the BPO maintained the large PABX's, they were never allowed to
make or install
Customers would purchase a PABX from the manufacturer of their
would design and install a system to suit the customers needs.
The BPO would oversee the installation and then take on the
for the equipment.
Click here for extension users guide
(PDF) - CD users only
Click here for Plessey PABX No. 4
Click here for GEC PABX No. 4
- Click here for pictures
of the GEC PABX No. 4 ACD
Visit a PABX 3
The PABX 4 has virtually an unlimited capacity and
uses the same standard types of automatic equipment, power plant
and extension telephones as the PABX 3. The capital cost
- which must be met by the customer - is greater than that of a
PABX 3 of the same size, but it offers more facilities and uses
a desk-type, cordless, manual switchboard in contemporary
It incorporates several novel features which will, if fully
utilised, produce a marginal saving in operator time, but with
some loss of personal service to the extension users. The
differences in facilities and degree of personal service between
the PABX 3 and 4 need careful assessment and consideration
before a choice is made. The Telephone Manager will gladly help
and advise on the relative merits of the two systems in
As with the PABX 3, the customer must buy the equipment from an
approved contractor who will also be responsible for installing
it. The Post Office provides the extension wiring and
telephones, and maintains the whole installation.
Apart from the operator’s switchboard the PABX 4 is similar in
design and construction to the PABX 3 already described; the
automatic apparatus is mounted on the same type of open racks,
and a similar type of power plant is provided. The main
difference is the provision of the cordless, desk-type
switchboard, and for this reason the PABX 4 is often called the
cordless PABX. Dependent automatic installations in separate
premises can be associated with a PABX 4, as with a PABX 3.
In common with the other systems described in this booklet, the
PABX 4 provides a normal range of PABX facilities, including
extension-to-extension dialling; extension-to-exchange dialling;
connexion of incoming exchange calls by the PABX operator;
through clearing; hold for enquiry; operator call-in for
assistance; automatic transfer of calls; and night-service
Some special features are:-
- Selected extensions can be given priority calling to ensure
that their calls are dealt with by the operator before calls
from other extensions.
- Incoming calls are put through to extensions by the PABX
operator by the use
of press-buttons. If a called extension is engaged the
operator can leave the incoming call to be connected
automatically when the extension is free.
- The operator can speak to an exchange-line caller or to the
called extension without being overheard by the other person.
- Selective restriction on calls can be applied so that
particular extensions can be prevented from making and/or
receiving exchange calls, or having exchange calls transferred
from other extensions. Inter-PBX line barring can also be
- Facilities are usually provided for the operator to use
press-buttons instead of a dial for outgoing exchange calls.
- Calls over inter-PBX lines from other switchboards may be
received by the PABX operator or, when signalling conditions
permit, may be received direct by the extension required.
Conversely, in most cases extensions can dial directly to
other switchboards, and sometimes
- if the distant switchboard is another PABX - directly to the
Night-service can be provided in one of the following ways:
- Incoming calls on exchange or inter-PBX lines cause suitably
sited bells to ring continuously until any extension answers
by lifting the handset and dialling 8. If necessary, the call
can be transferred to another extension in the usual way.
- Incoming calls ring the bells of certain designated
extensions, any of which answer simply by lifting the handset.
If all designated extensions are engaged the next incoming
call may ring an overflow bell or bells, or inject a warning
tone on to a preselected designated extension. All designated
extensions retain normal extension facilities.
- Each exchange line on which night service is required is
connected to an extension in such a way that the extension may
answer all incoming calls on that line while retaining all
normal extension facilities. A warning tone is given on any
such extension which is engaged on an internal call when an
incoming exchange call arrives, and when the handset is
replaced the extension is re-rung automatically.
- Each exchange line on which night service is to be provided
is connected to a nominated extension in such a way that the
extension becomes equivalent to a direct exchange line and
loses all extension facilities. Only one extension can be
connected to each exchange line.
One or more positions are provided, depending upon the size of
the PABX. They are cordless, desk-type, floor-standing,
contemporary in appearance, with compact styling, and adaptable
for use in any shape of room since they do not have to be
assembled in line as a suite. Maximum use is made of automatic
techniques to reduce the amount of manual operating work.
