Issue 1, 30.8.38
PRIVATE MANUAL BRANCH EXCHANGES
Details of Cordless Switchboards
This Instruction describes the details of, and the facilities provided by "
Switchboards, CB 935, CBS 364 and CBS 484." These switchboards are of the
cordless, table type and are issued in four sizes, see Table 2. They are for use
on the systems shown in Table 1.
||System on which used
||C.B. (manual) or Automatic
||C.B.S. No. 1
||C.B.S. Nos. 2 and 3
2. Equipment, Capacity, Dimensions and Weight
Table 2 gives the capacity and equipment of the four sizes of switchboards and
Table 3 their dimensions and weights.
The wiring is complete for the capacities quoted in Table 2.
EQUIPMENT AND CAPACITY OF CORDLESS SWITCHBOARDS
|Size (all types)
||Conn ccts Equipment
|1 + 3 (4)
|2 + 4 (6)
|3 + 7 (12)
|3 + 9 (12)
NOTE On the 3+7 (12) switchboards, plain face-plates on wooden dummy
indicators are fitted on extension positions 8 and 9, and key spaces - of the
same dimensions as the other keys - are fitted in the corresponding positions on
the key panel.
|Switchboard CBS 484 1+3 (4)
||Switchboard CBS 364 2+4 (6)
|Switchboard CB 935 3+7 (12)
5. Transmission data
The maximum permissible resistance of exchange line, plus extension, is given in
the following E.I.s :-
TRANSMISSION, Telephone, B 3503-C.B. manual and automatic areas
TRANSMISSION, Telephone, B 3505-L.B. telephones in C.B. (manual) and automatic
areas TRANSMISSION, Telephone, B 3550/1-L.B. exchange areas
TRANSMISSION, Telephone, B 3590-Interswitchboard extensions.
6. Design and equipment
These switchboards are designed to stand on a table or shelf. The front of the
board is hinged at the bottom and the to removable, to afford access to the
interior of the board. When the front of the board is lowered for inspection,
the weight of the front panel is taken by strips of webbing fixed to the sides
of the boards. The handle of the generator projects on the right hand side, and
the wiring is led into the switchboard through a hole in the left-hand
7. Night alarm
A buzzer is included in the equipment of the switchboard, to indicate when
either a calling or clearing signal is received.
8. Night service
On certain of the connecting circuits, night-switching keys are provided, the
operation of which enables the corresponding connecting circuit to be used for
connecting any extension to any exchange line, for night service. The number of
night-switching keys is limited to the exchange line capacity of the board. The
operation of any night-switching key will disconnect the night alarm from all
the exchange-line indicators, , and also the supervisory relay on the connecting
Ringing current for local circuits is provided by a hand generator fitted in the
10. Operator's telephone
A standard instrument without bell-set, as appropriate for direct exchange lines
in the same area, is used for operating the switchboard.
11. Designation strip
A transparent-fronted designation strip is fitted between the indicators and
keys. The circuit designations are marked on a paper strip placed under the
12. Termination of exchange and extension lines on switchboards
(a) Exchange lines
These are terminated on drop-type indicators, which are operated by
ringing current and must be restored by hand. (The indicators fitted in the
latest issues of these switchboards are "Indicators No. 3700A," which
are not operated by dial impulses, as were "Indicators No. 2200A"
previously used, see B 5901 and B 5903.) Each exchange line is connected to keys
arranged in a column vertically below the exchange-line indicators. The
lowermost key in each column is used as a HOLD key, except on "Switchboards, CBS 364" on which a HOLD key is unnecessary, see para. 14.
These are terminated on doll's eye indicators, which restore automatically when
the extension clears. Each extension is connected to keys arranged in a column
beneath the indicator similarly to the exchange lines-the lowermost key is used
as a RING key and is individual to its associated extension.
(c) Operating telephone
This is similarly connected to keys situated on the right-hand side of the
13. Connecting circuits
The keys in the other rows are commoned horizontally to form connecting
circuits, which are separate for each row, consequently the movement in the same
direction of any two keys in the same row connects together the two lines
concerned. The keys lock in all positions, with the exception of the RING
positions on the lower half of the bottom row of extension-line keys.
A special holding key or circuit is not provided on "Switchboard, CBS
364", as the operation of any exchange-line key disconnects the clearing
earth from the exchange line. Any exchange line can, therefore, be held when
required, by throwing the corresponding exchange-line key of a disengaged
15. Supervisory signals
The switchboards are of the positive clearing type, the extension calling
signals being also used as supervisory signals when an extension is connected.
Through-clearing on exchange connexions is provided; on an exchange-to-extension
connexion, the replacement of the extension receiver causes the clearing signal
to be given on the extension indicator at the P.B.X. and, simultaneously, at the
main exchange. Individual supervision is not provided on local connexions; a
double-clear is given when the receiver is replaced at one of the extensions on
"Switchboard, CBS 364," while on "Switchboards, CB 935 or CBS
484" clearing signals are not given until the receivers are replaced at
In C.B.S. No. 1 areas, if the main-exchange operator clears the connexion before
it is cleared at the P.B.X., the clearing signal at the P.B.X. is released. This
is a recognised disability and any case in which complaint is made by a
subscriber, should be referred to the E.-in-C. (S1), when suitable instructions
will be issued.
On "Switchboards, CB 935 and CBS 484," the HOLD key may be kept in
the operated position to enable the extension user to "call-in" the
P.B.X. operator during the progress of an exchange call without breaking down
the exchange connexion, The subscriber's attention, however, should not be drawn
to this facility, unless "calling-in" facilities have been
18. Prohibition of exchange service on private wires
The circuit arrangements for a private wire prevent connexion to an exchange
19. Power supply data
On Switchboards, CBS 364 current is not drawn from the P.B.X. bus-bars during
conversation, but only when a calling or clearing signal is given. On
"Switchboards, CB 935," however, current is drawn during conversation
on local connexions, as well as when calling or clearing signals are given, but
not during conversation on exchange connexions. On "Switchboards, CBS
484," current is drawn during conversation on both exchange and local
connexions, but this current is considerably less than that drawn when calling
or clearing signal are given.
The CB935 (1+3) is made by British Ericsson - their model N105.