plessey1.jpg (6468 bytes)PLESSEY
PABX 20 + 100 / PB 100

20 + 100 PABX
PB 100

pb100.jpg (24099 bytes)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.

The PABX provides full flexibility  in numbering arrangements. By appropriate wire strapping, any  are one of 10 serial groups (100 to 199, 200 to 299 etc.) can be allocated to extensions, leaving the  initial digit of each remaining group to be selected, as desired, for use  as access codes to exchange lines,  tie lines, attendant-assistance and  night-service circuits.

Extensions can be connected without the need for consecutive internal-directory or equipment numbering.

These are basically the same as those of earlier designed PABXs but are augmented by new arrangements to improve efficiency
and convenience of service.

1 Extension classification is in four categories, each determined by appropriate wiring straps:

  • Unrestricted.. with full traffic facilities.
  • Trunk barred.. with full traffic facilities except dialled trunk calls (special trunk discrimination equipment is required).
  • Semi-restricted.. with full internal traffic facilities and able to receive incoming exchange calls. Outgoing exchange calls are made with the assistance of the PABX attendant.
  • Fully restricted.. debarred from originating or receiving exchange calls.

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:

  • Automatic switching of incoming exchange calls to predetermined night extensions.
  • Signalling of incoming calls at one or more separate bells placed at strategic points, and calls answered by any non-restricted extension by dialling an appropriate digit.

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.

Special Service
Dictation recording, message registration, message waiting, loudspeaking telephones with keycalling of selected lines, staff
location and secretarial schemes.  Also multi-frequency pressbutton "dial" telephones if parent public exchange is suitably equipped.

PABX Equipment
pb100con.jpg (11146 bytes)The 20+100 PABX is accommodated in two floor-type cabinets A and B, intended for wall fixing. Both are identical in size and general appearance and capacity wired to accept add-on modules as requirements demand.

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.
Castors incorporated beneath each cabinet permit easy location within the office. Foot supports are also fitted, adjustable by hand; these enable the cabinets to be set plumb irrespective of floor irregularities, and also ensure complete PABX stability in combination with the integral wall-fixing lugs at the top of each cabinet.

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.

pb100x.jpg (31350 bytes)Power Equipment
The working voltage is 45-55v (50V nominal). If a reliable a.c. mains supply is available a wall mounted battery eliminator can be supplied, designed for 100-125V, 200-250V, 40-100HZ input. Alternatively, a battery with charger and control unit can be provided.

Plesseyphones or Etelphones are recommended for extension use. However, any 2 wire auto telephone may be used provided its dial has a 2:1 break/ make ratio and pulsing speed of 10 pps. Auto telephones (3 wire) with single press buttons are required for extensions connected for continuing exchange-line service during mains failure.

Line Limits
Maximum extension-line loop (including the telephone) is 1000 ohms at 50V.
Exchange and tie-line loop resistance may be up to 1000 ohms  depending upon the limits imposed by the remote exchange equipment.

Dimensions and Weights

Equipment Item   Height in (mm)   Width in (mm)   Depth  in (mm)   Net weight lb (kg)
PABX cabinet (50 lines)   73 (1854)   42 (1067)   22 (559)   1050 (476.2)
Attendant's switchboard   71 (191)   14 (362)   13.5 (343)   23 (104)


Plessey Publication No. 7161/1 (1971)

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Last revised: October 10, 2019