1,091,576. Substation circuits. INTERNATIONAL STANDARD ELECTRIC CORPORATION. July 9, 1965 [July 10, 1964], No. 29207/65. Heading H4K. In a V.F. key sender utilizing a transistor oscillator, V.F. signals for e.g. signalling an operator's position, may be sent during a call by depressing a key which shunts the loop with a high resistance and then after a predetermined delay connects the oscillator to the loop. The delay ensures that a register is connected up in order to receive these signals. Whenever a digit key T1 ... 0 is depressed, switch Tg connects the line a, b to the oscillator T, L, C, and disconnects the speech circuits SpK. During a call capacitor C3 is charged by line current and T is non-conducting. If the operator recall button RT1, RT2 is operated, the loop is effectively opened due to the very high shunting resistance of R4 and capacitor C3 discharges, thereby preventing T from conducting until the discharge is complete. Normally, one of buttons T1 ... 0 is operated with RT1, RT2 so that a V.F. signal is sent as soon as T conducts. Alternatively (Fig. 2, not shown), simultaneous depression of two of the digit keys may be effective automatically for closing RT1 and RT2.
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