TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Cycle of Events on Receiving a Code
Taking the same code as used in the description of the 4001-A Selector Key,
viz.: 3-11-3, the sequence of events is as follows :
On operating the selector key the battery relay of the system is operated,
connecting the line battery to the line, and remaining operated during the complete
revolution of the key. The initial operation of the battery relay sends an isolated
impulse (not included in the 17 coding impulses) which energises all selectors on the
line giving a flick to the holding spring, thereby releasing any selector which might
have been held on a coding pin. The first-set of impulses from the calling key are
reproduced by the impulsing relay which gives three impulses of alternating polarity
to the line. These three impulses advance all the selectors three steps. Then, although
the battery is held on the line the condenser in series with each selector (see Fig. 1)
stops the flow of current and the armatures of all selectors release. The code wheels
of all selectors, then return to normal except those coded with three as the first digit,
these are held in position by the holding spring K (Fig. 10). All selectors are then
advanced by the second set of impulses, 11 in this case.
Of the selectors that were held in an advanced position before, only the one
called will now have a code pin in the second pin position to be engaged by the
holding spring, because there would be only one station on the line with the first digit
of three and second digit of eleven. The selectors that returned to normal after the first
digit and which have a first digit of 11 will be in position to be held by the holding
spring. All other selector code wheels will restore to normal during the pause of
approximately one second after the second digit.
A total of 14 impulses have now been received and the selector under consideration
is therefore held at the 14th step. The third set of impulses, three in number,
advances all selectors three steps.
Only the selector that had its code pins in the code wheel set for 3-11-3 (the
selector called) will have reached the permanent code pin or ringing position, that is,
a total of 17 steps from the normal position. In this position the contact spring on the
top of the code wheel will be directly over the first ringing terminal and making
contact with it. Some of the selectors may be held on the first code pin, but will not
have advanced to the third or ringing code pin. It will be seen therefore that all
selectors respond to the impulsing but only the one coded to correspond with the
code transmitted reaches the ringing position. During the ringing period of approximately
two seconds, a ring-back tone is heard by the Controller or other receiver
across the line while the call bell is ringing.
Referring to Fig. 1, the ring-back circuit is as follows: L1, .02 μF capacitor
through the selector contacts, ringing battery, bell contacts, .02 μF capacitor, L2,
controller's telephone and back to L1.
Page 27