TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 46
The operation of Relay LC completes a circuit for the driving of line switch SC as
described above for an ordinary call, and also operates Relay BR. Line switch SC will
drive, and its wipers SC5/6, and SC7/8, will control the operation of Relay IS. Relay
IS controls Relay SR as before, the latter sending impulses to line. From line switch
bank contacts 5 to 22, Relay IS will be operated on odd bank contact numbers and
released on the even bank contact numbers causing Relay SR to send 17 reverse
battery impulses to line. All selectors will be stepped to the ringing position and the
bells will continue to ring until the Controller restores the “General Call” key. Actually
line switch SC steps on to SC bank contact 40 when its drive circuit is broken at
SC1/2 bank contact 40 through GCA1 being operated. It should be noted that GC4,
GC5 and GC6 contacts connect 50 volt positive battery to line switch SC bank
contacts 22, 24 and 26 to maintain the circuit for Relay IS, whilst GC3 provides 50
volt positive to SC bank contacts 40, 42 and 44 for the same reason.
When the “General Call” key is released Relay GCA is released, and at GCAI
restores the drive circuit for line switch SC which again drives until it reaches its home
position. When line switch SC wipers 5/6 reach SC bank contact 46 Relay IS releases
and causes Relay SR to release, thereby sending a final impulse to line to clear down
the selectors. When it reaches SC bank contact 49, Relays LC and GC are released at
SC wipers 3/4. When line switch SC reaches its home position, its drive circuit being
broken at wipers SCI /2, as already described.
Prolonged Ringing
If it is desired to prolong the ringing on a normal call, the Controller will operate
the “Long ring” key before dialling. This will cause Relay GCA to operate, and at
GCAI remove the 50 volt positive from line switch SC1/2 bank contact 40. Line
switch SC will step, as before described, until it reaches this contact, when it will
stop because the drive circuit is broken. Relay IS will remain operated and relay SR
will continue to apply battery to line. When a sufficiently long ring has been given,
the Controller will restore the “Long ring” key, which will release Relay GCA. On the
fall back of GCA1, 50 volt positive will be applied to line switch SC 1/2 bank contact
40 and restore the driving circuit, allowing line switch SC to drive home to its normal
position as before. Relay LC is released when SC wipers 3/4 reach SC bank contact
49.
Time Sending
To send a time signal, the Controller operates the circuit Speak key and a Time
Sending key, which completes a circuit for Relay TS. Relay TS operates and locks
over its own contact TS1 and Line switch SC3/4 bank contacts. Relay TS at TS3
completes the circuit for Relay LC and TS2 connects positive to Line switch SC bank
5/6 contact 28. The operation of Relay LC causes Line switch SC to step in the
manner described for an ordinary call. Relay IS will operate and release on alternate
steps of Line switch SC from SC bank contacts 5 to 27, thus sending 23 reverse
impulses to line via Relay SR contacts. This will step all selectors to the time signal
position. Relay GCA is also operated by the time sending key, removing the positive