Patent GB-741306-A (GB)
Title: Improvements relating to automatic telephone systems
Inventor: WARMAN BLOOMFIELD JAMES
IPC: H04Q3/42
Filed: 1952-12-29
Abstract: 741,306. Automatic exchange systems. SIEMENS BROS., & CO., Ltd. Dec. 24, 1953 [Dec. 29, 1952], No. 32894/52. Class 40 (4). The initiation of a call is registered at the exchange by the reversion to an ineffective condition of an electronic device individual to the calling line. In the embodiment described, a line-finder common control circuit contains an impulse-counting chain of thermionic valves comprising one valve for each of a plurality of line circuits; this chain is normally in continuous cyclic operation, and the initiation of a call causes the valve corresponding to the calling line to fail to fire in its turn. A line-finder is then set to the calling line under the control of the counting chain which is stepped on by impulses reverted from the line finder until it registers a predetermined total. The drawings show a line circuit, Fig. 1, a line-finder, Fig. 2, a line-finder common control circuit, Figs. 3 and 4, and a substation circuit, Fig. 5. The switches used are high-speed motor-driven uniselectors controlled by latch magnets. At a substation, Fig. 5, two condensers QA and QB are normally kept charged over the line wires to the voltage of the negative battery connected to the negative line wire in the line circuit, Fig. 1. When the subscriber lifts his handset to make a call, the switch-hook contacts sw1 and sw2 are operated so that the charges on the condensers QA and QB assist the negative battery to fire a coldcathode gas-filled diode VV, initially making the negative line wire positive with respect to the positive line wire to the extent of twice the battery voltage. This fires a cold-cathode gasfilled triode VL in the line circuit. Condensers QA and QB are rapidly discharged and valve VV is extinguished. In the line-finder common control circuit, Figs. 3 and 4, a coupling switch having wipers FC1, . . . , FC6 is normally standing on an outlet to a free line-finder which is thereby preselected, relays TC and TS being energized. While relay TS is energized, a relaxation oscillator incorporating a cold-cathode gas-filled viode VT operates continuously, sending impulses to a closed chain of counting valves VCA, ... , VCZ which fire in turn, each valve in firing extinguishing its predecessor. Each counting valve is associated with a particular line circuit, and a priming relay PR is energized during normal operation of the counting chain. The output of the relaxation oscillator is connected to the control electrode of a valve VST through a delay network DN which is arranged to ensure that the impulse affects the control electrode of valve VST at the same time that it affects the cathode through the firing of a counting valve, so that valve VST does not fire. When a call is initiated, the firing of valve VL in the relevant line circuit causes a voltage drop across a resistor YD which reduces the anode voltage of the associated counting valve (VCZ as shown) to an ineffective value. This counting valve then fails to fire in its turn and the impulse from the relaxation oscillator fires valve VST, which brings up a relay ST. This relay completes the anode circuit of a valve VW, switches the input of the counting chain from the relaxation oscillator to an " impulse converter " comprising two hot-cathode gas-filled triodes VX and VY, connects the anodes of the counting valves to positive battery, and earths a start wire WS connected to a group ofregistercontrollers. A free register-controller is thereby engaged and brings about the operation of a relay X in a circuit over wiper FC5, removing the earth connection from wire WS. A line relay A (not shown) is also operated from the register-controller and completes a circuit over wiper FC4 for the latch magnet LM of the preselected line-finder which thereupon moves offnormal. The P bank contacts are connected to positive and negative battery alternately and the P wiper is connected over wiper FC2 to the input of the '' impulse converter " which isarranged to produce one positive impulse on its output wire WP (at present connected to the counting chain) for each change of polarity on its input. The counting chain is thus stepped on in accordance with the movement of the linefinder. The order of the valves in the counting chain is the reverse of that in which the linefinder reaches the outlets to the corresponding line circuits, so that when the last valve in the chain is fired the line-finder is on the outlet immediately preceding that to the calling line. At the next step the resulting impulse fires valve VW and brings up relay TA, which breaks the latch magnet circuit and stops the line-finder at the calling line. A relay R operates and extinguishes the line circuit valve VL by connecting earth over wipers FC1 and CO. Relay ST is released, followed by relays ST, TC, TS, TA, X and R. A circuit is completed for latch magnet CLM, and the coupling switch FC moves on to preselect the next idle line-finder, being stopped on the relevant outlet by the operation of relay TC to battery over wiper FC3. An offnormal line-finder or associated first group selector is marked busy by the operation of a relay LBR. If the coupling switch fails to find an idle line-finder, it is stopped in its last position by the operation of relay TC. If a line-finder reaches its last contacts, a relay TF operates to stop the switch and complete circuits for an alarm buzzer BZ and a lamp AL. If, on the other hand, the preselected line-finder fails to move from normal within a predetermined time after the operation of the start relay ST, a relay FS is energized when a high-capacity condenser has acquired a sufficient charge, and relay TA operates to initiate release of the common control circuit. Modified forms of line circuit, Figs. 6 and 7 (not shown), and line-finder, Fig. 8 (not shown), may be used with the normal substation circuit of the loop-calling type.