Multiplex systems, a sort of transition between conventional and modern addressable systems, were often used in large buildings and complexes from the mid to late 1970s into the late 1980s. Early on, these systems were programmed to function as large conventional systems. Gradually, later installations began to feature components and features of modern addressable systems. These systems were often capable of controlling more than a building's fire alarm system (i.e. HVAC, security, electronic door locks...) without any type of alarm or trouble condition present. While the main panel was the brains of the system and could be used to access certain functions, fire alarm controls were usually accessed through transponders.