PAGE No. 2


January 1962

This improved ‘House Exchange’ system, known as the ‘Keymaster’, adds significantly to the special services and features offered by the new extension-plan arrangements recently introduced by the Company. Designed for the B.P.O., the Keymaster gives maximum flexibility of services and basically provides access from any one of five local stations to one c.b./auto exchange line or P.B.X.-extension line, and intercom between stations. The system includes lamp signalling and, in addition, provides for the connection of a single long-line station in lieu of one local station.

Operation is from a 50-volt d.c. supply derived from a battery or power unit.

The house Exchange system, designed in two versions (1 + 5 and 2 + 10), was first introduced into service in the early ‘30’s. Since this time it has provided a very satisfactory answer to the problem of meeting intermediate service needs in large private residences and small businesses where telephone requirements have expanded beyond the extension-plan stage, but do not justify the expense of a P.A.B.X. installation. As might be expected with any telephone system, no matter how useful or functionally efficient, there comes a time when new ideas in apparatus design and the availability of smaller components and more versatile telephones suggest improvements which can usefully be made. For this reason, and because of the continuing demand for this type of service, it was decided that a major revision of the existing system be undertaken, commencing with the 1 + 5 version.

Figure 1
The new Multiple Station Telephone and predecessor

In general, when re-designing any telephone equipment which the customer both sees and uses, it is logical that effort should be directed to improving its appearance, making it available in colour, and reducing its size. To these aims must be added the always desirable objectives of easier operation and maintenance as well as more simplified and economical installation. Most important, the new design must offer improved communication facilities.

The new system, known as the ‘Keymaster’ and designed for the B.P.O., fulfils these requirements. The ‘Keymaster’ accommodates up to five stations interconnected by multiple cable and, basically, provides intercom services between stations and access from any station to one cb/auto exchange line or p.b.x.-extension line. Provision is also made for the inclusion of a non-multiple 2-wire station in lieu of one multiple station. This useful feature permits communication to a remote location within or beyond the curtilage of the subscriber’s premises where the cost of extending the multiple cable would be prohibitive.

Key-switching telephones of Plan-Etelphone type, suitably modified for multiple and non-multiple working, are used in the interests of standardization, appearance, colour range and transmission performance. Because of the telephone’s basic design, all individual switching and signalling arrangements, including lamp signalling, are incorporated in each multiple-station instrument, thus eliminating the previous need and inconvenience of desk-mounting units accommodating lever keys and drop indicators. In addition, this arrangement has allowed the common equipment for a purely multiple-station system to be accommodated in a single wall-mounting unit of small dimensions. Even when the long-line 2-wire station is introduced, the auxiliary signalling relays required can be combined with the common equipment in a similar but larger unit. Besides these advantages, further economies have been obtained and both maintenance and installation simplified by the use of the instrument’s desk-block, which dispenses with the need for junction boxes or distribution cases.

Figure 2
Internal view of Multiple Station Telephone showing the monitor/restrict-access relay with cover removed

Operation of the system is from a 50-volt d.c. supply derived from a battery or an external a.c. mains-driven power unit.

A summary of the facilities offered by the ‘Keymaster’ is listed below, new and improved service features being indicated in italics.


  1. Direct calling between multiple stations by d.c. buzzer.
  2. Direct calling from multiple station to the 2-wire station by a.c. bell, ringing being derived from a transistor-ringing unit common to all multiple stations.
  3. Calling of multiple stations from the 2-wire station via selected multiple station.
  4. Lamp indication at all multiple stations of intercom call in progress.
  5. Conference.


  1. Direct access to the exchange from multiple stations.
  2. Direct access to the exchange from 2-wire station.
  3. Answering of exchange calls direct at multiple stations; and at 2-wire station under night service conditions.
  4. Holding of exchange calls at multiple stations for purposes of information and transfer calls.
  5. Holding of exchange calls at 2-wire station and transfer with the assistance of a selected station.
  6. Restricted access to exchange from any station.
  7. Series-secrecy on exchange connections.
  8. Visual indication of ‘exchange ringing’, ‘call answered’, and ‘exchange held’ conditions at multiple stations.
  9. Re-call of p.b.x. or exchange operator from multiple stations.
  10. Universal night service.
  11. Access from all stations to exchange line in event of power failure.
  12. Exchange-bell cut-off at any multiple station.
  13. Audible engaged test at multiple station when any called station is occupying exchange line.
  14. Monitoring facilities from any multiple station.
  15. Extension bells and buzzers.
Figure 3
Key mechanism/terminal-assembly unit

The new multiple-station telephone, illustrated alongside its predecessor in Figure 1, has six press button keys conveniently arranged in 1:1:2:2 formation above the dial. These consist of a bell off key, a locking ‘exchange’ key, and four non-locking intercom keys of the familar ‘two-in-One’ pattern. Key legends are clearly marked, and the numeral inserts 1 - 4 in the intercom keys are removable and inscribed with the figure 5 on the reverse side to permit easy re-arrangement of station numbering.

