1A2 Key System
From Wikipedia, the free encyclopedia
The 1A2 Key System is an analog multiline business key telephone system. Unlike more modern multiline systems, every telephone line serving a particular phone is wired into that phone, and electromechanical switches (the "keys") switch the lines in the phone itself. The control unit handled the ringing, line indicators (key lights), hold functions, intercom, and music on hold via modules. The original and largest manufacturer of 1A2 systems was Western Electric, later known as AT&T Technologies, Lucent Technologies, and now Avaya.
This system consisted of a central control unit and a number of specialized telephone sets. Each line to the telephone sets was routed using six wires:
- Two wires (one pair) carried the actual telephone line
- Two wires (a second pair) carried control information for that line
- Two wires (a third pair) carried current to a lamp installed at the telephone
A telephone set could contain five, 12, or many individual telephone lines. A common five-line keyphone, often called "KV set" would be connected using 25-pair cable and an Amphenol 50-position "MicroRibbon" connector. The lamps installed at the telephone sets allowed the user to instantly determine the status of all of the individual telephone lines that "appeared" at that set:
- Lamp off — The line is idle
- Lamp steady on — The line is in use for a call
- Lamp flashing slowly — The line is ringing with an incoming call
- Lamp winking fast — A call on the line is "on hold"
A user could select any of the lines simply by pressing the appropriate line button and picking up the handset. A caller could place a call "on hold" by pressing the red "hold" button. This would place the call on hold and then mechanically release the depressed line button, allowing the user to select another line.
An individual worker or executive might have a set with one or a few lines "appearing". The system attendant (receptionist) might have a set with many lines appearing so that they could monitor the status of all incoming lines simultaneously.
These systems also supported manual buzzers, intercom lines (with or without selective ringing), music on hold, and other simple features. The features were provided on a line-by-line basis by the selection of particular Key Telephone Units (KTUs) plugged into a pre-wired backplane in the central control unit. The central control unit also provided power for the entire key system (including ringing voltage). A mechanical interrupter in the power supply provided the pulsing voltages for the various lamps, buzzers, and ringers in the system.
Compatible 1A2 equipment was manufactured by a number of vendors including Western Electric, Northern Telecom, and Automatic Electric (GTE).
The 1A2 system is uncommon nowadays but some very large installations are still in use due to the high cost of replacing them. 1A2 systems are also used at radio stations because being analog, they are easily patched into the radio transmitter for putting callers on the air. Additionally, due to the electromechanical nature of the system, it remains functional in the event of a power failure, with the exception of certain features such as Hold and Line Use Indicators, therefore it is commonly used in police precincts and other emergency response offices.
The 1A2 system has largely been replaced by the AT&T Merlin, AT&T Spirit, and AT&T Partner systems. These are all now sold and/or serviced by Avaya.
