This exchange provided a capacity from 50 to
200 lines and was introduced around 1937. This type of exchange provide the
following usual standard facilities:- Coin-box ccts; Automatic metering on local
calls; Supervisory signals on junction calls in either direction (controlled by
the switch-hooks of the Uax sub); NUT; PBX auxiliary working (on the PBX
final-selector units only); Busy tone is transmitted when all free outlets on a
group-selector level are busy.
The following
modified standard facilities were also provided:- Busy tone of standard pitch,
but of 1 second on, and half second off, with no busy flash; Ringing current,
generated by a 16 Hz vibrator, having a period of half second on, 1 second off;
Ringing tone being supplied from the ringing current, and having the same
cadence; No metering on junctions; Forced release applied to PG held group
selectors; Busy tone transmitted when all circuits are
busy.
The
following facilities are omitted:- Dialling tone; Provision for party-lines,
omnibus junction working, and earth-return ccts; Interworking to other Uax’s, No
‘Trunk Barred’ indication on calls to the parent exchange; Operator hold; Trunk
offering; Uax junctions cannot be terminated on auto equipment at the distance
end; No automatic release under CSH conditions.
This type
of exchange was assembled out of three types of racks, which were enclosed in
the usual sheet steel enclosed cavity racks. The three types of unit were:-
The
linefinder unit (A1). Each linefinder unit provides 50 subs line circuits,
consisting of three relays per circuit, and the associated meters. There are
eight linefinders (of 50 line, uniselector type) associated to a group selector,
of pre-2000 type. The linefinders are allotted in sequence by a uniselector
allotter. Contacts 1-8 of level two (of the group selectors) are cabled to the
eight PBX finals in the PBX final unit. Contacts 1-8 of level three being
similar cabled to the eight regular finals in the non-PBX unit. Contact 1-8 of
level four are cabled out to the linefinder connection strips, but are not
normally used. The group selector multiple is continuous
throughout all the linefinder units, whereas the linefinder multiple is only
continuous within each unit. An overflow meter is provided in each linefinder
unit, which operates when no free linefinders are available. Busy tone is
returned if all outlets on a group selector level are
engaged.
The Final
selector units are of two types. The ‘unit, final selector A2’ is the PBX final
selector unit, which deals with the traffic from the first two-linefinder units,
and therefore caters for the first 100 lines of the exchange. A fully equipped
unit contained eight 2-10 PBX hunting final selectors, twelve junction relay
sets, the ringing and tone sets, and other common equipment. The numbering range
of this unit is 200-299, with PBX groups on any level. The PBX groups cannot
have 0 as the last digit of the number.
The
‘unit, final selector A3’ is the regular final selector unit, which deals with
the traffic from the second 100 lines connected to the exchange. It contains
eight regular finals and fourteen junction relay sets. The numbering range of
this unit is 300-399, with no PBX working on any line. The multiples of each
unit are not extended to any other unit.
As normal
the cabling of the various circuits is done to connection strips mounted at the
top of the units, with the subs multiple cabled back to the open type
MDF.
For
junction working, group selector contacts 1-5 on levels 7-9, and all contacts on
level 0, are connected to the linefinder unit junction connector tags.
The group selectors are designed in such a way that only one junction
route can be accommodated on each level, with the parent exchange on level 0.
The junctions can be arranged for bothways working, or incoming only. Each
junction is associated with a junction relay set, and is connected to the
linefinder banks via a subs line cct (therefore each junction also needs a subs
line cct to allow incoming calls, and for the purpose of guarding the junction
during the interval which occurs between the replacement of the receiver and the
removal of the plug at the manual exchange). The total number of junctions
outlets available is 25, but if the exchange has only the PBX final unit the
number is restricted to 12 (the amount of relay sets that can be accommodated in
one final unit is 12), 14 becoming available on the addition of the regular
final unit. (The ‘spare’ relay set being used when more than the normal amount
of junctions is required)
Coin box
lines use prepayment boxes, and local calls are completed automatically. When a
junction call is made a discriminatory tone signal is received on the depression
of a key associated with the junction at the manual
exchange.
As normal
a fault test number was provided, and if there are no faults inverted ringing
tone is returned. If NU tone is heard, it indicates that either a fuse has
blown, a selector has failed to restore to normal’ or the charging plant has
failed. No tones are heard if the ringing equipment has
failed.