Patent GB-593049-A (GB)
Title: Improvements relating to automatic telephone systems
IPC: H04Q3/42
Filed: 1945-05-04
Abstract: 593,049. Automatic exchange systems. SIEMENS BROS. & CO., Ltd., and LONG, D.P. May 4, 1945, No. 11439. [Class 40 (iv)] In an automatic exchange system in which control apparatus, common to a number of selectors, is taken into use with a selector and released after a conversational path has been set up, if no impulses are received within a predetermined time, the common apparatus is released by operation of the switching relay and the selector is held. busy, the relay remaining operated as long as the calling condition continues. An embodiment is described in which a first selector, Fig. 1, is of the type in which the incoming wires are switched directly through to the wipers and is associated with a common coutrol circuit, Fig. 2. Line and release relays A, B energize upon seizure and operate dialling relay C and coupling relay K, the latter connecting up the control circuit. The impulses now received step switch MA, after which C releases. If a second digit is required to complete the selection and if the first digit denotes a group appearing in the full line arcs the test circuit is immediately completed over a strapping of MA3 to a wiper of MB3, MB4 at normal and test relay T operates in series with PA which reoperates C and substitutes MB for MA. The second impulse train steps MB in a similar manner. If, however, the first digit denotes a group appearing in the dotted line arcs, the selector drives to the first position of the dotted line arcs before T operates in series with PA. When a free outlet is found relay T operates followed by TA and connects up switching relay H which locks to wire p and brings about release of the control circuit. If no free outlet is found the wipers come to rest on the last contacts in the arc and T operates in series with LC, the circuit including contacts of MA8 if only one digit is employed. This substitutes busy tone for dialling tone and operates an overflow meter over MA5, MB7, MB8. If a caller does not immediately dial after the selector is seized, an S pulse operates relay S over MA6 in the normal position and this relay locks over its right-hand winding. If pulses have still not been received by the time a Z pulse is received over the normal contact of MA7, a circuit is completed for TA over winding III which operates WS and H, thereby releasing the control circuit and closing an alarm circuit. WS remains held over normal contacts of arcs -2 and +2 and the subscriber's loop circuit and holds H. A discriminating signal may be sent to the following selector when required by connecting earth to the positive wire over banks MA2, MB2. If the subscriber dials a dead number, relay S is operated over strappings between terminal S and the appropriate contacts of banks MA4, MB5, MB6. Then when the wipers are positioned on their last contacts following the unsuccessful search, LC operates and NU tone is connected over s1. In order to prevent a selector being wrongly seized before H has fallen back after a previous connection, owing to the series of switching relays connected in parallel to the p wire presenting a low resistance to a testing relay, H is provided with a high resistance operating winding and a second winding of higher resistance in series with the first in the holding circuit. Specification 593,050 is referred to.