No. 52 PAGE No. 33

A Small Economic P.A.B.X.
Circuit Development, Engineering Department

This attractive 2 + 7 private automatic branch exchange has been designed to meet the essential requirements of small organizations - simplicity of operation and installation, minimum maintenance and, above all, economic as well as efficient telephone service.

Combined external and internal telephone service as provided by modern private automatic branch exchanges can frequently lead to increased business efficiency at lower cost. This fact is widely accepted by small and large organizations alike, and a wide variety of equipment has been developed to meet customers’ different requirements.

Recently a small-capacity PABX accommodating 2 exchange lines and up to 7 internal stations has been added to the available range. The new design provides most of the facilities offered by larger installations at a cost well within the budget of small organizations and is suitable for use as an exchange in its own right with direct connection to the public exchange, or as a subordinate exchange to another PABX, or a PAX. In either of these auxiliary roles the design constitutes a switching unit for a group of persons with a close community of interest, affording them the additional facility of communication with the parent-exchange stations via ‘exchange lines’.

Figure 1
Routing of extension-to-extension call
Figure 2
Routing of extension-to-exchange call
Figure 3
Routing of enquiry call
Figure 4
Routing of transfer call

Because the exchange is designed to occupy standard office space, special attention has been paid to making it compatible with the appearance, ease of operation and small-size characteristics of the subscriber’s other office equipment. Emphasis has also been placed on simplicity of installation and maintenance - two requirements that assume more than usual importance when skilled staff is not available.


The exchange permits the setting-up of three simultaneous calls, i.e. two external calls and one internal call.

Stations engaged on exchange-line calls can make enquiry calls to other stations without occupying the internal link (connector) and can transfer exchange line calls to other stations. During enquiry, the exchange line is held and the caller prevented from overhearing.

An intrusion facility permits urgent enquiries to be made or exchange calls to be offered for transfer when the wanted station is engaged on an internal call. Other than this, full secrecy is afforded on all calls.
A single non-locking pushbutton on each telephone instrument serves for originating or answering public exchange calls, for access to the enquiry (callback) link associated with either exchange line, and for regaining the exchange line after enquiry. No mis-operation of this button can affect service at other stations.

Internal calls are signalled by the instrument bells, and incoming exchange-line calls by an externally mounted bell serving both lines. This bell may be sited at any convenient point within hearing range of a designated answering station, or stations. Additional call bells up to a maximum of three may be paralleled in if required.

During mains failure, or night service conditions (i.e. with PABX switched off) the exchange lines are directly connected to two of the stations.

Internal Connection
Figure 1 illustrates an internal connection. The calling station seizes the link or connector when the handset is lifted and, on receipt of dial tone, dials the single-digit number of the required station. The uniselector of the connector responds by stepping to the contacts associated with the required station line and, if the line is free, ringing current is applied to the wanted station’s telephone. When the handset is raised in reply to the call, the ringing is disconnected and the connector provides transmission battery to the calling and called lines.
The ‘link occupied’ indication is the absence of dial tone, unless both exchange lines are also engaged, when ‘system busy’ tone will be heard.

External Connection
The path taken by an external connection is shown in figure 2. If the call is outgoing, removal of the handset and depression of the instrument button causes the station to be connected to one of the exchange lines and to receive exchange dial tone, provided both lines are not engaged. The instrument now operates as a direct extension on the exchange and calls may be made in the usual way, with privacy from other stations.

If the call is incoming from the public exchange the call bell rings and the answering station removes the handset and depresses the instrument button to take the call.

Enquiry Call
Assume station 2 is engaged on exchange line 1 and wishes to make an enquiry call to station 7; the path taken is as indicated in figure 3. Station 2 depresses the instrument button momentarily to connect a holding loop across the exchange line and also to seize the enquiry (callback) selector associated with exchange line No. 1. Dial tone is returned and the originating station dials the wanted subscriber’s number (7) causing the seized selector to step in sympathy and ringing to be applied to the called line, if free. Connection between the two stations is established immediately on pick up by Station 7 and conversation can take place in complete secrecy from the exchange subscriber.

Figure 5
General view of exchange when installed

On completing the enquiry, station 2 can resume the exchange-line conversation by again pressing the instrument button momentarily; the enquiry selector is then freed.

