Patent GB-707504-A (GB)
Title: Improvements in or relating to electronic dialling circuits
IPC: H04Q3/52
Filed: 1952-05-20
Abstract: 707,504. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. May 20, 1952 [May 23, 1951], No. 12722/52. Class 40 (4) In a system in which connections are established over gas-discharge tubes, a counting chain responds to the dialled impulses and in each counting position applies a firing potential to the tube of the line associated with the corresponding digit through an integrating circuit which is such that firing does not occur while the chain is being stepped, but does so only at the end of the impulse train. Specification 699,941 is referred to.  Initiation of call.-When the subscriber at station AA lifts his handset, closure of the line loop results in a strong negative pulse at point A, which pulse is applied to all the tubes such as BA leading to the connecting circuits such as VC. One such tube leading to a free connecting circuit fires, thus increasing the potential of point A and reducing that of point Q. The former prevents other tubes BA of the calling line from firing, and the latter reduces the potential of the firing electrodes #<SP>1</SP> . of the tubes connecting other lines to the seized connecting circuit. Digit dialled.-As the impulse contacts of the dial (not shown) are shunted by an impedance, the loop impedance is increased for the duration of each " break" so that a positive pulse occurs at A at the beginning of each " break " and a negative pulse at the end, these being of relatively small amplitude. These pulses are transferred via C9 to the grid of tube B 1 , and are repeated with opposite polarity to point U. The negative pulses are without effect whereas the positive ones each fire B 4  but not B 5 , which requires pulses of larger amplitude. The tubes B 2 , B 3 , B 4  ,B 5  all have impedances in their cathode circuits which render them unstable so that any tube fired by a pulse is doused when the pulse ends. Each time B 4  fires, a positive pulse is applied to tubes B 11 , B 12 , . . . , B 1 n which constitute a conventional counter. Normally all tubes of the counter are non-conducting, B 11  being primed by voltage divider R 31 , R 32 . The first pulse therefore fires B 11  ; the potential of point W rises and that of the cathode of B 11  rises also due to the drop in R 11 , so that B 12  is primed. Thus successive pulses fire B 11 , B 12 , B 13 , ... , each, on firing, dousing the preceding tube. Assuming that the digit " 3 " is dialled, B 13  will be conducting at the end of the train. The firing electrodes # 2  of the tubes BA, BB, BC and the like are each connected via an integrating network such as R  2 a, R  3 a, C 2 a, C  3 a to the cathode of the tube B 11 , .. corresponding to the number of the line with which it is associated. This network prevents the tube such as BA from firing during the reception of an impulse train, but when the train ends the relevant counting tube remains conducting and the firing electrode of the corresponding tube (in this case BB) is raised to the firing potential. Called line busy.-One of the other tubes (not shown) connected to point B will be conducting and the consequent rise in the potential of point B prevents the tube BB from firing. With a tube of the counter conducting, rectifier g 1  is conductive and busy tone is reverted over transformer TG to the caller. The caller hangs up and a strong positive pulse is applied to the cathode of BA. This may douse BA immediately, but even if it does not, a strong positive pulse is received at point Q which is repeated with reversed sign to point U, and thence via rectifier g, to B 3  . This strong negative pulse fires B  3  followed by B 2  which sends a strong negative pulse to the anode of BA to douse it if it is still conducting. Firing of B 3  also sends a strong positive pulse via g 2  to fire B 4  and B 5 , the latter with delay due to the presence of R 5 ,  C 4 . Firing of B 4  steps the counter on one, and that of B 5  produces a strong positive pulse at point W which douses the fired counting tube, whereupon the counter is restored to its normal condition. Called line idle.-Tube BB fires so that the potential of point Q decreases further and tube B 1  transfers a positive pulse to point U thus firing B 4  and B 5 . The counter is thus stepped on and then restored to normal. Rectifier g 1  is cut off, since point W is then at earth potential, and busy tone is not reverted. On the reply of the.called party, a negative pulse occurs at point Q which is repeated by B 1  to point U as a positive pulse. This. fires B 4  and B 5  whereupon B 11  fires and is immediately doused. First party release is provided, clearing by either party producing a strong positive pulse at Q to initiate release.