Patent GB-710071-A (GB)
Title: Electronic switching system
IPC: H03K19/12
Filed: 1948-12-10
Abstract: 710,071. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Dec. 10, 1948 [May 15, 1948], No. 31957/48. Class 40 (4) In an exchange system of the pulse multiplex type a common network of n (14 or 16 as described) conductors is provided over which communication signals are transmitted as modulation on pulses carried by various combinations of the n conductors, each combination being indicative of a particular subscriber, whose line circuit is arranged to respond only to that combination. General description.-The 4-digit exchange Fig. 1 comprises a group of primary networks 114 . . . 121 and a secondary network 132, 133, 135, 136 the latter busbars of which are interconnected (132 to 135 and 133 to 136) and may also be connected to a distant exchange. In a primary network each subscriber 101 ... 106 is allotted a time channel in a distributer cycle determined by an individual tapping on the delay line 117. A calling subscriber's line circuit 107 ... 112 transmits pulses in busbar 118 which seize a line-finder 122 ... 124 and also a register sender 128 or 129. The dial impulses, transmitted as modulations on the primary pulse train, set the register sender which thereupon translates the digits into a binary code and transmits them in the form of pulse or no pulse over 16 (or 14) busbars to set one register in the selector. The code of the calling subscriber is also transmitted over busbars 119 to set another register in the selector. When the selector is set it emits a train of pulses which suppresses the input to the sender so that the latter may be released for further use. The called subscriber's code is transferred, in the selector, a time channel seized in the secondary network and passes over busbars 132, 135 to the decoder driver 137 which translates the codes into a form which can be accepted by the subscriber's line circuit. This code consists of pulses on a combination of 16 (or 14) bars out of a total of 32 (or 28). The called line circuit emits ringing current which is tripped in that circuit by the subscriber's reply. The calling subscriber's speech fed into the selector over busbar 118 is carried by the pulses comprising the called subscriber's code and thence via the decoder driver to the subscriber's line circuit where it is gated in and then returned to an amplifier in the common " connector " 138, whence it returns to the line circuit for transmission to the called subscriber. The called subscriber's speech modulates pulses received from the connector 138 the modulated pulses being returned to the connector where they modulate the calling code which is fed to it by the selector over busbars 133 and 136 and which passes thence to busbars 132. In the connector the modulated calling code pulses are slightly delayed to avoid interference with the called subscriber's code which is also carried on busbar 132. The modulated calling code then passes via the decoder driver 137 to the subscriber's line circuits, the calling subscriber's line circuit receiving the pulses passing them for amplification to the connector 138 and then receiving them therefrom for transmission to the calling subscriber. Each line circuit is arranged to feed to a busy tone. connector 139 two sets of pulses one set for a conversation in progress and the other for all pulses intended for the line circuit. If then a call to a busy line circuit is attempted over a particular time channel, an unpaired series of pulses will pass to the busy tone connector which thereupon modulates the calling code in that time channel with busy tone. In the following description the first digit of each of reference numeral indicates the figure in which it is to be found. Operation of line-finder.-When the calling subscriber's loop is completed, the resulting positive potential appearing at point 210 fires the gas tube 223. The resulting bias in its cathode resistor applied to the left hand side of rectifier 214 overcomes the bias fed over resistance 215, thereby causing the rectifier to pass pulses received from the delay line 117 over lead 212 to lead 213 and thence over busbar 118 to the line-finders 122 ... 124. The pulse series is first fed to all line finders in turn over the leads 402, the last of these leads being connected to a common return start lead 404 which enables each of the line finders for seizure by the pulse series. When a linefinder seizes the pulse train its output over lead 402, and therefore over lead 404, is suppressed so that no other line finder can subsequently seize this pulse train. To ensure that a second pulse series does not seize the line finder the input from the common return start lead is blocked off after one distributer cycle, it being arranged to scan the line finders with a' 9 kc. pulse input so that they become available for seizure only when these pulses coincide with an incoming pulse train (whose frequency is 10 kc.). This ensures that in