Patent GB-645640-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: HUGHES JOHN
IPC: H04Q3/00
Filed: 1948-07-22
Abstract: 645,640. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. July 22, 1948, No. 19585. [Class 40 (iv)] In a final selector provided with trunkoffering facilities and having access to line circuits of known type, Fig. 1 (not shown), in which the line relay holds in series with the cutoff relay when the line is busy, if the wanted subscriber is engaged in a local connection the final selector test circuit is opened to allow the local connection to be released and is then completed again to seize the wanted line under the control of a differential relay TD, Fig. 2, one winding of which is energized when the selector is seized while the second winding is controlled in accordance with the condition of the wanted line. An operator seizes an idle final selector, Figs. 2, 3, over a group selector multiple GSM, applying resistance battery to the M lead to energize the left-hand winding of differential relay TD, which is without effect at present. Relay A operates over the calling loop to energize relay B, which busies the incoming P wire and brings up relays BB, C. The dialling impulses of the tens digit are followed by relay A and hence by vertical magnet VM, raising the wipers to the required level and operating vertical off-normal contacts N1-4. At the end of the tens digit impulses dialling relay C releases and relay E switches the impulsing circuit from vertical magnet VM to rotary magnet RM, also applying earth to the M lead, releasing relay TD and reenergizing relay C. The units digit dialling. impulses now cause rotary magnet RM to drive the wipers to the wanted line, operating rotary off-normal contacts NR1-4. After impulsing, relay C once more releases and completes a test circuit over the right-hand winding of test relay H to the P wiper. Relay E releases slowly. If the wanted line is idle, resistance battery on the P wiper operates relay H, which applies earth to the P wiper to seize the line, the call then proceeding normally. If the wanted line is engaged, earth is found on the P wiper and test relay H remains inoperative. The release of relay E energizes differential relay TD over its left-hand winding and removes the short-circuit from relay C, which again operates to open the test circuit so that it does not prevent the release of the wanted line. Relay TO energizes, applying busy tone from lead 13 over relay A, which acts as a transformer, and energizing from earth on the P wiper the right-hand winding of relay TD in opposition to its left-hand winding, so that TD releases. The operator momentarily depresses a trunk-offering key to unbalance a differential relay OC, which brings up a supervisory relay D and ring cut-off relay F which disconnects busy tone and completes a speaking connection between the operator and the conversing parties. The operator now offers the trunk call and both parties clear. The removal of earth from the P wiper releases the right-hand winding of differential relay TD which operates to release supervisory relay D and complete a second test circuit over the right-hand winding of test relay H which now operates. A second operation of the trunkoffering key again momentarily unbalances differential relay OC to release relay F, which rings the wanted subscriber over leads 15, 16 and releases relay TO, bringing up relay J to apply ring-back tone from lead 14. When the called subscriber answers, completing his line loop, a lightly adjusted contact f4 is opened so that relay F can operate fully to disconnect ringing and ring-back tone and to complete the talking circuit. The supervisory relay D re-energizes, followed by relay E which disconnects relays TD, J. The latter releases more slowly than the former, a battery pulse being applied to the M lead in the interval. In a local call this pulse would be used for metering. The call now proceeds as usual. In order to ensure that the second test circuit is not applied to the wanted subscriber's P wire until his line circuit is completely released, the differential relay TD may be made slightly slow to operate by causing the flux due to the righthand winding to be a little greater than that due to the left-hand winding. The selector may be of the type described in Specification 391,123.