Economy of buttons has been obtained by applying secondary functions to the ‘exchange’ key, which is mechanically coupled with the cradle-switch and all intercom keys. The mechanical arrangement of these two types of keys is such that on the depression of the ‘exchange’ press-button to originate or answer an exchange call, an associated group of plungers and springsets operate and the ‘exchange’ key locks to the ‘speak’ position. When any intercom key is subsequently pressed for the purpose of a local information call, a plunger in the operated group releases to restore the ‘exchange’ key button and cause a ‘hold’ condition to be applied to the exchange line. The released plunger is re-engaged with its group when the ‘exchange’ key is again pressed to the ‘speak’ position to return the station user to the exchange caller. From this position the ‘exchange’ key may be restored and its plungers fully released on replacement of the handset at the conclusion of the call, or may be pressed again beyond the ‘speak’ position to provide a further function, i.e. operator recall. An important feature of this function is that ‘recall’ is possible only in the sequence described and the operation is rendered ineffective should the exchange key be pressed in error direct to the over-press position.

To prevent unnecessary operation of the ‘exchange’ key while the handset is on its rest, a locking device is included in the key unit.

The bell/off key may be used as a non-locking key or be locked in position and subsequently released by means of its sliding cap to provide the switching functions for any one of four facilities. Simple strap adjustment in the telephone introduces the key into circuit.

As a non-locking key it serves to restrict access from any station to the exchange line, the key being connected to an add-on relay unit in the telephone to be restricted. This unit, shown in Figure 2, mounts above one of the exchange-bell gongs in the instrument and comprises a miniature-type relay using comb-operated springs with twin contacts.

As a locking key it may be used to control a similar relay unit for monitoring purposes. In this application, the relay unit is mounted above the bell as before, but incorporated in the same telephone as the controlling key. A further use is diverting incoming exchange ringing to the 2-wire station for night-service working, the required switching function being performed usually at the last multiple station in the system. Finally, as its designation implies, the bell/off key may be utilized to silence the instrument’s exchange bell. This facility is of considerable convenience where, for example, a station user wishes to be undisturbed by unimportant calls or where it is essential to prevent an unstaffed station from being fruitlessly rung during night-service. Usually the bell/off facility is denied to one station to ensure that one bell is permanently connected to line.

To facilitate connection of the key unit to the main telephone circuit, three terminal strips, each comprising six terminals, are mounted horizontally on a framework secured to the rear of the telephone chassis by a spring plunger. The assembly, together with the key mechanism, is shown in Figure 3, and the whole may be removed from the instrument as a unit.

Positive lamp signalling is ensured by the provision of simple signals, the use of different coloured lamps and lenses allowing adequate penetration of light.

The two lamps seen in the instrument face provide a total of four signals. The right-hand lamp, termed the ‘intercom-busy’ lamp, provides a single signal and emits a steady white glow when any station’s handset is removed from its rest. The corresponding ‘exchange-line’ lamp gives three red signal indications as follows:-

  1. An interrupted signal at ringing frequency on receipt of an incoming exchange or p.b.x. call and until the call is answered.
  2. A continuous glow when the incoming call is answered.
  3. A flashing signal at approximately 3 p.p.s. when a user ‘holds’ an exchange call while making an information call. The signal becomes steady when the user returns to the ‘held’ line or when a second station takes over the call.

The instrument is usually connected to the desk block by a 20-way cord. At installations where stations are not continuously staffed, however, an instrument may be equipped with a 25-way cord to enable an additional buzzer or exchange bell or both to be provided at particular locations on the premises.

Figure 4
Cord-termination plate before connection to desk-block base

The desk terminal block consists of a mounting plate, a cord termination plate and a moulded case and cover. As illustrated in Figure 4, the instrument cord conductors are permanently connected to metal tags fixed on the upper and lower sides of the insulating plate. Associated tag screws, held captive to prevent complete withdrawal, serve to align the plate quickly and clamp the tags to corresponding metal bushes pressed into the case. The bushes are knurled to ‘bite’ the case moulding and eliminate any possibility of movement from excessive screw tightening. In addition, the bushes are ‘tapped’ on the underside to provide screw terminations for the multiple cables. Two cord and cable entries are provided at opposite ends of the block and this number can be increased to four by use of ‘break-ins’ in the cover.

