Intercommunication Telephones with Exchange Facilities
July 1934

The type of telephone system which should be installed within an organization will naturally depend upon the type and number of calls to be made by the various departments. These calls may, in general, be split up into two classes - internal calls taking place between the departments within the organization, and external calls to and from departments and the main public exchange.

Up to the present time a subscriber has been faced with three alternative arrangements to cater for his telephone requirements;

1. to install a private intercommunication system when the majority of calls are internal, and have one or two public service telephones to cater for the few external calls.
2. to install a public service private branch exchange (P.B.X.), when a large number of calls from the various departments are external, and where an operator must be allocated to deal with these calls and (3) to install an intercommunication system as (1) and also a P.B.X. as (2).

It has long been felt that there is a definite demand for a system which will combine all the facilities of the above, and offer additional advantages due to the combination. It must, however, be simple in operation, provide maximum flexibility with regard to additions and alterations, and be easy and cheap to maintain in close co-operation with the British Post Office Engineers, the Ericsson Company has now devised a system by means of which all the facilities of a P.B.X. and a direct intercommunication system are combined in a single instrument.

The instrument, as illustrated, follows the usual intercom practice of providing push buttons for establishing the necessary calls. The mechanism is enclosed in a Bakelite moulded casework of distinctive appearance and registered design, and incorporates a much improved type of cradle and switch which has been designed with a view to eliminating the possibility of breakage. As will be seen from the various illustrations, the layout and arrangement give maximum accessibility to all parts of the instrument for inspection and maintenance purposes. The complete button mechanism may be removed as a unit, leaving in situ the spring banks and cabling. Each spring bank is removable as a unit, and the relays are mounted on separate brackets so that they can be readily detached and thereby provide complete access to the relay springs. The relays are of the latest Post Office 600 type.

As a large percentage of normal intercom adjustments is in connection with the buzzers, it was deemed desirable to incorporate these in the external connecting blocks where they may be adjusted without interference with the instruments proper. As a final safeguard and to facilitate installation, the connecting block is arranged in the form of a plug and jack so that the complete instrument may be removed at will.

The following facilities are standardized for the system:-

1. The same instrument is used irrespective of the type of public exchange.
2. Any instrument may be used as the master station.
3. Exchange calls are secret.
4. Engaged tests on 'busy' exchange lines.
5. Exchange lines may be 'held'.
6. Exchange calls may be transferred direct from one station to another.
7. Supervision on exchange lines may be given at any selected positions.
8. Trunk offering facilities at master station.
9. Exchange calls may be barred to any stations at the discretion of the master station.
10. Calling and clearing are direct from any station to all types of public exchanges.
11. Local calls are non-secret.
12. Conference facilities on local calls.
13. Engaged test on stations engaged on exchange lines.
14. Simple external extension instruments may be used, working on two lines only.
15. Power supply obtained over leads from nearest exchange.
16. Night working. A second choice master station may be given.
17. Independent release of each exchange line button.
18. One size case but two equipments, namely 1 exchange with 5 extensions, and 2 exchange with 10 extensions.

The above will be better understood by a description of the system and its operation.

The instruments are installed on the multiple or tee system which is economical in cable and provides maximum flexibility for future alterations and additions. Any one station is selected and called the master station, and at this point is fitted an auxiliary apparatus unit containing the necessary exchange line call-indicating equipment. Two types of auxiliary apparatus units are supplied, one when the public exchange is C.B. working, and one when L.B. working. In the case of L.B. a suitable strapping arrangement provides the necessary line conditions for C.B.S. No. 1, C.B.S. No. 2., and Magneto. In the case of automatic working each instrument is, of course, fitted with an automatic dial. The system has a working voltage of 18 volts and upwards, and this may be supplied by means of a power lead from the nearest exchange. When there is no power available at the main exchange -  this would generally apply to magneto working - a small capacity local battery is required. To give a good factor of safety, all parts have been designed to operate on 12 volts.

An Auxiliary Unit	Auxiliary Unit interior	Auxiliary Unit Interior

The use of the same instrument irrespective of the type of public exchange is very desirable from a stores point of view and also for maintenance purposes, as it would he undesirable to have a different type of instrument to install and maintain for each type of public exchange service.

The facility of being able to make any station the master station also tends to simplification of types and stocks, and provides maximum flexibility for night working and alterations, temporary or permanent.

An important feature is that all exchange calls are secret to the station using the line, and great care has been taken to ensure that no interference or intrusion can take place when a station is engaged on an exchange line. Twisted pair conductors are provided in the cable and cords to guard against cross talk.

