Plessey mobile crossbar exchanges are designed to provide up-to-date PABX service at short notice. Available in two capacities (20 exchange lines/120 extensions, and 30 exchange lines/240 extensions) they find wide application in situations where buildings are not available for installing normal static PABX equipment, or where emergency and temporary arrangements have to be made to provide a special or supplementary service. The advantages offered by this type of equipment are of particular significance to certain overseas areas where existing transportation and installation facilities pose many problems.

Both versions offer the same attractive range of facilities (including full press button operation) and each in its standard design is compatible with telephone exchange systems employing 10 pulses/second loop-disconnect signalling. Switching is based on proven 5005 crossbar principles, and traffic handling capability is high,. nominally 0.17 erlangs per extension.

Each exchange is housed in an articulated trailer designed for self standing on site and for towing by any tractor vehicle fitted with a universal coupling. Once on site the mobile exchange merely requires a source of mains power and connections made to extension telephones and the associated public exchange.

Standard Facilities
Automatic and unrestricted extension-to-extension calls.

DC press button sending (CCITT code C) for all extensions.

Assistance calls by sending a single digit, e.g. '0'. Calls to and from the public exchange via the PABX operator.

Direct access from the extension to public exchange by sending a single digit, e.g. '9'.

Barring extensions from direct access.

Trunk barring.

Fully barred extensions.

Hold for enquiry on exchange-line calls.

Add-on conference on external calls.

Automatic transfer of external calls between extensions.

Discriminating ringing for internal and external calls.

Calling or re-calling the PABX operator into an exchange-line call.

Group hunting for 20 extension lines (4 groups of 5 lines) in every 120-line unit. The numbering of each group need not be consecutive.

Continuing incoming exchange-line service for 15 selected extensions in the event of power failure.

Bothway service provided for one extra extension.

Lamp and tone supervision at the switchboard.

Direct key access by operator to pre-selected groups of exchange lines.

Common-answer and/or selected-extension night service.

Engaged exchange lines and operator-attended groups of lines visually indicated at the PABX switchboard.

Extension of alarms to the PABX switchboard.

Audible alarm cut-off at switchboard.

Press button operation for all calls set up by the PABX operator.

Faulty-line isolation by the PABX operator.

First-party release.

Standby rotary-dial operation on external lines for PABX operator.

Line lockout.

Standard ringing and tones.

Selective answering of incoming calls by the PABX operator.

Inter PBX connections.

Service-analysis metering.

Operator release of incoming external calls answered but not extended.

Serial external calls to extensions, with special supervision by the PABX operator.

'Ring when free (or camp on busy)' applied on calls built from the operator to engaged extensions.

Delay recall on calls extended by the operator but not answered.

Operator intrusion on established calls. Warning tone section given.

Call splitting by operator.

Through clearing via the automatic equipment on both exchange/inter-PBX lines.

Public exchange operator recall at the switchboard.

'Call-waiting' indication at the PABX switchboard

Trailer
The trailer is of insulated multi-layer construction suitable for long use in a wide range of environmental conditions, including dry and dusty locations. Ventilation fans and heaters serve to regulate interior temperatures. Air-conditioning units are available for extreme conditions. A single external door gives access to the front of the trailer where the switchboards are located, and a double door provides entry to the automatic switching room at the rear. An inner door serves rooms.

A dry powder fire extinguisher is fitted in each room, and a wardrobe provided for operator use.

Robust 'elephant' legs incorporated in the trailer structure, jack down to ground level to provide maximum on-site stability and allow the towing vehicle to be uncoupled with complete safety.

Access to the trailer is by means of sturdy wooden steps, arranged to hook securely over the horizontal metal rail below each outer door. During transit the steps are stowed beneath the trailer.

Automatic Switching Equipment
This is accommodated on shelves within double-sided, floor-fixing cabinets 6ft 6in (1 .87m) high, 4ft 41/2 in (1.33m) wide and 1 ft 6in (0.46m) deep. Cabinets are of sheet and angle section steel construction and produced with a dual smooth/texture finish in hemp beige, loam brown and lacquer red.

