PAGE No. 3

January 1964

A comprehensive telephone system is described integrating services that can be selected on a unit basis to provide system arrangements having wide application. It caters for up to 10 telephone stations and functions as a small private automatic branch exchange (PABX), an auxiliary private automatic exchange (PAX) or a multi-line system.

THE value of a well-planned telephone system is widely recognized in both industry and commerce as a significant step towards higher business efficiency. Many organizations, however, expanding beyond the direct-line telephone stage frequently find this difficult to achieve, for with expansion comes the problem of assessing precise communication needs consistent with future growth and the particular nature and routine of the organization concerned; whether the system under consideration should be one providing internal service, external service or a combination of both.

As a ready and economical solution to this problem, a system of considerable versatility has been developed - 'Multiphone'. Designed on the building-block or modular principle with relatively simple circuit units, the new system can be developed or varied in its application by simple addition or omission of units. In this way, the most effective installation can be provided to meet specific requirements at lowest cost.

Multiphone serves up to 10 telephone stations and embodies three system arrangements, permitting it to function as a self-contained or auxiliary 4-line sub-attended private automatic branch exchange, a satellite private automatic exchange, or a multi-line system giving access to 4 external circuits.

External circuits terminating in the system may be of the same classification or a combination of different types. These include direct public exchange lines; extensions to a PAX, PBX or PABX; private wires to other switchboards; control circuits to key points and radio links to remote stations.

Space requirements for the equipment are low. This feature, essential in any system, has been made possible largely through the use of single and twin press-key switching units which, together with transmission and supervisory components, are accommodated in each station telephone. The key units, because of their diverse switching applications, have substantially reduced the need for much of the common relay-switching equipment usually associated with fixed layouts offering service arrangements similar to Multiphone. For this reason, no floor standing rack-mounting equipment is required; all relay switching as well as related 24-volt power unit apparatus is housed in a compact central-control cabinet which may be mounted in any position convenient to the subscriber, on table or wall.

Because all modules in a fully equipped control unit combine to provide PABX services (i.e. internal and external) this method of working is described first, followed by some of the many possible arrangements for connecting Multiphone to suit particular requirements.

Multiphone as a self-contained PABX
Multiphone in this capacity is intended primarily for use in businesses where activities and layout of premises demand a compact system. Small companies working from office suites or large open rooms are in this category, as well as certain units of larger concerns that operate with a high degree of autonomy, for example, a research section or municipal architect's department.

Fig 1
Multiphone station telephone equipped for 2 + 10 PABX working, without optional line exclusion facility
Fig 2
Multiphone station telephone accommodating extra press buttons for 4 + 10 PABX working, including line exclusion facility

To meet different requirements, the PABX may be arranged in two ways to give access to a maximum of two or four external lines. The 2 + 10 and 4 + 10 station telephones employed in these optional arrangements are shown in Figures 1 and 2. As in all Multiphone applications, the station telephones are physically and electrically similar, and differ only in the number, type and disposition of press-buttons employed.

No switchboard operator is required for the handling of incoming calls at the PABX, since these are dealt with at any predetermined station or group of stations. Outgoing calls can be originated from any station and, in addition to incoming calls, can be held for the purpose of information calls on other lines or transfer to any station. Calls may be repeatedly transferred throughout the system.

An important feature is that service on external circuits can be arranged in three separate ways.

  1. To allow any station to enter an engaged line at the request of the originating station.
  2. As (a) above, but to permit any station (up to the number of external circuits installed) to have exclusive use of one pre-allocated line. This exclusion feature is suitable for principal stations and is controlled by pushbuttons in the selected station telephone.
  3. As (a) above, but to allow any selected group of stations priority of access to any pre-allocated line, the order of priority being determined by the particular arrangement of the stations in the external-circuit multiple. For example, station No. 1, on seizure of the line and operation of a control key, can exclude all succeeding stations 2-10. Similarly, station No. 2 can exclude stations 3-10 and so on.

Communication between stations is by selective dialling of one-digit codes over a single transmission link. Simple strap adjustment in the control equipment speedily converts the link from non-secret to secret working according to requirements.

Both internal and external calls are signalled visually by combined line-and-busy lamps and audibly by a.c. ringers. Audible indication of intercom calls is given by the instrument ringer and, for external calls, by ringers located convenient to the answering station or stations.

Because the system operates from a mains-operated power unit, provision is made to maintain external service on failure of mains supply. In this event, each station continues to have access to all external lines, incoming calls being signalled audibly at the answering station or first station in an answering group.

