Patent GB-722714-A (GB)
Title: Improvements in electrical switching arrangements suitable for use in signalling systems
Inventor: DAIN GORDON TAYLOR
IPC: H04L12/00
Filed: 1952-11-20
Abstract: 722,714. Electric control systems. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. Nov. 20, 1952 [Nov. 28, 1951], No. 27942/51. Class 40 (1) [Also in Group XL (b)]  An electrical switching arrangement for effecting a switching operation when potential of one polarity which is normally steadily connected to each of a pair of signalling lines is replaced by the steady connection of potential of the opposite polarity to both lines, but not in response to changes in polarity on either or both lines of less than a predetermined time, comprises two rectifiers connected in series in opposite senses across the lines and the junction point of the rectifiers connected to the grid of a thermionic valve, the anode circuit of which has associated with it a static delay network for controlling the operation of a relay in such manner that it takes place only when the normal polarity remains reversed on both lines for predetermined minimum period. As applied to a telegraph exchange system in which one of the lines. L1, L2 from a subscriber station can be associated with a regenerative repeater RG at the exchange, spacing (negative) potential normally flows over both lines L1, L2 and via rectifiers MR1, MR2, poled as shown, holds the valve V1 non-conducting. Capacitors C1, C2 charged from the anode circuit V1 and from a circuit in parallel render valve V2 conducting so that a, third valve V3 is held non- conducting and a relay z/1 in its anode circuit is not actuated. When the polarity on L1 is changed to positive to initiate a desired connection, V1 remains cut off by the negative polarity on the line L2. A short positive impulse on the line L2 to indicate that dialling can commence makes VI conductive, but the time constants of the circuits R2, Cl and R3, C2 are such that the valve V1 must conduct for at least 120 ms. before the valve V2 is cut off. If the desired station is busy, the engaged signal, which comprises an impulse of positive polarity for 1,200 ms., immediately renders valve V1 conducting and after 120 ms. the valve V2 is cut off and valve V3 conducts, so that relay z/1 is operated. After the termination of the engaged signal, negative potential on the line L2 cuts off the valve V1 and after a period of 400 ms. the valve V2 again conducts, cutting off the valve V3 and releasing the relay z/1. In response to the engaged signal, the operator at the calling station restores the circuit to normal condition in which negative potential is again applied to the line L1. If the called line is free, a steady and sustained positive potential on the line L2 renders V1 conducting and later V3, so that at contact Z1 the regenerator RG is permanently connected to the line L1. During signalling the negative signal elements cut off the valve VI, but in no case for a period long enough for C1, C2 to charge to a value sufficient to make V2 conducting and bring about the release of the relay Z. At the end of a transmission a long spacing signal cuts off the valve V1 for a time sufficient to allow C1, C2 to be charged to a value to make V2 conducting and thus cut off the valve V3. If the system uses negative instead of positive as " mark " the circuit arrangement is modified by the inclusion of a valve having its grid connected to the junction of the rectifiers MR1, MR2 and its anode connected to the grid of the valve V1. For single-current working, with positive mark, an additional resistor is connected from the grid of the valve V1 to negative potential, and for negative mark the rectifiers will have to be reversed. In a modification, a single to double current conversion circuit could be employed. One conversion arrangement comprises the connection of each line to earth via a capacitor shunted by two resistors in series connected in series with a diode having its cathode earthed. The junction points of the resistors are connected respectively to the rectifiers MR1 and MR2. For negative " mark " it is necessary to reverse the diodes.