Private Automatic Branch Exchanges
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.
(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.
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.
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.
The attendant can intrude into any established connection to offer a waiting exchange call.
This enables the attendant to 'speak to extension' or 'speak to exchange' without overhearing by the other party.
Incoming callers wishing to speak to several extensions in succession can receive special supervision by the attendant.
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.
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.
Tandem switching of inter-PBX lines.
Dictation recording access.
Attendant controlled conference.
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.
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.
Where press button operation is catered for, press button telephones employing CCITT recommended SSMF4 signalling are recommended.
PUBLICATION No. 7665, 4/76
Last revised: October 10, 2019