The switchboard has a small desk with a sloping face panel on
which are press-buttons, switches and either display lights or a
translucent screen. Each approved contractor may produce his own
version of the manual switchboard and although these conform to
certain basic requirements, they may differ considerably in
appearance. When making an outgoing call from the PABX
switchboard the operator is automatically connected to a free
exchange line by pressing a button. The call is set up through
the automatic equipment and when established is disconnected
from the switchboard. The PABX is designed to meet the needs of
offices where extensions are allowed to dial their own exchange
calls, and it is not as easy for the operator to obtain them as
it is on a PABX 3: the operator cannot connect an out going
call, on behalf of an extension, until the extension handset has
been replaced, and the extension therefore has to be rung back.
Presentation of calls to the switchboard
All calls to the switchboard are presented approximately in
their order of arrival to operators. Under one system there are
separate lights to indicate an exchange call, a call from an
extension, a call from another PBX, etc, and if more than one
light shines operators can select which type of call to answer.
In another system all incoming calls share the same light and
the type of incoming call is indicated by another light after
the operator has answered the call.
Numbering arrangements and dialling codes
These are similar to those used on the PABX 3 except that 8 is
usually reserved for night service.
These are similar to those on the PABX 3.
Standard dialling, ringing, engaged and number unobtainable
tones are used and, in addition, there is a ticking tone which
indicates that the operator has come in on an engaged line.
Exceptionally, a distinctive ringing tone may be used for
internal calls if required.
The same considerations apply to PABX 4 accommodation as for the
PABX 3. There is some saving in the size of the room needed for
a multi-position switchboard because the cordless boards can
take up to 30 per cent less floor space than that needed for a
similar number of PABX 3 switchboard positions, but the
automatic equipment will usually need more room. A table showing
typical room sizes is given overleaf.
As with the other types of PABX there are various optional
facilities that can be readily provided. Some of these are
Conference telephone calls between up to ten extensions, or
between one exchange line and up to five extensions.
Key callers to enable selected extensions to call up to twenty
pre-determined extensions by pressing the appropriate key.
Staff location systems under the control of the operator. Call
queuing for calls to busy extensions. Reception of incoming
exchange line calls direct on to selected extensions. Remote
controlled dictation systems.
Typical room sizes for PABX 3 and PABX 4 installations
The dimensions shown are intended to be a guide only; the sizes
quoted are approximate and some adjustment may be necessary
following a more detailed investigation of requirements.
of switchroom (ft)
equipment room (ft)
of battery room (ft)
floor area (sq. ft)
|100 extensions with 2 positions
||12 x 11
||14 x 13
|200 extensions with 3 positions
||14 x 11
||16 x 14
|400 extensions with 4 positions
||16 x 11
||22 x 18
|500 extensions with 4 positions
||16 x 11
||24 x 18
||13 x 6
|600 extensions with 5 positions
||18 x 11
||25 x 22
||14 x 6
|800 extensions with 7 positions
||23 x 14
||27 x 26
||16 x 6
|1200 extensions with 10 positions
||29 x 14
||32 x 27
||17 x 6
|100 extensions with 1 position
||12 x 10
||22 x 14
|200 extensions with 2 positions
||15 x l0
||24 x 18
|400 extensions with 3 positions
||18 x l0
||28 x 2l
|500 extensions with 4 positions
||21 x 10
||26 x 24
||19 x 6
|600 extensions with 4 positions
||21 x 10
||26 x 26
||19 x 8
|800 extensions with 5 positions
||24 x 10
||31 x 26
||19 x 8
|1200 extensions with 6 positions
||27 x 12
||43 x 36
||28 x 8
Taken from GPO Descriptive Leaflet DLD 400 (1/68)
Article taken from
Post Office Telecommunications Journal
Introducing . . . .
