|
Descriptive Leaflet TELECOMMUNICATIONS
INSTRUCTIONS
C MARKETING - INSTALLATION
3 Internal
E400l
Private Manual Branch Exchange No.
2/2
(P.M.B.X. No. 2/2)
Description
GENERAL
The Switchboard, PMBX, No. 2/2A is a lamp-signalling
cordless switchboard with three
connecting circuits and has a capacity for two exchange lines
and six extensions. The
switchboard has been designed to stand on a table or similar
support. The later version,
Switchboard, PMBX No. 2/2B differs only in internal detail,
e.g., electronic buzzer in
lieu of the earlier mechanical type.
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 (or 96) 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 (manual) and automatic
exchange areas, and also with Unit, Auxiliary Apparatus, CB 506
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
1. The colour of the switchboard is two-tone grey and a matching
telephone No. 706 is
normally used as the operator's telephone. The switchboard is 8
in wide by 5 in high by 10
in deep and it weighs 12 lb. It has a metal chassis and an
acrylonitrile-butadiene-styrene
copolymer (ABS) cover.
2. The lamp cover strip is removed by moving 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.
3. The designation strip is removed by applying firm outward
pressure to the lug at each
end. This releases the fixing clips, which slot into the front
panel on each half of the
designation strip mounting. The paper label, which is printed on
both sides, can then be
removed for marking as required. When replacing the designation
strip care must be taken
not to damage the panel surface. The two parts of the
designation strip are not
interchangeable.
4. The cover is held in position by two angle brackets which
pull the cover to the rear by
means of the two screws mounted on the flange at the rear of the
baseplate. To remove the
cover the screws, which are captive, are unscrewed. 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.
5. 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.
6. The relays in the switchboard are standard PO 600 and 3000
type. The plastic dust
covers for the relays are removed by releasing the screws
situated on the side supports of
the rear panel. On early models the 600 type relays are
positioned on the baseplate and
their dust cover is removed by releasing the single screw fixing
in the centre.
7. A terminal block with 70 terminals is mounted on the base.
The conductors of a
Connector No. 1038A are terminated on 38 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 and
STD meters. The remaining terminals 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. See Drawing
SD113.
Cordage enters the switchboard via the three grommet entry slots
on the flanges at the
rear of the base. When viewed from the rear the right-hand slot
carries the cord for the
operator's telephone, the centre the main 38-way cord and the
left-hand slot the STD meter
cordage. As STD meters will not always be required, a dummy
grommet is supplied for the
left-hand slot.
8. A Headset No. 1 may be fitted in addition to the operator's
telephone. When this is
required the operator's telephone is changed to a Telephone No.
710 the cover is replaced
by a Cover, Switchboard, No. 1A, Grey and a Jack No. 84C is
fitted.
FACILITIES
1. Circuit connections
Four-wire extension circuits may be connected on all extension
positions. Private
circuits, inter-switchboard circuits, inter- switchboard
extensions and 2-wire extensions
must, however, be connected on extension positions 4-6.
A Unit, Auxiliary Apparatus, No. 96A (or 96) should be provided
on all 2-wire extensions
and terminated in accordance with Diagram N1126.
Private circuits etc require the provision of a Unit, Auxiliary
Apparatus, No. 97, 98 or
99.
2. 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 relay AC operates to an
incoming ring. A resistor which is connected in series with the
exchange line lamp has the
same value as the low- resistance winding of the pilot relay.
This resistor is provided to
reduce the variation in brilliance between exchange and
extension lamps. 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 relay AC is
via the pilot relay. The
pilot relay has a low-resistance winding which is connected in
parallel with the operating
coil when the relay operates in order to reduce the amount of
series resistance added by
the relay.
3. 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.
3.1 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. In the case of
2-wire extensions the control
is by a supervisory relay in the Unit, Auxiliary Apparatus, No.
96A (or 96) (see Diagram N1102
explanatory figures).
