A SMALL DESK-TOP KEY-OPERATED P.M.B.X. SWITCHBOARD
July 1961

The new cordless desk-top switchboard described has capacity for two exchange lines and six extensions, and is the first of 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, small size and better facilities, including lamp signalling and power unit operation, are some of the interesting features of the new switchboard, the overall design of which is such as to merit serious consideration when a simply attended switchboard is required at reasonable cost.

The importance of cordless-type private manual branch exchanges has long - been recognized as an economic means of providing exchange and local connections in many business organisations served by a small number of lines. Up to now, the B.P.O. have met these requirements with a range of switchboards of various sizes, the largest having capacity for three exchange lines and nine extensions. Indicators are used for signalling and supervision, and ordinary lever keys for switching. These boards, similar in general appearance, are housed in wooden cabinets and present an unduly bulky and outmoded appearance incompatible with present day commercial office equipment and furnishings.

A new range of small desk-top cordless switchboards of most modern and efficient design has therefore been developed in conjunction with the B.P.O. The new switchboards, in three sizes, have capacities for two exchange lines and six extensions (2 + 6), three exchange lines and twelve extensions (3 + 12), and four exchange lines and eighteen extensions (4 + 18). The smallest of these is described.

The 2 + 6 is intended to replace the existing 2 + 4 version. The capacity has been raised from four to six extensions, because it has been found that the traffic can be handled on three connecting circuits, the same number as provided on the older design, thus increasing the usefulness of the switchboard. Despite its increased capacity, the new switchboard is considerably smaller than its predecessor, the increase in the number of extensions together with improved facilities being provided in the most economical way by the use of a new type of extension circuit employing a 4-wire line. Other factors contributing to its reduced size are, the use of a separate operator's telephone of the popular Etelphone type (B.P.O. 706); small 1,000-type switchboard keys, lamps instead of indicators, and the deletion of the hand generator, which has been replaced by an external a.c. mains-operated ringing supply unit.

The 4-wire extension line, used for the first time on this type of board, is simple in principle and consists of two conductors in addition to the normal speaking pair. The extra wires are used to enable an earth connection via a cradle-switch contact in the extension instrument to control the line lamp independently of the speech pair. From this it follows that the P.B.X. re-call facility can be easily added by fitting a press-button key on the extension telephone, the key having a contact in parallel with the contact of the cradle-switch. Flashing the line lamp without disturbing the line loop or earthing the line is thus obtained.

The additional cost of the extra line wires is low and more than offset by the reduction in components in the switchboard and, since most local telephones are a short distance from the switchboard, a definite economic advantage is secured.

Figure 1 General View of 2 + 6 P.M.B.X. Switchboard without operator's telephone

In some installations, it may not be convenient to have a 4-wire extension line as, for example, when long extensions over lines in the public network are used. For these, a relay unit is utilized to convert the 4-wire circuit to 2-wire. However, when a long extension using a 4-wire line is required, the limiting loop resistance of the speech pair is 500 ohms, a very satisfactory limit made possible by the use of a new calling lamp - the P.O. lamp No. 2/45.

Parallel-feed connecting circuits designed by the B.P.O., have been adopted for extension-to-extension conversations and are suitable for use with the 706type (Etelphone) or less modern instruments of the 300 series.

Below is a summary of the main facilities provided by the new switchboard:-

1. Lamp signalling on all lines.
2. Exchange-to-extension line limit of 1,000 ohms, or 900 ohms if 4-wire to 2-wire auxiliary unit is used.
3. Extension-to-switchboard line limit of 500 ohms speech pair loop resistance on 4-wire lines.
4. Separate clearing signals on extension-to-extension connections.
5. Secretarial hold - permitting an exchange line to be held and the operator to speak on another line without the held exchange-line caller overhearing.
6. Auxiliary units - for connecting private wires and inter-switchboard circuits; no alteration to the switchboard's internal wiring being necessary.
7. Mains fail - when a power unit is employed and a mains failure occurs, an established exchange-to-extension connection is not interrupted and, additionally, incoming calls are signalled on the operator's telephone.
8. P.B.X. recall, regardless of line polarity, and without the need for a relay in series with the line on both exchange-to-extension and extension-to-extension calls.
9. Night service - allowing exchange lines to be left through to any selected extension at night.

PHYSICAL DESIGN
The new switchboard shown in Figure 1 is approximately 9.5' wide, 6.1' high, and 11' deep (24 x 15.5 x 28 cms) and is normally provided in two-tone grey with matching operator's Etelphone.

