| Descriptive Leaflet TELECOMMUNICATIONS
INSTRUCTION
C MARKETING - INSTALLATION
3 Internal
E4003
Private Manual Branch Exchange No.
2/4
(P.M.B.X. No. 2/4)
Description
GENERAL
The Switchboard PMBX, No. 2/4A is a lamp-signalling cordless
switchboard with seven
connecting circuits and has a capacity for four exchange lines
and eighteen extensions.
The switchboard has been designed to stand on a table or desk.
Each extension telephone is normally connected to the
switchboard by four wires (A, B, C
and an earth). External extensions may however, where necessary,
be provided on a 2-wire
basis by fitting a Unit, Auxiliary Apparatus, No. 96A at the
PMBX end of each such
extension. Similarly inter-PBX circuits can be connected via a
suitable Unit, Auxiliary
Apparatus, e.g. No. 97, 98 or 99. The switchboard can be used in
CB and automatic exchange
areas, and also, with Unit, Auxiliary Apparatus, CBS 536 fitted
at the public exchange, in
CBS No. 1, 2 and 3 areas.
Power and ringing supplies are normally obtained from
mains-operated units but may be
derived from secondary cells when standby power is required.
PHYSICAL DESIGN
The switchboard is coloured two-tone grey and a matching Telephone
No.
706... is normally used as the operator's telephone. The
switchboard is 22 in wide by
9 in high by 15 in deep and it weighs 56 lb. It has a metal
chassis and an
acrylonitrile-butadiene-styrene copolymer (ABS) cover.
The lamp cover strips are removed by sliding the phosphor-bronze
clips at each end inwards
towards the centre of the switchboard and pulling forward. On
removal of the lamp cover
strip the two lamp-jack fixing screws are exposed. By unscrewing
these screws the lamp
jack is drawn forward. The lamps can then be removed without the
use of a lamp extractor
or removal of the switchboard cover. The individual lamp prisms
are a push fit and can be
readily removed by pressure on the rear. A non-metallic
instrument should be used for this
purpose to prevent scratching the prism. A new lamp prism can be
inserted from the front
by the application of slight pressure.
There are two designation strips fitted on the face panel, one
for the extensions and the
other for the exchange lines. The extension designation strip
(Strip, Designation, No.
40A) is removed by applying pressure on the lower edge and
sliding the strip upwards
towards the lamp cover strip. This disengages the fixing clips
attached to each half of
the designation strip mounting from slots in the face panel. The
paper label, which is
printed on both sides, can then be removed for marking as
required. With the extension
designation strip removed access is given to the label for the
exchange lines, which can
be removed by sliding it out of the right-hand side of its
mounting.
Subscriber's private meters of the cyclometer trip type (Meters
No. 21B, Grey) can be
provided on the face panel in the space normally occupied by the
exchange line designation
strip. When these are required the existing designation strip
(Strip, Designation No. 41A)
is removed by pivoting the locking projection on the under side,
and is replaced by a
narrow strip, (Strip, Designation, No. 35A), which covers the
upper part of the meter
front. In addition a Strip, Designation, No. 34A is fitted above
the FREE LINK signalling
lamps. When replacing designation strips care should be taken
not to damage the panel
surface. For details of metering schemes see E5001 and for the
application of these
schemes to the switchboard see Diagrams N1008 and N1009. Metering
connections within the
switchboard are made on the two auxiliary terminal strips fitted
on the key panel adjacent
to the exchange line keys.
The cover is held in position by an angle bracket which pulls
the cover to the rear by
means of the screws in the flange at the rear of the baseplate.
To remove the cover the
screws (which are captive) are unscrewed, and the latching
bracket springs clear of the
projections on the inside of the cover. The leading edge of the
cover fits under the front
of the face panel and adjustable plates, which are positioned on
each side of the panel
form dust seals.
The chassis consists of three sections, the front and rear
panels hinging on the
baseplate, forming a triangle when the chassis is closed' By
releasing the screws at the
apex, the hinged panels can be opened outward to give access to
the wiring and components.
