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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.〔S.E. Bush, ''Advances in the 1A2 Key Telephone System'', Bell Laboratory Record, October 1970, pp. 259〕 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. ==Components== 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.〔AT&T, Bell Systems Practices, Section 518-215-410, ''Service - 1A2 Key Telephone System Panels, 583 and 584-Type''〕 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.〔AT&T, Bell System Practices, Section 518-215-424〕 The most commonly used telephone sets for the 1A2 systems were modifications of the Bell System standard 500-series telephones for rotary dial systems, and the 2500-series Touch-Tone desk sets. 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. Specifically, such telephone sets were the types 565 (up to 5 lines), 630 (17 lines), 631 (29 lines), 830 (9 lines), and 831 (19 lines). Telephone sets could be either rotary dial models, or be equipped with Touch-Tone keypads. A power supply was either mounted within the panel or separately nearby. The power supply provided 24 VDC for relay operation, filtered 24 VDC for talk battery (intercom and direct-line services), 10 VAC for lamps, 18 VAC for buzzers, and 90-110 VAC at for ringers.〔AT&T, Bell System Practices, Section 167-466-101〕 Lamp and signaling voltages were routed through a mechanical interrupter, to create lamp flash (incoming line), lamp wink (hold), and interrupted buzzer and ringing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1A2 Key Telephone System」の詳細全文を読む スポンサード リンク
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