Patent GB-740450-A (GB)
Title: Improvements in or relating to automatic switching systems
Inventor: CURTIS GERALD
IPC: H04Q3/00
Filed: 1952-08-29
Abstract: 740,450. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. July 13, 1953 [Aug. 29, 1952], No. 21717/52. Class 40 (4). In a system in which equipments such as linefinders are taken into use in turn, they are rendered " busy after use " by a switching device which is maintained operated over a lead connected from a common circuit in parallel to all the equipments, until all the equipments are busy (either genuinely or "after use") whereupon all the switching devices of free equipments are released. A 200 line group is served by two groups of finders, the first finder group giving priority to the first 100 lines and the second finder group giving priority to the second 100 lines. Figs. 2 and 3 illustrate a finder-selector link whose finder belongs to the first group. This finder is normally seized over the chain circuit LSA, Fig. 1, and 1HSC, Fig. 2, in response to a call from a subscriber in the first hundred lines. It may however be used as a reserve finder for a call in the second hundred in which case it is seized over chain circuit LTB, Fig. 1, and 2HSC, Fig. 2. The subscriber's lines of second hundred are connected up by relay CO which is brought up on seizure over the chain circuit 2HSC. Similar normal and receive seizing chains LSB, LTA are provided for the finders of the second group. Two allotter circuits, one of which is shown in Fig. 1, are provided, so that if the one in use goes faulty, the other is switched in. Seizure and setting of line-finder. Relay GB is normally operated over lead LGC provided there is at least one available line finder in the two groups. GB holds GC operated. A call from a subscriber in the first hundreds group causes the placing of a starting earth on lead K1 to bring up XA, SA. The latter brings up PA. A starting earth is thus applied to both the normal and reserve chains LSA, LTA over relays TA, WA respectively. If an available line-finder is associated with both chains both TA and WA commence to operate but TA is faster than WA and in operating disconnects WA and the reserve chain. If only one chain has an available finder then either TA or WA operates and disables the other. Assuming TA operates then a first group finder/selector link is seized and line relay A therein operates in the seizing circuit. The hold relay B follows to energize the latch magnet LM of the finder LF which hunts until it finds the calling line, whereupon the test relay HX operates to stop the switch. HX removes a short circuit around BAA, BS which come up. The former brings up HA, releases HX and connects it to the group selector P-lead, connects A to the calling loop, reverts holding earth on the line finder P-lead and reverts dial tone. Relay BS switches the start chains 1HSC, 2HSC through to the next available finder and releases XA in the allotter. Relay BS locks to lead LGB and stays operated until released by the allotter. When the calling subscriber's cut-off relay operates the starting earth is removed from K1, SA releases bringing up the slow-to-operate relay MA which releases PA and MA follows. The allotter is now ready to deal with another call. Setting of group selector. The first dialled digit is repeated by line relay A to the group selector vertical magnet VM after which HA is short-circuited and releases to connect up the rotary stepping circuit. When a free outlet is found the test relay HX operates in a circuit containing make contacts of the rotary interrupter springs RMC. The test relay in operating opens its own operating circuit at back hx1 but is enabled to operate fully by the charging current for condenser QA, until it locks over front hx1. HA operates on completion of the rotary hunting, switches through and releases A. Relays B, BAA then release but HA, HX hold to earth reverted over the P-wire from the succeeding selector. On termination of the conversation, earth is removed from the P-wire to release HX, HA, a battery pulse being applied to the release alarm lead RS, the release alarm circuit then reverting earth over RAE to step the wipers of the group selector out of the bank whereupon it reverts to normal. The line-finder is then homed. If the group selector fails to home within a predetermined time battery applied from RM over the lead RAE brings up an alarm. If all outlets are busy the selector steps to the eleventh position where springs S operate the test relay HX to stop the switch and revert busy tone. Barred calls. "O " level calls proceed as previously described unless this type of call is barred to the calling subscriber in which case he has his D-wire earthed, which earth is applied over post springs npa to shunt the test relay HX until the selector reaches the eleventh position in which busy tone is reverted as previously described. Spare levels. A call to such a level proceeds as for all outlets busy but post-springs npb cause the reversion of N.U. tone instead of busy tone. Standstill release. If no dialling impulses are received, AY comes up to a pulse over the 30-second S-lead and 30 seconds later AZ comes up to a pulse over the Z-lead to release B to restore the final selector link to normal. In the subscriber's line circuit the line relay releases but the cut-off relay locks over the subscriber's loop and raises an alarm. Likewise if the finder cannot find the calling line, e.g. due to a fault in the test circuit, AZ comes up to a pulse on the 6. second S lead and 6 seconds later BS operates to a pulse over the Z-lead thereby releasing A, which releases the link, and extending the chain circuit to the next available link. Blown fuse. All relays of the link release but AY comes up to the making of the fuse contact GSF to extend the chain circuit and bring up an alarm over lead FAR. Restoration of the links when the last available is in use is effected by the release of GB which is no longer held over LGC. GB releases slow release relay GC and also all the BS relays held over lead LGB. The consequent restoration of earth on LGC by all the free links allows GB to reoperate when GC releases. GC then reoperates and the allotter circuit is normal. Replacement of faulty allotter. If on receipt of a starting earth on K1, SA operates, but there is no consequent operation of TA ot WA, then AR comes up to a pulse on the 6 second S lead and 6 seconds later BR comes up to a pulse on the Z lead. BR locks and operates BA which operates an alarm over lead STA and changes over the chain circuits and the LGC lead to the spare allotter therein. The'spare allotter then continues in operation unless it too goes faulty - when it operates in a similar manner and unlocks BR in the original allotter so restoring control thereto.