1A2 Key System

1A2 Key System

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 original and largest manufacturer of 1A2 systems was Western Electric, later known as AT&T Technologies, Lucent Technologies, and now Avaya. Compatible 1A2 equipment was also manufactured by a number of vendors including Western Electric, Northern Telecom, and Automatic Electric (GTE).

Systems that have replaced 1A2 equipment include the AT&T Merlin, AT&T Spirit, and AT&T Partner systems. These are all now sold and/or serviced by Avaya.


The system consists of a central control unit, known as a KSU (Key Service Unit), and a number of specialized telephone sets. The KSU provided DC and AC power for system functions, lamp control, intercom services, and any specialized functions that might have been needed for a specific installation such as tie-line, paging, or other features. Pulsating signals for buzzers, lamps, and internal ringing were provided by an electromechanical interrupter, contained either within the power supply itself or inside the KSU.


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 (known as 'A-Leads') for that line
* Two wires (a third pair) carried current to a lamp for the specific line key position on the phone.

A telephone set could contain anywhere from two to over twenty-nine individual telephone lines. Most key telephones (often called 'keysets') with up to nine line positions are connected to the system using a single 25-pair cable and an Amphenol 50-position "MicroRibbon" connector. Keysets with up to 19 line positions used a 50-pair cable, where the big sets with 29 line positions used 75 pairs (three connectors). There was even a 'Call Director' style phone made, at one time, which had over 30 line key positions, and used 100 pairs (four connectors).

Each of the keyset cables was usually run back to the wiring closet, or whatever central location where the KSU had been installed, and terminated on a connection device known as a 66 block or punch block. The blocks most often used to terminate these station cables were the type 66M1-50. Each of these blocks could accept two 25-pair cables (50 pairs, total) for termination.

Cross-connect wire jumpers, consisting of three twisted pairs (the six wires referenced above) would then be run between these blocks and the larger distribution connecting blocks within the KSU.

Very large installations of 1A2 systems had multiple wiring closets fed by branch cables extended from the central closet where the KSU was located. An example of this type of installation would be a multi-story building. The KSU and incoming lines might be in the basement, and each floor would have a branch wiring closet of its own where the phones for that floor were connected.

User interface

A user could select any available phone line 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. Doing so would also mechanically release the depressed line button, allowing the user to select another line.

An individual worker or executive might have a set with just 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.

Optional components for the 1A2 could also provide a function called 'I-Hold,' where a call could only be retrieved off hold at the phone that originally placed the line in the hold mode. 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.


Audible signals (most often ringers or buzzers) could be handled in one of two ways. The first is that the ringer in a specific telephone set could be hardwired to one specific phone line. This had the advantage that the phone would ring any time a call came in on that one line, even during a local power failure, but it also had the disadvantage of limiting ringing to that one line. No other lines could be connected to that ringer without causing problems.

The second method, sometimes known as 'common audible,' utilizes the internal circuitry of the KSU's 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. This has the advantage that you could route the ringing signal for any given line to any phone, or combinations of phones, but it also had the disadvantage of being non-functional during a local power outage.

As for buzzers, they were not usually designed to accommodate the 100 volt/20-30Hz ringing signal used by telephone bells. Instead, they usually took advantage of low-voltage AC (10-18 volts). Unfortunately, this had the same limitation as the second signaling method mentioned above, in that it was not functional during power outages.


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 (half second on, half second off) — The line is ringing with an incoming call
* Lamp winking fast — A call on the line is "on hold"


The 1A2 system is uncommon today, but some very large installations are still in use due to the high cost of replacing them. There are also several systems in use by collectors of 'vintage' telephone equipment. 1A2 systems are also very popular, still, with radio stations. This is because, being analog, they are easily patched into the radio transmitter for putting callers on the air.

Unlike most electronic key systems or PBX's, 1A2 systems remain partially functional in the event of a local power failure. The telephones themselves are still able to make and receive calls, assuming the phone company's central office stays alive, but the system is unable to provide any sort of visual (lamps) or audible (buzzers or ringers) signaling during a power outage. The 'Hold' function and intercom services would also be inoperable.

Despite the fact that the 1A2 systems have widely been replaced by more recent electronic key systems or PBX's, the simple and modular design of the 1A2's components provide a degree of versatility and reliability that few of its modern successors can match. This is why 1A2 equipment may be seen in service in places like emergency operations centers, and older police and fire stations.

See also

* RJ21 -- The 25-pair connector used for 1A2 systems

Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Key telephone system — A key system or key telephone system is a multiline telephone system typically used in small office environments.Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however… …   Wikipedia

  • Business telephone system — Early model 464G Western Electric key telephone set A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private …   Wikipedia

  • AT&T Merlin — 5 button telephone (voice terminal) manufactured in early 1985 The AT T (also Lucent, now Avaya) Merlin telephone system was introduced in late 1983, branded American Bell Merlin. After the breakup of AT T in 1984, it was rebranded AT T Merlin.… …   Wikipedia

  • Avaya Unified Communications Management — Developer(s) Nortel (now Avaya) Operating system MS Windows, and Linux Type Unified Communications Configuration and Management Avaya Unified Communications Management in computer networking is the name of a collection o …   Wikipedia

  • Avaya — Inc. Type Private Industry Telecommunications Founded 2000 Headquarters …   Wikipedia

  • Centrex — This article is about the type of telephone exchange. For the British police training agency, see Centrex (police training agency). For the Gazprom related group of companies, see Centrex Europe Energy Gas AG. Typical standard phone used with… …   Wikipedia

  • Avaya Application Server 5300 — Avaya Aura AS5300 The Avaya Aura Application Server 5300 or Avaya Aura AS5300 in telecommunication and computer networking is a secure Unified Communications (UC) system designed and manufactured by Avaya. The system provides integrated IP… …   Wikipedia

  • Avaya IP Phone 1140E — in telecommunications is a desktop IP client manufactured by Avaya for unified communications. The phone can operate on the SIP or UNIStim protocols.[1] The SIP firmware supports presence selection and notification along with s …   Wikipedia

  • Avaya Communication Server 2100 — The Avaya Communication Server 2100 or CS 2100 (former name Succession softswitch) [1] in computer networking, telecomunications and IP telephony is a carrier grade, IP soft switch or VoIP server solution for large campus and geographically… …   Wikipedia

  • Avaya Definity — This article is about the Avaya Definity PBX. For the Definity digital film recorder, see Definity. An Avaya G3si PBX with front cover removed (view from the top). The Definity was the last in a line of circuit based PBX hardware manufactured… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”