ERICSSON BULLETIN
No. 9 PAGE No. 20


Unattended P.A.B.X's 50 Extensions
July 1936

The term Private Branch Exchange or P.B.X. is applied to a telephone switchboard situated at a subscriberís premises to fulfil the dual purpose of supplying public exchange service to a number of extensions over one or more exchange lines, and of giving a means of internal communication between these extensions. The widespread use of automatic telephony both for the public service and for private exchanges has its equivalent for this type of board also, and is called the P.A.B.X.

A well-known type of P.A.B.X. is that in which internal and outgoing exchange calls are completed automatically from the callerís dial, while incoming exchange calls are dealt with by a manual operator or attendant. As outgoing exchange calls are usually charged for individually it is generally inadvisable to allow all extensions to have direct public exchange access for fear of misuse, and it is therefore the usual practice to confine this facility to certain privileged lines, all other outgoing calls being controlled by a P.A.B.X. attendant.

For incoming exchange calls it is always necessary to complete a call through the medium of an attendant at the P.A.B.X., not because there is any technical difficulty in avoiding such service, but because the telephone number supplied from the public directory is only able to direct a call to the P.B.X. establishment, where after the local knowledge of an attendant is required to complete it to any desired individual or department.

The object of an unattended P.A.B.X. is not therefore to dispense with manual aid, but to accomplish as many functions as possible by automatic means, thereby eliminating the manual switchboard and reducing supervision to such an extent that it becomes the minor duty of one or more persons occupied with other employment.

The unattended P.A.B.X. described herein operates from a 24-volt source and has a maximum capacity of 50 extensions and any desired number of exchange lines (a 4 + 50 P.A.B.X. is shown to the right).

The equipment consists of uniselector switches of the Ericsson heavy-duty pattern and Post Office 3000 type relays.

Apart from the switches and line relays, all equipment is mounted on jack-in units, each local link and exchange equipment being provided with a separate mounting.

The ringing and tone units consist of relays and a pole-changer mounted on two jack-in units provided with covers to conform to the general design of the board.

All equipment is mounted on substantial frames and efficiently shielded against dust.

The exchange operates on the register principle, where all calls are set up from register units.

The equipments for local calls and exchange calls are self-contained and function independently, except for the momentary use of a local linefinder to promote an outgoing call to the public exchange, the additional equipment necessary to combine the two services being confined to the registers.

An attendantís board for dealing with incoming exchange calls can usually be dispensed with but in cases where it is
necessary such a board will be of a very simple nature.


The automatic switchboard is divided into a unit for internal calling equipment, including the registers, and a unit for exchange line equipments. It is therefore, economically possible to provide a purely internal service by using one unit, and to add the exchange line unit if and when required, the extra equipment provided in the registers for both services being relatively inexpensive.

Local calls are made over link circuits designed for the introduction of many desirable facilities, circuits being available for preference extensions, loud speaker extensions, 2-party lines, internal P.B.X. groups, round call, conference, and tie lines to similar exchanges etc. Distinctive tones are provided for dial, busy and ringing; faulty lines are deflected from the registers, and both calling and called extensions are given immediate release from a connection.

Each extension line will consist of a pair of wires, the necessary functions being provided without the use of an earth connection. Extension instruments are of standard pattern, the dial being used for making enquiry calls, transfer calls and also exchange calls where permitted.

An exchange line equipment consists of two SO-point switches and a jack-in relay unit. This equipment serves calls in both directions and is designed for working with an automatic or a C.B. public exchange. An equipment required to operate to a magneto exchange would have another form, as ring-off signals are required.

A short description of the operations will best explain the facilities provided.

Outgoing exchange call from an extension with exchange access
When the handset of the telephone is lifted, the caller is extended to an idle register over any available local linefinder and dial tone is heard. To make an exchange call the subscriber dials a special digit. This will cause an available exchange line to be selected by the register and then one of the switches on the exchange unit will function as a linefinder to find the calling line, thereafter releasing both the local linefinder and the register. The caller is then extended to the public exchange, from which dial tone will be heard and the call can be completed by dialling the directory number of the required line.

Outgoing exchange call from an extension without exchange access
This extension will make a local call to the attendantís telephone, i.e. a normal extension with exchange access, and after supplying the necessary information will hang-up and await a call back. The attendant may then complete the call in the manner described above and after a reply has been received he can hold the line and make a local connection to the originating caller. The attendant can then speak to the extension and transfer the exchange call to the original caller.

It is also possible for the attendant to extend the connection to the public exchange and then let the caller complete his connection.

Incoming exchange calls in the daytime
When the P.A.B.X. is called from the public exchange, an idle exchange line of the group will be automatically selected and ringing will be sent out. This ringing will energize a relay at the P.A.B.X., causing the exchange line to be extended to a register which will automatically connect the call to any one of a group of normal extensions selected to operate as attendantís lines, the the register then being released. The selected extension is automatically rung with a special period of ringing to indicate that it is an exchange call. When the call is answered, the ringing is tripped both at the P.A.B.X. and the public exchange and the caller is through to the attendant. On the callerís requirements being stated, the attendant will hold the exchange call and the attendantís line will be deflected to a register. When the required extension has been dialled and the call has been answered, the two can speak together without overhearing to the exchange line. If the called extension is not the most suitable for dealing with the caller, the attendant can revert back to the exchange line and then make a further local call. After the attendant has received an answer from the wanted extension, he will transfer the exchange line to the extension and release his own line. As such calls are equally as urgent as the original exchange call, a special ringing period is sent out. If the wanted extension is engaged, a tone is introduced on the line to advise the extension that an exchange call is waiting. When the wanted extension replaces the receiver the bell is rung from the attendantís connection as soon as the previous call is released, and when the ring is answered the extension is connected with the attendant.

Enquiry and transfer calls
When an extension is connected to an exchange line, whether in an incoming or an outgoing direction the latter can be held
while a local call is made, and the exchange call can also be transferred to another extension and re-transferred from there to any other extension an indefinite number of times.

Night Connections
At night time the exchange line group may be segregated into individual lines at the public exchange so that a caller can be connected through to a particular extension by dialling a night connection number specified in the telephone directory.

When a key is thrown at the P.A.B.X., the registers will cause each individual exchange line to extend incoming calls to its specified extension, this extension being decided by the cross connection of a single wire.

If such a call meets an engaged extension, the P.A.B.X. subscriber will be warned by a tone and by replacing the handset will be released from the previous call and then rung from the exchange call.

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Last revised: May 17, 2003