1A2 Key Telephone System

The 1A2 Key Telephone System is a business telephone system developed and distributed by the Western Electric Company for the Bell System.

The 1A2 Key Telephone System was a modular system that provided flexible solutions for a variety of telephone service requirements. It provided multiple users with control over multiple telephone lines without the requirement for an operator, system attendant, or receptionist. Each user could select a specific telephone line to place calls on, or to answer calls, and manage those calls by placing them on hold or transferring them to other stations. The system provided options for station-to-station signaling and intercom, and music-on-hold. The control functions were operated directly on each telephone instrument with a set of push buttons (keys) that had lamps installed internally to provide visual indication of line status.

Introduced in 1964, the 1A2 system represents a stage of key telephone systems development at Bell Laboratories that started in the late 1930s with the 1A Key Telephone System, and was an improvement over the 1A1 system introduced in 1953.

Compatible 1A2 equipment was manufactured by competing vendors, such as Northern Telecom, Automatic Electric (GTE), ITT, and Stromberg-Carlson. The successor technologies to the 1A2 Systems include the AT&T Merlin, AT&T Spirit, and AT&T Partner systems.

The 1A2 Key Telephone System was produced to provide flexible solutions for widely varying telephone service requirements in businesses and enterprises.

The 1A2 system used a modular plug-in construction concept that permitted many configurations using the same basic components. A typical system consisted of a basic metal mounting frame, the Key Service Unit (KSU), also called a panel, with card-edge connectors and mounting brackets for components and punch-down blocks for interconnecting cabling. 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. Each central office telephone line connected to the system required at least one KTU.

The mounting panels varied according to the size and complexity of the telephone system. Typical early 1A2 systems used the Type 583 and 584 panels. The 584C panel contained an interrupter and 13 KTUs. The 583 panel did not have the interrupter and held 15 KTUs. For smaller installations, panels were available that housed all components, including the power supply and connecting blocks. Typically these panels supported only four to six central office lines.

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