656,559. Automatic exchange systems. MERCER, R. Jan. 16, 1948, Nos. 1480 and 21125. [Class 40 (iv)] In an automatic exchange system the switching devices used in establishing a complete connection between a calling and a called line are arranged in distinctive routes such that each route comprises switching devices in tandem with switching devices of one stage each individual to a distinctive first group of calling lines, and switching devices of an immediately succeeding stage each individual to a larger second group of calling lines and connected to multipled outlets of those corresponding switching devices of the first stage which are individual to those first groups of calling lines which form the second group. In operation an idle route is selected under control of the dialled digits and a register controller then controls the operation of the switching devices in the route in order to establish the connection. The switches in the main train may be either wiper or cross-bar switches or sets of discharge tubes analogous to cross-bar switches. Detailed description, Figs. 1-8. The invention is described with relation to an exchange serving 10,000 subscribers, the line equipment of three of which (Nos. 1111, 1211 and 1511) is shown in Fig. 1. The lines are divided into 100-line groups, each of which has access to a group (e.g. ten) two-digit selectors such as DS11A-K (Figs. 5-7); and are further divided into 500-line groups each having access to three impulse responding equipments IRE1, IRE2, IRE3 (Figs. 1-4). In both cases access is had over cross-bar equipment (Figs. 1-3) serving a 500-line group and comprising three hold magnets HM1, HM2 and HM3 corresponding to IRE1, IRE2 and IRE3 respectively; ten hold magnets, e.g. HM11A-HM15K corresponding to selectors DS11A-DS11K, for each 100-line group; and a prepare magnet, e.g. PM1111 for each line. Station No. 111 initiates a call. The energization of line relay LR1111 (Fig. 1) grounds the start lead SL11 common to the 100-line group, which is wired to the first impulse responding equipment IRE1. If IRE1 and IRE2 are busy the start leads SL11-15 are extended by their hold relays B1 and B2 (not shown) to IRE3, so that assuming that such is the case and that IRE3 is idle, relays P3 and X3 are operated. Relay X3 locks to SL11 over front P3 brings up Y3 which holds P3 over its lower winding, and grounds the chain circuit CC (Fig. 1) which extends over a front contact of the first operated line relay (LR2 of LR1111) thus operating the corresponding prepare magnet (PM1111). On the operation of HM3 over front Y3, cross-bar contacts CBC1111X3 are closed, whereupon cut-off relay CO 1111 pulls up over the lower contact, and the calling subscriber's loop is extended over the upper contacts to operate impulse relay A3. Relay B3 follows, holds CO1111, P3 and HM3, disconnects the upper windings of P3 to T3, and extends the start leads SL11-15 to the next impulse responding equipment if such exists. Relays X3 and Y3 release in turn. Dial tone is connected up over a front contact (not shown) of B3 and is disconnected by an off-normal contact (not shown) of two-digit switch DS3 (Fig. 4). Relay A3 responds to the dialled impulses and repeats them over IL3 to an impulse regenerator DR3 (Fig. 4) preferably of the type described in Specification 458,095, and through the winding of C3 to step the vertical magnet VM3 of switch DS3. On the operation of C3 two-step relay 103 pulls up over its lower winding and closes its "X" contacts only. At the end of the first digit when C releases, ID3 operates fully over both windings in series to ground on hold lead HL3. Thus the second digit steps RM3 and the wiper W3 of DS3 is set to a position corresponding to the first and second digits dialled and is there connected to a test lead, e.g. TL11, which indicates the busy or idle condition of all the final selectors serving subscribers' lines having those first two digits. Two-step relay 2D3 operates partially at the beginning of the second digit and fully at its end, thus disconnecting RM3 so that subsequent digits are received by DR3 only. Relay 2D3 also connects wiper W3 and test lead TDS3 to the common operate lead COL3 of the cross-bar switch CBS3 over rectifiers R3A and R3B respectively; and grounds lead PML which is extended over front P3, thus operating prepare magnet PPM3 of CBS3. The final selectors, e.g. FS11A-FS11K, each of which is represented by a rectangle in Figs. 7, 8, may be of any known type which responds to the two final digits, but are provided with an additional contact set on the hold relay which is shown in the rectangle in each case. The fixed contact of each of these sets is connected through an individual unidirectional resistance or rectifier to a bus-bar lead from the bus-bar cable EC common to the exchange which comprises as many leads as there are final selectors in a group. A common driven set of cams ACKC (Fig. 4) is arranged to apply ground