By P. A. Marchant
THE NEW PABX 4
A standard system has been introduced for the PABX 4 to overcome
the problems created by the need to deal with a number of
Six types of Private Automatic Branch Exchange are at present in
use by the Post Office. They are the PABX 1 (with a
cordless switchboard and automatic equipment for a maximum of 49
extensions); the PABX 2 (a similar equipment but with a cord
switchboard); the PABX 3 (automatic equipment with a cord manual
board for large installations); the PABX 4 (a cordless type also
for large installations); and the PABX 5 and 6 (small,
fully-automatic systems catering for a maximum
of 20 extensions and which need no switchboard operator).
All these PABXs are of standard design except the PABX 4 which, in
the past, has been supplied by using manufacturers’ own
proprietary designs. Although design approval by the Post Ofﬁce
has ensured that acceptable equipment and operational practices
have been used, inevitably differences exist between them.
To overcome the problems arising from the need to deal with ﬁve or
more variants, a standard system has now been introduced. It will
be known as the PABX 4, the previous models being distinguished by
the manufacturers’ preﬁx - for example PABX STC 4. To
provide the facilities required of a modern PABX, the switching
system of the PABX 4 is necessarily complex and can be only brieﬂy
The design follows conventional step-by-step switching principles
but some novel features have been included. The equipment is
extensible from the basic unit of 50 extensions to a total of some
7,000 extensions, with no limitations on the number of manual
positions, exchange lines and private circuits which can be
The basic design covers all the facilities normally required by a
PABX customer. In addition, special facilities can be
provided although these are not all as yet covered by the standard
design and will continue to be supplied in a proprietary form for
the time being. Equipment practice also follows the
Satellite PABXs can give considerable line economy and they have
been catered for in the new design, full facilities for satellite
extensions being achieved mainly by the addition of small
registers at the satellite. This, with other design features,
ensures that an extension connected to a satellite now enjoys the
same facilities as it would if connected to the main PABX. In
addition, there is no need for special routing codes to and from
the satellite or for different operating instructions.
Although the objective of a cordless system is to reduce operator
control of calls to a minimum, the switchboard still retains a
position of importance in the system. Telephonically it is
the shop window of the PABX. From a design aspect, cordless
switchboards have the advantage that functional requirements are
no longer the major consideration. Operators are not
now required to sit closely in line facing a formidable array of
multiple jacks. Instead, they can sit at attractive desk-type
cordless positions with simple press-button controls with an
illuminated display of operating information presented
automatically as required.
Switchboard presentation is important and variety in design, which
allows customers a choice, is a distinct advantage. This has
been achieved in the PABX 4 by giving the manufacturer freedom in
design while providing the essential operational and engineering
requirements of the Post Office. All the switchboards have
been approved by the Council of Industrial Design and constitute
the standard range offered by the manufacturers. Other
designs which may be offered to suit special requirements are not
The functional part of a cordless position is little more than an
assembly of keys and lamps used to control and supervise the
automatic equipment. The switchboard provides no through
connection for any call. Controls are routed from the
automatic equipment to the switchboard as and when the operator
calls for them. The presentation of calls to the operator,
the routing of calls from the operator and the ﬁnal clear down of
connections are all performed automatically. This
simpliﬁcation of operator control and supervision is achieved by
adding automatic equipment to perform the functions previously
The cost of this additional equipment can be offset if the
facilities provided by the cordless system are fully used to allow
savings in switchboard equipment and operating staff.
The two main components of the switchboard are the display screen
and the control keyboard. By means of an illuminated display, the
former tells the operator of calls awaiting answer, the class of
call, route information, line identiﬁcation, supervision of calls
in progress and provides other miscellaneous information.
This information is displayed by lamps mounted at the rear of a
non-reﬂective glass screen, with a suitably marked negative ﬁlm
interposed between lamps and screen.
All keys on the keyboard are of the press button type.
Located centrally are the connecting circuits on which calls are
held while being progressed by the operator. Normally six
are provided, the maximum being eight. On the left of the
keyboard are call selection and miscellaneous control keys and on
the right a keysender for use on both internal and external calls.
The dial is for emergency use only and is normally hidden from
Incoming calls to the PABX 4 may be operator-connected or
dialled-in directly to the extensions. Normally, all incoming
exchange calls are operator-connected as are inter-PBX calls if
they cannot be given dialling facilities. Where dialling is
provided the arrangements follow standard PABX practice.
