844,749. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Dec. 20, 1956 [Jan. 14, 1956], No. 1342/56. Class 40 (4). A group of outgoing relay sets is served by a number of impulse regenerators less in number than the number of relay sets in the group, the connection of a regenerator to a relay set being initiated immediately the relay set is seized, incoming digits being directly repeated until the regenerator is connected, subsequent digits then being repeated via the regenerator. The regenerator is preferably of the mechanical type described in Specification 458,095. The invention is applicable to systems comprising zone switching centres serving group switching centres, the latter serving minor exchanges possibly with dependants. The outgoing relay sets are arranged to be accessible over three different routes, viz. from local selectors, a local manual board, or a " transit access " path seizable from an incoming relay set connected to trunks incoming from a group switching centre or other zone centre. Only where access is obtained over the last path is the regenerator called in. Incoming relay set, Fig. 2.-An incoming call over the four wire trunk CEA is heralded by an XY combination of frequencies followed immediately by a Y frequency. These operate correspondingly lettered relays. Relays XY together bring up HSY, MC, MP. When X falls back DS operates over its middle winding (and holds over its lower) and B follows to apply holding ground to the P-wire of the local switch train ICS in case the call is to a local subscriber. At the end of the Y frequency HSY, MP, MC release to bring up DC. The first dialled digit received as a train of X pulses operates A to step the discriminating uniselector DS. If the first digit denotes a local call, the corresponding terminal on bank DS1 is strapped to terminal LDL and relay LD is operated to extend the two-wire side of the hybrid HDA to the local selector chain ICS, and subsequent digits received as X-pulses are repeated as loop impulses to this chain by relay A. In the case of a transit call or a call to a manual board the DS1 terminal is strapped to terminal TCL. Corresponding terminals of bank DS2 are strapped to terminals TS8 of a high-speed hunting switch TS which terminals correspond to outgoing relay sets of trunks or junctions going in the required direction. The earth applied via DS1 operates latch-magnet LM for the high-speed switch TS. When a free marked outlet is found T operates to stop the hunting, short-circuits a lowresistance winding to busy the outlet and brings up H. The latter permanently busies the outlet; connects contact xl over TS6 to lead PS for onward repetition of succeeding dial impulses; earths lead DL to seize the outgoing relay set; and operates FW to disconnect the hybrid and establish a through four-wire path. The end of dialling signal consists of XY frequency followed by X. Operations then follow as for a seizing signal except that when Y relapses it completes a circuit for an opposing winding of DS which thus releases thus permitting DC ultimately to release to apply resistance battery to lead DL as an end-of-dialling signal. If the register-sender at the previous exchange sends the end of dialling signal as soon as it has transmitted all the digits it contains, then the signal on DL may be used to initiate a forced release timing period which may be nullified if subsequent digits (preceded by the appropriate preparation signal XY-Y) are received in time. Outgoing relay set, Fig. 3.-Seizing earth over a transit access path TP on lead DL operates TC. The set is caused to transmit the preliminary XY-Y frequencies by means not shown. TC operates FWA which disconnects the hybrid HDB and prepares the through fourwire connection. Relay TC also places busy earth on the P-wire of the local access path LSM and operates ET to busy the relay set against seizure from the manual board. Relay TC earths the start lead to a series of chain-connected relays associated with the regenerators IRRS, the hunting switch of the first free regenerator being started to hunt for the calling relay set marked over the T-wire at tc6. When the hunting is successful, relay RC operates, followed by DT, which causes digits received on lead PS to feed the pulsing-in lead PI of the regenerator. The regenerated impulses received over lead PO key the X-frequency modulator CX to transmit X-frequency pulses to line CEB. If the first digit commences before the hunting is completed the impulses (with DT being unoperated) received over PS operate relay A which directly keys the modulator CX. The dialling relay CD then delays the operation of DT until the end of the digit should the finder be successful during that time. At the end of dialling, battery on lead DL operates ED to effect the transmission of the end-of-dialling signal (over contacts of relay SPB which operate at this time) by means not shown. If the relay set is seized over a local access path LSM, relay ET operates over the P-wire to guard against seizure over the other paths and pulses received over PU feed the X-frequency modulator CX directly. If seizure takes place from a manual board, S operates over the sleeve and brings up SS followed by ET. The relay set is thus guarded from seizure 6 over the other paths. Battery over the ring of the jack brings up RR and DT follows to bring up AA in the dialling loop and short out RR. The dialled impulses now release AA to pulse the X-frequency modulator CX. In each of these cases arrangements, not shown, are provided for sending preparation signals and end of dialling signals from the outgoing relay set. The relay set may also be used for trunks (e.g. zone centre to zone centre) in which impulse regeneration is not required. In this case lead 10 is strapped to lead 12 instead of lead 11 so that incoming impulses over lead PS feed the X-frequency modulator CX directly.