January 1960

The Subscriber Attended P.A.B.X. described in Bulletin No.22, January, 1951 has successfully met the needs of business organizations for many years and has been modified from time to time to keep pace with new requirements.

The P.A.B.X. described in this article is a new version designed to give more facilities and reduce maintenance.

THE Company's standard Subscriber Attended P.A.B.X. System for five exchange lines and 20 extension lines described in Bulletin No. 22 has been re-designed and the new version is now in production. Some of the new facilities provided are as follows:-

  1. Lock-out type extension line circuits to prevent switching equipment being held unnecessarily under P.G. and C.S.H. alarm conditions.
  2. The passing on of linefinder and exchange finder start conditions to a second circuit after a predetermined delay when the first circuit seized is faulty. This allows the call to be completed and prevents any uniselector driving continuously.
  3. The dialling of only two digits for extension calls and not as previously, two or three digits.
  4. The return of NU tone instead of busy tone when a spare or locked-out extension number is dialled.
  5. Right-of-way (intrusion~ facilities are now included as a permanent feature.
  6. The safeguard against the loss of an exchange call during transfer.
  7. Transistor ringing and tone supplies.
  8. Test jack points to enable circuits to be busied manually.

As is implied by its title, the Subscriber Attended P.A.B.X. does not involve the use of a manually operated switchboard. Certain extensions are designated to answer calls incoming from the public exchange and transfer them to other extensions as required. Any extension may be so designated but the function is usually restricted to four or five extensions at the most.

In addition to the foregoing, any extension may be:-

  1. Allowed full access to the public exchange.
  2. Prevented from having direct access to the public exchange but permitted to receive transferred exchange calls.
  3. Allowed right-of-way (intrusion facility) on extension-to-extension calls.
  4. Allowed right-of-way when consulting another extension during a public exchange call or when transferring an exchange call to another extension.

Any extension may be fully barred, i.e. restricted to extension-to-extension calls only; this cannot of course be applied to the extensions designated to answer incoming exchange calls.

The exchange line circuits are normally arranged for working to either automatic or C.B. public exchanges. Simple internal modifications can readily be applied to convert the circuits for either C.B.S. or magneto working.

Access to the public exchange is gained by dialling a single code digit. The exchange line circuits can, if necessary, be arranged in two groups with an access code digit for each group.

Connection to an external telephone system other than the public exchange, with direct access from all. extensions can be obtained by:-

  1. Utilizing a second group of exchange line circuits, incoming calls being answered by the designated extensions.
  2. Replacing the second group of exchange line circuits with tie line circuits which allow direct 'in dialling' to extensions.

Extension lines are numbered 20-39 whilst digits 8, 9 and 0 are available for allocation as access codes to exchange or tie line circuits.

Fig 1
Trunking and start outline

Internal Call

As may be seen from Fig. 1, when an extension removes the handset to originate a call, a start signal is routed to a free connector circuit to cause the linefinder (LF) to self-drive to the outlet associated with the caller's line circuit. At the same time, a thermal relay is activated in the connector circuit thus enabling the start signal to be passed to another free circuit after approximately 5 seconds should the first linefinder fail.

Normally the extension will be switched to the connector and dial tone returned within one second. It is therefore convenient to continue to use the thermal relay, arranged in this case to operate after approximately 20 seconds, as a guard against the caller failing to dial or causing the inter-train pause to be excessive. Should either of these contingencies arise, the connector is released and the caller's line circuit is locked out until the handset has been replaced. Switching equipment is thus prevented from being held indefinitely should a P.G. line fault occur.

When the requisite two digits have been dialled to position the connector switch (CS) on the required outlet, the called extension line circuit is tested. If the extension is already engaged, busy tone will be returned to the caller, but if the line circuit is locked out or the number dialled is spare, NU tone will be returned. When the called extension is found free, interrupted ringing current is applied to the line and the caller receives ring tone until the called extension answers.

At the end of the conversation, if the caller replaces the handset first, the connector is immediately released and the called extension line circuit is locked out until the handset is replaced. In the event of the called extension replacing the handset first, the thermal relay is again brought into use to enable the connector to be released and the calling line circuit locked out within approximately 20 seconds.

