PABX No. 7
0 - Assistance
Exchange lines Service to the public exchange is
given over bothway lines. The PABX calling condition is an
earth on the B wire. Enquiry and transfer facilities are
available on all exchange calls. Exchange line relay-sets
are suitable for connection to CB and automatic public
exchanges. To operate to a CBS public exchange a Unit,
Auxiliary Apparatus, CBS 536 must be fitted at the public
exchange in each PABX exchange line, and Relay-sets SA 8541 at
the PABX modified as detailed in F1071, par 23. Requests
for connection to other types of exchange should be referred to
Introduced in 1969
Post Office Telecommunications Journal
By H. F. Edwards
THIS IS THE NEW PABX7
The Post Oﬂice is introducing a new private automatic branch exchange - to be known as the PABX 7 - which is intended for rental by customers who need something larger than the largest PABX 1 (capacity of 10 exchange lines and 49 telephone extensions), but are not likely to need more than 20 exchange lines and 100 extensions.
Hitherto, these customers have had to buy from a contractor one or other of the approved large-type PABXs 3 or 4 which start with a 50-extension capacity but can be enlarged in stages up to several thousand extensions.
Although highly adaptable, these large systems tend to be expensive (in terms of cost per extension) in the smaller sizes and manufacturers have produced cheaper PABXs, for sale abroad, by applying the simple “marker” switching system, already used for the PABX 1, to a 100-extension capacity design.
The Post Ofﬁce has adopted a design by Standard Telephones and Cables Ltd., has arranged with the company for a few modiﬁcations to be made and placed an initial order for 200 to be supplied during 1969 and 1970.
Although the PABX 7 will be suitable for the majority of customers, it has some facility limitations at present, for example, incoming exchange calls cannot be connected over inter-PBX extensions to other PBXs. Later versions will have the full range of facilities.
There has for long been a demand from customers for a larger PABX available on rented terms, particularly from those whose businesses have outgrown the capacity of 49 extensions of the No. 1. Consequently, almost all the ﬁrst 200 PABX No. 7s are already reserved and there will be no difficulty obtaining renters for as many more as can be obtained in the next few years. More-over, there are so many thousands of PMBXs (manually-operated branch exchanges) of this capacity still in use that an annual demand for some hundreds of No. 7s can be expected.
The PABX 7 utilises equipment of the “Strowger” type already widely used in PABXs Nos. 1-6, but more economically. The large PABXs 3 and 4 use the rather expensive two-motion selectors for all calls made from extensions and the PABX 4 uses them also for incoming calls to extensions.
The PABX 7 (like the PABX 1), apart from momentary seizure and release in exchange calls, uses them only for extension-to-extension calls and calls dialed in from other PBXs. “Dial 9 for exchange” calls are made and incoming calls via the operator are connected by the cheaper single-motion selectors (uniselectors) used as lineﬁnders in a marker system.
If the PABX 7 had used only two-digit numbers there would have been none left for the above services so a three-digit numbering system is used in which all extensions have the ﬁrst digit 2 and the extension range is therefore 200-299.
The remaining ﬁrst digits are used for: 7 - Direct lines to another PBX; 3, 4, 5 and 6 - Direct lines to additional PBXs; 8 - “Dial-8” night service; 9 - Public exchange; 0 - PABX operator; (1 is not used).
The PABX 7’s facilities are similar to those of the PABX I but the No. 7 has the advantage, for some users, that it can be connected over privately-rented direct lines to more than one other PBX (for extension calls only on the present design).
The capacity has so far been quoted, for simplicity, as 20 exchange lines plus 100 extensions, without any mention of the capacity for inter-PBX lines. In fact, the ﬁgure of 20 represents the total capacity for outside lines whether to exchange
or to other PBXs. Inter-PBX lines with dialing facilities also eat into the capacity for 100 extensions. For example, a PABX 7 with three such lines would have its capacity reduced to 17 exchange lines and 97 extensions.
The design of the PABX 7 is interesting in that the ﬁrst of the two cabinets accommodating the equipment comprises a self-contained PABX of 9 outside lines and 50 extensions capacity - almost the same as that of the largest PABX 1.
It therefore offers an attractive alternative to the PABX 1 for future use. Whereas an exhausted PABX 1 cannot be extended and must be replaced by a PABX 7 or other larger PABX, an exhausted PABX 7 ﬁrst unit can be extended by adding a second unit to bring its capacity up to 20 + 100.
It is possible, therefore, that eventually PABX 7 ﬁrst units may take the place of 10 + 49 PABX 1s. For the ﬁrst few years, however, while the supply is limited, all PABX 7s will be allocated to customers who need to use both units immediately.
The physical dimensions of each of the two cabinets of the PABX 7 are much the same as those of the single cabinet of the 10 + 49 PABX 1.
The ﬁrst unit is actually a little smaller and lighter but the second unit is somewhat wider than the PABX 1, although no heavier. It may be possible, on redesign, to reduce the width and so minimise difficulty in handling through doorways and so on.
The PABX keyboard, about the size of a type-writer, pale grey and green and angular in the modern style, is entirely push-button operated, except for the dial used for making outgoing calls.
The calling lamps and labels are enclosed in the push-buttons so that the keyboard is similar to the new one being introduced for the PABX 1. Since the two boards will then be practically identical in operation, no fresh training problems should arise
when an operator goes from one to the other.
An early version of the PABX 7 has been in use since March, I967, serving the Brighton Head Post Ofﬁce. A few others of an early pattern are now being installed to meet the pressing needs of subscribers who have been waiting some time for them.
The ﬁrst of the main order are expected to become available early in 1969.
Last revised: December 14, 2015