Patent GB-913617-A (GB)
Title: Automatic telephone exchanges
Inventor: CROWTHER GERALD OFFLEY
IPC: H04Q3/52
Filed: 1960-09-29
Abstract: 913,617. Automatic exchange systems. MULLARD Ltd. Sept. 29, 1960, No. 33459/60. Class 40 (4). In an automatic exchange in which connections are established over cold-cathode tube matrices, a counting chain of cold-cathode tubes which counts the dial pulses passes a pulse by way of a delay stage to a busy-tone generator, thus causing operation of the generator after a time delay. In the ten-line exchange described three link circuits, termed the X, Y and Z channels (Fig. 1), are provided, each having a control circuit CS, and each subscriber's exchange equipment (Fig. 7) includes three tubes such as X1, Y1, Z1 associated with the three channels, respectively, the cathodes of which are commoned, as at K1. On the initiation of a call by a subscriber the closure of his loop lowers the potential of the speech tube cathodes. The right-hand triggers (Fig. 1) of the tubes of channels not in use have a positive bias applied thereto cyclically via the points C, and the coincidence of this bias and the lowered cathode potential fires one tube, say X1. The resulting negative-going voltage step at point A of channel X causes the negative-going pulse gate NG (Fig. 3) of the control circuit CSX (Fig. 8) to open, together with the negative-going direct voltage gate DG. The latter passes a pulse to the channel lock-out stage LO which removes the positive bias normally applied via point C to the right-hand triggers of the tubes X1, X2 ... , thus busying the X channel. The  gate NG applies a positive-going pulse to the trigger electrode of a tube in the counter primer stage CP which fires ; and DG supplies a delayed negative-going step to the same trigger electrode to disable subsequent pulses from NG. Operation of the stage CP prepares the counter chain CC for operation. When the caller dials the wanted number a train of positive-going pulses appears at point A (Figs. 1 and 8) the first pulse passed by the positive-going pulse gate PG (Fig. 4) and pulse-shaping stage PS (Fig. 8) fires the first tube of the counter CC and douses that of CP. Succeeding pulses fire the counter tubes in succession, the relevant speech tube being fired at the end of the train. The first pulse is also passed by the counter chain CC to a release bias stage RB (Fig. 9) in which it fires TRB, thus priming a tube (not shown) in a counter release stage CR (Fig. 8). On firing of the speech tube the negative-going pulse produced at point A passes gate NG to the trigger of the tube in stage CR which then fires and releases the counter chain CC, dousing the fired tube therein. In the called line circuit (Fig. 7) the rise in potential at the junction of R8 and U1, which occurs on firing of the speech tube, charges capacitor C8 so that TJ conducts on the positive half-cycles of ringing tone which are thus applied to the called party's bell. On the reply of the called party negative battery round the loop prevents further conduction of TJ. Called line busy.-The positive-going output from TRB (Fig. 9) is applied via a busy tone delay network BD to the trigger of TBT in the busy tone control stage BT, the delay of the network being greater than the maximum dialling period. Tube TBT will thus be fired and busy tone from transformer S will be applied to point B (Fig. 1) after a short interval if the wanted line is busy. Firing of TBT also releases the counter chain. If the called line is free, firing of the CR stage tube in response to that of a speech tube douses tube TRB before busy tone is reverted. Belease.-On, the replacement of each handset a positive-going step is produced at point A. These pass via gate PG to the pulse delay circuit PD and main release circuit MR (Fig. 8), a tube in the latter is fired and the resulting negative-going step is applied to point B to douse the speech tubes. All links busy.-Rectifier Q5 (Fig. 7) conducts to the negative potential round the calling loop and exchange busy tone is reverted to the caller. Specifications 913,618 and 913,619 are referred to.