GB489107A - Improvements in or relating to telephone systems - Google Patents

Abstract

489,107. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO.. Ltd., and PEARCE, O. A. Jan. 19, 1937, No. 1629. [Class 40 (iv)] Bidirectional finder switches are controlled by a chain start circuit to which periodic impulses are delivered as long as a common start relay is energized, the latter being connected in a bridge circuit in such a way that in the event of simultaneous calls by subscribers wired to different levels the bridge becomes balanced when a corresponding number of finders have been successively started. Line relay L by its operation marks the position of the calling line in the marking bank and vertical bank multiples M, VW and connects battery over resistance YA, which is common to the lines occupying a level, to the common start wire PSL, whereupon relay PS energizes and connects up a sluggish selfinterrupting relay SL which delivers impulses to a chain start circuit as long as PS remains up. Assuming that no other finder is in use, relay A energizes before SR and holds over B (lower winding), which pulls up after SR has removed a short-circuit, connects battery over resistance YB to common wire X, and extends the chain start circuit. The finder hunts vertically to the marked level, whereupon VT energizes, disconnects A and makes a fresh circuit for B (upper winding). Rotary hunting then takes place and switching relay HA or HB energizes and short-circuits B. Cut-off relay K operates to ground in the group selector and L falls back. Relay PS is bridged across wires PSL, X and the pairs of resistances YA, YB and YC, YD are equal. In the case of a single call, the bridge is balanced when B operates and PS falls back, so that only one finder is started. In the case of simultaneous calls on different levels, battery is connected to wire PSL over several resistances YA in parallel, and PS remains operative until a corresponding number of finders have been started by the successive operations of SR. In the case of simultaneous calls on the same level, a second finder is not started until the first one switches through. Any finder may be artificially busied by shifting link T to the lower position. In a system arranged as in Specification 366,143 in which first choice primary finders are connected directly to group selectors, while last choice primary finders are connected thereto via secondary finders, all the finders are identical, and a distributing frame IDF is provided for making connections either to a group selector or to the banks of secondary finders. The secondary finders are controlled over a chain start circuit by relay SS, which is connected in a bridge circuit similar to that of PS. Specification 450,019 also is referred to.