Patent GB-615448-A (GB)
IPC: H04Q3/00
Abstract: 615,448. Automatic exchange systems. TENCH, F. (Automatique Electrigue Soc. Anon.). Dec. 15, 1945, No. 34009. [Class 40 (iv)] In a multi-exchange system in which connections are set up over tandem exchanges by the transmission by the calling party of a prefix digit followed by the digits designating the required exchange, .the latter digits are stored at each tandem exchange in a register which retransmits the necessary digits for seizing an appropriate junction forming part of a direct or indirect route according to the availability of outlets, and, at a subsequent stage, retransmits unchanged the digits designating the required exchange. Specification 615,504 is referred to. A typical portion of the system (Fig. 1), which is designed for use in conjunction with a railway network, comprises a main exchange M having direct junctions to five primary exchanges 1A ... IE, some of which are inter-connected by direct junctions. The main and primary exchange each act as the centre of a zone which is designated by a single digit. Primary and secondary exchanges are each designated by a further single digit and subscribers' lines by four digits, while for tertiary exchanges, e.g. 3K, two digits, of which the first is 8, are used. Subscribers at tertiary exchanges extend a connection to the associated primary or secondary exchange by dialling the digit 1. Dial tone is transmitted when a calling party removes his hand-set and may comprise a morse code signal. In the case of an inter-zone call, dial tone is again transmitted from the exchange with which the called party is associated and this tone also may be morse code or a recorded verbal signal to indicate that the call has been routed to the desired exchange, while in calls to tertiary exchanges a further continuous dial tone may be transmitted from the tertiary exchange. Calls may be set up automatically to the public telephone network by dialling O, this facility being available to certain privileged subscribers only. Subscribers engaged in calls to the public network may make local enquiry calls while holding the public exchange line. Service lines terminating at the operator's position allow her to set up calls to any party on the network without passing through a second position. The trunking arrangements are illustrated in Fig. 3. Preselector-Fig. 4. This is standard for all exchanges and is of the no-normal type that advances one step on release. Cut-off relay K energizes when an idle line is found and connects up switching relay M, both these relays being held by earth fed back on the c wire before the slow-to-release line relay L has relapsed. The connection made to terminal 4 associated with the d wiper depends upon the status of the subscriber. If the subscriber is unrestricted, 4 is disconnected; if he has the additional facility of breaking into an existing connection, 4 and 5 are strapped; while for restricted subscribers 4 and 3 are strapped. These features' will be described later, as will. that of discrimination on incoming calls from the public network for which terminals 1 and 2 are strapped. If the holding earth is not fed back, e.g. on account of a blown fuse, the re-operation of L holds M and closes an alarm circuit over terminal 6. First group selector, IGSI-Fig. 5. This selector may function either as a first group selector IGSI, incoming first group selector IGS2, or incoming first group selector for public network calls, IGS3 (Fig. 3). Relay A operates on seizure and is followed by H, HA and AS, dial tone being reverted from terminal 11. The first digit is repeated by relay A to the vertical magnet, AS releasing at the end of the digit to connect up the rotary magnet which steps to the first outlet in the selected level. If this is busy, the magnet interacts with AS and hunts for an idle outlet to which CN operates, switching through the speaking leads and holding HA. Busy tone is transmitted from terminal 12 by the operation of 11th step contacts SI. if all outlets in a level are busy. If the calling party does not commence dialling within 30 seconds, PL and MF operate to remove earth from the incoming c lead and place the pre-selector under fault conditions. If the selector is used for incoming public exchange calls, terminals 14 and 15 are strapped and relay P is not provided, so that on the operation of CN the incoming d lead is extended to the next switch. In normal use with a restricted subscriber, the operation of HA is followed by full operation of P to the full earth on the incoming d lead, so that if the selector is stepped to any but the first level (local calls), the operation of the normal post springs brings up AS which interacts with RM to step the switch to the 11th step where AS releases and busy tone is reverted over SI. In the case of an unrestricted subscriber, no earth appears on the d lead so that P is not operated, while resistance earth in the case of a priority subscriber operates