The selection of any function at a station is by single or twin-type press-buttons located in front of the instrument handset cradle. A typical press-button arrangement is shown in Figure 3. Buttons are logically arranged for convenience of operation and easily identified by designations clearly impressed in white characters on the face of each button. External-line buttons are marked numerically and the remaining buttons by letters; 'H' indicating the 'hold line' function and 'I' the intercom link. Control buttons X and O (when fitted) are for line exclusion purposes.

Fig 3
A typical press button arrangement

A simple system of interlocking and trip release is employed between buttons and handset switch-hook, and between the buttons themselves. Buttons H and O are non-locking; all other buttons lock when depressed and release to normal upon restoration of the handset. In addition, with the exception of button X, any button when depressed releases any other previously operated.

Any station telephone may be easily adapted for line exclusion; on site if necessary. The operation merely entails minor revision of connections in the telephone, the release and withdrawal of the two inner single buttons held captive inside the instrument case, and their replacement by buttons of twin type. No further action is called for, because the simple springset assembly required for provision of this optional feature exists in all telephones.

Signalling lamps are designed to give satisfactory display even under conditions of direct sunlight and are of two types, clear and coloured, to assist ready identification of all calls held and in progress. Clear lamps are located in front of associated external line press-buttons and below these to the left is the intercom lamp which is individually mounted and, in contrast, emits a red signal when illuminated.

For simplicity, signals are confined to three in number, these being a steady glow for busy lines, a rapid 'wink' for incoming calls, and a slow 'wink' denoting a line held. Both winking signals are markedly different, the rapid wink having equal on-off periods of approximately 0.5 second whilst, in the slow 'wink', current is removed from the lamp for about 0.1 second in every two seconds.

Occupation of the intercom link is shown by a steady glow on the 'intercom' lamp at all stations. This indication occurs as soon as a caller enters the link by removing the telephone handset and depressing the locking button I.

Connection to any station is established on dialling the single-digit code assigned to the called station. On completion of dialling, audible indication of the incoming call is given by a burst of ringing of approximately five seconds duration on the instrument ringer. Simultaneously, the steady glow condition at the called station changes to a rapid wink.

Removal of the handset and depression of the 'I' button at the called station establishes connection with the intercom link and causes the visual indication to revert to the steady 'link-occupied' signal. When the call is concluded, the 'I' buttons restore at both stations as the handsets are individually replaced and, when both handsets are at rest, the intercom lamp is extinguished.

Any station can initiate an outgoing call direct by pressing any button associated with a free line. The call is completed in the normal manner and, during its progress, the associated line-and-busy lamps at all stations glow continuously.

On an incoming call, the incoming ringing signal operates lamp-flashing relays in the central control unit to apply the rapid wink signal to the appropriate line-and-busy lamps at each station, identifying the calling line. At the same time, an audible signal is given by the external ringer at the answering station. If there is more than one answering station in the system arrangement, the ringing signal received at the first station is repeated to the remaining answering stations by a transistor ringing generator in the control equipment. According to the particular requirements of an installation, interrupted or continuous ringing can be employed for the repeat signal.

Depression of the correct line button (shown by the winking lamp), switches the external line through to the answering station telephone. Ringing ceases and the winking signal changes to a steady glow, indicating to all stations that the call has been answered and a busy condition exists.

If the answering station wishes to hold the line, perhaps to make an enquiry call to an intercom station or external subscriber, the non-locking hold key is depressed. This action restores the exchange-line button, applies a hold condition to the line initially seized, and causes the associated line lamp to wink rapidly, serving to remind the user that the exchange caller is waiting to resume conversation.

With the hold condition applied, the enquiry line can be entered by operation of button I or any free external-line button, whichever is appropriate, and an information call originated. The enquiry call is made with privacy from the waiting caller and, on its completion, a return to the held line can be made by re-pressing the button initially operated. This automatically disconnects the enquiry line and causes the winking signal to revert to a steady glow.

If the enquiry call is to an internal station, this station can pick-up the external call by operating the external-line key above the winking lamp. This operation switches the take-over station to the external line and a steady glow condition replaces the winking signal, indicating the interception of the call. On seeing this signal change, the transferring station replaces the handset.

Stations equipped for line-exclusion can originate or answer an external call in secret on one line only. When the particular line is picked up, secrecy from succeeding stations connected to that line is imposed on depression of button X. At the conclusion of the call, replacement of the station handset releases both buttons and restores the excluded line to general use.

When a principal station wishes to transfer a call after secrecy conditions have been applied by button 'X', all excluded stations must be restored to line. This is accomplished by momentary operation of button 'O', its action releasing button 'X', thus restoring the cut-off stations and allowing transfer of the call to be completed in the normal manner.

