TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 40
The total number of code settings which are applicable is 66, as shown in Code Table
No. 1, page 25. The possibility of requiring more than 66 selectors on one line is
remote but should it be required a system can be supplied catering for the larger
number of code settings by increasing the total number of steps in the code.
Three line switches are incorporated in each selective circuit, SA for registering
the first digit, SB for registering the third digit, and SC for controlling the impulses
sent to line. Several control positions may be multiplied together each using the same
selective apparatus. Where a controller is required to handle several lines a method of
indicating incoming calls is necessary. This is effected either by providing magneto
generators at way stations or by a central battery system with loop calling.
Facilities for general call and time sending are available controlled by individual
lever type keys.
A prolonged ring can be given on any call, so that, instead of the usual short ring
of two or three seconds duration, ringing will be continued until the controller restores
the long ring key.
The Schematic Figure 13 shows the supply for calling lamps designated CBX-
CNX and that for the busy lamps BBX-BNX. These supplies may be A.C. or D.C.
and of a value to suit local conditions. It is usual to maintain the calling lamps from a
battery to cover the possibility of mains failure whereas the busy lamps can be fed
from A.C. only in order to minimise battery drain when the main supply and presumably
charging supply have failed.
Incoming Calls
When a way station makes a call by operating his call key, a loop is connected
across the lines. This operates relay TC, Fig. 13, as follows :- Line 1, BR3, BC2,
BCA3, TC winding d-e, to 50 volts negative. Line 2, BR1, BC4, BCA2, TC winding
b-a, to 50 volts positive.
Relay TC is locked over 50 volts negative, TC winding e-d, BCA3, BC2, TC1,
YL resistor, TC2, BC4, BCA2, TC winding b-a, to 50 volts positive. The controller's
call lamp is illuminated from CBX, TC3, call lamp TCL to CNX and this lamp
remains alight until the controller operates his speak key to accept the call. Operating
the speak key operates relay BC via 50 volts positive, speak key, BC winding to 50
volts negative. BC operating operates BCA and breaks the circuit for TC which
releases.
The release of TC extinguishes the call lamp TCL and the operation of BCA
lights all busy lamps BL via BBX, BCAL BL and BNX. The busy lamp LP on the
relay panel (mounted on the apparatus rack) associated with the speak key operated
is also illuminated indicating to the maintenance staff that that particular circuit is in
use.