No. 22 PAGE No. 12

5 + 20 Sub-attended P.A.B.X.

Rapid and efficient telecommunication is one of the essentials of modern times. It is as necessary within the business organization as it is in the public telephone network and is being achieved by increased automatisation, but even when private and public exchanges are automatic the fullest benefit is not experienced if connections from the former to the latter have to be established through a local manual switchboard.

This disadvantage is obviated in the sub-attended P.A.B.X. which is completely automatic and incorporates the following facilities:-

(1) Access to the public exchange is obtained by dialling a single digit.
(2) Extensions can be completely or partially barred exchange facilities.
(3) Incoming exchange calls may be answered by any one of any number of designated extensions.
(4) Extensions with full facilities can originate, answer, or transfer exchange calls.
(5) An extension engaged on an exchange call can consult another extension without releasing the external connection.
(6) A transferred exchange call can be further transferred to any other extension having exchange facilities.

Fig. 1
5 + 20 P.A.B.X. Equipment Rack

Critical design, together with the use of twin type relays and 25-point British Post Office type uniselectors, has made it possible to produce this equipment, incorporating every essential facility, at a competitive price.
Employment of the twin relay, performing the functions of two standard relays but occupying the space of one, has enabled the equipment to be mounted on a rack of unusually small dimensions, the height being only five feet and the width approximately two feet six inches. The potentialities of the uniselector as a switching mechanism have been fully utilized, so that the P.A.B.X. has a maximum capacity of 25 lines.  Of these, up to twenty are used for internal extensions and any number of the remaining five for the circuits giving access to the public exchange (exchange links).
The connecting circuits for internal calls (local links) are equipped as required, the standard rack, illustrated in Fig. 1, accommodating a maximum of three, but where the traffic loading is exceptionally high special provision can be made to equip extra links.

The extensions are numbered in three groups, as follows, the prefix digit, or digits, determining the group:-

01 to 09 inclusive - (9 numbers)
11 to 19 - (9 numbers)
001 and 002 - (2 numbers)

Extension instruments are provided with a push button for use in connection with consultation or transferred calls. Instruments for extensions without exchange facilities need not have a push button.

In the following description, an extension dealing with a public exchange call is termed an “exchange“ extension.

Internal Call
The extension lifts the handset and when connected to a local link receives dial tone. After dialling the number required, ringing tone is received if the line is free, or busy tone if engaged. When the call is established, the first extension to replace the handset at the conclusion of conversation releases the connection.

Outgoing Exchange Call
The preliminary digit 9 is dialled and connection to the public exchange line is immediately established. If all lines are engaged, busy tone is received, but, if the extension cares to wait, connection is automatically made when a line becomes free. Should a barred extension attempt to make an external call, busy tone is received.

Incoming Exchange Call
Attention is drawn to an incoming exchange call by the ringing of a special bell, or bells, positioned within hearing of all designated extensions. The first of these extensions to answer the call is connected automatically to the exchange line concerned. Under no circumstances can an incoming call be answered by a barred extension or by dialling the digit 9.

Consultation Call and Transfer
An extension engaged on an exchange call and wishing to consult another extension, momentarily operates the button on the instrument. This “holds“ the exchange line and connects the exchange extension to a local link, thereby enabling the wanted number to be dialled.

When the called party replies, the exchange extension can either complete the transfer simply by replacing his handset or, after having finished the consultation, which incidentally cannot be overheard on the exchange line, can release the local link and resume the original exchange call by again operating the button.
Should the wanted extension be engaged on an internal call, busy tone is received by the exchange extension but an intrusion tone is superimposed upon the conversation of the local call to give warning that an exchange extension is calling. On hearing the tone, both local extensions should replace their handsets, when the one required will be automatically rung and, on answering, will be connected to the exchange extension.

If the wanted extension is engaged on an exchange call, ”number unobtainable“ tone

Fig. 2—Skeleton Circuit. 5 ± 20 P.A.B.X. System.
is received by the exchange extension and the intrusion tone is not connected.

In Fig. 2 the circuit of the P.A.B.X. is reproduced in simple skeleton form so that the association of the elements can be readily appreciated.

Internal Call
When the handset is lifted, the extension line circuit responds to the loop and connects earth to the ST lead of the common local link start chain. A free local link is selected and its line finder (LF) searches for the calling line. When connection is made, dial tone is heard and the calling extension dials the wanted number. The dialled impulses position the connector switch (LS) and the local call is completed over the link LF1 to LSI.

Outgoing Exchange Call
A local link is obtained and the extension dials the preliminary digit 9 to position the connector switch. From bank LS2 of this switch, earth is connected to the exchange link start chain, through relay EC and bank LF2 of the line finder. (The inclusion of this bank allows a pair of terminals for each extension to be placed in the STE lead so that any extension can be barred exchange calls by omitting the strap between the two terminals).