P over its first step only, connecting resistance earth to the outgoing d wiper. Second group selector and discriminator selector repeater-Figs. 6 and 7. On seizure 2A operates, followed by 2H and 2AS, and HS hunts for an idle junction to the satellite exchange, CS pulling up on finding to complete an impulsing loop thereto. The incoming digit is repeated by 2A1 to the vertical magnet of the selector and by 2A2 to the satellite exchange. If the connection is for a local subscriber, the first digit is 1-6 and the corresponding vertical contacts are cross-connected to terminal 35, so that when 2AS releases at the end of the train RM is energized and the wipers step to the first outlet. If this is busy, 2AS and RM interact until an idle outlet is found, when 2CN operates and switches through to the final selector. Relay 2A falls away followed by 2H and CS, and releases the junction and the satellite equipment. If the connection is to be extended to the satellite, the vertical bank contacts will be strapped to terminal 34, so that 3CN pulls up when 2AS releases, and extends the calling loop to the junction circuit while 2A and 2H fall away and the selector goes home. If HS has not found an idle junction before the group selector is stepped off normal, BB pulls up and releases CS, so that if the call is for the satellite exchange, the operation of 3CN serves only to revert busy tone from terminal 33, but a connection to a local subscriber may be completed over the group selector wipers. Final selector FS1-Fig. 8. Relay 1A pulls up to the seizing loop followed by 1H and 1AS. The first digit is repeated to 1VM, 1AS releasing at the end of the first train to bring up RO which transfers the impulsing circuit to 1RM for the second digit. As soon as the wipers step on to 'the first outlet of the level, 1AS is re-operated over 1NR2 so that the release of RO at the end of the horizontal movement connects test relay 1CN during the .release time of 1AS to the c wiper. If the line is busy, 1CN is not operated and the release of 1AS causes the reversion of busy tone from terminal 29. On an idle line 1CN operates, connects up the battery feeding relay B, and transmits ringing current from terminals 26, and 27 or 28. On the reply of the called party, RT pulls up, trips the ringing and completes the battery feed to the called party, so that B operates and is followed by BR which reverses the battery connections to the incoming leads. Release is under the control of 1A. However, if the called subscriber clears first, release is delayed for approximately 5 seconds, to avoid a false clearing signal when the called party replies to the call, by a thermal relay TH which is connected up on the release of B and which controls 1A through VT. If the final selector is taken into use by a priority subscriber, the resistance earth on the d wiper of his preselector is extended and operates DF over its first step to close DF1 and DF2. If the required line is busy, the calling subscriber dials the digit 1, PR operating fully at the end of the impulse to connect a special tone from terminal 30 to the called line. Both the subscribers engaged on the existing connection clear on hearing this tone, and the final selector then switches through the required line. An incoming public exchange call causes earth to be extended over the d lead to operate DF fully, so that the right-hand winding of 1CN is connected in parallel with a resistance and 1CN will operate only if the called party's cut-off relay is shunted as is the case for subscribers entitled to receive such calls. In all other cases, busy tone is reverted. Furthermore, the operation of DF causes a different frequency ringing current from terminal 28 to be employed. Relay VT operates on the reply of the called party, cutting out the feed bridge at the final selector, battery feed being supplied from the position equipment. Since DF is fully operated on incoming public exchange calls, the operator completing the connection may camp on a busy line by dialling the digit 2. The first impulse operates PR and the second AT which releases PR to prevent special tone transmission. The final selector switches through when the existing connection is released. If desirable, the operator may, while camping, dial the digit 1 to cause tone transmission. If the final selector has access over certain levels to tertiary exchanges, arc and, wiper e are provided while the associated contacts in the d bank are earthed. It is arranged that the second digit transmitted to the final selector in such cases is always 1 so that the wipers step to the first outlet in the selected level. If this is busy, non-operation of 1CN allows the earth on the d wiper to operate 1RM, RO operating in the same circuit followed by AS which cuts the drive. This interaction continues and the selector hunts over the level until 1CN operates to an idle line. The junction line to the tertiary exchange is accessible over an impulse repeater which