Improvements in or relating to telephone systems

845,101. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Feb. 10,1956 [March 11, 1955], No. 7157/55. Class 40 (4). A hunting circuit is provided in common to groups of private branch exchange lines, whereby on a call to any one of the P.B.X.'s the idle or busy condition of the lines in that P.B.X. is indicated to the hunting circuit and the first idle line is taken into use. The system described in Specification 813,338 is adapted to provide for P.B.X. working. Each line circuit of the P.B.X. is divided into two parts as shown in Fig. 1, an originating line circuit OLC and a terminating line circuit TLC, connected over a distributing frame LIDF to a line finder/final selector PS which may be of the cross-bar type. No line circuit is connected to that position in a 100-line block appropriate to the directory number of the P.B.X. Instead the marking bias and pulse appropriate to that position are strapped to leads MB, MP of the individual group equipment IPGE, Fig. 2, which leads are signalled by the exchange controller when a call is made to the P.B.X. to fire VKGDN. This tube fires the group marking tube VKGM to mark the trigger electrode of tube VKEBB, Fig. 1, in each of the terminating line circuits of the P.B.X. group. VKGM also primes VKMBL and fires VKGC and VKGS. VKGC signals the exchange controller EC (not shown) that the call is to a P.B.X. and the exchange controller signals over lead HFL to strike VKHDA, Fig. 3, in the P.B.X. common equipment. VKHDA then fires VKMG in each group equipment, Fig. 2. Each tube VKMG applies a triggering pulse to the related tube VKMBL but only that tube in the chosen P.B.X. group is primed and fires to strike VKML, Fig. 1, in all the terminating line circuits of the selected P.B.X. group, the associated tube VKM firing only if the particular line circuit is busy. Any tube VKM which fires primes and back-biases the tubes VKRO and VKSZ, respectively, in the related stage of the hunting circuit, Fig. 3. High-speed hunting. After a delay determined by R21, C21 tube VKHDA, Fig. 3, strikes VKHDB to pulse the trigger electrodes of VKSZ, VKRO of the first stage of a highspeed hunting circuit such as is described in Specification 765,928. If the corresponding line circuit is idle VKSZ strikes followed by VKSZA which pulses common lead SZ to fire VKSZB, Fig. 1, in all the line circuits. VKEBB and VKLPTB strike to provide a temporary guard on the B conductor. VKLPTB marks lead AM to indicate the location of