TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas
TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas
TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas
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here that from SC bank contacts 29 to 39, Relay IS is held by direct positive battery<br />
connections to these bank contacts. SC bank contacts 41, 43 and 45 are also connected<br />
direct to positive, whilst SC bank contacts 40, 42 and 44 are connected to<br />
positive via TS2.<br />
It will be remembered that line switch SC stepped under the control of Relay<br />
SD. When it has stepped to SC bank contact 49, Relay LC will be released at SC<br />
wipers 3/4. LC4 releases Relay BR, which at BR2 and BR4 disconnects the impulsing<br />
circuit and removes the battery from the lines. LC3 completes the driving circuit for<br />
stepping the line switches SA and SB to normal from 50 volt negative through the<br />
operating magnets, self drive contacts, wiper arms SA1 and SBI, G4, LC3, to 50 volt<br />
positive. When the wiper arms reach the normal position, these driving circuits are<br />
broken.<br />
Line switch SC continues to drive under the control of Relay SD (until it<br />
reaches its home position) from 50 volt negative, SC driving magnet SD, wiper arms<br />
SC 1/2. At the home position, Relay SD releases since there is no 50 volt positive<br />
through LC2.<br />
At the conclusion of the conversation the Controller restores his speak key<br />
which releases relays BC, BCA and the circuit is ready for further calls.<br />
In order to avoid false operation of relay TC (when BC releases) due to the<br />
charging of the line capacity, the auxiliary relay BCA is introduced which follows BC<br />
but is slow to release. After the release of BC and during the release time of BCA<br />
(approximately ¼ second) the line is charged to the normal voltage and polarity over<br />
contacts BCA 2 and 3, and resistor YR. When BCA finally releases the resistor YR is<br />
replaced by the windings of the relay TC but by this time the charging current to the<br />
line will have ceased or dropped to such a small value that it cannot give a false<br />
operation of TC relay.<br />
It should be noted that at the instant when BC releases the line may be completely<br />
discharged (as it would be at the end of a long call) or it may be holding a<br />
charge which has been imparted by the signalling battery (as it would be if the call is<br />
released immediately after signalling). In either of these cases the device described<br />
will safeguard the TC relay from false operation.<br />
General Call<br />
No dialling is necessary with a "general call." A "general call" is given by sending<br />
out one continuous train of 17 impulses, thereby operating all telephone selectors to<br />
the ringing position past all code pins. To make a "general call," the Controller<br />
operates the <strong>Circuit</strong> Key to the "Speak" position, and then the "General Call" Key.<br />
This latter key operates Relay GCA via 50 volt positive, "General Call" key, "Speak"<br />
key, Terminal LR, Relay GCA to 50 volt negative, and also Relay GC via 50 volt<br />
positive. "General Call" key, "Speak" key, Terminal GC, Relay GC to 50 volt negative.<br />
Relay GC locks over its second winding via 50 volt negative, Relay GC, GC1,<br />
Line switch SC bank 3 and 4 to 50 volt positive. A circuit to operate Relay LC is set<br />
up via 50 volt negative, Relay LC, GC7, line switch SC banks 3/4, to 50 volt positive.<br />
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