GB541768A - Communication exchange systems - Google Patents

Abstract

541,768. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. June 18, 1940, No. 10549. Convention date, July 24, 1939. [Class 40 (iv)] In a call-finder arrangement, each individual line circuit comprises three high resistances J, K, L connected as shown, and the existence of a calling condition is' marked by potentials applied by way of the loop to a common start wire k and to the individual c wire, thermionic valves being employed for detecting such potentials. The resistances K, L are substantially equal and may be of the order of 50,000 ohms, while the resistances J are of higher value. When a call is made, the negative potential applied over the three resistances in series to the grid of the valve V which is normally earthed through a high resistance G brings about the release of relay X and the operation of relay Y, thus completing the stepping circuit for the finder connected up by the allotter N. A common test circuit T which is similar to the start circuit S is associated with the finder through the allotter N, and when the calling line is reached the negative potential over K, L causes the release of X<SP>1</SP> and the operation of Y<SP>1</SP>, which stops the finder and connects up switching relay R. The latter grounds the c wire and thus restores the start and test circuits to their normal condition. When the finders serve a group of 100 or 200 lines, instead of all the lines being connected in multiple to the common start wire k, it is preferable to divide the group into sub-groups of 5 to 20 lines with a separate wire k<SP>1</SP> for each sub-group, the various wires k<SP>1</SP> being connected in succession to the start wire k by a constantly rotating commutator. In an extreme case, an individual wire k<SP>1</SP> may be provided for each line, and in such an arrangement the resistances J may be omitted. In a modification, the start and test circuits are combined in a single circuit ST, in which relay Z<SP>11</SP> which is operated to start the finder transfers the control valve V<SP>11</SP> to the test circuit and holds in series with slow-release relay Y<SP>11</SP>until the calling line is found, when it relapses followed by Y<SP>11</SP>. The starting arrangement may be used in a system in which testing is performed by a simple bridged test relay. Other arrangements of finders may be employed, for example, a number of finders may be started simultaneously, or a finder having nearest access to the calling line may be started. Moreover the finder switches may be replaced by finders of relay or cross-bar type.