When all connections are completed and the terminal plate assembled, the case is located over the threaded spigot of the mounting plate and retained in position by the cover-fixing screw.

Because this instrument uses a 2-wire line, lamp signalling and direct calling of all multiple stations is not feasible. For this reason the 2-wire station telephone, while resembling the multiple-station instrument in general appearance and size, includes no lamps, has fewer press-button keys and is associated directly with a selected multiple station to obtain assistance when a call to the remaining stations is desired.

The two press-button keys provided consist of one locking ‘extension’ key for access to the selected station, and a non-locking ‘exchange’ key. Both keys correspond in type to the line keys in the multiple-station telephone but differ in function. Internally, the visible differences are the omission of the key-unit terminal assembly, and the replacement of the d.c. buzzer by an a.c. bell, which is dual purpose, serving both for signalling intercom as well as exchange-line calls.

A common or combined services unit is the only additional switching apparatus required in the system. The common-services unit (Figure 5) is l2.1” long x 7.25” high and 4.75” wide (308 x 186 x 121 mm.) and contains a transmission-feed relay for the intercom line and three 3000-type relays for ringing and lamp signalling purposes, together with a cabling terminal strip. The combined unit (Figure 6) is a similar all-metal unit, but is four inches wider to include the 2-wire station’s auxiliary equipment. This consists of six switching and signalling relays, all of 3000 type, and a 25-cycle transistor ringing unit, which is an optional extra for use when no suitable ringing source exists on site.

Fig. 5 Fig. 6

The power unit is also an optional extra. This unit is a battery eliminator used as an alternative to a battery to provide the 50-volt d.c. supply for the system. It is designed to operate from 200/250volt 50 hz mains and incorporates a transformer, choke, fuses and terminal strip. The unit, for wall or rack mounting, has a louvered pull-off cover.

All multiple stations are inter-connected by a 21-wire cable. The exchange line terminates on the desk-block at the first station (see Figure 7 (a) ) and is continued from this point in the multiple cable to the desk-block at Station 2 and so on, to terminate finally in the common-services unit, to which the power unit cable is also connected.

When the 2-wire station replaces a multiple one, its cable terminates, together with the power-unit and multiple cables, in the combined services unit as shown in Figure 7 (b).


When a call between multiple stations is originated, the caller lifts the handset and the individual ‘intercom-busy’ lamps glow steadily at all multiple stations, giving visual warning that the intercom line has been seized. On momentary operation of the appropriate intercom key, a signalling earth is applied to the called line to sound the d.c. buzzer in the telephone of the wanted station. Removal of the handset at the called station completes the through connection, and the call proceeds in the normal manner, the intercom-busy lamps remaining lit until the replacement of both handsets at the termination of the call.

To call the remote 2-wire station (when fitted), the same operating procedure and visual signalling conditions apply as for a multiple-to-multiple station call. Because this station uses a common signalling a.c. bell, however, a suitable ringing current is necessary in place of the normal earth calling signal used between multiple stations. This is provided automatically by the transistor ringing generator in the combined services unit when the relevant calling key is pressed. Connection between the two stations is established when the 2-wire station user lifts the handset and operates his ‘extension’ key. Replacement of the handsets restores both telephones to normal, the ‘extension’ key at the called station being released automatically.

All intercom calls from the 2-wire station circulate via the selected multiple station and are originated by pressing the locking ‘extension’ key. A distinctive signal is then given by the instrument buzzer sounding continuously at the selected station, thus permitting ready identification of the calling station. Meanwhile, tone is passed to the caller and the appropriate lamp signal is given at all other stations.

The attendant at the selected station responds by lifting the handset, thereby disconnecting both buzzer and tone from the line, and proceeds to call the wanted station. On reply, the call is set up and the selected station withdraws from the connection by replacing the handset.

If any station (multiple or 2-wire) is called while engaged on an exchange line, the calling signal is diverted at the engaged station to operate the buzzer in the caller’s telephone during the period the relevant intercom key is pressed, thus providing an engaged test.

As there is a common talking path for intercom purposes, conference calls can be set up from any multiple-station telephone with any group of stations by simply pressing the appropriate intercom keys.

Unrestricted Station

Any multiple station with full facilities can originate an exchange call direct by pressing the ‘exchange’ key. During progress of the call the ‘exchange-line’ lamps glow continuously.

An incoming exchange call is signalled visually by the associated lamp at all multiple stations, and audible indication is given by each instrument exchange bell, except at restricted stations or where the ‘silent call’ facility is provided and in use.

