FMM-101 is a miniature monitor module (a mere 1.3" (3.302 cm) H x 2.75" (6.985 cm) W x 0.5" (1.270 cm) D) used to supervise a Class B (Style B) circuit of dry-contact input devices.
This unique and patented device has been helping stop false fire alarms around the world for more than 25 years, without restricting legitimate alarms.