PAGE No. 3

The 22 Line P.A.X.
July 1936

In a previous issue of 'The Ericsson Bulletin' a description was given of register controlled P.A.X's, covering boards having capacities of 35 to 400 lines. Where the number of lines required is less than 20 it is found more economical to depart from the register control principle and for this reason the Ericsson Company have developed a board, which is particularly suitable for small installations, and is operated on straightforward linefinder-connector principles.

The board provides for a maximum of 22 lines with three connecting relay sets, and employs 3,000 type relays and single motion switches (uniselectors) of approved British Post Office design. Ringing and tones are generated by a special relay group which is started and stopped automatically as required and is particularly economical in current consumption.

A 24 volt supply is required for operating the board and this may be derived from primary or secondary cells, or when an A.C. supply is available it may be operated from an eliminator. Wide voltage limits are possible and satisfactory operation is guaranteed between 20 and 30 volts.

Telephones of any pattern using B.P.O. standard dials giving 10 I.P.S. may be used and lines varying between 0-1000 ohm loop resistance can be accommodated. When desired, long line equipment can be provided, so that lines having a greater resistance than 1000 ohms may be connected. The minimum permissible wire to wire insulation for an extension line is 50,000 ohms.

Fig. 1 shews a view of a P.A.X. with covers in position, while Fig. 2 shews the same P.A.X. with covers removed. The board illustrated is equipped for 15 lines and two connecting circuits. It should be noted that wiring is always provided for the full capacity to enable additional equipment to be added with a minimum of trouble.

Fig 1 & Fig 2

Located at the top left hand corner is the fuse panel carrying the necessary fuses for feeding the various circuits on the switchboard; to the right of this, connection strips are fitted for terminating the lines. Immediately below the fuse panel and connection strips a uniselector shelf is fitted for accommodating the connector and line finder switches; two connectors are shewn to the left with a spare bank position for the third, whilst the two switches on the right are the line finders, the spare bank on the right being the position allocated for the third line finder when fitted. At the extreme right-hand end of this shelf is fitted a link strip provided for the purpose of busying the links when it is desired to carry out routine tests, etc.

Below the uniselector shelf are fitted three mounting plates carrying the line equipment and allotter relays. It will be noted that the line relays are fitted under individual covers; this is to avoid cross-talk, since these relays are utilized for feeding current to the transmitters as well as fulfilling the usual function of providing a calling signal. The bottom shelf accommodates the connecting relay sets and, on the right, the ringing and tone relay set all of which are mounted as jacked-in units. As will be seen, ample space is available below these relay sets to accommodate additional equipment for any special services that may be required, and in certain instances it is found convenient to mount the power equipment here.

The total overall dimensions of the board are 4' 6" high by 2' 1" wide by 10" deep, and when desired it can be mounted close against the wall to save floor space, arrangements having been incorporated so that access to the wiring is possible from the front. This has been accomplished by arranging the equipment on hinged shelves which swing open for inspection purposes.


(1) Using Primary Cells.
This arrangement is recommended only in cases where no electric supply is available. Twenty-three primary batteries are required having a capacity of 500 ampere hours.

(2) Using Secondary Cells. This scheme is suitable where there is a D.C. supply. Two batteries of secondary cells each having a capacity of 20 ampere hours are required, either being suitable for operating the equipment for 2-3 days. The charging current is provided by inserting suitable voltage dropping resistances in the mains leads.

(3) Operating direct from A.C. Mains.
This is the most satisfactory method and has many advantages, some of which are as follows:-

(a) No charging plant is required.

(b) No batteries are required.

(c) The initial cost is lower

(d) The maintenance costs are reduced since the system is fully automatic and does not require any attention.

(e) Considerably less space is required.

The equipment required for this method of operation is a battery eliminator which consists essentially of a transformer, a Westinghouse metal rectifier and 'smoothing' equipment consisting of condensers and choke coils, etc. The transformer primary is tapped and can be used on supply voltages varying from 100-250 volts. Full details of these and other eliminators are given elsewhere in this issue of the Bulletin.


(1) Automatic intercommunication between any two parties on the system.

(2) A low pitched continuous tone termed 'dialling tone' to indicate that dialling may commence.

(3) A low pitched interrupted tone termed 'ringing tone' to indicate that the called party's bell is being rung.

(4) A high pitched continuous tone to indicate that the called party is engaged.

(5) Release of the connection is dependent upon the calling party.

(6) Preference facility.

(7) Battery testing is employed throughout so that the blowing of a fuse automatically renders busy the circuit or circuits served by the blown fuse.