Individual shelf covers are of clear flame retardant plastic, affording convenient external viewing and good mechanical protection. Each pivots on metal hooks, and carries magnetic rubber tape to ensure an effective seal with the cabinet framework. During inspection of the equipment, a removed cover can be conveniently parked on the hooks of an adjacent cover.

If necessary, a shelf can be swung forwards and downwards to a self-locking position, giving full access to components.

Three types of shelf are used, equipped with (a) a crossbar switch with up to 16 bridges (b) a crossbar switch with 10 bridges and line circuit relays and (c) relays exclusively.

Jack in relay sets of 10, 20, 30 and 40-relay capacity are accommodated in the cabinets, as required. Fuse panels and test jacks for battery and common services are mounted at the top of the cabinets.

Lift-out panels between cabinets and at the end of a suite give easy access to cabinet-wiring tag blocks without need to expose the switching equipment. All connecting wires to the tag blocks have gun-wrapped solderless connections, and PVC insulated wiring is used throughout. Cable entry is at the top of the cabinets.

Miscellaneous Equipment
The following equipment is also accommodated in the automatic-equipment room:

Wall-and floor-mounted distribution frame.

Battery.

Rectifier.

AC distribution box.

Fuse panel.

Line-test case.

DC/ DC converter unit.

Service meter cabinet (240-line exchange only).

First aid case.

Switchboard(s)
One table top switchboard is provided for the 120 line exchange and two for the 240 line exchange. The switchboards are of all-metal construction and attractively finished in textured hemp beige and loam brown.

An operator controls up to 20 exchange lines arranged in groups of 10. Control keys are provided with in built supervisory lamps, simple press to release and non-locking actions, and press buttons in contrasting colours. An acrylic screen in the switchboard turret gives a light-emitting-diode display of the exchange line groups currently controlled by an operator and the identity of the exchange lines engaged.

The rotary-dial is provided as a standby for exchange line calling in the event of the Keyset becoming inoperative, e.g. if a sender fault exists.

Switchboard dimensions are 8.45in (0.21 m) high, 14in (0.36m) wide and 16in (0.41 m) deep.
Weight: 33.75lb (15.3kg).

Out-of-order Cabinet
This equipment is located within easy reach of the operators) and provides a quick and convenient means of preventing seizure (from an extension or a switchboard) of any exchange or inter-PBX line that becomes faulty. Operation of an appropriate key isolates the faulty line and causes visual indication to be given at the cabinet and on the switchboard display screen'. busy conditions are returned to the public exchange.

Exchange Dimensions and Weights
Cabinet dimensions are: 14.76in (0.38m) high, 23.7in (0.6m) wide and 10.24in (0.26m) deep. Weight: 44lb (20kg).

Telephones
BPO approved press button telephones are recommended for extension use, available in desk and wall-mounted versions and suitable for operation in temperate or tropical climates.

Ringing and Tones
Readily recognised tones are used to indicate the progress of an originated call, and two different ringing cadences to enable an extension user to discriminate between an incoming extension-originated call and an incoming public-e change or inter-PBX call.

Power Plant
Both mobile exchanges operate from a d.c. supply of nominally 50V negative and within the limits of 46V to 54V. The supply is derived from a 24-cell battery (251 Ah for the smaller exchange and 309Ah for the larger) floated across the output of a rectifier unit. The output voltage of this unit is closely controlled by a transductor control system and smoothed to CCITT standards. The rated output current exceeds the busy-hour load to an extent that keeps the battery fully charged, ensuring maximum reserve in the event of a.c. supply failure.

A positive 50V supply for press button signalling and metering is derived from a wall-mounted dc/dc converter unit, consisting of main and standby 3A converters with regulated outputs of 48V to 52V under all working conditions.

Fans, heaters and lights are fed from a central a.c. distribution unit which also supplies the 240V a.c. supply to the constant-potential float charger.

Dimensions and Weight

Taken from Publication No. 7650, 3/76

Last revised: November 24, 2025