Figure 2 Rear View of Switchboard with cover removed

The one-piece cover, which is formed in impact resisting thermo plastic and polished, is easily detached from the base to give full access to the key panel, relay plate and other components. The plastic-coated key panel and the relay plate are respectively hinged to the front and rear of the switchboard's base; the chassis, when closed, forming a triangular rigid construction, with the lamp panel conveniently positioned at the top as shown in Figure 2.

The hinges enable the key panel and relay plate to be opened like a book, and Figure 3 shows the high degree of accessibility afforded by this method of construction, all components, terminals and wiring being in full view.

The new keys (1,000 type) are fully described in Bulletin No. 35, p. 38, and are of miniature pattern, having comb-operated relay-type springs and wedge shaped handles.

The lamps are housed behind a Diakon lens-strip also of new design and, when the strip is removed by releasing phosphor-bronze clips at each end of the cover, the lamp-jack fixing screws are revealed. These are unscrewed to draw the lamp-jack forward and allow the lamps to be easily removed without using a lamp extractor or removing the switchboard cover.

The relays are either 3,000 or 600 type, fitted with clear plastic dust shields. Resistors are small, wire wound, ceramic enclosed and the capacitors are of tubular type mounted on tags fixed into the rear panel of the chassis.

Figure 3 Switchboard Chassis Opened

A 70-way terminal block is mounted on the base to provide connections for the operator's telephone cord and also a 38-way cord for exchange and extension line connections. The remaining terminals provide convenient strapping and connection facilities for auxiliary relay units used to convert certain extension circuits into 2-wire extensions, private wires, or inter-switchboard lines. Should the switchboard be mounted in a fixed position, the cord may be omitted and the cables terminated direct on the block.

CIRCUIT DESCRIPTION
Some of the innovations have been briefly mentioned, but before proceeding in more detail, a brief outline of the circuit employed in the old pattern board is necessary to bring out the reasons for, and the advantages of, the new design.
The previous circuit has indicator calling signals operated by the extension loop. When the extension is switched through to the exchange, a 'series relay is necessary to detect when the extension loop is removed, and a contact of the relay re-operates the line indicator via connecting keys to give a clearing signal.

The same 'series' relay performs a similar function on extension-to-extension calls, with the disadvantage that no clearing signal is given until the handsets are replaced at both extensions. Also on exchange calls, the line loop resistance is reduced by the resistance of the 'series' relay, and 'operator re-call' is dependent upon the exchange line polarity if an earthing-button method is used.

In the new switchboard, these drawbacks have been overcome by separating the supervisory and re-call functions from the speech pair, whilst still using the same lamp for both calling and supervisory purposes. This has been made possible by the use of a four-wire line for extensions, two wires constituting the speaking pair, and the other two controlling the line lamp when the connecting key is operated. The circuit diagram in Figure 4 illustrates in simplified form the essential features of a single exchange line, one extension line and a connecting circuit.

Figure 4 Simplified Schematic of the New 2 + 6 Switchboard

Extension Calls
When the extension handset is removed to initiate a call, a calling lamp. lights via the extension loop and the common pilot relay P operates. Contacts P1 and KA (normally operated) complete the buzzer circuit to give an audible alarm. Should two or more extensions call simultaneously, overhearing between extensions across the common pilot relay impedance is prevented by the low impedance shunt of the electrolytic capacitor C1. The call is answered in the usual way by operating the appropriate KX (extension) and KO (operator) keys in the same free connecting circuit to energize both the extension and operator's telephones in parallel via the associated feed relay L.

The bottom 'throw' of each vertical row of extension connecting keys is a 'ring' key, not shown in the diagram, and the appropriate KR key is pressed to ring the required extension. A second extension may be switched in parallel by operating the associated 'extension' key in the same connecting circuit; that is, in the same horizontal row. On completion, the operator restores the 'operator' key, leaving the two extensions connected to the parallel feed via L relay coils, with the 'exchange' key KE normal.

At the end of the conversation and as the extension telephone handsets are individually replaced, the associated calling lamp lights to provide separate clearing signals. The clearing circuit is from earth, via CS auxiliary cradle-switch contact, to the lamp on each extension line.

Exchange Calls
After answering a calling extension, it may be required to switch the call through to the exchange, and this is done by operating a free exchange line connecting key (KE) in the same connecting circuit as the extension. Contacts KE1 and 2 change over the line pair from the feed relay coils to the exchange line, the calling extension telephone then being energized from the exchange battery, with no series supervisory relays in circuit. Supervision of the exchange call is dependent upon the extension cradle-switch earth via the fourth wire to light the extension lamp.

Following usual practice, the exchange-line connecting circuits are arranged to prevent two exchange lines being connected together.

