PAGE No. 20

Register Controlled P.A.X’s
July 1933

The research, technical development and manufacturing experience gained in perfecting the equipment in use on the modern public telephone service is incorporated in the new Private Automatic Exchanges manufactured at our Beeston Works, and the facilities which are offered, or may be added on these boards, gives them the flexibility necessary for meeting the conditions likely to be put forward by varied business requirements.

Three types of switch-boards are manufactured,  covering a range of from 35 lines to 400 lines or more. The two smaller boards are non-extensible and have a capacity for 35 lines and 50 lines respectively, inclusive of all services. These exchanges operate from 24 volt batteries, or where A.C. mains are available from battery eliminators specially designed for this purpose. The largest switchboard is built up in 50 line units to any capacity from 50 to 400 lines for purely local traffic, but when required to work as a P.A.B.X. in conjunction with another exchange or other exchanges the ultimate capacity will be reduced by 50 lines for each 6 junctions or tie lines. This board operates from a 50 volt source.

PAX 35 PAX 35 with switch gate open PAX 50

The equipments for producing the various tones and the interrupted ringing are composed of standard relays for the most part, and a pole changer. They are mounted on jacked-in units of conventional form and incorporated in the general design of a switchboard. 

The illustrations of the boards show compactness and accessibility of apparatus, the pleasing symmetry of equipment and the robust construction. 

The various pieces of apparatus are protected at the front and rear with dustproof covers. The non-extensible boards may be mounted close against a wall to save floor space.

The boards operate on the register control principle. The relays are of the P.O. 3000 type, and the switches are of the Ericsson uniselector heavy duty types for 25 and 50 lines. The disposition and function of the various pieces of apparatus are described with reference to the views of the 35 line board.

In the top left hand corner is the fuse panel, to which the battery or eliminator leads are connected. The fuses are capable of division into two groups to allow smoothed current to be applied to the transmission circuits when a battery eliminator is employed as the voltage source. To the right of this panel the connection block for the incoming subscribers lines is mounted. The external wiring from this block will consist of two wires for each instrument installed. Below this is mounted a gate on which all the uniselectors are fitted. This gate is pivoted on the left, and fastened on the right by two thumb screws. At the back of this gate a sheet of metal is fixed to the rear of the framework to prevent the ingress of dust.

The gate can be swung outwards to expose the switch multiple wiring. A glass fronted cover latched over this gate permits observation of the switches and protects them from dust. The first four switches from the left on the top and bottom rows are the line finders and connectors respectively of the four links and the next three are the link finders and the tens and units switches of the two registers.

The mounting plates below these switches contain the line relays, extra relays for preference subscribers and 2 start relays.

The equipments for the four links are mounted on jack-in units on the next row and the two registers and the ringing and tones sets on jack-in units along the bottom row. The units of the large switchboard are completely enclosed in steel cabinets with detachable doors both front and rear.

The dimensions of the switchboards are as follows:-

Type Height (ft. ins.) Width (ft. ins.) Depth (ft. ins.)
P.A.X. 35 5' 6" 2' 1" 10"
P.A.X. 50 5' 6" 2' 6" 10"
P.A.X. 50/400 (each section) 5' 9" 2' 6" 1' 6"

The 35 line board has 2 registers and a maximum capacity for 4 links; the 50-line board, 2 registers and 6 links; and the 50/400 board 2 to 6 or more registers per board, 7 line finders and group selectors and 6 connectors per 50 line unit. In all cases the switch banks with wiring are completed for these maximum figures but the switch mechanisms and link relays on the jack-in equipment may be supplied to traffic requirements.

A straightforward numbering scheme is adopted in each case; for the 35 line board it is 10 to 44, for the 50 line board 10 to 59 and for the 50/400 board 100 to 499.

PAX50/400 - two sections shown making a 100 line equipment

As already explained, the relays are of the standard P.O. 3000 type. Among the many technical points of this relay the outstanding features which recommend it for service and maintenance are the twin contact springs and the method of applying tension to the individual springs against a buffer block to ensure correct individual contact pressures. In every case the relays on these switch-boards operate to standard adjustment charts which preclude the use of two-step relays.

The voltage variations from eliminator on varying load are excess of those of a secondary battery and to cover these variations the 24 volts boards are designed to operate satisfactorily at a voltage varying between 20 and 30 volts and the larger boards between 40 and 60 volts.

The subscribers’ instruments used in conjunction with these switchboards may be of the standard type fitted with the P.O.

