No. 52 |
PAGE No. 33
A Small Economic P.A.B.X.
This attractive 2 + 7 private automatic branch exchange has been designed to meet the essential requirements of small organizations - simplicity of operation and installation, minimum maintenance and, above all, economic as well as efficient telephone service.
Combined external and internal telephone service as provided by modern private automatic branch exchanges can frequently lead to increased business efficiency at lower cost. This fact is widely accepted by small and large organizations alike, and a wide variety of equipment has been developed to meet customers’ different requirements.
Recently a small-capacity PABX accommodating 2 exchange lines and up to 7 internal stations has been added to the available range. The new design provides most of the facilities offered by larger installations at a cost well within the budget of small organizations and is suitable for use as an exchange in its own right with direct connection to the public exchange, or as a subordinate exchange to another PABX, or a PAX. In either of these auxiliary roles the design constitutes a switching unit for a group of persons with a close community of interest, affording them the additional facility of communication with the parent-exchange stations via ‘exchange lines’.
Because the exchange is designed to occupy standard office space, special attention has been paid to making it compatible with the appearance, ease of operation and small-size characteristics of the subscriber’s other office equipment. Emphasis has
also been placed on simplicity of installation and maintenance - two requirements that assume more than usual importance when skilled staff is not available.
Stations engaged on exchange-line calls can make enquiry calls to other stations without occupying the internal link (connector) and can transfer exchange line calls to other stations. During enquiry, the exchange line is held and the caller prevented from overhearing.
An intrusion facility permits urgent enquiries to be made or exchange calls to be offered for transfer when the wanted station is engaged on an internal call. Other than this, full secrecy is afforded on all calls.
Internal calls are signalled by the instrument bells, and incoming exchange-line calls by an externally mounted bell serving both lines. This bell may be sited at any convenient point within hearing range of a designated answering station, or stations. Additional call bells up to a maximum of three may be paralleled in if required.
During mains failure, or night service conditions (i.e. with PABX switched off) the exchange lines are directly connected to two of the stations.
The ‘link occupied’ indication is the absence of dial tone, unless both exchange lines are also engaged, when ‘system busy’ tone will be heard.
On completing the enquiry, station 2 can resume the exchange-line conversation by again pressing the instrument button momentarily; the enquiry selector is then freed.
If a station dialled for the purpose of enquiry is busy on an internal call, the calling station can, if the enquiry is of sufficient importance, intrude on the connection by dialling the intrusion digit ‘1’. Busy tone is disconnected and the calling station teed into the connection. To warn the conversing stations of the intrusion a distinctive tone is superimposed on the line.
If station 7 is busy on the local link, station 2 can still offer the exchange call by using the intrusion facility. Should station 7 agree to accept transfer, station 2 requests both interrupted stations to replace
The same provisions ensure exchange-line service at these stations when the system is otherwise inoperative due to mains failure.
The complete exchange equipment, including tone, ringing and power supplies is housed in an attractive brown and cream enamelled sheet-metal cabinet fitted with removable hinged wrap-around covers (see figures 5 and 6). It occupies small space, being only 2 ft. 4.75 in. high, 1 ft. 2 in. wide and 1 ft 0.5 in. deep (i.e. 73 cm x 35.6 cm x 31.75 cm) and can be installed in any dry and reasonably dust-free situation, either wall-mounted or free-standing on a desk or bench.
As illustrated in figure 7, the components are mounted on two vertical back-to-back frameworks, the one on the right-hand side being hinged on the front edge to allow access to the wiring and screw-type terminal strips behind, Cables for connection of the station telephones, the call bell and mains supply are brought in through the base of the cabinet via an aperture in the rear face of a shallow plinth.
The apparatus includes heavy-duty uniselectors, 3000-type relays, and strip-relay units’ which are used exclusively for the line circuits. Space is available for an additional three strip relays, permitting the system to be extended to 10 stations at installations where traffic load is light.
The strip units can be seen in figure 7. They consist of up to five relays mounted on a common yoke which also serves as the core-iron and primary anchorage for the main components. The design of
these relays ensures a high degree of contact reliability with unvarying contact pressure under all circuit conditions. No spring or armature adjustment is necessary under any circumstances; should the springset become damaged it can be freed from the assembly for replacement merely by withdrawing a single screw. When reassembled with the new springset unit, the relay will automatically attain correct adjustment. Detachment of the springset also permits removal of the coil, which may be replaced with similar ease.
Last revised: October 15, 2019