3.2 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 (or 96) 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.
4. Holding of exchange calls
A separate HOLD key is provided below the connecting circuit
keys of each exchange line.
Operation of the HOLD key disconnects the exchange line from the
connecting circuit,
applies a 430 ohm loop to hold the exchange line connection and
reconnects a transmission
feeding battery. This permits the operator to speak to an
extension on the the connecting
circuit with complete secrecy from the subscriber held on the
exchange line.
5. Exchange and extension limits
For transmission and signalling resistance limits see TI E0110.
6. Night service
The NIGHT SERVICE key should be operated whenever the
switchboard is left unattended
overnight or for long periods. This prevents locked lamp signals
on the exchange lines
should an incoming call be received during this period. With the
NIGHT SERVICE key
operated the call is indicated on the switchboard only for the
duration of the incoming
ringing signal. Any selected extension can be extended on night
service including those
fitted with a Unit, Auxiliary Apparatus, No. 96A (or 96). If a
non-standard night service
switching arrangement is required an earth extended to terminal
NS can be used for
operating any switching relays which may be necessary.
7. Prohibition of exchange service on private circuits
A prohibition relay is connected to the P terminal of extensions
4 to 6 when a Unit,
Auxiliary Apparatus is fitted thereto and strapped to provide
exchange prohibition. The
contacts of this relay are arranged to disconnect the private
circuit if an attempt is
made to connect it to an exchange line.
8. 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.
9. Lightweight headsets
A Headset No 1 may be provided in addition to the operator's
telephone by minor
modification to the switchboard. When this facility is required
the operator's telephone
is changed from a Telephone No. 706 to a Telephone No. 710.
Additional spring-sets in the
telephone disconnect the headset when the handset is removed
from the rest. (The
significance of the use of the Telephone No. 710 is purely
mechanical as the gravity
switch has a greater mechanical advantage to operate the
additional change-over springs
required).
10. 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
second exchange line can be connected to a selected extension to
give normal exchange
access.
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 25 Hz 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 N1102 and for battery float system
installations from Diagram N2332.
CIRCUIT DESCRIPTION AND OPERATION
Diagram SA7163 is the schematic diagram for the switchboard and a
detailed circuit description
is given in Diagram Notes SA7163. Diagram N1102 gives details of the
apparatus, circuit
elements, installations 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
N1102.
On all internal extensions three wires plus an earth wire are
required for each telephone.
-
A 41-wire cable to a Box, Connection, No. 5A
should be terminated on a Connector No. 100A
in accordance with N1102 and in the manner shown on Drawing
SD113. A Pliers
Crimping No. 1 is used for this puropose.
-
The cabling between the switchboard and the
Box, Connection, No. 5A must not exceed 100
ft to avoid an excessive voltage drop under full load
conditions.
-
The extension instruments should be terminated
in accordance with Diagram N806.
-
STD meters, if required, should be provided in
accordance with E5001 and Diagrams N1003
and N1004.
-
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 Q0060.
-
A lightweight headset, if required, should be
fitted in accordance with Works Spec S(W)
2064.
(This instruction succeeded EI Telephones, PBXs, B1051)
Drawing - 91508/0
and 91508/1.
Specification - S629.
Label No. 397 -
Exchange and extension designation.
Click here for metering
schemes and apparatus required - Telephones, PBX, B3104.
Click here for an article
in
the Ericsson Bulletin
User Guide
An extract from
The Post Office Electrical
Engineers' Journal
Volume 54, Part 1 - April 1961
A New Small Cordless P.M.B.X.
Switchboard
by C. M. HALLIDAY and E. J. LIDBETTER
The first of a new range of private manual branch exchange
switchboards of the cordless type has a capacity for two exchange
lines and six extensions. It is a lamp-signalling switchboard of
compact design incorporating new features and the power supply is
derived from a separate mains-driven unit supplying 50 volts d.c.
and 25 c/s a.c.