A right-of-way extension receiving busy tone after dialling may intrude on an engaged extension by dialling digit 1. This causes a speech path to be connected to the engaged parties and intrusion tone to be transmitted to indicate the presence of the third party.

If a private conversation is required, the engaged extensions should be requested to replace their handsets. The required extension will then be rung and the call proceeds normally.

Outgoing Exchange Call
Having removed the handset and received dial tone the originating extension dials the relevant code digit to gain access to an exchange line circuit. This positions the connector switch and, as shown in Fig. 2, enables relay EL to operate. This relay is interconnected with the EL relays of other connector circuits in a manner that permits only one to operate at a time. The arrangement prevents the possibility of a wrong connection when a second group of exchange lines or tie lines is equipped and calls to both groups occur simultaneously.

Fig 2
Outgoing exchange call switching elements

The operation of relay EL routes a start signal to a free exchange line circuit. Should there be no free circuit available the caller will receive busy tone, but normally the operation of relay EC prevents the return of tone. The exchange finder (EF) self drives to the outlet associated with the calling extension's line circuit, the switch being arrested by the operation of relay KE. Relay EF in the connector also operates to cause the connector to release and when this has been proved the extension is connected directly to the exchange line circuit.

If the public exchange is automatic, dial tone will be returned and the required subscriber's number may then be dialled. Alternatively, if the public exchange is manual the appropriate signal is applied to call the operator.

Should a barred extension attempt an outgoing exchange line call, the operation of relay EB prevents EL operating and causes NU tone to be returned.

Incoming Exchange Call
On an incoming call, relay CR operates to ringing current from the public exchange. As shown in Fig. 1, bells are rung to attract the attention of the designated extensions and the first to answer causes the exchange finder (EF) to self-drive to the outlet associated with the line circuit concerned. The extension is thus connected directly to the exchange line circuit.

Consultation Call and Transfer
An extension engaged on an exchange call may consult, or transfer the call to, another extension. The extension instrument transfer button is momentarily depressed to cause relay TR to operate and divert the extension to an auxiliary line circuit. (Fig. 3). TR also connects a holding loop to the exchange line. A connector is seized and the required extension number dialled as for an internal call.

Fig 3
Consultation and transfer switching elements

At the end of a consultation call the instrument transfer button is again momentarily depressed, relay TR releases and the connection to the exchange is re-established.

When the exchange call is being transferred the required extension having been obtained as for a consultation call the calling extension replaces the handset. Relay KG releases and, provided that no other transfer switching is taking place, relay TP operates. The exchange finder self-drives and is arrested on the required outlet by the operation of relay KE. The EP relay releases because of the low resistance shunting effect of KE and causes the connector to release; relay KG operates, TP and TR release and the transfer is complete.

An exchange call may be repeatedly transferred.

A right-of-way extension making a consultation or transfer call may intrude on an engaged extension in the manner already described; the ring and answer procedure must however be completed before the handset is replaced to transfer the call.

Exchange Finder Safeguard
A thermal relay is fitted in each exchange line circuit to prevent prolonged hunting of the finder. The circumstances in which this safeguard is used and the actions that result are as follows:-

  1. When an outgoing call is being attempted and the finder fails to locate the relevant outlet, the exchange start signal is passed to another free circuit.
  2. On an incoming call, if the finder fails to locate the required extension the call is diverted to the extension selected for mains-fail working.
  3. When a call is being transferred and the finder fails to locate the relevant outlet, owing to a circuit fault or to mis-operation on the part of an extension, the transfer is abandoned and the designated extensions are rung as for an incoming call. The re-establishment of the exchange call is thus ensured.

Continuous 25 c/s ringing current is generated by means of a transistor relaxation-type oscillator. A parallel-tuned transistor oscillator is used for the 400 c/s tone requirements. The absence of moving parts in the ringing and tone generators gives obvious advantages when compared with non-static devices. Less maintenance is necessary and radiated interference is virtually eliminated.