Any type of public exchange may be called direct from any station by removing the micro-telephone and depressing the appropriate exchange line button. Should the line be already engaged, the buzzer will operate to give a busy signal. It is desirable to test the line, before removing the micro-telephone, by depressing the button and getting the above test signal.

The replacement of the micro-telephone will restore all depressed buttons and give the clear direct to the exchange in the standard manner. This is a very important feature as the same method of operating exchange calls is utilized irrespective of the type of public exchange, and ensures a clear being given in the case of magneto exchanges.

It was, of course, a comparatively easy matter in the case of C.B. and C.B.S. exchanges to arrange automatic call and clear, but quite a different arrangement had to be introduced for magneto exchanges. In the case of the latter the problem was solved by making use of the plan devised for the new group service scheme. This consists of a small relay set fitted at the magneto exchange to provide the necessary calling and clearing conditions. It is this small modification at the exchange that made it possible to standardize the instrument and also reduce the number of auxiliary units to two, namely, C.B. and L.B. as already mentioned above.

Calls incoming from the exchange are received by means of an indicator on the auxiliary apparatus unit at the master station ; the micro-telephone is removed and the appropriate exchange line button depressed. Should the call be for another station, it may be transferred direct. To do this, the required station is rung by depressing the local button to its fullest extent.

This operation will release the exchange line button, but, due to the construction of the latter, one of its spring banks will remain operated and ' hold' the exchange line. When the required station answers, the number of the exchange line calling, i.e. No. 1 or No. 2, will be given by the master station, whereupon the former will depress the appropriate button, and as the exchange line is being held busy the call buzzer will operate, and give a tone which is fed back over the local lines to the master station to indicate that the call has been taken up. The master station now replaces his micro-telephone and the call is connected direct to the second station. These calls may be further transferred to any other station, in a similar manner, and without reference to the master station.

The feature of being able to 'hold' an exchange line is very valuable when it is required to obtain information from another station whilst engaged on the exchange line. During the time the local call is taking place, the exchange line is held but the distant party cannot hear the local conversation; re-engaging the line, by re-pressing the exchange button, automatically restores the local button to normal.

Trunk offering facility to the master station, or supervision to a chief, is given by means of a small strap in the instrument. The removal of this strap allows the instrument to cut-in on an exchange line when it is engaged by another station.

Under certain conditions it may be desirable to allow some stations to use the exchange line only at the discretion of the master station. A very slight alteration in cabling, and a press button fitted on the master station auxiliary unit gives this facility; only when this button is momentarily depressed can the exchange line be used by the particular stations selected.

It is sometimes convenient to be able to release the 'hold' on an exchange line without replacing the micro-telephone. For this purpose a small lever is fitted near each exchange button; by pressing this lever to one side the hold section of the button will be released without in any way interfering with the rest of the instrument.

Local calls between the various stations are made in the standard method of push button instruments, i.e. to call another station the micro-telephone is lifted and the appropriate button depressed to its fullest extent. This operates the calling buzzer at the distant station. The call is answered by removing the micro-telephone at the called station.

If during a local call, it be found necessary to bring in other stations, the calling party depresses a special conference button on his instrument and can then call individually each of the required stations. Each station provides its own transmission bridge so that the transmission is not impaired by the conference as is usually the case.

Should a local call be made to a station which is already engaged on an exchange line, the buzzer at the calling station will operate to give an engaged signal.

On some installations communication is required at a point some considerable distance from the main installation, and where the length of multiple cable which would be required becomes excessive. To cater for this condition, arrangements are made to provide what is known as external extension working. Only two wires are required between the extension and the main cabling, and a standard telephone is used at the extension premises. The extension can call to any one selected position, and at this position a small auxiliary unit is fitted. Any station on the system can, however, call direct to the external extension, and the latter may also be switched through to the public exchange.

Calling from the external extension is by means of an indicator on the auxiliary unit, and this same indicator is also used as a clearing indicator for exchange calls switched through to the external extension.

Arrangements have been made so that, for night service or similar reasons, the exchange calls may be transferred to a station other than the master station. To do this an auxiliary unit is fitted at the selected station, straps are permanently removed from the master station unit, where, by the simple operation of a key all calls are passed through direct to the other in the case of an external extension, only the calling indicator is extended to the second choice master station, and this indication is incorporated on the exchange line auxiliary apparatus unit, thus the external extension can be given night service facilities by operating the key in the auxiliary apparatus unit.

From the above somewhat brief description it will be seen that the new Ericsson intercommunication system with exchange facilities, incorporates all the advantages of a normal intercom system, together with those of a P.B.X. Engaged tests are given where required, the use of an operator is reduced to a minimum and every station has the use of the equivalent of two telephones which in this new Ericsson system are actually combined in one instrument.

See also Bulletin No. 11

Last revised: October 14, 2019