INTRODUCTION
The requirements for small manually-operated telephone
switchboards for use in subscribers’ premises have, for many
years, been met by a range
of cordless switchboards, all of which are of a similar basic
design. Private manual branch exchanges (P.M.B.X.s) of this type
have a maximum capacity of three exchange lines and nine extensions;
lever keys are used to interconnect the circuits, and the signalling
facilities are provided by indicators.
A new range of cordless switchboards is now being designed to
supersede the existing series, and the first of these new designs,
the P.M.B.X. No. 2/2A, has a maximum capacity of two exchange lines
and six extensions. The type of switchboard that it supersedes has a
maximum capacity of two exchange lines and four extensions. The new
switchboard has been given a modern appearance, approved by the
Council of Industrial Design, so that it will harmonize with other
new designs of subscribers’ apparatus. Lamp signalling is used and
the connecting keys are of a new miniature type.
The number of extensions that can be connected to the new
switchboard has been increased to six compared with four on the
existing design, as it has been found more economical to have a
higher extension to exchange line ratio for a switchboard of this
size. It has been confirmed nevertheless that the traffic-carrying
capacity of three connecting circuits will suffice even with the
increased number of extensions, and therefore no change has been
made in the number of such circuits.
The adoption of the 4-wire extension principle has enabled all the
requisite facilities to be provided in the most economical manner.
Briefly, this principle consists of running an extra wire plus an
earth lead to each extension telephone for supervisory purposes. A
4-wire/2-wire conversion relay-set has been designed for use on
external extensions that need to be connected via the
public-exchange local-line network. All the extension line circuits
on the switchboard can be used for 4-wire extensions, while
extension circuits No. 4 to 6 can, by simple rearrangement of cords and
straps within the switchboard, be used for 2-wire extensions,
private circuits, or inter-switchboard extensions. In addition to
the conversion relay-sets necessary for 2-wire extensions, auxiliary
units are also required for private circuits and inter-switchboard
extensions.
The power supply for the switchboard is derived from an external a.c.
mains-driven power unit supplying 50 volts d.c. and 25 c/s a.c. for
ringing purposes.
Fig 1
FACILITIES PROVIDED
The main facilities provided by the new switchboard are as follows:-
-
Lamp signalling for exchange lines and extensions.
-
A transmission and signalling limit of 500ohms for an extension
circuit and 1,000ohms for an exchange-to-extension connexion when
connected to an exchange with equipment for 1,000-ohm lines.
-
Press-button recall on extension-to-extension and
extension-to-exchange calls under all conditions.
-
Individual clearing on extension-to-extension calls.
-
Holding of an exchange call by the P.M.B.X. operator while
speaking to an extension on the same connecting circuit, the
conversation being inaudible on the exchange line.
-
Connexion of private wires and inter-switchboard circuits
without modification to the permanent wiring of the switchboard.
PHYSICAL DESIGN
The new switchboard, which is shown in Fig. 1, will normally be
provided in two-tone grey with a matching Telephone No. 706 as the
operator’s instrument. The switchboard has a metal chassis and a
plastic cover, and its dimensions are approximately 8 in. wide by 5
in. high by 10 in. deep. The design of a compact item has been made
easier by the adoption of the 4-wire extension principle and by the
introduction of lamp signalling to replace the indicators used on
the present type of switchboard. In addition, a new miniature key
(the 1,000-type key) has been used instead of the standard lever
key. The springs of this new key, which are comb-operated, are
similar to relay springs and have twin contacts. The key handles are
small and wedge-shaped (see Fig. 2).
The keys, lamp jack and the circuit-designation label-strip are
mounted on the panel in such a manner that all screw heads are
concealed. When the lamp-jack lens-strip is removed by releasing the
phosphor-bronze clips at each end of the cover, the lamp-jack
screw-heads are exposed. By releasing these screws the lamp-jack may
be drawn forward to permit lamps to be changed without the use of a
lamp extractor or the removal of the switchboard cover.