to these leads (AE-KE) at distinctive instants in a cycle, the motor either running continuously or being started on the initiation of a call. The cam set may be replaced by a relay chain if desired. As shown in Figs. 5-7, the leads AE-KE are also connected to the corresponding two-digit selectors. Thus if any two-digit selector or final selector is busy an intermittent earth is connected to that one of the test leads TDS or TL corresponding to the group to which the selector belongs by the hold relay of the selector. The trunking arrangements in the exchange are such that two-digit switches have access only to final selectors having the same suffix as themselves and a call from any substation has the choice of as many routes as there are two-digit selectors in a group, e.g. 10. The cross-bar contacts CBC11A (Fig. 1) give access over cable DS11A to two-digit selector DS11A (Fig. 5), of which only five sets of contacts out of 100 sets are shown. Each set of contacts, e.g. 11, is connected by a cable, e.g. 11A, to the corresponding final selector, e.g. FS11A, similar cables being employed for the other two-digit switches. Assuming the called station is No. 1167, wiper W3 (Fig. 4) rests on contact 11 after the first two digits are dialled so that test lead TL11 is connected to COL3 over R3A, and test lead TDS3 is connected over front P3 to TDS11 (call originated in group 1100). One terminal of the right-hand winding of each of the hold magnets HM3A-K of the cross-bar switch CBS3 is connected to the corresponding one of leads AE-KE and the other terminal is connected over a back contact of the magnet to COL3 which is also connected to battery via resistance R3 over front SD3 and back CR3. At instant A in the cam cycle (i.e. AE grounded) HMA3 will operate partially to close its X" " contact provided there is no ground connected to COL over R3A or RB3 at that instant due to either DS11A or FS11A being busy. Groups B-K are similarly tested during the remainder of the cycle. Assuming DS11A and FS11A to be idle, HM3A closes its " X " contact and completes a circuit for its left-hand winding from ground on HL3 in series with CR3. Relay CR3 disconnects battery from COL3 to prevent operation of any of the magnets HM3B-K, and HM3A in conjunction with PPM3 connects the output impulse leads OIL3 to the speaking leads of DS11A and the hold lead HL3 to the hold lead of DS11A. Magnet HM3A by operation of its right-hand contacts connects AE to the side of R3A and R3B respectively remote from COL3 thus extending ground to leads TDS11 and TL11 at instant A so that DS11A and FS11A test busy to other impulse responders even though their hold relays are not yet operated. Earth extended by the operation of CR3 to DR3 results in the completion of a loop across leads OIL3 to which A11A of DS11A (Fig. 5) pulls up, followed by B11A. The dialled digits are then retransmitted over OIL3 by DR3, A11A repeating the first digit through C11A to vertical magnet VM3A. Two-stage operation of ID11A transfers the impulsing circuit to RM3A which receives the second digit, similar operation of 2D11A transferring leads OIL3 to the speaking wipers of DS11A and thence to FS11A (first two digits " 11 "). The next two digits retransmitted by DR3 set the final selector in known manner. It is assumed that battery feed, busy test, and ringing or busy signals are provided by the final selector in any known manner. When DR3 has completed retransmission of the digits OCR3 is operated and connects booster battery in place of earth to the hold lead extending over front B3, CBC1111X3, non-linear resistance NLR1111 and PM1111 to battery. NLR1111 which prevents PM1111 operating to earth on the hold lead reduces its resistance and PM1111 operates. Resistance battery is thus connected to a lead common to the 100-line group leading to HM11A-K, and as the hold wire of each of the selectors DS11A-K is connected to the corresponding magnet HM11A-K, HM11A operates and locks up independently of PM 1111. The resulting closure of cross-bar contacts CBC11A connects the calling subscriber directly to DS11A and thence to FS11A. The operation of OCR3 also removes earth from front A3 whereupon B3 releases followed by HM3 and IRE3 restores to normal. Release. When the calling party replaces his receiver the impulse relay in the final selector releases followed by the hold relay which disconnects earth from the P wire. Relay 2D11A in the two-digit selector DS11A relapses, release magnet ZM3A homes the switch over off-normal contacts N, HM11A and CO 1111 fall away and the equipment is released. Additional features not illustrated. Timed release may be provided for permanent loop conditions; simultaneous seizure of a route by two impulse responders may be prevented by feeding the battery to the COL leads through a chain circuit common to all the impulse responders including contacts of the relays corresponding to 2D3 and CR3 (Fig. 4) so that only one respond