1A2 Key System

A typical rotary dial key telephone: the Western Electric eighteen button Call Director, manufactured from 1958 to the early 80s.

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 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.

Components

The earlier 1A2 technology consisted of Key Telephone Systems (Bell Systems Practice 518-215-100). Most Key Telephone Systems at that time (as late as 1980) consisted of key telephone units (KTU) which provided circuit features such as line interface, dial intercom, direct line circuits, etc. Also part of the system were the mounting facilities that hold the KTUs, and power supply. The common telephone sets at that time were the 565 (5-lines), 630 (17 lines), 631 (29-lines), 830 (9-lines), and 831 (19-lines). If the telephone set number started with a '2', such as '2565', then it was Touch-Tone.

The power supply provided 24VDC for relay operation, 24 VDC filtered for talk battery (intercom and direct-line services), 10VAC for lamps, 18 VAC for buzzers, and 90VAC at 20Hz for ringers (Bell System Practice 167-466-101). Lamp and signaling voltages were passed through an interrupter (KS-15900 L1) to create lamp flash (incoming line), lamp wink (hold), and interrupted buzzer and ringing. The mounting facilities varied according to the size and complexity of the Key Telephone System. Typical of early 1A2 systems was the extensive use of 583 and 584-panels to hold the KTUs. The 584C panel contained an interrupter and 13 KTUs. The 583 Panel did not have the interrupter and held 15 KTUs (Bell Systems Practice 518-215-410).

For smaller installations Key System Panels could be purchased that housed all components, including the power supply and connecting blocks. Typically these panels supported only six lines and a dial intercom (Bell System Practice 518-215-424).

Five pairs of wires were required for each line: The central office tip and ring, the station (telephone instrument) tip and ring, the A and A1 (control) leads, lamp and lamp ground, and the ring (signaling) pair. The cable to the telephone instrument required fewer pair: station tip and ring, A and A1 pair, and the lamp lead. The A1 ground was only used on the first button, it was not terminated afterward. The Yellow-Slate (grey) pair was usually used for the ringer (Bell System Practice 502-541-407).

Wiring

Each line to the telephone sets was routed using six wires:

25 Pair Color Code Chart as used by 1A2 systems

• 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.

Audio

Audible signals (most often ringers or buzzers) could be handled several 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.

The third method that was sometimes used because it had the advantages of both of the previous methods, was a combination of the first two methods. A set of relays that were powered by the KSU power supply continuously. The common audible ringing signals from the KSU would run though the energized relays to certain phones that would also ring if there was a power failure. The phone lines that terminated at the KSU were also terminated at these relays and in the event of a power failure, the relays would de-energize and switch the phone lines to the ringers of selected phones.

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.

Visual

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"

Advantages

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 studio equipment 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

External links

• Telecommunications Virtual Museum -Retrieved 29 July 2011
• 1A2 Key System explanation -Retrieved 29 July 2011
• The Power of 1A2 -Retrieved 29 July 2011
• 1A2 Key System Advantages -Retrieved 29 July 2011