No. 49 PAGE No. 43

The 2 + 5 Keymaster
Circuit Development Engineering Department
JULY 1964

Low cost, compactness and ease of operation are highlights of this new key-system-design. Elaboration of equipment is avoided without restricting the potential applications or narrowing the range of facilities offered.

IN the intensive setting of the modern compact business, the surrender of office space to a switchboard and the allocation of staff duties for its surveillance, tend to become equally unacceptable. The development of’ key systems may be seen as a series of efforts, progressively more effective, to provide a basis of communication free from these disadvantages.

The success of a given system is therefore to be judged by two criteria. Firstly, by its compactness; the aim should be to introduce only a condensed minimum of equipment external to the station instruments. Secondly, by its simplicity of operation; this is especially important since the task of handling traffic may in practice fall to varying grades of staff, senior as well as junior.

As a basis of design the Plan-Etelphone conception of station instrument has a number of favourable features. Sufficient mechanical switching flexibility exists to permit all the main functions of a moderately sized system to be brought under direct control of the button-operated springsets without resort to external switching apparatus.

There is a comparable degree of flexibility in the physical layout of springsets and signalling lamps within the instrument. The ideal layout is one in which the pattern of buttons and lamps itself suggests to the user what the system is capable of doing and how it may be made to do it. With a reasonable approach to this ideal, operation soon becomes practically instinctive, the only conscious rapport between hand and eye being that necessary in dialling.

A study of these considerations in the development of the 2 + 5 ‘Keymaster’ has resulted in a notably compact system, simpler to operate than its predecessors and of unusually low installed cost. The facilities offered include all those likely to be required by an intermediate-sized business.

There is provision, under normal circumstances, for a maximum of five stations, each equipped with the instrument shown above. Instruments are available in ivory, two-tone grey and black, and in forms suitable for operation in auto or manual CB areas.

The station button arrangement shows the logical separation of intercom and exchange line functions, the former being associated with the four ‘CALL' buttons arranged symmetrically around the dial and the latter with the 2 : 1 : 1 : 2 button formation in line above it. A bell cut-off toggle switch is positioned below the dial. Operationally, the exchange line button formation is divided into two groups of three buttons, each consisting of the single ‘EXCH’ button, giving access to the exchange line concerned, and the two ‘twinned’ buttons adjacent. These latter respectively operate in association with the ‘EXCH’ button, to give a ‘cancel’ facility and to produce a ‘HOLD‘ condition on the line. Each button-group is mechanically independent of the other. Within a group the ’EXCH‘ and ’HOLD‘ buttons, both of locking type, mutually inter-trip, i.e. the depression of either button releases the other. The ‘X‘ button releases both, and is itself non-locking.

To prevent loss of exchange facilities due to’ button fiddling’ when the instrument is not in use, replacement of the handset renders locking buttons mechanically inoperative as well as restoring any which are locked down. Intercom call buttons are non-locking and can be operated with the handset on rest; a useful feature because intercom signals may be required for subsidiary purposes, e.g. for summoning secretaries or announcing break periods.

Visual signalling of incoming exchange line calls is by means of the white lamp mounted in front of each ‘EXCH’ button, this lamp also serving to give supervisory indication when calls are in progress. The adjacent red lamp, in front of the ‘HOLD‘ button, indicates a ‘ held’ condition on the line.

Audible signalling of incoming exchange line calls, on either line, is by means of the instrument bells at all stations where the bell cut-off switch is not operated. It is usual to arrange, by strapping in the appropriate instrument, for one station bell to remain operative at all times.

With the system as normally installed, exchange lines are series-multipled from station to station and series secrecy is thus afforded, stations usually being arranged in order of priority from the exchange end of the multiple.

If the requirements of an organization are such that two or more stations commonly need to enter exchange lines together, the series multiple arrangement can be replaced by a parallel one.

