programmed logic from the sequence-of-operations matrix, drives notification, supervises every circuit, and reports. Modern panels integrate the CPU, power supply and charger, addressable loop controllers, notification appliance circuits, relay outputs, and a communicator. Older conventional panels use zone modules.
Classes and styles
Class B circuits (Style B initiating, Style Y notification) are two-wire with an end-of-line device; a single open disables everything beyond the break and produces only a trouble. Class A circuits (Style D, Style Z) return to the panel so a single open does not disable the devices; Class A is required in high-rise and other critical occupancies. Addressable loops work the same way: a Class A signaling line circuit with isolator modules keeps the rest of the loop alive after a short. The panel must supervise initiating circuits, notification circuits, and the loop for open, short, and ground faults and annunciate trouble.
Wiring
Article 760 governs: power-limited fire alarm cable, separation from power circuits, support and protection, termination in listed enclosures, and identified conductors documented on as-builts.
Worked example and trap
On a 40-device conventional zone, an open at device 25 removes all 25 downstream devices but shows only a trouble. A short across the zone produces an alarm, because the short looks like the end-of-line current path bypassing the devices. Candidates often assume a short is always a trouble. Equally common: an end-of-line resistor installed at the panel, which destroys supervision of the zone.