A free exchange link is selected and its finder switch searches on bank EF2 for the local link marked on bank LF3 of the line finder by the operation of relay EC. When contact is made, relays LK and ER operate to break the drive circuit of the EF switch, and to release the local link, respectively.
The exchange link is now connected to the calling extension via bank EF1 and the external call can be made.

Incoming Exchange Call
Ringing current from the public exchange operates relay PC which closes the circuit for the special exchange call bells, and switches the ST lead through to relay STE in the chain associated with the exchange link through which the call has been received. The first designated extension to answer causes the relay to operate and the EF switch of the link to search for the line circuit concerned.

Consultation Call
Operation of the instrument button by the exchange extension causes contact TF in the exchange link to complete a connection over the EL lead to the associated exchange line circuit. The latter is similar to the extension line circuit and, over the STF lead, selects a free local link which searches for the contact on bank LF1 to which the particular ME lead is wired. When the two circuits are connected, relay contact EP opens to prevent the irregular seizure of another exchange link over banks LS2 and LF2 should the digit 9 be dialled. The exchange extension can now dial the wanted number and, at the end of the consultation, another operation of the push button releases the local link and reconnects the exchange line.

To effect a transfer, the exchange extension establishes connection to the desired party, as for a consultation call, and then replaces the handset, thereby causing battery to be connected to a contact in the LF4 bank of all local links. This contact occupies the same relative position as the contact in the LF1 bank to which the ME lead of the particular exchange link is connected.
Chained contacts ensure that only one exchange link at a time is able to mark its associated contacts in the LF4 banks, consequently, in only one local link is the LS3 wiper at battery potential.
Bank LS3 is wired to bank EF3 in such a manner that contacts in each, representing the same extension, are connected, and it is arranged that the EF switch of the exchange link searches over bank EF3 to find the contact marked by battery.
When the circuit is established relay LK operates to break the drive and the EF switch is thus stopped on the contact, on bank EF1, connected to the extension line circuit marked by the LS switch of the local link.
Relay ET operates in series with relay LK to release the local link ; relay TF in the exchange link is released and the exchange line is thereby transferred to the second extension. The latter may make a consultation call to a third extension and if necessary transfer the exchange call once again.

Fig. 3
Rear View of Rack showing Plug and Jack Connections on Relay Plates

Test Line
The incoming line from extension 01 passes through a test jack, thus permitting this number to be used for test purposes.

Mains Failure
Normally, power for the P.A.B.X. is obtained from the mains via a rectifier unit which is convenient and requires little or no attention. In the event of a mains failure the internal system will be temporarily put out of action but all exchange lines are automatically connected to selected extension instruments so that contact with the public exchange system is maintained.

Unused Numbers
Should an incorrect prefix digit, or digits, be dialled, busy tone is returned.

The ringing current and low-frequency ring and dial tones are generated by a vibrator-converter of a type commonly employed in P.A.B.X. equipment, while the higher frequency tones are obtained from a self interrupting relay.
The busy and intrusion tones are of the same frequency as the continuous N.U. tone but are interrupted at different periodicities, in order to give a distinctive character to each tone.

The equipment will function over a range of from 46 to 52 volts, and with extension line loop resistances up to a maximum of 500 ohms.
Instruments fitted with the transfer press button require an earth connection which may be obtainable near the instrument, but, if this is not possible, earth potential can be supplied from the P.A.B.X. equipment over a third conductor, the resistance of which should not exceed 250 ohms.

Fig. 4. View of Uniselector Equipment with cover removed

The equipment is mounted, as illustrated in Fig. 1, on a rack of angle-iron construction which has two brackets at the top rear (Fig. 3) for supporting it from the wall, close proximity to the wall being made possible by the fact that all the equipment is readily accessible from the front.

The relays are strip mounted and, with the exception of those on the two topmost plates, are wired to plugs which mate with
jacks terminating the rack wiring, thus plates of relays can be easily removed for attention.

An enlarged view of the upper portion of the rack equipment is shown in Fig. 4 from which it may be seen that the switches are mounted on a hinged shelf fastened by two thumb screws and are grouped to facilitate straight multiple wiring.

Above the switches are two fuse panels, and three terminal blocks for the connection of external cables. Facilities for barring exchange calls from extensions, and for cross-connecting to suit various public systems are also given on these blocks.

Maintenance is simplified by the straightforward design of the circuit and by the accessibility and comparatively robust construction of the components, which are protected by front and rear dust covers. These advantages, combined with compactness, low cost and the fact that tropical finishes are provided throughout, make this P.A.B.X. equipment well worthy of consideration for use in small business organizations anywhere in the world.

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Last revised: June 22, 2003