Descriptive Leaflet -
Descriptive Leaflet -
Introduced in 1966 - made by GEC.
C MARKETING - INSTALLATION
Branch Exchange No. 6
(P.A.B.X. No. 6)
Scope of Instruction
This Instruction describes the facilities and equipment of the
|Equipment Cabinet and Attendants
No. 1A with Telephone No. 710)
The PABX 6 is an unattended PABX, i.e. it is not attended by an
operator, and incoming exchange calls are answered by certain
designated extensions and transferred to the required extension.
It has capacity for 5 exchange lines, 20 extensions and 4
connecting circuits. Two inter-switchboard circuits may be
provided in place of two exchange lines, if required.
The Equipment, PABX 6 consists of a cabinet 3ft 9in wide, 5ft
4in high and 1ft 2in deep, which contains the basic equipment
and wiring necessary to enable relay-sets, Uniselector
mechanisms etc, to be fitted to give the required number of
exchange lines, extensions, connecting circuits and
|Equipment Cabinet Closed
||Equipment Cabinet with covers
The following general facilities are provided as standard.
Further details are given in the relevant circuit Diagram Notes.
(a) Designated extensions
These are pre-determined extensions which are able to answer
incoming exchange calls. Any extension may be so designated but
the maximum number of designated extensions for which provision
is normally made is eight. This number may be increased to
twelve if necessary at the expense of night-service facilities.
This reduces the number of possible night-service arrangements
by the capacity of one night-service relay.
(b) Non-designated extensions
These are extensions which are unable to answer incoming
(c) Incoming exchange calls
An incoming exchange call rings common bells strategically
placed in the building. When the handset of any designated
extension is lifted the exchange call is automatically connected
(d) Outgoing exchange calls
Access to an exchange line is obtained from an extension by
pressing the telephone instrument button. For a short period
during the setting-up of this connection a connecting circuit is
employed, but if all the connecting circuits are busy, an
alternative method of switching is used to ensure access to a
free exchange line.
(e) Exchange call barring
Designated extensions may be barred outgoing exchange calls.
Non-designated extensions may be barred incoming or outgoing
(f) Extension-to-extension calls
These are dialed direct using connecting circuits. The
connecting circuit is held for the duration of the call. An
extension may also dial another extension or a switchboard
operator over an inter-switchboard circuit, dependent upon the
type of P.B.X. at the distant end.
(g) Inquiry calls
While an extension is engaged on an incoming or outgoing
exchange call, inquiry calls may be made to any other local
extension, or, over an inter-switchboard circuit, to other PBX
operators and extensions by pressing the instrument button and
dialing the appropriate number. During the period of the inquiry
the exchange line is held. On completion of the inquiry the
original extension may return to the exchange call by
re-pressing the instrument button.
An extension having made an inquiry call to a second extension
while holding an exchange call may transfer the call to the
second extension. This is done by asking the second extension to
hold while the handset of the originating extension is replaced.
Transfer does not take place if the inquiry has been made to an
extension which is barred exchange calls, or to an extension or
operator over an inter-switchboard private circuit. If for any
reason transfer does not take place, the original extension is
(j) Ring when free
If a designated extension makes an inquiry call and finds the
required extension engaged, the designated extension will
normally return to the exchange line and inform the subscriber.
If the subscriber decides to hold, the designated extension may
re-dial the required extension number followed by the additional
digit 1 and then replace the receiver Provided that the required
extension is not barred incoming exchange line calls, the
exchange call is their 'parked' on the required extension line
which is automatically rung when it cleats from the previous
call. The exchange call is connected through when the handset on
the required extension is lifted.
(k) Trunk offering
A designated extension wishing to transfer an exchange call to
another extension may, on making an inquiry call to the
extension, find that it is engaged. The designated extension may
by dialing an extra digit 1 obtain access to the extension to
offer the exchange call. During the time that the designated
extension is intruding in this way on an established call a
'warn' tone is automatically applied to that call to indicate
(l) Night service
By operating a night service key which is located on an
allocated designated extension telephone the normal extension
conditions may be altered, e.g. extensions which are designated
by day may be made non-designated by night or vice versa.
Similarly extensions which are barred exchange calls by day may
be given the facility by night or vice versa. It is also
possible under night service conditions to arrange for
additional or different call bells to be operated on incoming
(m) Power failure
The power supplies are obtained from a mains-driven power unit.
