2+4 or 3+9
with automatic or C.B. exchange lines
|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
- Positive supervision.
- Overcall (provided on type Nl22A only) to allow incoming calls to be answered when all
connecting circuits are engaged.
- Hold exchange.
- Night extension.
- Audible alarm.
- 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
When the switchboard is for connection to a C.B. exchange, the dial is replaced by a
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.
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
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.
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.
Etelphones (catalogue sheet 7145) or Plesseyphones (catalogue sheet 7159) are recommended,
offering high transmission performance over extension-to-exchange loops of up to 1000
Width: 21.5 in (541 mm)
Height: 7.3in (196mm)
Depth: 14in (355mm)
N121A 321b (14.5kg) net
N122A 40lb (18.1 kg) net
Taken from the Plessey Publication No. 7127/1 (1971)
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
All were supplied in two-tone grey.