TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

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One segment is flat and in close contact with the impulse wheel and keeps K1
and K2 closed, whilst the other has a bent up portion which clears the wheel by
approximately ¼ inch and engages the insulated piece on K2, thus raising the spring
sufficiently to keep the contacts open during this period. The setting of the segments
is done by counting the first number of the code from the first tooth (remembering
that the top of a tooth is one impulse and the space between the teeth is also one
impulse) in a clockwise direction and the last number of the code from the ringing
position in a counter-clockwise direction. As the total number of impulses is 17 for all
codes, it follows that if the first and last digits are correct the space between the two
segments must give a number of impulses equal to the middle number of the code.
During the inter-digital pauses the condition of K1, K2 contacts (open or closed)
must be maintained, therefore, if the digit ends on a tooth use a flat coding segment,
if in a space use a bent-up segment. Thus, two of either style or one of each will be
required for each key as indicated in the following table.
Referring to Fig. 7, the key is shown set for the code 3-11-3 using a flat and a
bent up segment. When the impulse wheel rotates K1 makes contact with K3,
through the impulse wheel, for a comparatively long period before impulsing. This
gives a wide margin for the operation of the battery relay before the impulsing period
commences. The initial operation of K1, K3 connects battery to the line (over the
battery relay contacts) and transmits an impulse. This isolated impulse advances all
selectors one step which clears them from any code pins, on which they may have
held during a previous transmission and prepares them for the code about to be sent.
K1 then moves to the top of the first tooth which closes K1, K2 K1 still in contact
with K3) thus operating the battery reversing relay and transmitting the first impulse
of the code. K1 then drops into the space between the first and second teeth, opening
K1 and K2 contacts, so releasing the reversing relay, thus transmitting the second
impulse. (This procedure is repeated for as many impulses as there are in the digit).
The third and last impulse of the first digit is made by a closure of K1, K2 contacts,
therefore they must remain closed during the inter-digital pause, so a flat segment is
used as shown. Counting counter-clockwise from the ringing position, for the third
digit, we find that it starts on the top of a tooth which indicates that the second digit
ended in a space, we therefore use a bent up segment in the position shown. With the
two segments in position the second digit automatically becomes 11 because 17 is the
total impulses.