In the event of a failure of the supply the exchange lines are
automatically switched to predetermined extensions which then
function as normal exchange lines on the public exchange. Any
calls which are in progress at the time of the failure are lost.
On restoration of the power supply any call in progress from a
predetermined extension is maintained until the call is
(n) Inter-switchboard circuits
Two inter-switchboard circuits may be provided to give access to
other PABX's or PMBX's over two routes or one route.
The total number of exchange lines which may be provided is
reduced by the number of inter-switchboard circuits.
Access to an inter-switchboard circuit from an extension is
obtained by dialing a code digit.
Tandem dialing over inter-switchboard circuits is not provided.
When the PABX 6 is connected to a public exchange with S.T.D.
and subscribers private metering facilities, total call meters
may be provided for each exchange line and for extensions as
required. Provision is made to accommodate these meters in the
cabinet. The operation of these meters is dependent upon the
application of 50 c/s pulses to the exchange line at the public
(p) Connection to exchanges
The PABX 6 may be connected to either Automatic, CB, CBS or
(q) Attendant's telephone (optional)
The attendant's telephone consists of a modified Telephone
No. 710 mounted on an Answering
Unit No. 1A (grey). This apparatus looks similar to
a Planset No. 625 fully assembled.
An Article from
Post Office telecommunications Journal
By A. H . Hearnden
THIS PABX NEEDS
New unattended small private automatic branch exchange which
dispenses with the need for the services of an operator and
allows incoming exchange calls to be answered at extension
points and re-directed, as required will soon become generally
available. It is the PABX 6, an improved version of the PABX 5
which was introduced in 1963.
The PABX 6 has a maximum capacity of 20
extensions and five exchange lines. Two inter-PBX lines
can be terminated in place of two of the exchange lines.
The equipment, including the mains power unit and ringing
converter, is contained in an elephant-grey steel cabinet 3ft
9ins wide, 5ft 4ins high, and 1ft 2ins deep. The switching
equipment employs Post Ofﬁice standard components such as relays
and uniselectors which act as linefinders and final selectors.
The extension numbering range is 20-29 and 30-39.
Extensions are classified as “designated,” that is able to
answer incoming calls, or “non-designated” and may be allowed or
barred outgoing exchange line calls. Non-designated
extensions may also be allowed or barred transferred incoming
exchange line calls. Calls between extensions have first party release.
To make an out going exchange call the extension user presses
the button on the telephone to obtain the public exchange
dialing tone. Connections to other PBX’s are made via the
interconnecting private wire lines by first dialing a single
routing digit. In the absence of an operator, an extension which
has answered an incoming call must be able to hold it for
enquiry to another extension and to transfer the call if
necessary. Enquiry access is open to all extensions (including
barred extensions) or to an extension or operator over an
To make an enquiry call the telephone button is depressed and
the appropriate extension or inter-switchboard line number is
dialed. A holding condition is applied to the exchange
line during the enquiry and by depressing the button again, the
original extension can return to the exchange line at the end of
Transfer of the exchange call to the second extension is
effected by simply replacing the handset on the attending
telephone. If the second extension is busy when an attempt is
made to transfer a call, intrusion is possible by dialing a
further digit “1” which overrides the busy condition.
Another facility offered by the PABX 6 is that an incoming call
can be parked on an engaged extension which, having previously
been offered the call, will be re-rung as soon as the call in
progress is finished. During the waiting period no tone is
returned to the caller and no indication is given to the
extension that another call is waiting.
The provision of night service on an unattended PABX differs
from that on other PABX’s in that it is essentially a method of
altering the classification of extensions to suit night staffing
arrangements. This is achieved by a night service key
fitted on one designated extension which alters the designation
and barring condition of the other extensions as required.
Additional bells can be added for night service. If the
mains supply fails each exchange line is diverted to a
predetermined extension which the functions as a direct exchange
Since it may fall to the lot of a typist or secretary to answer
the bulk of incoming calls on a unattended PABX a special
attendant’s telephone has been designed to cater for this
requirement. The telephone concentrates two designated
extensions at one station and provides facilities for holding a
call on either line. During busy period the user can
handle incoming traffic without having to have two designated
extension instruments on her desk.
GPO Picture of the PABX No. 6
This is a standard PABX No. 6 with no meters
GEC Pictures of the PABX No. 6