The thermoplastic cover of the switchboard is vacuum-formed by deep
drawing p.v.c. sheet and is polished after forming. The leading edge
of the cover slots under the front of the face panel and adjustable
plates on the sides of the panel form dust seals. The cover is
removed by releasing the captive screws at the rear on the base.
The chassis consists of three sections; the front and rear
panels, hinging on the base plate, form a triangle when the chassis
is closed, as shown in Fig. 2. By releasing the screws at the apex
the hinged panels can be opened outwards to permit inspection of the
wiring and components, as shown in Fig. 3. To conserve space in the
switchboard, small wire-wound resistors and tubular capacitors have
been used and are mounted on insulated tags fixed direct into the
rear panel of the chassis. Plastic dust covers are provided to
shield the 600-type and 3,000-type relays.
A connexion block with 70 terminals is mounted on the base. Some of
the terminals are used for the connexion of the operator’s
telephone, subscriber’s private meter and a 38-conductor flexible
cord for the exchange lines, extensions and miscellaneous
terminations. The remainder of the terminals are provided to permit
rearrangements of straps and cords when 2-wire extensions, private
circuits or inter-switchboard extensions are required, thus avoiding
the modifications to the permanent wiring that are necessary in some
applications on the present type of switchboard. The free end of the
38-way cord is terminated on a multi-way plug which fits into a
wall-mounted jack cabled to the connexion box. If subscribers’
private metering is required, a Meter No. 19, as used on direct
exchange lines, is associated with the switchboard.
ELECTRICAL DESIGN
A new parallel-feed transmission bridge has been adopted for
economic reasons. For 50-volt working the transmission bridge
consists of a 300 + 300-ohm coil with an 0.1uF capacitor across the
output to give improved side-tone balance. This combination is
suitable for all extension-to-extension calls up to a signalling
limit of 500 ohms, with either 300-type or 700-type telephones.
Non-removable relay shields fitted to the transmission-bridge relays
increase the crosstalk attenuation between circuits to at least 75
db.
A new lamp (Lamp No. 2, 45 volts) has been developed for use on the
switchboard. It has a reasonably flat lumens/resistance response
over the range 0-500 ohms line-plus-telephone loop resistance, and
the use of this lamp has obviated the need for a line-signalling
relay.
No hand generator is fitted in the switchboard; the ringing supply
is derived from a frequency-division mains-operated unit producing a
25 c/s a.c. output.
The switchboard circuits have been designed to operate from a
nominal 50-volt d.c. supply but are capable of operating from a
supply in the range 45-55 volts, and this has permitted economies to
be made in the design of the power unit. Arrangements have been made
to ensure that exchange calls already in progress are maintained if
a mains failure occurs. It is recognized and accepted, however, that
extension-to-extension and inter-switchboard calls will fail under
such conditions, but it is considered that little inconvenience will
be caused thereby, since failures of the mains supply in this
country are generally few and of short duration. Under mains-failure
conditions night-service arrangements are adopted. At installations
where a break in service cannot be tolerated, however, the
switch-board will be operated from a floated-battery system.
FIG. 2 - CHASSIS OF SWITCHBOARD CLOSED
FIG. 3 - CHASSIS OF SWITCHBOARD OPENED
CIRCUIT DESCRIPTION AND OPERATION
The 4-wire extension principle is used on all internal extensions
and can be applied to external extensions within the curtilage of
the subscriber’s premises. Where external extensions are connected
via the local-line network it will generally be desirable and more
economical to use a 4-wire/2-wire conversion relay-set. For all
extensions without a conversion unit the exchange-to-extension
transmission and signalling limit is the same as that of a direct
exchange line, since a relay is not required for supervisory
purposes. On external extensions for which conversion units are
required this limit is reduced to 900 ohms. The difference is due to
the resistance of the supervisory relays in the conversion unit.
