The PB120 crossbar exchange is a double-unit equipment designed to provide modern PABX service for organisations requiring up to 120 extensions and 20 trunks (classified as public-exchange or inter-PBX lines). Each unit can accommodate 60 extensions and 10 trunks (provided in groups of 2) and either unit can be partially equipped for 30 extensions and 2/10 trunks. Provision is made for six simultaneous internal calls per 60 line unit.

The equipment is modular, highly standardised and uses plug-in design techniques for both cabling and equipment to facilitate installation and maintenance and allow easy on-site expansion.

One desk top switchboard is provided with all the necessary equipment to cater for the ultimate size of the exchange, which has a traffic-handling capability of 0.14 erlang per extension.

The system employs simple 2-stage switching based on well-tried 5005 crossbar principles and operates in the conventional rotary-dial mode. Alternatively, the exchange can be arranged for exclusive press button or dual dial/press button operation.

Extension Classifications
Extensions can be arranged in any of sixteen classes of service by appropriate wiring straps. The main classes of service are as follows.

(a) Unrestricted access to the public exchange.

(b) Restricted direct public-exchange access to the local-fee area.

(c) Barred direct access to the public exchange. Calls are set up by the switchboard attendant.

(d) Barred from making and receiving public-exchange calls.

(c) Unrestricted access to both inter-PBX routes (A and B).

(f) Restricting direct access to either inter-PBX route.

(g) Barred direct access to both inter-PBX routes.

Extension Facilities
Extension calls
Extension-to-extension calls are unrestricted and automatic. A 3-digit numbering scheme is employed.

Enquiry calls
Extensions can hold an established public-exchange call while making an enquiry call to another extension.

Add-on conference
The enquiring extension can allow entry of the exchange-line caller into the enquiry connection, thus permitting a 3-way conference.

Automatic transfer
Enquiry-held exchange calls can be automatically transferred to the enquired extension on handset replacement at the enquiring extension.

Attendant assistance
Extensions can obtain assistance from the switchboard attendant in the setting up and transfer of exchange calls.

Discriminating ringing
By simple strapping arrangement two distinctive types of ringing can be provided to enable extensions to differentiate between internal and exchange calls.

Switchboard Facilities
Press button signalling

This facility is provided at the attendant's switchboard for the setting up of calls to extensions on the PABX.

Direct key-access to exchange lines.

Extension and trunk-busy display.

Call supervision by lamps and tones.

Audible and visual indication of incoming calls.

Delay recall

Any call extended by the attendant and not answered within a predetermined time is automatically reverted to the switchboard.

Ring when free

This condition is automatically applied on calls from the attendant to engaged extensions and is automatically applied on calls from the attendant to engaged extensions.

Trunk offering

The attendant can intrude into any established connection to offer a waiting exchange call.

Call splitting

This enables the attendant to 'speak to extension' or 'speak to exchange' without overhearing by the other party.

Serial calls

Incoming callers wishing to speak to several extensions in succession can receive special supervision by the attendant.

Local access

This allows the attendant to make calls to extensions on the PABX.

Audible and visual alarms

Exchange prompt-alarm conditions are extended to the switchboard.

System Facilities
Group hunting

Up to 12 extensions per 60-line unit can be formed into a maximum of four hunting groups. Any call made to a group number rings the first free telephone in that group; numbers need not be consecutive.

Night service
Incoming calls from the public exchange during the night-service period can be routed to selected extensions or signalled by special bells and answered from any non-barred extension keying the access code.

First-party release
On internal calls the connection is released when either party clears.

Line lockout
This occurs if a call does not proceed within the permitted time following handset pick-up. Lockout also occurs if the line is faulty or if the caller fails to clear at the end of a call.

Inter-PBX working
Calls can be made to or from other private exchanges automatically or via the switchboard attendant. Inter-PBX lines have enquiry, add-on and transfer facilities as on public-exchange lines.

Power-fail fallback
In the event of an exchange power failure, selected extensions are switched automatically to public exchange lines and function as direct subscribers to the public exchange.

Optional Facilities
Press button (SSMF4) or dual dial press button operation.

Tandem switching of inter-PBX lines.

Staff location.

Dictation recording access.

Emergency reporting.

Traffic measurement.

Attendant controlled conference.

Equipment Cabinets
The automatic equipment is housed in double-sided metal cabinets mounted on castors for ease of movement. The cabinets are of welded construction 1.32m high, 1.12m wide and 0.56m deep. Fully equipped cabinets weigh approx. 440kg, but to facilitate installation in difficult locations, plug-in equipment can be removed, leaving a residual weight of approx. 100kg.

One cabinet contains all equipment except power for a 60 line system, while the other cabinet houses the equipment necessary to extend the system to its full capacity. Since all equipment is plug-in, cabinets can be sub-equipped at initial installation, line units being provided in groups of 30 and trunk connections in groups of 2.

The cabinet is enclosed on four sides by lockable, sheet-steel, lift-off door panels, including a separate right-hand front panel to allow access to the line-termination field without disturbing the equipment areas. The cabinet contains an in-built 'quick-connect' type distribution frame that also mounts sockets for connection to the attendant's switchboard and interconnections between cabinets. Cable entry is at the top of the cabinet.

Attendant's Switchboard
The desk top attendant's switchboard is of pleasing design, having a neutral colour scheme of loam brown, hemp beige and lacquer red. Dimensions are 215mm high, 362mm wide and 408mm deep.

The switchboard is equipped to handle the maximum capacity of 20 trunks, on a key-per-trunk basis. Calls to the public exchange are extended by use of the dial, but if the public exchange caters for SSMF4 signalling, the press button Keyset can be used for outgoing signalling as well as for its normal function of setting up calls to internal extensions. Trunk-busy display is provided on depression of the Busy Test key, illuminating the key of each engaged trunk. The design concept of the attendant's switchboard is based on the dark-board principle, i.e. no visual signal is given at the switchboard unless some action is required by the attendant.

The switchboard is equipped with a lamp panel for the display of busy extensions. the full complement of 120 extension-busy lamps is provided. Also provided is a double jack for plug connection of a handset or lightweight headset.

Telephones
BPO type telephones are recommended for extension use. Alternatively, telephones of comparable performance can be used, having a dial with a 2:1 break/make ratio and a pulsing speed of 10 p/s.

Where press button operation is catered for, press button telephones employing CCITT recommended SSMF4 signalling are recommended.

Line Limits
Maximum extension line loop resistance (including the telephone) is 1200 ohms. Exchange and inter-PBX line limits can be up to 1500 ohms, depending upon the limits imposed by the remote exchange.

Speech Attenuation
Internal call 0.5dB at 1000HZ.
Exchange call 0.5dB at 1000Hz.

System Reliability
Features of the exchange design are a considerable reduction of common-control equipment and the use of crossbar switches incorporating self-steering control relays. The switches test all available paths open to a particular call and steer to the marked objective. If there is a free path it is selected and a faulty switch is automatically bypassed. The common controls are easy to maintain and the system as a whole has a high degree of fault tolerance.

Power
The PB120 operates from a nominal 50V negative supply and within the limits of 46 and 54V. A mains operated power unit or a battery with charger and control unit is available.

PUBLICATION No. 7665, 4/76

Last revised: October 10, 2019