The removal of the receiver operates the line relay which marks the calling line selectable in the banks of the line finder, and at the same time causes the allotter to allocate a free connecting relay set. The line finder switch associated with the allotted connecting relay set now hunts for the calling line, and when this is found dial tone is transmitted to the calling party to indicate that dialling may commence. Unlike the usual systems of this type, the line relay remains connected to the calling party and is used for impulsing and transmission purposes. When the dial is operated the line relay responds and steps the connector switch, associated with the connecting relay set in use, to the desired line. If the called party is free, ringing is connected and ringing tone is transmitted to the calling party. When the called party answers the ringing is cut-off and the two parties can converse. If the called party is engaged, busy tone is transmitted to the calling party who should then replace the receiver and call later. Provision is made so that selected parties can gain access to engaged lines. This facility can be readily incorporated by a simple strapping on the line connection strip.

The Trunking arrangements are shewn in diagrammatic form in fig. 3.

The 22-line board has been so designed that the initial cost is at a minimum, but nevertheless the equipment details and circuit arrangements are such that all the additional facilities enjoyed by users of the larger P.A.X's can be readily added without alteration to the standard board. The special services now available are as follows:-

Round Call or Person Finder Facilities
Provision can be made for a round call to be given for any one of 10 selected parties. Lines requiring this service are given special numbers apart from their normal directory numbers. The digit 7 is usually allocated for round-call working and the dialling of this digit connects the calling party with the round-call equipment, at the same time releasing the regular connecting relay set and leaving this free for use on other calls. The special number allocated to the wanted party must now be dialled into the round-call equipment and this causes a number of bells or buzzers to sound a code signal round the offices or factory. The wanted party can answer the call by removing the receiver from any instrument and dialling '8'. This action automatically places the wanted party in communication with the caller and stops the code signals. The replacement of the receiver by the calling party releases the connection.

Tie Line Working
The digit '9' is usually allocated for this service, and when dialled, access is obtained to the tie line relay equipment, and the line to the distant P.A.X. is seized. Further impulsing by the calling party is then repeated to the distant P.A.X. to select the required number on that P.A.X. Particular care has been taken in the design of these tie line circuits so that it is possible not only for two 22-Iine P.A.X's to be coupled together via tie lines, but also a 22 line board may be coupled to a register controlled P.A.X. without any modifications being necessary. Other tie lines can be provided to suit special cases when desired. A calling party with preference facilities can gain access to the tie line while it is engaged. A warning tone can be introduced to indicate to the parties already engaging the tie line that intrusion has taken place.

Automatic Conference Facilities
This enables conference facilities to be provided for any 9 selected parties. Such lines are given single digit conference numbers apart from their normal directory numbers.

The conference circuit occupies one position on the connector multiple and access is gained by dialling 6.

Any non-conference line can be appointed to set up a conference connection.

The initiating party can call all the required lines consecutively without disconnecting between calls.

When a conference call is in progress any party can release from the conference by momentarily replacing his handset.

Any conference party can join a conference connection by dialling the number of the conference unit followed by digit '0' and his own conference number.

A connecting circuit is taken into service only whilst a conference call is being set up.

Non-conference lines can also be asked to join a conference connection temporarily for consulting purposes.

When a conference call is made to an engaged line, the caller is advised by tone that this particular line is engaged, and a short tone is also induced on the engaged circuit to indicate that one of them is wanted on the conference circuit.

An engaged conference party can be recalled.

A conference party who is engaged on another call can be transferred to the conference connection.

Manual Conference Facilities
Two schemes are available:-

(a) All parties requiring conference facilities are provided with a key, the operation of which switches their line on to a common speaking circuit. Each line is operated independently.

(b) Certain master stations are provided with a set of keys associated with the parties with whom it is desired to hold conference. The operation of a key causes ringing to be connected to the line associated with it. The operation of a number of these keys causes each line to be rung, and when the various parties answer they are connected to a common speaking circuit and the conference can proceed.

Loud Speaker Facilities
Any line can be fitted with a loud-speaker and an additional facility is usually provided in the connecting relay set to transmit busy tone when the distant party clears. This warns the party at the loudspeaker instrument to restore the amplifier key.

Discriminating Ringing Equipment
When desired, provision can be made so that selected parties when making calls will give a distinctive ring to the wanted party. This feature is useful for the managers or other important personages and ensures a quick response to their calls.

2 -Party Line
in certain instances it may be desirable to provide two telephones on one pair of wires. These telephones can have full automatic facilities by fitting a relay switching group. Either of the parties can call the other by dialling a special digit, allocated for revertive service, which releases the connecting circuit and rings the wanted party. When the latter replies, speech can take place between the two parties.

Every effort has been made to incorporate all the features known to be in popular demand. Other facilities can however be incorporated when required.

It should perhaps be pointed out that, owing to the simplicity and reliability of these boards, they can readily be adapted for use as public exchanges in small rural areas since the current consumption is particularly low and a minimum of attention is required.

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Last revised: October 14, 2019