Patent GB-694092-A (GB)
Title: Electrical switching system
Inventor: FLOWERS THOMAS HAROLD
IPC: H04Q11/04
Filed: 1947-10-27
Abstract: 694,092. Automatic exchange systems. POSTMASTER GENERAL. Oct. 21, 1948 [Oct. 27, 1947], No. 28657/47. Class 40 (iv). In an automatic switching system in which a plurality of switching circuits is provided to inject signals into a common channel on a pulse multiplex basis, incoming signals determining the pulse position to be used in the multiplex cycle are applied via a gate circuit, in common to all the switching circuits to initiate the operation in a marked one of said circuits of a pulse generator which thereupon produces a continuous train of pulses in the appropriate pulse position, these pulses being then fed back via a common channel to the gate circuit which thereupon suppresses incoming pulses in this pulse position to prevent them actuating pulse generators in further switching circuits. Two valve distributors 1 ... 10, 11 ... 20, (Fig. 3a), have their valves cyclically and synchronously enabled by pulses PI... P10, (Fig. 4), from the pulse generator PG, the first distributer passing incoming and the second passing outgoing signals, each valve pair such as 1, 11 being connected to the two wire line T over a hybrid transformer T1. On closure of a calling subscriber's loop +ve potential is applied to the corresponding incoming distributer valve which thus feeds - ve pulses to valve 31 at that subscriber's allotted instant in the multiplex cycle which are repated as +ve pulses in the C and G leads. One finder (Fig. 3c) only is available at any one time, being marked by earth over wiper X of switch SA, which enables the valve 40 to respond to the first pulse over the C-wire. A - ve pulse is thereby fed to valve 46 which cuts off causing 47 to conduct and allowing network W4 ... 7 to initiate a high frequency current pulse from a source CG, this pulse being terminated by a K2 pulse, timed shortly before the instant allotted to the next channel, which restores the valves 46, 47 to normal. A portion of this pulse is rectified at D9 in a circuit having a long time constant the resulting output being applied to disable valve 40, thereby busying the line-finder against seizure by another subscriber. The remainder of the high frequency current passes through a delay line D.L. and emerges shortly before the next instant allotted to the calling subscriber when it is used to gate at valve 45 a K1 pulse (see Fig. 4), which is then fed to valve 46, so causing the pulse cycle to be repeated and a regularly recurring circulating pulse to be produced at the instant allotted to the calling subscriber in the multiplex cycle. This pulse is applied to the calling subscriber's input and output gates 41, 43 and 42, 44. The rise in anode potential of the valve 51 which is cut off initially by negative potential from D9 applied over a time constant circuit C44, R54 and held cut off by rectified calling subscriber's pulses on the G-wire, causes valve 53 to bring up relay E and step the marking switch SA; prepares the called subscriber's output gate 420,440; and causes 54 to conduct, thereby enabling valve 401. Rectified pulses from valve 41 release the normally operated relay A to bring up B. CO comes up and prevents extension of the next line finder marking earth until after the digit is dialled. Cessation of pulses at each of the dial breaks re-operates A to set the switch DS, dialling relay C and its slave CC releasing at the end of the train. During the release period of CC a marking applied over bank DS2 enables one of the valves 21 ... 30 so that it is caused to conduct by one of the P1 ... P10 pulses and produce an anode pulse at the instant allotted to the called subscriber. This pulse, applied over SM to valve 401, initiates a second regularly recurring pulse in a selector circuit (Fig. 3d) similar to that in the line finder circuit. This pulse is applied to the called subscriber's input and output gates 410, 430 and 420, 440, and pulses are fed over R to cause the appropriate distributer valve of the set 11 ... 20 to conduct, the pulses so produced being rectified by D1 to release the normally operated relay S to give a calling signal over wires L. When the called subscriber answers, his pulses over the G wire are accepted by 410 which cuts off 510 thereby enabling the calling subscriber's output gate 42 so that his distributer valve may be energized and his relay S released to give an answering signal. High frequency pulses from both circulating systems are injected into wires PS and rectified by D2, D3 to provide a blocking pulse for valves 32, 39 to suppress further pulses on C and SM at these times, thereby preventing further line-finders and selectors being seized by this call. Conversation takes place by amplitude modulation of the incoming pulses which pass via the gate 41, 43 (or 410, 430) to a demodulating network (such as 400), the resulting current amplitude modulating the outgoing pulses at gate 420, 440 (or 42, 44) which pulses are then demodulated by network 112<SP>1</SP> and thence transmitted to the subscriber's line. When the calling subscriber hangs up, pulses on wire G cease so that when C44 charges up valve 51 conducts to apply a long negative pulse to valves 47, 470 to interrupt the pulse circulation in their associated circuits.