PAGE No. 69

July 1963

The two cordless desk-top switchboards described complete a new range of cordless, B.P.O.-approved private manual branch exchange switchboards developed by the Company for use in subscribers’ premises. Attractive appearance, compact size and improved facilities including lamp signalling and power unit operation are some of the features of the new switchboards which have capacities for three exchange lines and twelve extensions (3 + 12) and four exchange lines and eighteen extensions (4 + 18).

Figure 1
The 3 + 12 Line Switchboard (N116)

In an earlier article a new type of BPO-approved cordless private manual branch exchange was described. This PMBX, equipped for a maximum of two exchange lines and six extensions (2 + 6) was the first of a new range designed to meet the various capacity and service requirements of small and medium sized business organizations. To complete the range, two further cordless PMBX’s of 3 + 12 and 4 + 18 capacities have been developed in conjunction with the BPO. Their design has been based on similar objectives as determined for the 2 + 6 in order to reproduce the desirable features of convenient size, attractive appearance, better service for extension users and easier operation for the switchboard attendant.

Increased facilities are offered by the larger switchboards but, despite this, economy of equipment has been maintained through the continued use of the 4-wire principle in the circuit arrangements. This principle requires one wire and an earth at each extension telephone in addition to the two wires provided for speech.

Although the switchboards are essentially for use with 4-wire extensions, provision is made for the inclusion of a limited number of 2-wire extensions, inter-switchboard extensions or private circuits.

The main features of the two larger switchboards are as follows:-

  1. Lamp signalling for exchange and extension calls.
  2. Automatic ‘hold’ of exchange calls.
  3. Follow-on-exchange call trap.
  4. Separate supervisory signals on extension-to-extension calls.
  5. ‘Operator recall’ on extension and exchange calls.
  6. Operator’s headset in addition to operator’s telephone, when required.
  7. Night Service.
  8. Mains-failure safeguards.
  9. 5 connecting links on the 3 + 12 switchboard and 7 on the 4 + 18.
  10. Connection of 2-wire circuits by simple rearrangement of cords and straps in the switchboard.
  11. A maximum line resistance of 500 ohms for extension circuits and 935 ohms for extension-to-exchange connections.
  12. Subscriber Trunk Dialling trip meters when required.
  13. Free connecting link lamp signals. (4 + 18 switchboard only.)
  14. ‘Overcall’ circuit to allow the operator to answer exchange and extension calls when all connecting links are engaged. (4 + 18 switchboard only.)

By use of similar constructional elements a standard shape has been made possible for all the PMBX’s in the range, and dimensional and weight characteristics have been contained within suitable limits. The 3 + 12 PMBX illustrated in Figure 1 has overall dimensions 17” wide, 8” high and 13” deep (43 x 20 x 33 cm) and weighs 33 lb. (15 kg), while the larger 4 + 18 switchboard shown in Figure 2 is 22” x 9” x 15” (60 x 23 x 38 cm) and weighs 56 lb. (26 kg). Both switchboards are normally provided in two-tone grey with matching operator’s telephone.

The light French-grey covers of each switchboard are fabricated in one piece by injection moulding and composed of a durable co-polymer plastic. Each cover drops easily into position and is firmly retained by its leading edge beneath the front of the face panel and by two screws at the rear of the baseplate.

Figure 2
The 4 + 18 Line Switchboard (N116)

The face-panel, mounting the keys, lamp jacks and designation labels, is finished in hard wearing elephant-grey plastic with an attractive leather grain pattern overall. Legends are in white and clearly impressed upon the plastic by a hot-stamping process
The keys are of the now well-established miniature type, and the colours of the wedge-shaped handles have been chosen to permit ready identification of the keys associated with a particular circuit. They are light ivory for the exchange-line and operator’s keys, and alternately light ivory with black inserts and French grey with black inserts on the extension positions. Above these the extension and exchange line lamps are mounted behind a Diakon lens strip, the arrangement being such as to permit quick lamp replacement, no lamp extractor or removal of the switchboard cover being necessary. The lamp-cover strip is simply withdrawn to reveal the lamp-jack screws which on release allow the lamp jack to be moved forward and the lamps withdrawn by hand.

When the cover of the switchboard is removed, a 3-section metal chassis is exposed. The face-panel and the relay plate which are respectively hinged to the front and rear of the baseplate can be opened in book fashion to afford a high degree of accessibility as shown in Figure 3. The relays, consisting of 3000 and 600 types are mounted on the rear plate under a clear plastic dust cover. Transmission relays are provided with non-removable relay shields to minimize crosstalk between circuits.

Some static components are mounted on the base together with the connection blocks. One type of connection block has soldered connections, while the other type is equipped with screw terminals and is used for the connection of the operator’s telephone and line circuits. Screw terminals are also provided to permit re-arrangement of the straps and cords when 2-wire extensions, private circuits or inter-switchboard extensions are required. Extension circuits 7 to 12 on the 3 + 12 PMBX, and 10 to 18 on the 4 + 18 PMBX can be used for 2-wire circuits.

The switchboards are provided with multi-way cords, the free end being terminated on small connectors that plug into wall-mounted jack assemblies. The 3 + 12 is equipped with a 68-way cord and the 4 + 18 with a 100-way cord.

On the 4 + 18 switchboard there are 88 keys to scan before selecting a free connecting link. For this reason this switchboard accommodates a free-link signalling system which permits easy selection of a free connecting link and therefore increases speed of answering calls. The free-link signals are given by amber-coloured lamps arranged vertically on the left-hand side of the face panel. Two lamps are provided for each row of keys, one for the upward movement and one for the downward movement.