If the switchboard is used to replace an existing installation, the
internal extensions will have to be wired for 4-wire-working and it
will be necessary to change the extension telephones to include the
additional auxiliary switch-hook spring-set.
A simplified circuit diagram of the new switchboard is shown in Fig.
4.
Extension-to-Extension Calls
When a call is originated at an extension by lifting the telephone
handset, earth via KX1, the telephone loop, KX2 and the calling lamp
operates the pilot relay, P, connected to -50 volts, and the
extension calling lamp glows when contact P2 operates and
short-circuits the high-resistance coil of the P relay. Contact PI
causes an audible alarm to be given if the alarm on key, KA, is
operated. If more than one extension is calling the switchboard,
overhearing between the extensions is suppressed by capacitor Cl.
The call is answered by operating the operator’s telephone key, KO,
associated with the chosen connecting circuit and the appropriate
connect-extension key, KX, and by lifting the handset of the
operator’s telephone. The transmission-bridge relay, L, feeds
transmitter current to both telephones.
If connexion to another extension is required, the operator
checks that the extension is disengaged by observation of the keys,
and calls the extension by the operation of the appropriate ringing
key, which connects 25 c/s ringing current to the extension. When
the called extension answers, the calling lamp glows and the KX key
associated with that extension is operated to complete the
connexion. The operator then restores key KO and replaces the
operating telephone handset. Either extension can recall the
switchboard by depressing the recall button on the extension
telephone. An earth (on
the fourth wire from the connexion box) is then extended to the C
wire (the third wire) and the calling lamp glows while the recall
button is pressed. When the call is completed and the extension
handset is replaced, an earth is extended via the auxiliary
switch-hook spring-set, SWA, to the C wire to give a clearing signal
on the calling lamp via key contact KX4. Individual clearing signals
are given when either extension replaces the handset.
Extension-to-Exchange Calls
If the calling extension requests connexion to an exchange line, the
connect-exchange key, KE, of a free exchange line is operated. This
disconnects the local 50-volt supply from the connecting circuit at
KE1 and KE2 and extends the extension to the exchange line. As there
are no supervisory relays in the connecting circuit (the third wire
being used for supervisory purposes) no additional relays are
required to prevent the calling lamp flashing to pulsing if the
exchange line is connected to an automatic exchange. The
exchange-line connecting-keys are, however, arranged as on the
previous type of switchboard, to prevent two exchange lines being
connected together. When the exchange call is completed the clearing
signal is given on the extension calling lamp only, when the
extension telephone handset is replaced.
Note: There are three connecting circuits
FIG. 4 - SIMPLIFIED CIRCUIT OF NEW SWITCHBOARD
Incoming Call on Exchange Lines
Incoming ringing current on the exchange line will operate relay AC
over one coil via capacitor C2 and relay contact MF1 normally
operated. Relay contact AC1 lights the calling lamp and AC2
completes a holding circuit for relay AC (to provide a locked
calling signal on the exchange line) and also operates the pilot
relay, P.
The circuit is arranged to ensure that, provided the alarm on key,
KA, is operated, an audible alarm is given at the P.M.B.X. even if
the exchange-line calling lamp becomes disconnected. The operation
of relay AC is delayed by the short-circuit maintained across the
hold coil by contact AC2. This avoids false operation due to line
surges when switching takes place. Specific safeguards have also
been incorporated to avoid the possibility of lost calls due to
misoperation of the switchboard keys. A calling signal is not
extinguished until the operator’s handset has been lifted and both
the KE and ICO keys have been operated. If an exchange-line key only
is operated with all the remaining keys in the same connecting
circuit normal, the L relay of the connecting circuit is operated
via KX3, K03, KE5 to earth at K04, and the calling signal is
maintained by contacts LI and L2 (contacts L3 and L4 are similarly
connected in the second exchange-line circuit). When key KO is
operated, relay L releases, but the ringing current is extended to
ring the bell in the operator’s telephone and this is continued
(unless the calling party clears) until the call is answered by
lifting the operator’s handset. The circuit thus ensures that a
signalling device is always connected across the exchange line until
a call is answered, and the need to connect a calling device
incorporating a capacitor permanently across the line is thus
avoided. A call can be extended to an extension as already
described, and the clearing signal is given on the extension calling
lamp when the handset of the extension telephone is replaced.