the event of two simultaneous calls at least 111 Á secs. must elapse between two such coincidences being presented to the circuit, by which time the circuit will have been so affected by the first coincidence as to be unaffected by the second. Thus the 9 kc. pulses and pulses from the' common return start lead 404 are applied to the coincidence circuit 503, 505 whose output is applied together with the input to a second coincidence circuit 507, 508. The resulting output is then fed through a delay line 509 whose delay is equal to the period of one distributer cycle, and whose output is applied via valve 510 to valve 507, to gate in the next pulse of the series and so on. The output from the coincidence circuit is fed out over a utilization lead 407 and also to an integrator circuit 515 which blocks the coincidence circuit 503, 505. The output from the delay line 509.is also applied via valve 511 to valve 501 to block the output to lead 402. Reception of digits.-The pulse train over busbar 118 is also used to seize a registersender 128 (or 129) via the sender pulse suppressor 130. As shown in Fig. 6, the input from the busbar 118 at terminal 600 in the pulse suppressor is fed over lead 602 to a series (two in this case) of finders similar to those described above. When the pulse series has been seized it is fed to valve 607A where it is modulated with dial tone and fed over busbar 120 to lead 243 of the subscriber's line circuits. In the calling line circuit the pulses are in synchronism with pulses fed from rectifier 214 over rectifier 240 and together they overcome the bias on rectifier 241 to feed the dial tone modulated pulses to filter 246, whence the dial tone is fed to the subscriber via the hybrid transformer 209. When the subscriber dials, the resulting drops in potential at point 210 are superimposed on the standing bias on the rectifier 214 so that the pulses on busbar 118 are modulated by the dial breaks. In the register sender, these pulses pass through filter 614 which removes the pulse component, and the dialling modulations are then differentiated, the resulting positive pips being inverted in valve 618 whereafter they actuate a flip-flop circuit 619,620 which times the delivery to condenser 625 of a measured charge, by valve  626, for each dialled impulse. The charge stored on the condenser 625 is thus proportional to the number of dialled impulses in the first digit. Valve 618 is enabled by the charging by the incoming impulses of condenser 612 whose time constant is such that the valve 618 is not enabled when it receives the first pip due to the initial reception of the pulse series. Valve 626 is enabled by bias provided by valve 627 which is controlled by gas tube 630 which is normally ignited. The output from valve 618 is also integrated at 635, differentiated at 636, and the pip thus produced at the end of each impulse train is used to transfer conduction successively to valves 631 ... 633, the preceding valves being extinguished. These valves enable successive circuits 622 ... 624 for registration of the subsequent digits on condensers therein similar to 625. During the dialling of the last digit the gas tube 633 cuts off valve 643 thereby permitting valve 644 to pass the pip from the differentiator 636, due to the end of the dial train, to valve 645 whereafter it is applied to valve 646 which in conjunction with the network 647 produces a pulse of length at least equal to the period of one cycle of the distributer. This pulse unblocks valve 648 to allow it to pass the next pulse of the incoming series via valve 649 to enable valves 650, 651 in each of the digit registers 621 ... 624, whereupon pulses of magnitude proportional to the charges on the condensers 625 are transmitted to a translator over leads 653. A pulse is also sent over lead 686 (with a very slight delay to ensure complete response of the selector )to identify the pulse series over which the called digits were received and so to ensure that only the selector associated with the seized line finder is set. The pulse from valve 648 is also applied after a slight delay to enable valves 628 in the digit registers to discharge the condensers 625 therein. Release of the register sender.-When the selector has been set it reverts a series of pulses, in synchronism with the calling line pulses, to the pulse suppressor 130 whereupon the latter pulses are suppressed in the lead 602 and the finder 603 releases, the register sender. In the latter, extinction of the last valve of the chain 630 ... 633 does not automatically trigger the first, but the bias on the first is such that it will fire when none of the others is drawing current through the common anode resistor 663. However to ensure that the valve reignites a positive pulse derived from valve 659 at the end of the dialling is applied with slight delay to its grid. In the case where the subscriber hangs up before completing dialling, the cessation of his pulse train causes the removal of a negative bias on valve 63