1A2 Key Telephone System
The control functions are operated directly on each telephone instrument with a set of push buttons (keys) that have lamps installed internally to provide visual indication of line status.
[1] Compatible 1A2 equipment was manufactured by competing vendors, such as Northern Telecom, Automatic Electric (GTE), ITT, and Stromberg-Carlson.
The 1A2 system used a modular plug-in construction concept that permitted many configurations using the same basic components.
The principal switching and control modules were constructed on printed circuit boards, called Key Telephone Units (KTU).
KTUs provided many system features, such as various types of line interfaces, dial intercom, music-on-hold, and alarms.
[2] For smaller installations, panels were available that housed all components, including the power supply and connecting blocks.
For key system operation these sets were equipped with a set of push-buttons (keys) and additional internal contact springs to control the additional operational features, resulting in a large variety of specialty telephones.
Most keysets with up to nine lines are connected to the system using a single 25-pair cable terminated with an Amphenol 50-position "MicroRibbon" connector.
A user could select any available telephone line by pressing the appropriate pickup key and taking the handset off-hook.
Key telephone systems also supported manual buzzers, intercom lines (with or without selective ringing), music on hold, and other features.
The cadence of the 'I-Hold' lamp signal was steady illumination punctuated by a series of rapid blinks (produced by a module called a 'flutter generator') every couple of seconds.
The telephones may still be used to make and receive calls when the central office is available, but the system is unable to provide visual or audible supervision, as well as hold functions and intercom services during power outages.
Another method, sometimes known as common audible, utilizes the internal circuitry of the KSU power supply, and circuitry in the individual key telephone units serving each line, to provide a separate and locally generated ringing signal for each phone line.