Interruptions are controlled by means of interacting relays and a miniature uniselector.

Following are details of the various tones and pulses.

Dial tone is obtained by feeding continuous ringing current via an attenuator network.

NU tone:- continuous, at 400 c/s.

Busy Tone:- 400 c/s; 800 milli-seconds ON, 800 milli-seconds OFF.

Intrusion Tone:- 400 c/s; 60 milli-seconds ON, 340 milli-seconds OFF, 60 milli-seconds ON, 1140 milli-seconds OFF.

Ring Tone is obtained by modulating the 400 c/s supply with the continuous-ring supply. The resultant tone is then interrupted in the sequence 400 milli-seconds ON, 400 milli-seconds OFF, 400 milli-seconds ON, 1200 milli-seconds OFF.

Interrupted Ringing periods are the same as those for ring tone.

Interrupted Earth, which is applied to the bells associated with designated extensions when an incoming exchange call originates, has a periodicity of 800 milli-seconds ON, 800 milli-seconds OFF.

The power supply for the P.A.B.X. equipment is usually fed from the mains via a suitable rectifier unit. Should the mains supply fail, the equipment will be out of service but contact with the public exchange can be maintained from selected extensions. These may be ordinary extensions within the P.A.B.X., provided that the public exchange is of automatic or C.B. type. Calls set up during a mains fail period are not disconnected when the power supply is restored.

Fig 4
5+20 Unit with battery eliminator at the bottom

The equipment is designed to operate from a nominal 50-volts d.c. supply but will work satisfactorily over the range 45 to 55 volts.

Extension lines with a loop resistance of up to 800 ohms including the instrument may be connected to the system. The earth connection required for consultation and transfer facilities may be obtained locally or by means of a third wire to the P.A.B.X. equipment.

Exchange and tie loop resistance is independent of extension line limits; the equipment will function with a 1,000 ohm loop but this may depend upon the efficiency of the distant exchange equipment.

As illustrated in Fig. 4, the equipment is mounted on a rack of pressed steel construction. The unit is approximately 5 ft. 9 in. (175 cm) high, 2 ft. 6 in. (76 cm) wide, and 1 ft. 2 in. (36 cm) deep when the outer cover is in the closed position. Two short angle brackets at the top enable the unit to be supported from a wall.

All the equipment is accessible from the front. The lower uniselector shelf can be opened as shown in Fig. 5 to reveal the connection strips, whilst the upper shelf is similarly hinged to enable the fuse panel to be exposed. The various circuits are strip mounted on jack-in type plates with the exception of the plates for the line circuits, but as the connections to these are formed into a looped cable arm, the plates can be withdrawn should it be necessary to gain access to the wiring side.

The hinged outer cover is of lift-off type and when closed, completely protects the equipment against dust, the rear cover being permanently sealed.

Power may be obtained from batteries and a suitable charger, but a mains rectifier unit which is adequate for most installations is usually provided and mounted at the bottom of the rack as shown in Fig. 4.

Fig 5
Lower Uniselector shelf swung down to expose connection strips

The stock unit as illustrated is fully equipped for 20 extension lines, 4 connector circuits (local links), 5 exchange lines and their associated auxiliary line circuits, and the ringing, tone and alarm relay set. However, a partially equipped version is also stocked, the rack being identical in size and appearance but equipped for 10 extension lines, 2 connector circuits, 3 exchange lines and the other common items. The upper uniselector shelf is not then provided but can be supplied complete with pre-formed cable and any other necessary equipment for intermediate requirements.

Apart from the extension instruments, the only other items of equipment are the d.c. bells to be fitted near the designated extensions, a small fuse alarm panel and an alarm extension panel, the last two items being optional.

Although many new features have been added to the system, every effort has been made to keep it as simple and economic as possible. Special consideration has been given to the maintenance aspect. Well established types of apparatus, including heavy duty uniselectors, 600-type, 3000-type, and standard high-speed relays are employed. All items have full tropical finish, and p.v.c. insulated wire is used throughout; the maximum reliability is thus ensured even under the most stringent climatic conditions.

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Last revised: October 15, 2019