To remove the possibility of the two exchange lines being connected together - a first necessity in system design - the station switching is so arranged that connection to Exchange Line 2 at a station instrument is via break-action springs in the ‘Exch1‘ springset. Connection to Exchange Line 1, on the other hand, is direct, through these springs only. Exchange Line 1 therefore has a form of switching priority over Exchange Line 2.

Full exchange line service is maintained during local supply failure, but supervisory lamp signals are absent.

The intercom system is non-secret and features a common speech-link. Audible signals are employed and are given by buzzers in the station instruments; signalling is selective, utilizing separate conductors in the inter-station cable.


Removal of the handset at any station causes the instrument to be directly connected to the intercom link. To establish a call it is only necessary to verify, by a brief listening check, that the link is free, and then to press the appropriate ‘CALL’ button.

The buzzer signal is given at the wanted station for the duration of the press.

Conference calls may be set up in a similar manner by calling the wanted stations in turn.

Exchange Lines
An exchange call is originated by removing the handset, selecting a free exchange line (associated white lamp extinguished) and pressing the corresponding ‘EXCH‘ button. The call may then be made through the external system in the normal manner; for its duration the associated white lamp is continuously lit at all stations.

If the call is not successful, it may be cancelled by operating the ‘X’ button momentarily and the ‘EXCH’ button re-pressed to originate a new call, without need to replace the handset. The same procedure may be adopted on conclusion of a call if it is required to take up the other exchange line or the intercom.

The ’hold‘ facility permits the concurrent handling by any station of up to three calls (both exchange lines and intercom) with an operating flexibility comparable with that given by a cordless switchboard.

When for instance an exchange call on Line 1 has been established, the station concerned may operate the appropriate ’HOLD‘ button in order to make an enquiry or answer a call on Line 2 or upon the intercom, or to enter the latter for the purpose of offering transfer of the Line 1 call to another station.

Having completed the Line 2 or intercom call, he can then return to Line 1 by re-pressing the associated ‘EXCH’ button or, if transfer of the Line 1 call has been accepted elsewhere he may relinquish it, either by replacing the handset or, if further calls are to be made, by pressing the associated ‘X’ button. In a similar way, Line 2 may be left ‘held‘ while a call on Line 1 or the intercom is being dealt with, and may subsequently be taken up again, or the call on it transferred.

While the ’HOLD‘ button is locked down, that is until the ‘EXCH’ button is re-pressed, the X button operated or the handset replaced, there is a glow on the associated red lamp at the holding station. The white lamp indication continues unchanged at all stations during the hold period and until the line is finally released.

In the concurrent handling of calls, conversation on any one of the three outlets from a station is private from waiting callers on the other two.

An incoming exchange call, in addition to ringing at selected stations, is signalled by a flash on the associated ‘,vhite lamps throughout the system, this flash being at the ringing interval. To take the call, the answering station removes the handset and presses the appropriate ‘EXCH’ button; bell signals then cease and white lamp indications change to a steady glow.

One selected station is normally designated as ‘answering station’ and the bell cut-off switch will normally be made inoperative here. If this station operator goes off duty, arrangements will be made to ensure that bell signals are received at a manned station elsewhere.


Any station may be barred exchange line access by ‘slipping’ the multiple at the station instrument; barring may be applied on either exchange line, or on both.

Exchange Outlets
Lines to an associated PABX, PBX or PAX may be substituted for one or both exchange line outlets. Because of the switching priority given to Exchange Line 1, where the two lines actually connected are of differing importance, it will be the recommended practice to associate the one of greater priority with this outlet.

The question of the relative priority of exchange lines is also of interest in relation to an optional feature of the system, that of suppression of intercom buzzer signals at one or more selected stations when exchange lines are in use. This provision may obtain when one exchange line is switched to a station, or when either line is switched.

The application of this feature will depend on organizational factors. It may, for instance, be of value when a principal regularly makes important calls on a higher priority line and must give his undivided attention to them. It will be less desirable to apply it with respect to a lower priority line as this may prevent or delay transfers of calls on the higher priority line to the station concerned. The actual arrangement adopted may be varied after installation by a simple re-adjustment of strappings.

