PAX 222



The Equipment caters for provision of up to 3 main units each providing for 100 lines, giving an ultimate capacity of 290 extensions; when the Equipment exceeds 200 lines capacity, and therefore comprises the 3 main units, a fourth unit, carrying final selectors giving access to the third 100 lines is provided..

The equipment is fitted with Connector relay sets which provide for Tie lines to other P.A.X’s & also arranged for the ultimate capacity of 290 lines. When Tie lines are provided the capacity is reduced below 200 or 290 depending upon the number and size of Tie line groups.

Where tie lines are provided the necessary junction circuits are equipped on a special unit, provided for the purpose.

The Numbering scheme for local extensions is 200-399 for the 200 line case; the numbering for the third 100 lines, when provided for, is 0400-0499.

As already stated, provision is made, if required, for working a number of groups of Tie lines to other Private Automatic Exchanges which may be of the type described in this Report, or of our 25, 50 or 100 line sizes, Types No. 40, 45 or 21 respectively. A maximum of 6 Groups of junctions is permissible; groups of 1 or 2 junctions may be catered f or without sacrificing any multiple numbers other than those actually used for junctions, for each Group of from 3 to 10 junctions it is necessary to allocate a level of 10 Connector outlets, spare outlets in such a level not being available for extensions. The numbering scheme for the 6 tie line groups allowed for is ‘04’ to ‘09’ on the 300 line equipments 5 groups only, ‘05’ to ‘09’, are permissible, the level of outlets reached by dialling ‘04’ being used for giving access to the final selectors in the 3rd hundred.

In addition to the ordinary facilities for local calling between any two extensions the equipment provides as standard a Right of Way facility whereby certain executives may, when desired, gain access to engaged extensions.

Code calling, providing for a maximum of 10 codes, is a facility which may be catered for, if required, by the addition of equipment mounted externally to the Units. Two multiple numbers must be allocated for this purpose, one for seizing the code Calling circuit, and one for answering the Code Call. The operation of the Code Calling circuit is described in a separate Report relevant to that circuit.

Since this system operates on the finder connector principle it will be realised that when a call is made a finder must be started in search of the calling line. Further, since more than one finder may be free, it is desirable, in order to keep the current consumption as low as possible, that only one shall search for the calling line, and therefore one particular finder must be selected to deal with the call. Moreover, as the switches used for finders are of the two-motion type, it is necessary, in order to reduce searching time and. circuit complications, to mark a particular level as being the one in which the callers line is situated.

The finder bank multiple has been “Slipped” on levels in order that failure to locate a calling subscriber in a lower level (due to any cause such as a broken wire, dirty contact etc.) shall not debar subscribers whose lines are in higher levels from making a call.

Taken from Report No. 4393 (October 1946)

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Last revised: June 20, 2011