PAGE No. 37

A 5 + 20 Indicator - Signalling Cordless P.M.B.X.
January 1964

Seven connecting circuits and an ‘overcall‘ facility are among the features of the switchboard available for Auto or GB working. The switchboard is in two versions, both identical in size and general appearance, but individually equipped for 5 exchange lines and 20 extensions and 4 exchange lines and 16 extensions.

GOOD appearance in office equipment was until recently regarded as an incidental consideration in design, quite subsidiary to the aims of utility. In consequence, almost the only examples of designs where progressive attitudes to styling found expression were those of equipment familiar to the general public. On the favoured side of the limitation was the telephone instrument, evolved out of all recognition over some forty years and, on the other, the cordless switchboard, virtually indistinguishable from its Edwardian predecessors throughout.

Traditional wooden construction and traditional component design imposed functional as well as aesthetic limitations on this type of switchboard. A maximum capacity of about three exchange lines and nine extensions, set by consideration of bulk, meant the generally unwelcome change to a floor-pattern board once the telephone needs of a business advanced beyond this capacity.

Fig 1
General view of the Indicator - Signalling 5+20 Switchboard

New constructional techniques have done much to remove these limitations, offering greatly increased capacity as well as transformed appearance. Several such redesigned switchboards have in fact been introduced as replacements for existing floor-pattern models, among them the example now described. This switchboard has two identically sized versions, respectively equipped for five exchange lines and twenty extensions, and four exchange lines and sixteen extensions with provision for adding-on to the full capacity. Both versions are arranged for two wire extension working and are available in auto or CB forms.

Fig 2
Rear view of switchboard with wrap-over panel removed
Fig 3
Wrap-over panel removed and face equipment panel lowered

The basis of construction is a light but rigid welded steel frame surmounting a hardwood base. The case has overall dimensions 23” wide, 12.5” high and 9.5” deep (58 x 32 x 24 cm), and is formed of grained plastic laminate, the top and back being a one-piece wrap-over. Figure 1 shows the general appearance. The tilted front panel and wrap-over are dove-grey in colour, the sides buff and the base black, bronze metal trims serving both to protect the edges and complete the overall visual effect.

Wedge-type keys and twin shutter indicators are employed throughout and are of well-established miniature patterns. The attendant’s telephone is electrically equivalent to the Etelphone and gives the same standard of transmission efficiency. A rotating magnet generator, having the low operating torque requirement characteristic of this design, is normally incorporated, but a transistorised ringing generator may be fitted as an alternative. Audible alarm is by an internal buzzer.

Figures 2 and 3 show the general accessibility of apparatus. The keys are mounted in separate vertical strips, readily removable from the panel for inspection, while the indicators detach individually, but are so arranged that contact cleaning can be carried out in situ. A similar ease of maintenance is provided for by plug connection of the dial and handset.

The miniature indicators show coloured numbering (exchange lines red, extensions green) on a white convex shield; the display being prominent and readily visible over a wide angle. As an aid to surveillance the indicator colours are repeated on the tips of their related keys, and the attendant’s keys are tipped black. The well-defined vertical association of keys with indicators, and the pleasant handling characteristics of the keys themselves, are other features contributing to an outstanding simplicity of operation for a switchboard of this capacity.

The equipment is fully tropicalised and proofed against the entry of dust and insects.

The key arrangement follows normal practice and provides for seven connecting circuits, each exclusive to a particular level of keys and direction of throw in the usual manner.

Downward operation of appropriate keys in the bottom row produces three operating facilities: ‘Hold Exchange’, ‘Ring Extension’ and ‘Overcall’. The ‘Hold Exchange’ facility may be used when a wanted extension is not immediately available for acceptance of an exchange call, or to permit enquiries to be made at the extension in privacy from the exchange line caller, the appropriate exchange connecting key being meanwhile restored. The ‘Overcall’ facility provides a means of answering further exchange line or extension calls when all connecting circuits are in use.

Extension indicators reset automatically on answering or clearing down, exchange indicators being manually reset. The termination of calls on inter-extension connections is signalled by the buzzer and the two indicators after both handsets have been replaced, and on exchange-to-extensions by the buzzer and extension indicator upon replacement of either handset.

Through-calling via the public exchange is available to extensions, and ‘revertive‘ calls, where the attendant is left to obtain the required exchange number and call the extension back, can be handled with minimum effort.

Any or all of the exchange lines may be connected to chosen extensions for night service, a key being provided to isolate the supervisory relays from battery and so eliminate current drain. Under night service conditions, no visual or audible alarm is given at the switchboard.

For the convenience of the attendant during normal working, the buzzer alarm may be cut off or transferred to a remote point by means of a further key. 

Transmission and signalling limits between the switchboard installation and the exchange are dependent upon the type of telephones used at extension stations. With Etelphone-type instruments, for example, the extension-to-exchange limit is as great as 1000 ohms.

The switchboard operates from a 24-volt supply which may be derived from a suitable lead-acid battery or, where a.c. mains are available, from a mains-operated power unit. Busy-hour current consumption is 0.84A.

The main advantage of a cordless switchboard is that the attendant can combine its operation with other duties in a more satisfactory manner than with the equivalent floor-standing board. This switchboard extends the advantage to larger organizations than hitherto, while offering appropriate standards of operating simplicity and appearance.

Click here for more information on the 5+20 switchboard (N115)

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Last revised: October 15, 2019