Exchange-Line-Hold Facility
A hold key, KH, is provided in each exchange-line circuit and these
keys have the dual function of applying a resistive loop to hold the
exchange equipment and of reconnecting a 50-volt supply via a
bridging coil to the connecting circuit, which is disconnected from
the exchange line so that the operator may use the same connecting
circuit to speak to an extension without the conversation being
audible on the exchange line. This facility is normally used on
incoming exchange-line calls. A single call held lamp common to both
exchange lines is provided to give a visual indication to the
operator when a hold condition is applied.
Mains Failure
As the switchboard has been designed to work from a mains-operated
50-volt d.c. supply unit, the circuits have been arranged so that
exchange-line service is maintained under mains-failure conditions.
A mains-failure relay, MF, has been included and is operated by the
50-volt supply. When a mains failure occurs relay MF releases and
contacts MF1, 2 and 3 connect the bell of the operator’s telephone
to the first exchange line so as to give an audible indication of an
incoming call. The operator can answer the exchange call under
mains-failure conditions by operation of the appropriate KE and KO
connecting keys and by removing the handset from the operator’s
telephone. Outgoing exchange calls can also be originated by the
operator in the normal manner. The operating instructions for this
type of switchboard will advise subscribers to connect the remaining
exchange line to a selected extension, as for night-service working,
should a mains failure occur, and the selected extension will then
have direct access to the second exchange line.
Press-Button Recall
By fitting a press-button key with a make contact in parallel with
the auxiliary switch-hook spring-set of each extension telephone,
press-button recall is effected by extending the earth on the fourth
wire to the C wire (the third wire), and recall can be provided on
all extension and exchange calls.
Night Service
Night-service arrangements are provided by the night service key,
KNS, which disconnects the pilot relay, the extension calling lamps
and also the AC relay locking circuit. Selected extensions can then
be connected to the exchange lines by operating the KE and KX keys
to a specific connecting circuit.
Private Circuits , Inter-switchboard and External Extensions
Using the additional terminals which have been provided for
extensions No. 4-6, private circuits, interswitchboard and external
extensions can be connected without modification to the permanent
wiring of the P.M.B.X. When an exchange-line key and an extension
connecting key are operated together in the same connecting circuit,
earth is applied to the extension P terminal from K04 via KE5 and
ICX5. This earth can be used to operate a relay in an auxiliary
unit. The contacts of this relay disconnect the line, thus providing
a prohibition facility to prevent the interconnexion of exchange
fines and private circuits.
An auxiliary switch-hook spring-set, SWB, controls relays in the
4-wire/2-wire conversion relay-set when this is provided. The
circuit of the relay-set is such that, when the operator intervenes
on an established exchange call, the exchange fine is held by a loop
and current for the extension is provided from the local
transmission feed in the relay-set. The SWB spring-set ensures that
calls are not inadvertently released by the operator entering the
circuit by operating key KO before lifting the handset of the
operator’s telephone.
CONCLUSION
It is confidently expected that the new cordless P.M.B.X., with its
use of modern materials, compact design and an increased range of
facilities, will be more attractive to the subscriber and to the
Post Office than the existing type of cordless switchboard which it
supersedes.
ACKNOWLEDGEMENT
The authors wish to thank Ericsson Telephones, Ltd. who, as the
liaison manufacturers, worked in close co-operation with the Post
Office on this development.
Additional Pictures
Rear view with case removed
Side view with case removed
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. 5A
|