For installations where Subscriber Trunk Dialling (S.T.D.) is operative, space is provided on the face panel for private trip meters, when required. These are used on S.T.D. calls and located in line with the appropriate exchange-line keys. Meters can be provided individually for each exchange line or one common meter can be employed to serve all exchange lines.

A further optional extra is the operator’s headset. When this is required the standard Etelphone used for the operator’s telephone is replaced by a Plan Etelphone, and a headset jack is provided below the face-panel in the lower left-hand corner of the baseplate. Changeover from the handset is arranged by auxiliary cradle springsets in the Plan-Etelphone so that when the handset is on its rest the headset is operative. Conversely, when the handset is lifted, the transmission path is automatically transferred to the handset.

Automatic Holding of Exchange Calls
When an exchange call is answered, an exchange holding loop is prepared, to be made effective when the attendant restores the ‘Operator Speak’ key. When this occurs a ‘Call Held’ lamp, common to all connecting circuits, glows as a visual reminder to the operator that supervision of the switchboard is still required. If the line is subsequently switched to an extension, the holding circuit is released and the ‘Call Held’ lamp extinguished when the extension answers the call.

If the operator wishes to release an exchange call without connecting it to an extension, the holding circuit is released when the exchange key is restored.

Exchange Call Trap
At the conclusion of an exchange-extension call, the clearing signal is given by the extension calling lamp as the extension telephone handset is replaced. This action operates a call trap relay to reconnect the exchange-line signalling relay across the exchange line and disconnect the exchange line from the connecting circuit. Thus a follow-on incoming call is trapped on the signalling relay and operation of the extension telephone bell prevented should the call not be immediately cleared down at the switchboard.

Free-link signalling
All lamps are extinguished in the idle condition. When an incoming call arrives, the first lamp glows and the operator answers the call using the associated connecting link. The free-link signal now steps to the second lamp to indicate that connecting link 2 should be used for the next call.

The sequence is always the same, but if any link should be taken out of turn the remaining lamps still glow in the same order, excluding the circuit taken out of sequence if it is still in use. When a circuit becomes disengaged, the free-link lamp glows again if it is the first free link in the sequence. When all seven connecting links are engaged the eighth lamp (the ‘overcall’ lamp) glows to indicate that all connecting circuits are engaged and any following call must be answered by means of the overcall facility.

Overcall enables the operator to answer a call when all 7 connecting circuits are engaged.

Figure 3
Switchboard with hinged mountings lowered

To answer an extension call under these conditions, the ‘Speak On Overcall’ key is operated to disconnect the ringing supply and extend the operator’s telephone to the extension ’Ring‘ key. The operator can now speak to the calling extension by holding operated the appropriate ‘Ring’ key. The ‘Speak On Overcall’ key must be restored before a normal call is answered.

An exchange call is answered by operating the ‘Speak On Overcall’ key and the appropriate ‘Exchange Overcall’ key. The operation of these keys extends the exchange line to the operator’s telephone and also applies an exchange holding loop to permit the operator to leave the circuit if necessary. The operator may wish to leave the connection to offer the call to an engaged extension and then, if required, break down the connection and extend the exchange call to the extension. When the call has been dealt with, the ‘Exchange Overcall’ key and the ‘Speak On Overcall’ key are restored.

Press Button Recall
Each extension telephone is provided with a press button key for recall purposes. Recall is available on all extension and exchange calls.

Night Service
Night service arrangements are provided by the Night Service key. The key disconnects the extension calling lamps and renders the exchange line call-trap circuits ineffective. Selected extensions can then be connected to the exchange lines by operating the appropriate ‘Exchange’ and ‘Extension’ keys.

Mains Failure
As the switchboards have been designed to operate from a mains operated power unit the circuits have been arranged so that exchange line service is maintained under mains failure conditions. The operator can answer or originate a call under mains failure conditions in the normal manner.

The exchange lines can be connected to selected extensions if mains failure should occur and the selected extensions will then have direct exchange line access.

For simplicity of design a parallel feed transmission bridge is employed for extension-to-extension traffic. The output of the bridge is capacitance coupled to line to give satisfactory side-tone balance, and crosstalk attenuation between circuits is increased to at least 75 db by screening shields on all transmission-bridge relays.

Improved lamp signalling and elimination of line signalling relays have been obtained by use of a newly developed switchboard lamp. This maintains a reasonably uniform level of illumination throughout the prescribed loop resistance for extension lines (see LINE LIMITS).

No hand generator is fitted in the new switchboards. Ringing can be provided by a 25-cycle transistor generator unit which may be included in the switchboard itself or be suitably arranged for wall mounting according to requirements. Alternatively, use can be made of a suitable ringing source if this exists on site.

The circuits of both switchboards are designed to operate from a nominal 50V d.c. supply and within the wide voltage range of 45 to 55 volts. Power is normally derived from a mains-operated power unit the size of which depends on the type of switchboard and the amount of auxiliary apparatus used. To meet all requirements, power units having outputs of 1, 2 and 4A are available.

Satisfactory operation is ensured on extension lines having a maximum loop resistance of 500 ohms. The exchange-to-extension transmission and signalling limit is 935 ohms but where 2-wire conversion units are required this limit is reduced to 850 ohms.

Click here for the GPO Switchboard No. 2/3

Click here for the GPO Switchboard No. 2/4

Click here for the Ericsson N116

Click here for the Ericsson N117


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Last revised: October 15, 2019