Taken from
Engineering supplement to the Siemens Magazine No. 86 (July 1932)


In the Engineering Supplement to this Magazine for December, 1931, a brief description was given of three new private automatic equipments having capacity for 10, 25 and 50 lines respectively. We have recently developed a larger type of P.A.X. which has capacity for 100 lines and is known as P.A.X. No. 21. This equipment employs relays and two-motion switches. There is also one uniselector (preselector) to act as a call-distributor.

The operating voltage range is from 20 volts to 30 volts. Finder-connectors can be fitted to provide for a maximum of 10 simultaneous calls. As in the case of our 50-line P.A.X., the rack for this equipment is of the floor type and is mounted on a chassis provided with rollers. A back-plate fixed to the wall carries a distribution frame which consists of two sets of terminal blocks. One set is used for terminating the cables from the various telephones, and the other set is used for terminating the line wires from the equipment. By means of cross connections between these two sets of terminals any line on the equipment can be connected to the line wires of any telephone. In this way it is possible to arrange the wiring of the telephones in the most economical manner. The arrangement also makes it possible for any telephone to retain the same number if it becomes necessary to move the telephone from one part of a building to another. Such a move often necessitates the use of a different pair of line wires, but by moving the cross connecting wires on the distribution frame the telephone can still retain its old number, thus rendering expensive directory changes unnecessary.

Fig. 1 Fig. 2

A hinged floor-plate, normally raised in front of the rack, can be lowered to the floor to provide a runway on which the rack may be drawn forward to permit access to the distribution frame and the rear of the rack. A locking device is incorporated in the runway arrangement which prevents the rack from tilting in any direction whilst being withdrawn or pushed back. The two-motion switches and fuse panels (see Figs. 1 and 2) are mounted on the front of the rack. At the rear of the rack the line relays and two relay sets containing the common equipment are mounted on gates which open outwards in order to give access to the wiring of the line relays and the shelves carrying the two-motion switches. Below the gates a number of terminal blocks are fitted; these serve as a concentration point for wiring from the switch banks and the line circuits. The cable to the distribution frame situated on the back-plate is also connected to this point (see Fig. 3).

The cover enclosing the equipment is of metal faced plywood; only the front is removed when the P.A.X. requires attention (see Fig. 4). The sides and top of the cover are attached to the rack, but the rear halves of the sides are arranged to swing outwards on hinges in order to permit of the opening of the gates, on which the line relays and common equipments are  mounted.

The overall dimensions of this equipment are:- 5 ft. 5 ins. high by 2 ft. 8 ins. wide, by 1ft. 11 ins. deep.

Like the smaller sizes of Private Automatic Exchange it has the smallest dimensions of any equipment 100 lines capacity on the market.

Fig. 3 Fig. 4

Dialling, ringing and busy tones are provided, each being similar to the corresponding tone used on public exchanges. Ringing is interrupted, but in a manner dissimilar from the public exchange ringing, thereby providing discrimination when a P.A.X. telephone is fitted near to a public exchange telephone.

Ringing current is normally provided by a relay pole-changer in conjunction with a transformer, but arrangements are made so that a ringing machine can be fitted, if loud ringing bells are to be operated from the equipment. If a ringing machine is supplied, it is mounted at the rear on the baseplate of the rack (set picture to the right).

As in the smaller sizes of Private Automatic Exchange the relays on this equipment are provided with twin contacts throughout, thus ensuring the utmost reliability in service.

In addition to the normal facilities, the following special services can be provided:-


Executive Right of Way
Executives who desire the facility for entering conversations in progress are provided with a press button beside the telephone. If the called line is engaged, the caller hears the busy tone, but on pressing the button the tone is cut off and the executive can speak to the wanted party. Any ten lines on the equipment may be given the facility to exercise right-of-way. Normally this facility would only be granted to senior members of any firm as it is permissible for such a caller to enter an already established connection and thus possibly hear part of a conversation. In view of the increasing demand for this facility, it is considered that an up-to-date equipment should be capable of providing this service when it is desired.

Limited Right of Way
In view of the fact that the special lines to which right-of-way has been granted can enter an established connection by pressing the right-of-way button, the question of preventing some of these lines from exercising this facility on the senior executives arises. For instance, the general manager of a firm would probably object to anyone entering a call on which he was engaged with, say, the works manager. The limited right-of-way facility makes it possible to prevent the exercising of the right-of-way facility on calls to certain groups of lines.