Fig 4
Typical application of system as an auxiliary PABX

Multiphone as an auxiliary PABX
In an organization served by a PAX or PABX, there may be a department or group of principals with sufficient community of interest to justify an auxiliary system to the main. Figure 4 shows a typical arrangement providing intercommunication between the group and the main and between stations in the group. The group, first disconnected at individual line circuits, is re-associated with the main equipment by a 10-station Multiphone PABX connected by two communal lines to a similar number of line circuits. Calls within the group are completed by one-digit dialling, and outgoing calls to 2-wire stations by dialling two or more digits dependent upon the capacity of the main PAX or PABX.

Because traffic requirements to and from the main are low, this system arrangement not only reduces dialling effort for the group but, more important, minimizes traffic load on the main and permits considerable line-circuit reduction to be realized.

A further example of Multiphone in this capacity is shown in Figure 5. This application is suitable where an existing PAX (or PABX) at the headquarters of an administration is some miles from an associated branch office or department. Connection between the two systems in this instance is made over two rented lines. In this way, the branch or department is provided with a complete local system fully integrated with the main.

Fig 5
An auxiliary PABX system over rented lines

Multiphone as a satellite PAX
Where a few extra stations are required on a PAX with no spare line circuits available, Multiphone station instruments may be substituted for individual groups of low-traffic PAX station telephones. For example, as illustrated in Figure 6, six Multiphone instruments, replacing an equal number of 2-wire telephones, are multipled to three PAX line circuits, thus freeing the remaining three line circuits for other uses. These circuits may themselves be utilized for the addition of further Multiphone stations to the PAX. In this application the intercom-selector unit is dispensed with, but lamp flashing units are included to assist identification of calling lines.

Fig 6
Typical application as a satellite PAX
Fig 7
Multiphone as a Multi-Line system

Multiphone as a Multi-line System
As illustrated in Figure 7, Multiphone can be applied to travel bureaux, booking offices and other organizations supplying information services of different kinds. With incoming lines multipled to all stations, a call can be answered by the first free attendant, ensuring even distribution of calls among members of the staff and quick speed of answer essential to good service.

If one of the lines is incoming from a private exchange within the organization, the provision of the hold facility can be advantageous despite the absence of intercom and transfer services. By its inclusion, a call can be held on any of the other three lines while an enquiry call is made to any station in the associated private system.


A metal cabinet 30' x 13.5' x 11.75' (74, 34, 29 cm) houses the central-control equipment, comprising switching relay sets, intercom-selector unit, power unit and transistor ringing generator. The cabinet, suitable for table or wall mounting, is arranged to be as nearly dustproof as possible consistent with ventilation.

All units are equipped with screw and lug fixings for quick assembly to a metal baseplate and upper structure. Power unit and ringing generator are mounted to the right, and relay units to the left, as may be seen from Figure 8, illustrating the complete equipment (cover removed) for a 2 + 10 PABX installation.

Up to 7 mounting plates can be accommodated for switching units consisting of four types; external-line, lamp-flashing, common-equipment and intercom-selector units. Each mounting plate is equipped

with screw terminals for jumper field interconnection of plates (see Figure 9) allowing speedy installation of a preferred arrangement and easy addition of units within the limits of the system.

Figure 10 shows one of the plates (the intercom-selector unit) in greater detail. Physically it consists of a 10-point miniature selector switch and associated major-type relays.

Power for the system is via a 3-point reversible shrouded plug which interlocks with the cover as a safety precaution. A range of a.c. supply between 110 and 250 volts at 50/60 cycles is allowed for by the transformer tappings.

Fig 8
General view of the control unit with cover removed
Fig 9
Rear view of the central control unit showing jumper field interconnection of switching units

All lines terminate at the central control unit. For each exchange line or similar external circuit connected to the system, 6 conductors are required, multiple-connected via the instrument terminal blocks at each station. When internal communication is included in a system arrangement, each station requires 5 directly-connected conductors to the control unit.

The maximum transmission signalling limit from each station to the distant external line terminal is 1000 ohms loop resistance, less a loop resistance of up to 35 ohms allowed between stations and the control unit. This limiting resistance represents a station line length of 220 yards (201 metres) 6.5 lb/mile cable.

The highlight of the new system's design is undoubtedly its wide flexibility in use and, for this reason alone, Multiphone should have wide appeal. In addition, it has the advantage that only the apparatus currently required by the individual customer need be installed, a feature ensuring that idle equipment as well as investment are kept to the minimum. And further, the design permits faster installation and changes and, owing to the inherent simplicity of the switching apparatus throughout, it presents a low-fault liability.

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Last revised: October 15, 2019