Additional buzzers and bells, if required, may be mounted externally to the station instruments at any convenient point.

System Variants
Considerable flexibility is possible in the arrangement of the system to suit differing requirements.

A typical secretarial arrangement can be made. One secretary serves two principals with intercom between all three, the secretary normally filtering exchange calls on both lines.

An arrangement permitting the connection of seven stations is to be considered. Two stations are divided into two close-proximity pairs. Intercom is provided between the pairs and the other stations, but not between stations in a pair. Individual intercom calling of one of a pair is by code.

Sharing by one station may also be imposed as a modification to the standard system if an ‘Operator Recall‘ or similar facility is required at station instruments, the necessary
button being installed in place of one of the ’CALL’ buttons.

In any of the above arrangements, full exchange line facilities can be given throughout.

The key springsets are contained in a single unit of box construction mounted between the handset rest pillars, and consist of eight banks of comb-operated contacts. To facilitate connection of the key unit to the main telephone circuit, it is wired by a short cable form to three terminal strips, each comprising six terminals. These are mounted horizontally on a framework locating at the rear of the telephone chassis and held in place by spring plungers.

The four lamps, of miniature type, are slide-mounted in a unit which is wired to the key unit and secured to the body by a single screw. Individual lamps are readily accessible. The lamp, key and terminal strip units with their inter-connecting wiring may be removed from the instrument as a complete assembly.

The lamps accurately align with the lens axes to give optimum light transmission. This, despite the inconspicuous blending of the lenses with the instrument body, is so effective as to make signals prominent even in direct sunlight.

To simplify subsequent rearrangement of intercom system numbering, the ’CALL‘ button caps are easily removable and can be interchanged with a fifth, spare cap numbered ‘5’, stored within the instrument body.

The desk terminal block consists of a two-sided terminal arrangement contained in a moulded case with detachable cover.

The common equipment unit contains a transmission feed retard for the intercom and two relays for ringing and lamp signalling purposes, together with a cable terminal strip.

An important feature of the system is the low operating voltage (6V) made possible by the absence of locally energized relays, and there is a correspondingly small power supply requirement; no energy is wasted in dropping resistors and the only drain on the local source is in supplying the low-consumption signalling lamps and the intercom transmission feed.

A battery eliminator, shown to the right, is normally specified for the system. This is capable of a maximum output of 1 amp at 6 volts and is suitable for use on a.c. mains in the voltage range 100-125 (5-volt steps) or 200-250 (10-volt steps). Its dimensions are 6.1 in. long by 4.6 in. high by 4.6 in. wide (15.6 x 11.6 x 11.6cm).

In the absence of a mains supply the system may be operated from dry batteries. In the interests of maximum economy it is then arranged, by suitable internal strappings in the instruments, for station signalling lamps to be inoperative while the handset is on rest.

The inter-station cable is connected to the instrument desk blocks and requires a total of only 17 conductors where there is a ‘straight‘ series multiple, i.e. when the position of stations along the cable from the exchange-end to the tail coincides with the required order of exchange line priority. If some other order of position is more convenient, a larger cable may be installed giving the necessary extra pairs for routing the multiple to and fro as required. Incoming exchange pairs may be taken to some intermediate station if this proves more economic of cabling; these pairs are again connected to the instrument desk block.

The common equipment unit is invariably connected at the tail end of the cable.

The maximum loop resistance between the first station and the common equipment is fixed by considerations of satisfactory local signalling performance. This limit is unlikely to be approached in any normal layout of offices, even dispersed within relatively large buildings.

Satisfactory transmission is achieved over exchange line loops up to 1000 ohms.

The facilities provided by the system are in many respects comparable with those hitherto associated with appreciably larger desk units and much more complex central apparatus.

Simplifications have been considered entirely from a user standpoint; in this the economics of an installation, its operating demand and its effectiveness in service have well-defined significance.


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Last revised: July 20, 2003