2+4 or 3+9
Cordless PMBX
Indicator signalling with automatic or C.B. exchange lines


n121a.jpg (12743 bytes) n122a.jpg (11287 bytes)
N121A - 2+4 N122A - 3+12

This compact switchboard, equipped for 2+4 (two exchange lines plus four internal extensions) or 3+9 lines, occupies no more space than a typewriter. It has a sloping front to afford maximum visibility and convenience for operating.  The styling is attractive and the colour two-tone grey. Accessibility for maintenance is a feature of the design.

  1. Positive supervision.
  2. Overcall (provided on type Nl22A only) to allow incoming calls to be answered when all connecting circuits are engaged.
  3. Hold exchange.
  4. Night extension.
  5. Audible alarm.
  6. Through dialling and through clearing.

Construction and Finish
The French-grey switchboard cover is a one-piece moulding of a durable copolymer plastic. It drops easily into position and is firmly retained by its leading edge beneath the front of the elephant-grey face panel, and by two screws in the rear of the baseplate.

Removal of the cover exposes a 3-section chassis which can be opened book-fashion to permit inspection of wiring and components. Each vertical row of keys is removable as a unit.

When the switchboard is for connection to a C.B. exchange, the dial is replaced by a dummy.

The equipment has full tropical finish, i.e. coils are impregnated and metal parts specially finished.  Wire conductors are insulated with p.v.c., which is moisture-proof and not subject to fungoid growth or attack by insects. The handset cord has a p.v.c. sheath, as well as p.v.c. insulation on the conductors.

Keys are N9300 miniature type, with wedge handles having inserts of contrasting colours in the tips so that an operated key is instantly discernible.
Totally enclosed twin-shutter miniature indicators are used; when operated, they reveal a white shield behind a green (extension lines) or red (exchange lines) number.

Extension-line indicators
Type N4831 are non-locking and self-restoring. The 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 dial has standard numbering (i.e. 1 to 0), a 2:1 break/make pulse ratio, and a plastic finger plate. The hand generator is of the rotating-magnet type, but this can be replaced or supplemented by a transistor ringing-current generator with associated ring-start facility.

A modern lightweight plug-in handset with rocking-armature receiver and coiled cord is included, but can be replaced or supplemented by a lightweight, plug-in headset if specially ordered. Also included are an alarm buzzer and cut-off key.

An internal terminal field permits direct termination of incoming lines. Alternatively, a flexible connecting cord with desk terminal
block can be supplied.

A miniature glass fuse is fitted at the rear of the base and can be replaced without removing the switchboard cover.

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: 0.4A for 6-line, 0.6A for l2-line board.

Extension Telephones
Etelphones (catalogue sheet 7145) or Plesseyphones (catalogue sheet 7159) are recommended, offering high transmission performance over extension-to-exchange loops of up to 1000 ohms.

Dimensions (Casework)
Width: 21.5 in (541 mm)
Height: 7.3in (196mm)
Depth: 14in (355mm)

Weights (approx.)
N121A 321b (14.5kg) net
N122A 40lb (18.1 kg) net

Taken from the Plessey Publication No. 7127/1 (1971)

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.  

Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: October 12, 2019