Patent GB-845101-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: ALLUM ROBIN DEVENISH
IPC: H04Q3/54
Filed: 1955-03-11
Abstract: 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 the line circuit with respect to the line finder/final selector stage as described in Specification 813,338. VKLPTB also fires VNIA to pulse class-ofservice lead CS. The switched H.T. on leads SHT is interrupted by control equipment (not shown) to extinguish the tubes controlled thereby. Busy tube VKEBB, however, remains conducting until the cross-point in the line finder/final selector is closed, when earth appears on lead A. If, instead, the first line circuit is busy the pulse from VKHDB fires VKRO of the first stage of the high-speed hunting circuit instead of VKSZA. VKRO passes a pulse to the second stage and so on until a stage is reached where the line circuit is idle and VKSZA strikes, or until VKRO in the final stage strikes when busy tone is returned to the calling subscriber. Trunk offering Trunk offering is performed on the last line-circuit of a group, in which the anode of VKM is connected to switched H.T. over contacts toc, Fig. 3, instead of directly. The exchange controller provides idle marking on the last line by signalling lead TOL to fire VKTOC, so energizing relay TOC. Tube VKM is thus extinguished so that the last stage of the high-speed hunting circuit finds the line marked idle over lead M, always assuming that an idle line has not been found at an earlier stage. The composite P.B.X. line circuit operates for originating calls in substantially the same manner as in Specification 813,338. Specification 765,928 also is referred to.