Patent GB-661182-A (GB)
Title: Pulse electronic switching systems
IPC: H04Q3/52
Filed: 1948-03-25
Abstract: 661,182. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. March 25, 1948 [March 26, 1947], No. 8756/48. [Class 40 (iv)] In an automatic exchange of the distributer multiplex type, electronic means are provided to render a busy subscriber inaccessible to a calling line and to revert busy tone thereto. Seizure and operation of line finder. Each subscriber is connected to a dynode 101 of the cathode-ray distributer 20 and when a subscriber Am lifts his handset the dynode is biassed so as to produce, at instants tm in the distributer cycle, pulses 71 on the cathode C, which are fed via valve 23 to the gate VG. The first pulse passes thence via the gate V2 to the circuit 300, which thereupon produces pulses 75 which block the gate V2 for the remainder of the cycle to busy the circuit 300 against other subscribers' pulses. The circuit 300 also produces pulses 78, 79 at succeeding instants tm, the former opening the gate V7 for the calling subscriber's pulses on wire 232 and the latter blocking VG to prevent seizure of subsequent line-finders by subscriber Am. Pulse 75 is also applied via valve V6 to open the gate V1 to pass calling pulses of other subscribers over wire 243 to subsequent line finders. The operation of the circuit 300 is maintained by the output pulses from V 7 . Selection. Dial tone is applied via the home contact of bank P<SP>1</SP>1 and V18 to gate V20 which is opened at instants tm by pulses derived from V7. Dial tone modulated pulses are thus passed over the grid of the distributer to the calling subscriber Am. Each dialled impulse causes the pulses 71 to increase in amplitude, the consequent lowering of anode potential of V7 operating through the low-pass filter 401 to cut off valve V11. Relay 402 in its anode circuit thus repeats the dialled digits successively to switches P1, P2, P3, after which relay 403 comes up. The banks P 1 iv, P 2 iv, P 3 iv of the digit switches connect up sections of delay lines LR1, LR2, LR3 in accordance with the dialled digit, the sections having respectively delays of 1 /10 , <SP>1</SP>/ 100 , <SP>1</SP>/ 1000  of the sweep time of the distributer, the total delay corresponding to the instant in the time cycle tn of the called subscriber An. Accurate synchronizing is effected by gating (e.g. at V13) the output of each delay line (e.g. LR1) by accurately timed pulses derived from the distributer sweep circuit 21. Called subscriber busy. Pulses at instants tn will be present on lead 244 either from the cathode of a valve such as V5 or from the delay lines such as LR1 ... 3 (via valve V9) according as the subscriber An is a calling or a called subscriber. Contact 432 of relay 403 operates late so that during a brief interval pulses from the delay lines and pulses from lead 244 are applied simultaneously to valve V8. The resulting anode pulses change over the valve V16b of the triggered pair V16a, V16b to the non-conducting state, thereby blocking valves V8 and V17b, so preventing communication in either direction between Am, An. Valve V17a is also unblocked to pass busy tone from generator 26 through the calling subscriber's output gate to the grid of the distributer. Application of ringing current and tone is prevented by the blocking of V22. When the calling subscriber hangs up pulses 76, which hitherto held condenser C 12 at a steady positive potential, cease, so producing a negative pulse on wire 36 to restore the pair V16a, V16b to normal. Absence of output pulses from V7 unblocks valve V 10, whereupon relay 404 operates to home the switches P1 ... P3. Called subscriber free. With 403 fully operated pulses at instants in obtained from the delay lines are repeated by valve V18b to unblock the called subscribers output gate V21. Ringing current from source 25 is applied through valve V22 to both grids of valve V18 which passes it to both calling and called subscriber's gates V20, V21 for application to the grid of the distributer as ringing tone and ringing current respectively. Pulses at instants tn are applied to lead 244 to block VG and so prevent the answering subscriber's pulses seizing another line-finder. When the called subscriber answers, his pulses 71 are applied to V8 simultaneously with pulses derived from the delay lines. The resulting drop in anode potential is used to block V 18, cutting off the ringing current and tone. Speech from Am modulates pulses at instants tm, these pulses passing through the input gate V7 to the demodulator F2, whose output modulates pulses at instants tn produced by the called subscriber's output gate for transmission to subscriber An. over the grid of the distributer. Speech from An likewise reaches Am through the input gate V8, demodulator F3 and the calling subscriber's output gate V20. Unwanted feedback between outgoing and incoming signals at the distributer is neutralized by feeding a portion of the outgoing signal on the grid 204 back in anti-phase to the incoming circuit over valve V23.