When an incoming call arrives for connection by an operator a
signal is given on all cordless positions. In all circumstances an
indication must be given of the class of call, that is, whether it
is an exchange, inter-PBX or an assistance call. Common
answering may be provided which means that the next call waiting,
whatever its class, is accepted by the operator.
Alternatively, the operator can select the class of call to be
answered. The operator accepts incoming exchange calls by
depressing an answer bar or key and selecting a free connecting
circuit. She then routes the caller automatically to the extension
required, by using the keysender.
When the call is established all switchboard contact is
automatically released from the connection. Up to this point
full supervision is given to the operator. lf for any reason
the call has to be supervised the hold key of the connecting
circuit concerned may be operated to prevent automatic release
from the switchboard. lf a called extension is engaged and
the caller decides to wait, the call remains connected under
operator supervision until the extension becomes free when it is
automatically rung and connected.
With minor exceptions, outgoing calls from the PABX can be made
automatically by the extensions. This is the ideal
arrangement if maximum advantage is to be obtained from a PABX and
particularly so with cordless working. Unfortunately this
ideal is not always obtained in practice mainly because freedom of
access to the outside network for all extensions does not always
suit the customer.
Where outgoing traffic is dealt with automatically the
arrangements follow normal PABX practice. The extension
dials the appropriate routing digit or digits to obtain connection
to the outside lines. If a call is to be connected by way of the
manual position “O” is dialled. The operator accepts the
demand and connection is made by reversing the call to the
extension concerned. When established, the connection is
identical to that for an incoming call.
Apart from extension-to-extension connections, all established
calls can be held while an enquiry is made of another extension or
transferred to another extension under the control of the
extension user. This is a common feature of cordless systems
where a high degree of extension control of calls without operator
intervention is desirable. The operator may.
however, be called in if desired and there is a built-in safeguard
which ensures that in the event of misoperation by the user an
outside caller is not left unconnected but is switched
automatically to the operator.
Another important facility which can be readily applied to large
cordless systems and which has been designed into the PABX 4, is
direct dialling into PABX extensions from the main exchange
network (DDI). This is achieved by allocating exchange
numbers to the extensions, so that a call is routed from the main
exchange through the PABX equipment directly
to the required extension without the intervention of the PABX
operator. This facility will undoubtedly become popular when
exchange multiple numbers become more readily available.
When fully used it could remove the only remaining essential
switching junction of the cordless switchboard operator and
thereby reduce the operating responsibilities to assistance and
With STD in mind, the PABX 4 provides an operational facility for
metering calls at the switchboard on a common exchange line basis,
or at the extension point. The switchboard provides for
meters at the right of the display panel which the operator can
associate with any outgoing call set up by way of the switchboard.
Association of the meter is automatic.
STD codes may be barred to individual extensions and, as with all
PABXs, access to the exchange network can be barred completely.
Tandem switching for private circuit networks, either manually or
automatically operated, can be provided and among the many other
facilities are full night-service arrangements, access to
dictation systems, supervisors and enquiry desks. A standard
system of keycalling, which gives immediate access between an
executive and a selected group of extensions
and can incorporate loud-speaking telephones, will be available
The optional features which have been provided for a number of
cordless installations in the past will continue to be available,
although the frequency of demand may not in every instance warrant
the development of standard arrangements. The optional
features include conference facilities set up by the operator or
controlled from selected extensions; emergency call arrangements
resembling an internal 999 Service; manual extensions giving
immediate access to the operator; short-code dialling (obtaining
frequently called numbers by dialling or keying a code of two or
three digits); night watchman’s patrol systems; staff location
systems; internal queuing systems for enquiry bureaux; calling
line identiﬁcation to the operator; special facilities for hotels
and the provision of automatic accounting of calls; and
press-button telephone working.
This list is by no means exhaustive. The new PABX is
expected to be closely integrated with the needs of industry and
commerce and not necessarily conﬁned to precise telephone
communication. No doubt the future will call for even more elastic
treatment but the PABX 4 equipment should be able to cope.
Click for an article from
the Ericsson Bulletin
on the ET P.A.B.X 4
with two enquiry desks on the left (Ericsson ET4
(Ericsson ET4 type)