PABX 20 + 100 / PB 100
|PRIVATE AUTOMATIC BRANCH EXCHANGES
20 + 100 PABX
This newly developed private automatic branch exchange is a register controlled crossbar system, providing improved service for installations requiring up to 20 external circuits and 100 internal extensions; external circuits may be of exchange-line classification exclusively, or any combination of exchange-line and private circuits.
Contrary to usual practice in exchanges of this size, the automatic switching equipment is accommodated in two cabinets instead of one, each with capacity for lines. As a result, the new design can fulfil the PABX requirements of both small and medium-size organisations at lowest cost. Plug-in modules simplify installation to single or double cabinet capacity, and also facilitate removal and re-use of the equipment in different locations. The cabinets are made for back-to-wall mounting to conserve floor space and equipped for easy mobility on site before being installed.
Attractive appearance characterises the overall exchange equipment, including the small, easy-to-operate switchboard for the PABX attendant. Colours and finishes are complementary, being selected to Crossbar blend with modern office decor so that the complete exchange can be located satisfactorily adjacent to office furniture. In this forms the PABX employs simple 2-stage switching based on the well-tried Plessey 5005 cross-bar switch. This uses precious metal contacts, ensuring consistent high-quality transmission, while the short contact travel results in high speed, vibrations, barely audible operation. Its simple relay like structure also ensures maxi mum reliability with minimum mechanical wear.
The exchange combines high traffic handling capacity (0.15 erlang per extension) with an acceptable grade work and subsequent extension up of service.
1 Extension classification is in four categories, each determined by appropriate wiring straps:
2 Extension-to-extension calls are unrestricted and dialled direct; connection is released when either party clears. Busy extensions are numerically displayed at the attendant's switchboard.
3 Outgoing exchange calls can be set up by the PABX attendant for any extension not fully restricted. Trunk-barred and unrestricted extensions can also make such calls directly by dialling an access digit, followed by the exchange subscriber's number (or instructing the distant operator if the exchange is manual).
4 Incoming exchange calls are received at the attendant's switchboard, digit keys being used to extend the call. After a call has been extended, all switchboard contact is automatically released from the connection. Busy exchange lines are visually displayed at the switchboard.
5 Private-circuit calls are obtained as in 3 and 4 above when standard exchange-line circuits are fitted for private-circuit application. If special private circuits (tie lines) are employed, calls can be dialled out and dialled in direct or via the attendant, no extension being barred.
6 Discriminating ringing indicates to an extension user whether an internal or an external call is awaiting answer.
7 Trunk offering enables the attendant to enter established connections; warning tone is given on line entry. After cleardown by the engaged parties, ringing is automatically applied to the wanted extension's line.
8 Hold exchange line by dialling digit 1, enables an enquiry call to be made to another extension in complete secrecy from the exchange-line caller. Alternatively, the originating extension can offer the exchange call to the called extension and transfer it by replacing the handset.
9 Add-on conference by repeating the digit 1 after an enquiry connection, the originating extension allows entry of the public-exchange caller to the enquiry connection, thus permitting 3-way conference.
10 Attendant recall on an exchange call is obtained by dialling digit 1 followed by the access digit allocated to assistance circuits.
11 Group hunting (optional) provided over a group of up to 10 numbers allocated to a group of extensions located in the same department or section; numbers need not be consecutive.
12 Night service in two versions, under control of the attendant's switchboard:
13 Exchange-line service maintained if mains failure occurs. Selected extensions (up to
the number of exchange lines provided) have telephones with an in-built pressbutton for
use during mains failure to gain access to an exchange-line.
System modules comprise fully wired crossbar-switch shelves and various relay sets which, after factory testing, can be packed and shipped as units ready for connection into the PABX.
The A and B cabinets together include provision for 10 local transmission relay groups (LTRGS) and 4 registers (6 LTRGs and 2 registers in the A cabinet and 4 LTRGs and 2 registers in the B cabinet).
Each equipment cabinet, excluding its front cover, is constructed in sheet and
angle-section steel, carrying a dual smooth/texture stove-enamelled finish in hemp beige
and loam brown. The overall colour scheme is completed by two lacquer-red resilient metal
panels which clip vertically into stainless-steel edging strips to retain the front cover,
this being composed entirely of clear, flame-retardant plastic. This simple clip-in cover
arrangement affords quick access and convenient external viewing of the main switching
equipment and ensures good mechanical protection and a dust-free interior.
The PABX switching equipment consists essentially of Plessey 5005 Crossbar switches in 10 and 16 bridge versions, together with newly developed high-reliability relays. Electronic circuits are used throughout for timing and pulse generation.
The crossbar switches are shelf mounted on the front framework. Each shelf and its components are clearly identified by designation markings and can be swung forward and downwards to a self-locking position for inspection purposes. A similar degree of access is afforded to the rear equipment (comprising plug-in relay sets, fuse panel, battery jacks and, terminal strips for line and inter-cabinet connections) the front framework being hinged, and castor equipped for smooth opening on the release of four captive screws. Cable entry is provided at the top and bottom of each PABX.
Dimensions and Weights
Plessey Publication No. 7161/1 (1971)
Last revised: October 10, 2019