The relays in the switchboard are standard PO 600 and 3000 type
and are mounted on the
rear panel. A metal cover, retained by a spring clip at each
end, is provided to protect
the relays.
Two terminal blocks (Blocks, Terminal Nos. 47A and 47B) with 55
terminals each are mounted
on the switchboard base immediately below the relay mounting
plate. The conductors of a
Connector No. 1100A are terminated on 100 of these terminals.
The free end of the
connector is plug-ended and jacks into a wall- mounted socket to
which the permanent
cabling is terminated. Terminals are also provided for the
connection of the operator's
telephone. The remaining terminals on the terminal blocks,
together with the tags on the
Strip, Connection No. 153/7B which is also provided on the base
of the switchboard below
the face panel, permit the termination of 2-wire extensions,
inter-PBX extensions or
private circuits using associated Units, Auxiliary Apparatus,
without modification to the
switchboard permanent wiring.
Cordage enters the switchboard via the two entry holes in the
flange at the rear of the
base. When viewed from the rear, the right-hand hole is for the
operator's telephone cord
and the second hole is for the main 100-way cord. A dummy
grommet is provided on the
left-hand side of the baseplate when viewed from the front. The
dummy grommet is removed
and a Jack No. 84C fitted when a Headset No. 1 is required in
addition to the operator's
telephone. Wiring is provided in the switchboard for the
connection of this jack. When a
Headset No. 1 is fitted the operator's telephone is changed to a
Telephone No. 710; for
details see Diagram N1104.
FACILITIES
Circuit connections Four-wire extension circuits may be
connected on all extension
positions. Private circuits, inter-switchboard private circuits,
inter-switchboard
extensions, and 2-wire extensions must, however, be connected on
extension positions
10-18.
A Unit, Auxiliary Apparatus, No. 96A should be provided on all
2-wire extensions and
terminated in accordance with Diagram N1128.
Private circuits etc require the provision of a Unit, Auxiliary
Apparatus No. 97, 98 or
99. Details of the particular N diagrams applicable to any
signalling group are given in
F0402.
Lamp calling on exchange lines and extensions
A Lamp No. 2-45V which has a reasonably flat lumen/resistance
response is used to
eliminate the need for a line-calling relay on extensions. The
lamp is connected in series
with the battery feed via the pilot relay and glows when the
extension loop is completed
by removal of the telephone handset at the extension. The pilot
relay is shunted by an
electrolytic capacitor to prevent overhearing between calling
extensions. The exchange
lines use the same type of lamp and a locked signal is given
when the signalling relay
operates to an incoming ring. In order to reduce the amount of
series resistance added by
the pilot relay it has a low-resistance winding which is
connected in parallel with the
operating coil when the relay operates. If the calling lamp of
an exchange line becomes
disconnected an audible alarm is given (provided the ALARM ON
key is operated) as the
holding circuit for the signalling relay is via the pilot relay.
Clearing and recall
The use of the 4-wire extension principle separates the
functions of transmission and
supervision. As a result it has been possible to arrange that
individual clearing and
recall signals are given by each extension lamp.
Clearing
A clear is given when the extension handset is replaced on both
extension-to-extension and
extension-to-exchange calls. On 4-wire extensions control is by
an additional gravity
switch spring-set in the extension telephone. On 2-wire
extensions the control is by a
supervisory relay in the Unit, Auxiliary Apparatus, No. 96A (see
Diagram N1104 explanatory
figures).
Press-button recall
This is a standard facility and a switch is provided in
accordance with Diagram N806 on each
extension telephone. Recall is provided on 4-wire extensions by
connecting the make
contact of the switch in parallel with the additional gravity
switch spring-set provided
in the extension telephone. On 2-wire extensions where a Unit,
Auxiliary Apparatus, No.
96A is provided the press-button is arranged to earth the
telephone loop and operate a
differentially connected relay in the unit; the relay contacts
operating complete the
extension signalling lamp circuit. The facility is effective on
both
extension-to-extension and exchange-to-extension calls.