If a station dialled for the purpose of enquiry is busy on an internal call, the calling station can, if the enquiry is of sufficient importance, intrude on the connection by dialling the intrusion digit ‘1’. Busy tone is disconnected and the calling station teed into the connection. To warn the conversing stations of the intrusion a distinctive tone is superimposed on the line.

Figure 4 illustrates the transfer of an exchange line call, again from station 2 to station 7. It is assumed that the latter is not already occupied on a call. Station 2 contacts him by the normal enquiry procedure and, upon transfer being accepted, replaces his handset. The exchange line is then automatically transferred to station 7.

If station 7 is busy on the local link, station 2 can still offer the exchange call by using the intrusion facility. Should station 7 agree to accept transfer, station 2 requests both interrupted stations to replace

their handsets, thus terminating the local call. This done, the accepting station receives ringing and, on picking up again, will be connected to station 2 by the normal enquiry path. The latter now replaces his handset to complete transfer.
If station 7 is busy on an outside call and therefore not accessible to intrusion, this fact is made known by the continuance of busy tone after the intrusion digit is dialled.

Night Service
A form of night service may be given by switching off the a.c. power supply to the PABX. The exchange lines are then connected respectively to stations 1 and 2 which in all respects function as direct extensions on the exchange. Because incoming ringing is signalled on the instrument bells instead of the call bell, there is complete freedom in the siting of the stations.

Figure 6
Hinged covers open, showing interior

The same provisions ensure exchange-line service at these stations when the system is otherwise inoperative due to mains failure.

Since it was considered essential that any reasonably competent technician, not necessarily a specialist in telephone equipment, should be able not only to install the system but take charge of it, special attention has been given to accessibility and to the choice of durable and trouble-free components of well-proven type.

The complete exchange equipment, including tone, ringing and power supplies is housed in an attractive brown and cream enamelled sheet-metal cabinet fitted with removable hinged wrap-around covers (see figures 5 and 6). It occupies small space, being only 2 ft. 4.75 in. high, 1 ft. 2 in. wide and 1 ft 0.5 in. deep (i.e. 73 cm x 35.6 cm x 31.75 cm) and can be installed in any dry and reasonably dust-free situation, either wall-mounted or free-standing on a desk or bench.

As illustrated in figure 7, the components are mounted on two vertical back-to-back frameworks, the one on the right-hand side being hinged on the front edge to allow access to the wiring and screw-type terminal strips behind, Cables for connection of the station telephones, the call bell and mains supply are brought in through the base of the cabinet via an aperture in the rear face of a shallow plinth.

The apparatus includes heavy-duty uniselectors, 3000-type relays, and strip-relay units’ which are used exclusively for the line circuits. Space is available for an additional three strip relays, permitting the system to be extended to 10 stations at installations where traffic load is light.

Figure 7
Covers removed and gate swung open for access to terminating strips and internal wiring

The strip units can be seen in figure 7. They consist of up to five relays mounted on a common yoke which also serves as the core-iron and primary anchorage for the main components. The design of these relays ensures a high degree of contact reliability with unvarying contact pressure under all circuit conditions. No spring or armature adjustment is necessary under any circumstances; should the springset become damaged it can be freed from the assembly for replacement merely by withdrawing a single screw. When reassembled with the new springset unit, the relay will automatically attain correct adjustment. Detachment of the springset also permits removal of the coil, which may be replaced with similar ease.

All items have tropical finish, and pvc insulated wires are used throughout. Maximum reliability is thus ensured even under the most adverse climatic conditions.

The PABX incorporates a mains rectifier unit, a.c. supplies of 100-125 or 200-250 volts 40/60 c/s being suitable for its operation. Voltage adjustment is by 5-volt steps in the lower range and 10-volt steps in the higher, selected by tappings on the mains transformer.

The maximum loop resistance of a station line, inclusive of the telephone is 500 ohms, whilst exchange-line loop resistances up to 1000 ohms are acceptable. Telephone instruments of Etelphone type are employed, ensuring a high transmission performance unaffected by differences in line loss between stations or from stations to the exchange.

This new unit represents a successful scaling-down of the modern PABX formula in size and cost, though not in capabilities. Maintenance, which under all normal circumstances will demand no more than periodic contact cleaning and lubrication of uniselectors, should not prove a liability even to organizations with minimum service resources.


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Last revised: August 04, 2003