The process of a call on a 2-digit board is shortly as follows:-
When the calling subscriber lifts his hand-set, the loop through the telephone energises his line relay. The line relay actually disconnects itself from the line wires and holds up in series with a common starting relay. This start relay causes the link finder switch of a free register to rotate and stop at a free line finder which has access to the calling line. The switch of this line finder will then rotate to find this calling line and when this is found the caller will be extended to the register, from which he will hear a dial tone of approximately 33 periods per second. When he then dials the wanted number, the impulses will be taken up by the tens and units switches of this register. The register will then endeavour to engage a common marking circuit and when this is obtained the connector associated with the engaged line finder will be set to the wanted number. The register will then be dissociated from this call and restored to normal in readiness for a further call and the caller will be deflected to the connector circuit. Should the wanted line test engaged, the caller will hear busy tone, consisting of a periodic note of high pitch but should the line be free periodic ringing will be sent to the wanted number and a periodic ringing tone will be sent back to the caller. When the call is answered, the ringing will cease and the two subscribers will be in speaking communication. This speaking circuit consists of an individual feeding relay for each subscriber inter-connected by condensers. When either subscriber re- places the handset, his line is immediately released, the last subscriber retaining the link receiving busy tone.

Preference Facility
When a preference subscriber makes a call to an engaged line, a warning tone is first sent to the subscribers in communication and then the preference caller is introduced into the conversation. Should the preference caller desire to withdraw and allow the original conversation to proceed, he may do so by replacing his handset, but if his call is private he asks the wanted number to hang up. This causes the line to be released from the previous call, after which the preference call will automatically ring the wanted line and be connected under normal conditions when the ring is answered.

Loud-speaker equipment may be introduced in place of any normal instrument without modification to the switchboards, the busy tone introduced on the line when the distant end clears obviating the risk of omitting to restore the amplifier key. 

The circuit design of the connector permits dial impulses to be sent over the connector multiple to control the setting of switches. This feature allows facilities to be added which may be of primary importance to some administrations, such as Tie-Line, Round Call and Conference working. These types of calls are controlled by equipment units additional to the standard switchboards.

Tie-Line Working
A variety of tie- line circuits provide for automatic both-way working to other exchanges of various types. A tie-line relay set is designed to be connected to the multiple of a group selector stage on the larger exchanges and to the conn ector multiple on the small exchanges. If more than one tie-line is required, the call will automatically connect itself to the first free tie-line of a group. The registers are wired up to make these junctions available by dialling a single digit. It is thus possible to have full automatic inter-communication between two P.A.X.’s of the types described, and furthermore the standard circuit is designed to allow manual supervision for tie-line working with any type of manual board.  

The Round Call or Person Finding facility supplies a means of rapid communication with a person who is within a building but not at his usual location. A number of persons holding important positions may be allocated round call numbers which will be dialled when they are unable to be found on their normal telephone numbers. Round call numbers are usually restricted to ten, but this figure may be exceeded when necessary.

The process of a round call is as follows:-
When a caller has been unsuccessful in finding the wanted round call subscriber at his normal telephone number he releases this connection and makes a new call by dialling the single digit allocated to the round call unit and then the round call number of the wanted line. This will immediately operate code calling equipments in various parts of the building. This code calling equipment may be of the type which sends out audible code signals or alternatively visible code signals with an audible tone to attract attention. The subscriber whose code is being transmitted is thus made aware that he is wanted and can quickly come into communication with the caller by dialling a specified digit from any convenient P.A.X. telephone in the vicinity.

Conference Calling is a means of collecting a number of subscribers on the same speaking connection for purposes of discussion or instruction. Two types of equipment are available for this purpose a manually operated and a dial operated system, the former being suitable for a P.A.X. with short lines and the latter for a P.A.X. having the subscribers spread over a larger area. Each subscriber given this facility has a special conference number in addition to the normal number and nine lines is a convenient maximum for this type of equipment.

Faulty Lines, etc.
If a line is connected to a register for more than a predetermined time, a thermo-relay comes into operation and transfers this line from the register to a holding circuit, where it will be retained until the fault or loop is cleared. One of the normal numbers must be used for this holding circuit, this number being wired to a jack on the switchboard for use as a normal line when the maintenance engineer is in attendance.

Click here for the PAX 35

Click here for the PAX 50

Click here for the PAX 50/400

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Last revised: October 14, 2019