Holding of exchange calls
Automatic holding of exchange calls is provided on all
connecting circuits. If, on an
exchange call, the operator restores the operator's key with no
extension connected to the
circuit, a previously operated hold relay connects a 910ohm
resistor across the exchange
line to hold the exchange equipment. The hold condition is
removed by the
re-operation of
the operator's key or the restoration of the exchange line key.
If, however, the exchange
line has been extended to an extension circuit, the holding
circuit of the relay is
dependent upon the earth extended over the C-wire from the
extension telephone auxiliary
gravity switch, and the hold condition is maintained on the
exchange line while the
extension handset is on its rest. The hold condition is removed
when the extension
answers. If the extension does not answer the hold condition is
removed either by the
re-operation of the operator's key or the restoration of the
exchange line key. Visual
indication of a call being held is given by the glowing of the
CALL HELD lamp which is
common to all connecting circuits.
Exchange and extension line limits
For transmission and signalling resistance limits see E0110.
Follow-on-call trap
When an exchange-to-extension connection is cleared down by
replacing the extension
handset the 'call-trap' relay is operated in the switchboard.
Contacts of this relay
disconnect the extension circuit from the exchange line circuit
and reconnect the
signalling relay to the exchange line. Therefore, if a
follow-on-call is received on an
exchange line which has not had its exchange line key restored
after a previous call, it
will be trapped on the signalling relay; the exchange line
calling lamp will glow but the
extension telephone bell will not ring.
Night service
The NIGHT SERVICE key should be operated and selected extensions
connected to the exchange
lines whenever the switchboard is left unattended overnight or
for long periods. Under
night service conditions the pilot relay is disconnected from
the extension calling lamps
to prevent these from glowing. The exchange line signalling
relays are also disconnected
to prevent the exchange line calling lamps glowing. A 200k ohm
resistor in series with a
2pF capacitor is connected across each exchange line to ensure
that the lines do not test
as disconnected should the NIGHT SERVICE key be operated without
the exchange lines being
extended to extensions. Operation of the NIGHT SERVICE key
connects an earth to terminal
NS of the switchboard which can be used as a switching condition
to provide non-standard
night service arrangements. Contacts of the night service relay
(which is released)
provide night service switching arrangements for any Units,
Auxiliary Apparatus which may
be connected to extensions 10-18. The operator's telephone bell
circuit is also
disconnected from the first exchange line when the NIGHT SERVICE
key is operated.
Prohibition of exchange service on private circuits
These circuits terminate on extensions 10-18 and a prohibition
relay is connected to the P
terminal of an extension on which a Unit, Auxiliary Apparatus is
fitted and strapped to
provide exchange prohibition. The contacts of this relay are
arranged to disconnect the
private circuit from an exchange line when both the exchange and
extension keys are
operated.
Audible alarm
This is switched on when the ALARM ON key is operated. An
additional bell or buzzer can be
provided, if required, by making use of the NA terminal. Any
type of bell or buzzer
suitable for 50v working may be used.
Lightweight headsets
A Headset No. 1 may be provided in addition to the operator's
telephone. When this
facility is required the operator's telephone is changed from a
Telephone No. 706 ... to a
Telephone No. 710... and additional spring-sets provided in the
telephone to disconnect
the headset when the handset is removed from the rest. The
Telephone No. 710... is used
because its gravity switch has the greater mechanical advantage
needed to operate the
additional change-over springs required.
MAINS FAILURE
Operation under mains failure if the power supply to the
switchboard fails at
installations supplied by a mains-operated power-unit, exchange
connections in progress
are maintained but extension-to-extension calls fail. An audible
indication of an incoming
call on the first exchange line is given by ringing the bell in
the operator's telephone.
Under power failure conditions the first exchange line is
normally used by the operator
for both incoming and outgoing calls. The remaining exchange
lines may be connected to
selected extensions to give normal exchange access.
Free-link signalling
A free-link signal is provided to assist in the selection of a
free connecting circuit.
Two lamps are provided for each row of keys, one for the link
connected by the upward
movement and the other for that connected by the downward
movement. In the idle condition
all lamps are extinguished. On receipt of an incoming call the
pilot relay operates and
the first lamp glows (connecting link A).
