Switching network interfacing telephone-exchange equipment with

Abstract

A line loop (a, b) serving two telephone subscribers with sets (U.sub.1, U.sub.2) each normally connected between ground and a respective line conductor, coupled at the exchange via a line transformer (T) to a voice path, includes a pair of line relays (RA, RB) respectively inserted in these conductors in series with two high-ohmic resistors (RR.sub.1 ', RR.sub.1 ') and two low-ohmic resistors (RR.sub.2 ', RR.sub.2 ') forming part of a switching network (CI) which interfaces the loop with call-control circuitry (CCT) at the exchange. Two selection relays (RE.sub.1, RE.sub.2) in the switching network normally connect the two line conductors to negative battery whereby an off-hook condition at one set, grounding the corresponding conductor, causes a current flow insufficient to operate the associated line relay but detectable by a line monitor (RI.sub.1, RI.sub.2) connected across the low-ohmic resistor thereof which generates an engagement signal (i.sub.1, i.sub.2) when that conductor is being addressed by a slow-rate scanning pulse (u.sub.1, u.sub.2). A resulting switchover of the exchange circuitry to fast-rate scanning energizes the respective selection relay and locks it operated to short-circuit the high-ohmic resistor, thereby actuating the corresponding line relay and disconnecting the other subscriber set from the loop. When the exchange detects a persisting off-hook condition in the absence of activity in an assigned time slot, it emits an isolating command (in) concurrently with the scanning pulse to set a flip-flop (FF.sub.1, FF.sub.2) causing the release of the selection relay.