Cordless PMBX
Indicator Signalling with Automatic or C.B. exchange lines

n115.jpg (28182 bytes)This indicator signalling switchboard extends the capacity range of desk-top cordless units to five exchange lines and 20 extensions. Seven connecting circuits are provided.

By the use of miniature keys and indicators, and lightweight structural materials, the size and weight are kept to the minimum.

An improved form of construction contributes to the switchboard's modern appearance and provides increased access to equipment for maintenance.


  • Through dialling and through clearing.
  • Positive supervision.
  • Hold exchange.
  • Overcall; i.e. the facility for answering incoming calls when all connecting circuits are engaged.
  • Night extension.
  • Audible alarm.

Construction and Finish
The pressed-steel frame, welded to prevent distortion, is mounted on a hardwood base. The front panel  can be swung down, as shown in the illustration overleaf, to afford access to the internal wiring. Each vertical row of keys is removable as a unit for spring inspection.

Replaceable plastic laminated panels enclose the equipment. The back and top are a single sheet which clips in at the top and can be removed to obtain access to the rear equipment after releasing three captive screws at the bottom. The switchboard is attractively coloured: the equipment panel, covered with grained dove-grey p.v.c. above the black bottom strip and base, tones well with the buff side and rear panels, which are outlined by bronzed metal strips.

The appearance is enhanced by black indicators and a black handset and cord.  A dial dummy instead of the dial
is provided on C.B. switchboards.

The equipment has full tropical finish; i.e. coils are impregnated and metal parts specially finished.

Conductors are insulated with p.v.c., which is moisture-proof and not subject to fungoid growth or attack by insects.

An internal terminal field permits direct termination of incoming lines. Alternatively, a flexible connecting cord and cable distribution box can be supplied.

Type N9300 miniature keys, and totally enclosed miniature twin shutter indicators are used. Extension line indicators Type N4831 are non-locking and self-restoring. Exchange line indicators Type N4832 operate and lock to ringing current and are restored by pressing down a projection at the top of the plastic front cover. The curved shutters open horizontally to display a red (exchange line) or green (extension line) number against a white background.

The handles of the associated keys are tipped with red or green inserts to match; this facilitates accurate operating and enables operated keys to be instantly discerned.

The dial has standard numbering and a 2:1 break/make pulse ratio.  The hand generator is of the rotating-magnet type, but can be replaced or supplemented by a transistor ringing unit if specially ordered. Relays and battery-feed retards are Type N30000 (BPO 3000).

Also included are an alarm buzzer and cut-off key, and a plug-in light-weight handset with capsule receiver and coiled cord. In lieu of, or in addition to, the handset, a lightweight headset can be supplied.

Power Details
Nominal operating voltage: 24V D.C. normally provided by a mains operated 1A power unit.
Alternatively, storage batteries with charging equipment can be supplied, if preferred. Busy hour current drain for fully equipped board: 0.84A.

Extension Telephones
Plesseyphones (catalogue sheet 7159) or Etelphones (catalogue sheet 7145) are recommended. For automatic working, dials are normally adjusted to transmit 10 p/s with a 2:1 break/make pulse ratio, and have standard numbering (i.e. 1 to 0). Other speeds and numbering can be provided to customer requirements.

Switchboard Dimensions
Height: 12.5 in (317mm)
Width: 23in (584mm)
Depth: 9.5 in (241 mm)

Weights (approx.)
Switchboard (fully equipped 5+20): 58lb (26.3kg) net.

Plessey Publication No. 7131 (1971)

Click for an article from the Ericsson Bulletin

Additional Information

The N121 (2+4), N122 (3+9) and N115 (5+20) switchboards were also by the Irish P&T.

They superseded the N111 (2+4) and N112 (3+9).

All were supplied in two-tone grey.  


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Last revised: October 12, 2019