Restricted Entry to Special Lines
It is possible to prevent certain groups of lines from calling lines in certain other groups whilst still permitting the latter to call the former. The advantage of this facility is that junior members of a firm can be prevented from making calls to certain senior members whilst the latter are, of course, permitted to make a call to any telephone on the system.

This facility enables
executives who have the right-of-way facility to hold a conference by telephone. In addition, any other two lines may be brought into a conference. This is a very desirable feature as one of the executives may be away from his office - possibly in one of the workshops - when a matter arises of sufficient importance to warrant a conference being called immediately. This can be done from the nearest telephone, and this telephone can then be connected to the ensuing conference.

Usually a special line is allotted for the conference number, which is marked engaged to all calls. Upon a conference being arranged, the parties to the conference each make a call to the conference number, On receiving busy tone the connection to the conference is established by pressing the right-of-way button. Each party is free to retire from a conference by replacing the receiver, make other calls and then re-enter the conference by calling the conference number and again pressing the right-of-way button.

In the case quoted above, however, where the conference is arranged from a telephone which has not got the right-of-way facility, the conference is arranged by asking the participants to call up the number of the line from which the request is made. In this case, the member of the conference who is using this telephone cannot retire and then re-enter the conference. If two subscribers, neither of whom has the right-of-way facility, are required to enter a conference, arrangements are made for a call to be established between such subscribers in the usual way and the participants in the conference having the right-of-way facility are instructed to call either of these telephones and then enter the connection by pressing the right-of-way button.

Double Exchange Working
This facility permits of the division of the lines on the exchange into two separate groups, neither of which can communicate with the other.

This provides extremely economical working for a business house which lets off a portion of its premises to some other business, or any case of a similar nature in which two separate small - telephone systems are desired. These groups of lines virtually become separate exchanges working on a common - group of switches using t h e same ringing and tone equipment and the same common battery (or other power supply unit). Thus a great saving is effected in initial cost, space and maintenance.

Code Call
In some establishments it is extremely desirable to be able to communicate immediately with executives or other important persons when they are away from their telephones. To meet this condition, a code call system can be provided. When a call to such a person receives no reply, the call is released by replacing the receiver. A special number allotted as the code call number is then dialled, followed by the required person’s code number. This operates call signals situated in various parts of the establishment. On hearing his call signal, the wanted person knows that someone wishes to speak to him. He then goes to the nearest telephone and by calling up a special number establishes connection with the caller. This facility is given by providing a code call unit in addition to the standard equipment. The connection to the code call unit is made via a finder-connector. Arrangements are made to enable the impulses which denote the person’s code to be extended to the code call apparatus.

As this facility is not often required, the code call unit is mounted separately from the standard equipment, the dimensions of which are thus kept at a minimum.

Junction Working
Many business concerns find it more efficient and economical to divide their business into two or more separate establishments. In the case of a firm where the staff have to interview many callers in connection with the business, it would be preferable to have the offices in some easily accessible position in the main part of a city. On the other hand, the factory or warehouse of such a business would probably be more economically run if situated in the suburbs. The junction working facility permits of two of these exchanges being linked together to meet such a case, Special equipment is necessary and it is mounted separately from the standard equipment. The number “00” is allotted for junctions. Any number of junctions up to the number of connectors fitted at the smallest exchange may be used. When it is desired to connect a line on one of these exchanges to a line on the other, “00” is dialled first, and then the local number of the desired line. The junctions permit of calls being set up in either direction.

Operation of Electrical or Electro-Mechanical Devices
This facility is given to enable some or all of the P.A.X. lines to control some device exterior to the P.A.X. by dialling a special number. For instance, the opening or closing of an electrically controlled lock, or the operation of fire alarm bells. These two cases are, of course, only two examples of the many uses in which this facility may be used.

Power Equipment
Usually the service on equipments of this size is of sufficient importance to make it necessary to guard against interruption due to failure of the public supply mains; for this reason the equipment is normally operated from a secondary battery. If, however, only a small number of lines is equipped and the maximum current demand (allowing for the starting current of the ringing machine, if one is fitted) does not exceed 6 amperes; the equipment can be operated quite satisfactorily from our Transrecter No. 3, which was described in the December, 1931, issue of this Engineering Supplement.

In conclusion, it can be fairly claimed that in its compactness and convenience, its neat appearance, great accessibility, its sound design, and in the facilities which it affords, the No. 21 P.A.X. Equipment occupies a conspicuous position in the front rank of modern telephone engineering products.


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