To answer the call, a station user presses the ‘exchange’ key, thus silencing the bells. Simultaneously, the lamp signal denoting the incoming call changes to a steady glow, and the answering station is switched to the exchange-line caller.

While occupying the exchange line, a station user may initiate an information call to any other unrestricted station by pressing the key associated with the required intercom station. This action releases the ‘exchange’ button to the ‘hold’ position, signals the called station and switches the calling telephone to the intercom line. Meanwhile, relay operations in the common services unit cause the flashing signal to be given on all ‘exchange line’ lamps to indicate that a call is being held.

When the wanted station answers, conversation takes place in complete secrecy from the exchange.

If the called station wishes to take over the exchange line call, he ‘picks-up’ the line by operating his ‘exchange’ key. All ‘exchange-line’ lamps then revert to a steady glow, whereupon the transferring station replaces his handset to remove his telephone from the line and complete the transfer.

If transfer of the exchange call to the called station is not required, the station holding the exchange call can switch from the local intercom circuit and re-establish connection to the exchange line by again pressing his ‘exchange’ key.

Restricted Station
Any multiple station may be barred direct access to the exchange line but may be allowed exchange calls at the discretion of a controlling station.

If an exchange call is allowed to originate, the ‘restrict-access’ key (i.e. bell/off) is initially maintained pressed at the multiple station designated for control. This causes the associated add-on relay in the restricted-station telephone to operate and permit access to the line in the normal manner. Following seizure of the line, all appropriate lamps are lit and, at the controlling station, pressure is released from the key and the handset replaced.

At the conclusion of the permitted call, the ‘restrict-access’ relay is automatically released on restoration of the caller’s handset.

2-Wire Station
When the non-locking ‘exchange’ key is pressed to call the exchange, the adjacent ‘extension’ key operates simultaneously and locks. The operation of the ‘extension’ key connects the station telephone to the combined services unit where relay operations, initiated by the momentary operation of the ‘exchange’ key, cause the telephone to be switched to line and the appropriate visual signal to be given at each multiple station.

With a call established on the exchange line, the user at the 2-wire station may perform the following sequence of operations:-

  1. Re-press the ‘exchange’ key to hold the line and simultaneously call the selected station. On reply, request the attendant to signal the wanted station for the purpose of setting up an information call, or to ask this station to take over the exchange call.
  2. Press the ‘exchange’ key a third time to revert to the exchange line.

During day service, incoming exchange calls for the 2-wire station are received and answered initially at any unrestricted-station telephone and transferred to the 2-wire station in the same way as for a transfer call to a multiple station.

The 2-wire station acknowledges the call from the transferring station by operating his ‘extension’ key and, when informed of the exchange call, presses his ‘exchange’ key to establish connection with the exchange-line caller. At the same time, the flashing lamp signal at all multiple stations becomes steady.

Since all unrestricted stations have direct access to the exchange line, and common signalling of exchange calls is provided, night service is automatic at these stations.

On installations where the 2-wire station is included, night service facilities can be given at this station on operation of the ‘night-service’ key (i.e. bell,off) in a selected multiple-station telephone. When the key is locked down, incoming exchange ringing is repeated by the transistor ringing generator in the combined services unit to sound the instrument bell at the 2-wire station. Audible and visual indications of the call continue to be given at multiple stations.

This simple method of establishing night service merely by extending a signal to the 2-wire station, enables incoming exchange calls to be received direct at the station even during normal working hours, if necessary, without sacrifice of day-service facilities.

Because the exchange line is connected in series to all stations, there is a form of priority in access to the line, and secrecy depends on the relative positions of the stations in the multiple, station 1 disconnecting stations 2 to 5, and station 2 disconnecting stations 3 to 5, and so on.

When a monitoring station wishes to listen-in to an exchange call, the ‘monitor’ key (i.e. bell1off) is manually locked down, thus causing the associated add-on relay unit to operate and extend the station to the beginning of the exchange-line multiple.

The system is suitable for connection to a 1,000-ohm exchange line and a 500-ohm 2-wire station line, with the proviso that the resistance from the 2-wire station to the exchange is not greater than 1,000 ohms.

For maximum local-signalling performance between multiple stations, it is recommended that the distance between the first and last multiple station should not exceed half mile, using 21-wire 6.5 lb. cable.

The circuits are arranged to allow all stations direct access to the exchange line under mains-failure conditions.

Units are currently under development to permit the use of a tie-line in place of a multiple station. This will allow two systems to be inter-connected, or the system to work in conjunction with a distant switchboard, using standard signalling methods,
i.e. A-wire earth, a.c. calling or balanced-battery calling.


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Last revised: October 15, 2019