The 1A2 Key System is an analog multiline key telephone system used in small businesses and homes. Unlike more modern multiline systems, every telephone line
serving a particular phone is wired into that phone, and electromechanical
switches (the "keys") select a line to be used.
A 1A2 system consists of a central control unit, known as
a KSU (Key Service Unit), phones, accessories, and cable.
The KSU has a power supply (either internal or nearby)
that converts the standard 110 volts AC to various AC and DC voltages necessary
for system operation. For example, lamps (light bulbs) operated on 10 volts AC,
ringing is about 90 volts AC, and intercom uses 10 or 20 volts DC.
Intermittent pulses of power to provide light flashing and
rhythmic ringing is provided by a motorized switch called an interrupter. It
uses a rotating camshaft with multiple lobes that caused metal contacts to make
and break contact to apply and remove power as needed. Later 1A2 KSUs used
electronic interrupters instead of electromechanical ones.
The "architecture" of the 1A2 system used six wires in
three pairs for each line going to each phone:
The first pair, known as "tip and ring" carried voices,
and the dialing signal from a phone/
The second pair, "A and A1," provided a switch closure
when a line key was pressed to energize a "line card" to turn on the lamps to
indicate that a line was in use.
The third pair, "L and LG" (lamp and lamp-ground) provided
the power for each lamp on a phone.
A1 and LG were both connected to the ground side of the
power supply, and there was no need to provide separate AI and LG wiring paths
to each line position in each phone. Some phones used one common ground wire
that was shared within a phone, to reduce the number of wires needed.
The most common 1A2 phone has six buttons. One button is
for Hold, and the other five could be used for a mix of lines, intercom and
features. It uses a 25-pair (50-wire) cable, but not all of the wires were
connected. There are many variations of the common six-button phone, with
modifications including a headset jack, amplified handset, and a key that
can rotate to control a function.
The 10-button phone has one horizontal row of buttons and
provides up to nine lines with the same 25-pair cable.
Similar phones with 20 or 30 buttons use 50-pair and
Early large 1A2 phone had vertical strips of six buttons
and were called "Call Directors." A model with 30 line keys used a 100-pair
Initially the 1A2 cables were connected to "terminal
blocks" near each phone by attaching wires to individual screws. This method was
labor-intensive and could easily lead to mistakes and malfunctions. In YEAR,
phones started to be equipped with 25-pair plugs. The were initially
manufactured by Amphenol, and today "Amphenol cable" is the general term for a
telecommunications cable with a 25-pair connector on it, even if the connector
is made by another company such as AMP or 3M.
Generally, each phone's cable was run back to the
central location where the KSU was located, and "punched down" onto a "66-type punchdown block" which connected to the KSU's circuitry.
In small installations, all of the cables could be
connected with the KSU cabinet. Larger installations used external blocks which were
connected to the internal block with 3-pair "cross-connect wire."
Large 1A2 installations could have several points where
phone cables came together, to limit the cable necessary to reach the KSU. The
blocks in these locations are called an IDF (Intermediate Distributing Frame).
Blocks near the KSU are an MDF (Main Distributing Frame).
Besides the power supply and interrupter, the other main
component in a 1A2 KSU is the "line card." It's a plug-in circuit board that
provides light control and hold operation for one line. The general family of
cards used for central office line operation is the "400 card," and various
models were made including 400C, 400D, etc. Later models incorporated advanced
circuitry that made them smaller and less expensive, and added features such as
music-on-hold and LED indication of card operation.
The same card format was also used for other functions,
such as the 401 manual intercom card.
KSUs were made with various capacities starting at four
lines. Big systems consisted of multiple "584" panels that could hold up to
13 line cards each.
To make a call, a 1A2 phone user selects an available
phone line simply by pressing the appropriate un-lit line button and picking up
the handset or activating a speakerphone. A caller could place a call "on hold" by pressing the red HOLD
button. Pressing the button also causes a depressed line button to pop up, and
the line card and interrupter would cause the lamp to flash to indicate a call
on hold. The flashing rhythm is different for ringing and holding.
People in an office can have different phone models with
different lines appearing on them. A small system where each phone has the same
lines is called a "square" system.
These systems also support buttons and buzzers, intercoms (with or without selective ringing), music on hold,
paging and other
features. The features were provided by the installation of
particular Key Telephone Units (KTUs) plugged into a socket in the KSU. Some
accessories were mounted outside the KSU.
Optional components for the 1A2 could also provide a function called I-Hold
or Exclusive Hold,
where a call could only be taken off hold at the phone where it was placed on
hold. 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 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 way one bell in a phone could ring on
multiple lines. It had the disadvantage of not working during a power
outage. Uninterruptable Power Supplies ("UPSes") were not commonly used with 1A2
systems, as they are with modern phone systems.
Some times ringers were connected to relays so that during a power failure, they
could be powered by the phone lines.
Buzzers used to indicate an intercom call were usually
powered by 10 volts AC from the KSU's power supply. Later phones and KSUs could
provide intercom operation, with announcements from speakers and hands-free
The lamps (light bulbs) in the phones allow the user to easily
determine the status of all of the lines that "appear" on a phone:
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 studio equipment for putting callers on the air.
Unlike most electronic key systems or PBX's, 1A2 systems remain partially
functional during a 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.
Although the 1A2 systems have mostly been replaced by more recent
electronic, digital, and IP phone systems, 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 is in service in
emergency operations centers, and older police and fire stations.
The photo shows a 4-line AT&T "shoebox" KSU. Photo from TelecomHistory.org