SWITCHBOARD
CB 935, CBS 364 & CBS 484

ENGINEERING INSTRUCTIONS
TELEPHONES
P.B.X.s
B 1050
Issue 1, 30.8.38

PRIVATE MANUAL BRANCH EXCHANGES
Details of Cordless Switchboards

1. General
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.

TABLE 1

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.

3. Wiring
The wiring is complete for the capacities quoted in Table 2.

TABLE 2
EQUIPMENT AND CAPACITY OF CORDLESS SWITCHBOARDS

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 side. 

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 circuit concerned.

9. Ringing
Ringing current for local circuits is provided by a hand generator fitted in the switchboard.

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 transparent strip.

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.

(b) Extensions
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 switchboard.

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.

14. Hold
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 connecting circuit.

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 both extensions.

16. Clearing
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.

17. Calling-in
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 specifically demanded.

18. Prohibition of exchange service on private wires
The circuit arrangements for a private wire prevent connexion to an exchange line.

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.

Last revised: December 20, 2003

FM