Patent GB-142488-A (GB)
Title: Machine switching telephone exchange systems
IPC: H04Q3/00
Filed: 1920-04-29
Abstract: 142,488. Western Electric Co., (Assignees of Goodrum, C. L.). Sept. 14, 1917, [Convention date]. Automatic exchange systems.-The hunting movement of a group-selector, or of a line-selector when making connexion with an idle line of a series allotted to a private branch exchange, is prevented if all the lines of the wanted group are busy, and is resumed when one of the lines becomes idle. Group-selector, Fig. 1. The line relay 101. slow relay 102, and release relay 120 are energized when the selector is seized. The first set of line interruptions operate the primary stepping-magnet 113 and energize a slow relay 114, which controls a side-switch magnet 115, so that, at the termination of the series. the side switch moves to its second position. If all the wanted lines are busy, the energized relays a, a<1>, a<2> of these lines connect ground to a contact 123, on which an auxiliary brush 108 has been set by the primary stepping movement, energizing a relay 117 which opens the circuit of the secondary stepping-magnet 116, and a busy signal is sent back by a transformer 118 to warn the calling subscriber to hold back the remaining sets of impulses. As soon as a line becomes idle, the relay 117 relapses and the secondary stepping-magnet comes into action. As soon as the test brush arrives on the ungrounded terminal of the idle line, the sideswitch magnet 115, which was first energized in series with magnet 116 and then over the test circuit 122, relapses and moves the side switch to its third position. The line conductors are now switched from relay 101 to relay 200, which energizes slow relay 201, which grounds the test circuit 121 so as to energize relay a and to lock the release relay 120. Final selector, Fig. 2. Upon the interruption of the busy tone, the subscriber sends in the remaining sets of impulses. The first set operates the primary stepping-magnet 206 and the second set the secondary stepping-magnet 211, the side switch moving in the interval to its second position under control of the slow relay 202, which energizes the side-switch magnet 204, during the passage of impulses. The stepping-magnets are energized at the end of each line interruption, instead of at the beginning as in Fig. 1. If the wanted connexion is to a private branch exchange having a series of main exchange lines, the selector has now been set on the first of these lines. If all the wanted lines are busy, the energized relays a<3>, a<4>, a<5> connect ground through a fourth brush 210 to a relay 214, which locks the side switch in its second position, and a busy signal is sent to line by a transformer 215 through an off-normal contact 216 closed by the second movement of the selector. When one of the wanted lines becomes idle the original energizing-circuit of relay 214 is opened, but this relay remains locked until a second circuit is opened by an interrupter 219; the interrupters of different selectors of the same group operate in turn, so as to avoid double connexions due to the simultaneous release of two waiting selectors. The relapse of relay 214 connects the test-brush 209 to a relay 222, which controls a self-interrupting circuit for the magnet 211. The selector therefore hunts until it finds an idle line whereupon the relay 222 relapses, and the magnet 204, which till then was kept locked by the relay 222, moves the sideswitch to its third position. Supply of ringing current. Ringing current is now supplied from a source 226 through a relav 203 and a contact of the side-switch magnet 204, which is energized over the side-switch arm 225. When the call is answered, relay 203 open-circuits the magnet 204, which moves the side switch to its fourth position, completing the talking connexions. Release. The release magnets 107, 230 are energized when the calling subscriber disconnects himself.