When this circuit is taken into use by the operation of the
appropriate exchange line or
extension line key, to answer an incoming call, the transmission
bridge relay operates.
The contacts of this relay disconnect the lamp associated with
connecting link A, and
connect the lamp of connecting link B which glows to indicate
that this is the next free
connecting link.
The sequence is repeated as further links are taken into use.
When all seven connecting
links are engaged the eighth (OVERCALL) lamp glows. Should any
further calls then be
received, they can be answered by the operator on the overcall
circuit.
Should a connecting link be taken into use out of turn this does
not affect the order or
the display of the signals, i.e. the lamp glowing will always
indicate the first free
connecting link starting with circuit A. If a circuit becomes
free the associated lamp
glows if it is the first free connecting link in the sequence.
Overcall
This enables the operator to answer a call when all seven
connecting links are engaged.
-
To answer an extension call the SPEAK ON OVERCALL key is
operated. An additional
transmission feed relay now provides current to the operator's
telephone and, in
operating, its contacts disconnect the ringing supply from the
extension ringing key and
extend the operator's telephone thereto. The operator can now
speak to the calling
extension by holding operated the appropriate ringing key.
-
To answer an exchange call the SPEAK ON OVERCALL key and
appropriate exchange line
OVERCALL key are operated. Contacts of the latter key disconnect
the additional
transmission feed relay and connect the exchange line to the
operator's telephone. The
transmission feed for the operator's telephone is now supplied
by the public exchange and
a contact of the exchange line OVERCALL key connects a bridging
coil to provide a holding
condition on the exchange line. This permits the operator to
leave the circuit and offer
the call to an engaged extension and then if required restore
the previously set-up
connection and extend the exchange call to the extension.
POWER AND RINGING SUPPLIES
The switchboard has been designed to operate from a nominal 50v
dc supply but is capable
of working in the range 45-55v. This supply is normally obtained
from a mains-operated
power unit but a battery float system is available for use at
installations where the
limited facilities available under mains-failure conditions are
insufficient.
A 25Hz ringing supply is provided by a mains driven ringing
converter at installations
using power units and by a DC operated ringing converter at
installations provided with
stand-by batteries.
Details of power and ringing equipment for mains operated power
unit installations may be
obtained from Diagram N1104 and for battery float system
installations from Diagram N2332.
CIRCUIT DESCRIPTION AND OPERATION
Diagram SA7165 is the schematic diagram for the switchboard and a
detailed circuit description
is given in Diagram Notes SA7165. Diagram
N1104 gives details of the
apparatus, circuit
elements, installation wiring and cabling instructions and of
the operator's telephone
circuit (with and without headset).
INSTALLATION
The switchboard should be installed in accordance with Diagram
N1104.
On all internal extensions three wires plus an earth wire are
required to each telephone;
four wires should also be provided for external extensions
subject to the restrictions as
detailed for Extension Plans 1A in C0010. For all 2-wire
extensions a Unit, Auxiliary
Apparatus, No. 96A should be provided and connected as shown on
Diagram N1127.
A 50 pair cable to a Box, Connection, No. 5C should be
terminated on a Connector No.
101/2A in accordance with Diagram N1104 and in the manner
shown on Drawing SD113.
The cabling between the switchboard and the Box, Connection, No.
5C must not exceed 100ft
to avoid an excessive voltage drop under full load conditions.
The extension instruments should be connected in accordance with
Diagram N806.
STD meters, if required, should be provided in accordance with
E5001 and Diagrams N1008 and
N1009.
For a mains-operated installation the power unit should be
installed in accordance with
Q0020.
For a battery float installation the rectifier and battery
should be installed in
accordance with Q0071.
The above picture shows from top to bottom, a
Unit, Auxiliary Apparatus, No. 96A (one each external
extension or inter-switchboard circuit), a Power Unit and a Box,
Connection, No. 5C
User Guide
Drawings - 91838/0,
91838/1, 91891, 91892, 91893 and 91923.
Label No. 431 - Exchange lines.
Label No. 430 - Extensions.
Made by Ericsson
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