Incoming ringing on either exchange line operates relay AC in series with all KE3 contacts in the connecting circuits, MF 1 (normally operated) and capacitor C2. Relay AC holds from the pilot relay battery to earth via the 'night service' key KNS, contacts AC2 and KE4. A separate contact, AC1, lights the calling lamp, so that should the lamp become disconnected, the call will still be signalled by the buzzer. The short circuit normally presented across the hold winding of relay AC provides a
slow-to-operate condition, thus guarding against false operation of the relay by transient voltages on the line caused by switching.

Incoming exchange calls should be answered in a similar way to extension calls. To guard against mis-operation, however, some novel features are incorporated. Firstly, if the 'exchange' key only is operated, the calling lamp remains lit and the ringing is not tripped. The circuit for this feature is provided by additional 'break' contacts on the extension' and ' operator' keys which, when all are normal in the associated connecting circuit, cause relay L to operate via its third winding; contacts L1 and 2 (L3 and 4 in exchange line 2) reconnect the AC relay previously disconnected by KE3 and 4. A second safeguard ensures that the operator must not only operate the appropriate 'exchange' and 'operator' keys to answer a call, but must also take up the handset of the operator's telephone. When the 'operator' key is thrown, relay L is released at K03 and incoming ringing is diverted to the operator's telephone bell which rings until the handset is lifted.

The holding circuit for exchange line calls has also been improved, the 'hold' key KH, provided in each exchange line circuit, having several functions. In addition to its primary purpose of applying a 200-ohm loop across the line to hold the exchange equipment, it disconnects the exchange line from the switchboard, connects the 50-volt supply via a bridging coil to the connecting circuit, and lights the 'call held' lamp. The operator can then converse with the particular extension user without the exchange party overhearing, the 'call held' lamp serving as a visual reminder to restore the 'hold' key later.

Power Supply and Mains-fail Arrangements
The switchboard is designed for 50-volt working primarily from a power unit connected to an a.c. mains supply. Where the electrical supply is unreliable and occasional interruption of extension-to-extension calls cannot be tolerated, a battery and charging arrangement is recommended.

An important feature is that in the event of a mains failure the installation is not isolated from the exchange. Established extension-to-exchange calls are not disconnected and incoming calls on the first exchange line are automatically routed to the operator's telephone. A 'mains-fail' relay MF, as shown, controls the switching of the first exchange line and is operated by the 50-volt supply, but releases on any interruption of this source; contacts MF1, 2 and 3 release and connect the bell in the operator's telephone to the exchange line. In addition to this, the second exchange line may be switched to a selected extension as for night service, thus allowing exchange calls to be originated and received at this point.

P.B.X. Re-call
All extension telephones may be fitted with a push-button key having one 'make' contact connected in parallel with an additional cradle-switch contact to give press-button re-call. Standard keys and cradle-switch contacts are available for easy addition to existing telephones of the Etelphone or 706 type.

Auxiliary Units
Extension lines 4-6 are provided with extra terminals for simple connection of auxiliary units to convert long extension lines, private wires and inter-switchboard junctions from 4-wire to 2-wire circuits.

Exchange prohibition is allowed for on the switchboard in that the P-terminal in a line circuit is connected to earth when an extension is switched to an exchange line. Earth from K04 via KE5 and KX5 to the P terminal can be continued to the unit to operate a relay to cause the disconnection of the line.

CONCLUSION
The new switchboard, with its low maintenance costs and exclusive design will undoubtedly fulfil the increasing demand in small businesses for a compact and attractive equipment which, because of its simplicity of management, offers the additional advantage of permitting the operator's services to be combined easily with other work. From all standpoints, but particularly that of efficient service, this switchboard, as all others in the new range, will meet the most exacting present day requirements.

APPENDIX
Perhaps it may be of interest to note, in brief, the salient points of the 3 + 12 which, together with the 4 + 18 switchboard, will form the subject of a subsequent article.

The 3 + 12, similar in style to the 2 + 6, allows six of its twelve 4-wire extensions to be used as desired with the same auxiliary conversion units mentioned above, and includes two outstanding facilities which are novel for a board of this type and size.

These are:-

Automatic Hold
This takes place when an incoming exchange call is switched through to the required extension, enabling the operator to withdraw from the circuit before the called extension answers. Thus, no 'hold' key is necessary and economy of operator effort obtained. Visual indication of the held call is also given.

Follow-on Call Trap
The exchange line circuit is arranged to prevent a follow-on call ringing the extension telephone bell. Indication of such a call is given to the switchboard operator by the intermittent flashing of the associated exchange line lamp, and also by buzzer when audible warning is desired.

Click here for the GPO P.M.B.X No. 2/2

Last revised: October 15, 2019