718,394. Automatic exchange systems. CO., Ltd. Jan. 27, 1953, [Feb. 8, 1952], No. In a cross-bar system the line finder and final selector functions are performed by two stages of crossbar switches arranged in groups, each group consisting of a single crossbar switch having its bridges divided between incoming and outgoing trunks, and a plurality of crossbar switches accessible thereto, each giving access to the same subscribers' lines as do corresponding switches of the other groups. General arrangement of line finder/final selector unit-Fig. 1. Each LF/FS unit consists of three 12 x 10 crossbar switches, two of the 12 select magnets providing for double group working in known manner. The unit caters for 400 subscribers, the first and second hundreds groups being connected to the primary switch PSAA, and the third and fourth hundreds groups to PSBA, so that each bridge BR1 to BR 10 in PSAA or PSBA terminates ten lines from each of the first and second groups, or ten from each of the third and fourth groups, respectively. Corresponding links (e.g. 1LA, 1LB) of each set of ten from the primary switches terminate in pairs on the double contact units of bridge BR1 of switch SSA and are multipled over the remaining bridges. Trunking. The number of units provided for each 400-line group would be a maximum of eight with each 200-line subgroup multipled throughout the A or B primary switches. Detailed description-Figs. 4 -10. On the initiation of a call by subscriber number 111 in the 400 line group 111-500, relay L (Fig. 4) pulls up to the calling loop and marks the relevant "hundreds" lead HC (1) (Fig. 5), "units-and-hundreds" lead U1C1 (Fig. 4) and individual lead 111 (Fig. 5). The designation of the "units- and-hundreds" leads are such that the figure following U represents the units digits of lines to which the lead is common and that following C their hundreds digit. Relay W (Fig. 5) pulls up to HC (1) (indicating first hundreds group) and operates group switching magnets PSAM (AA) to PSAM (AH) (Fig. 8) of the primary switches, and SSAM (A) to SSAM (H) (Fig. 6) of the secondary switches. Slave relay WA (Fig. 5) is operated, bringing up DS (Fig. 5) to disable X, Y and Z, and extending the ten leads U1C1 to U0C1 (Fig. 4) to the AUTOMATIC TELEPHONE & ELECTRIC 334/52. Class 40 (4). units relays 1RU to IORU whereupon 1RU pulls up to ground on lead U1C1. This relay brings up IRUW (Fig. 4) indicating that the calling line is in the first hundred group and that its units digit is "1" operates the select magnets 1SM (Fig. 9) in switches PSAA to PSAH, and energizes DSA (Fig. 8) to disable 2RU to IORU. Relay 1RUW extend the individual leads of the ten lines having hundreds and units digits "1" over leads DC (1) (Fig. 5) to DC (0) (Fig. 8) to the ten tens digit relays 1BC to 10BC, of which the relevant one, in this case 1BC, operates. This relay tests whether the bridges appropriate to the calling party (BR1) in PSAA to PSAH are busy, relays DA to DH (Figs. 8, 9) coming up over bridge magnet contacts of the busy bridges to indicate that the trunks of the corresponding secondary switches are not available for the present call. 1BC also operates select magnet ISM (Fig. 6) in each of the secondary switches A to H. Relay ST (Fig. 8), which pulls up in series with 1BC, operates CS to initiate atimingcycle; connects up relay A (Fig. 10), which operates after a delay (left-hand winding short-circuited); operates such of relays AGC to HGC (Fig. 7) as are associated with groups of trunks AOT to HOT which are unavailable or busy; and disconnects battery from the incoming test lead TIC (Fig. 10) from the subscriber's line marker MKR (not shown) to prevent incoming calls. Testing for free trunk. It is assumed that none of the groups of trunks are busy with calls to or from the 400 line group, so that AGC to HGC (Fig. 7) will all be down. Relay A (Fig. 10) operates and locks over its left-hand winding in series with AR. This last relay connects the five test relays 1TR to 5TR to the T wires of the five outgoing trunks accessible to secondary switch A, and if all are busy, no test relay operates, and B (connected up by AR) eventually pulls up. Relay B releases AR and brings up BR which tests the trunks from switch B in like manner. Trunk groups known to be unavailable not tested. If, for example, AGC is' operated, relay B is connected up at the same time as A and both operate, but B prevents operation of AR and trunks BOT are tested immediately. Free trunk tested. Assuming trunks 3, 4 and 5 of BOT to be free battery is present on T3, T4 and T5, and 3TR operates to the exclusion of 4TR and 5TR. This disconnects the operate path to relays A to H, and brings up T0. As a result, bridge magnet 3BM (Fig. 10) of secondary switch B and bridge magnet IBM (Fig. 6) of primary switch AB are operated, and the +, -, S, and M leads of the calling subscriber's line circuit are connected to the +, -, S, and P leads respectively of trunk 3 (BOT). The operated crosspoint of the secondary switch reverts holding earth to the caller's P wire to operate K and hold L. Relays 1RU, 1BC, ST, and W fall away, initiating release of the common equipment. The secondary bridge magnet holds to earth returned over the P wire of the seized trunk over its own contacts 3BM2 (B) (Fig. 7) and wire 3H (B), which leads directly to the bridge magnet. This same earth extends over the M wire of the link and lead 1H (AB) to hold the primary bridge magnet. Indicating class of calling line. The subscriber's S lead (Fig. 4) is connected to earth, resistance earth, or positive battery on terminal ORD, CB, or BT, respectively, according as the line is an ordinary one, a coin-box line, or one to which certain calls are barred. All trunks busy. Relays pairs A and AR to H and HR (Fig. 10) operate in succession and, after a delay, relay EC pulls up, releases any operated relay AR to HR, and relapses to initiate a second test. In the meantime, CS (Fig. 8) (operated by ST) has operated CO, and connected the charged condenser QA across its own winding, CS, disconnected by CO, eventually relapses after QA has discharged, disconnecting CO and releasing ST and 1BC. Thus, during the release time of CO, the common equipment may be taken into use for an incoming call. When CO finally releases, ST and 1BC re-operate, provided no incoming call has seized the common equipment. Permanent loop conditions. Timed release equipment in the outgoing trunk releases the bridge magnets, whereupon L releases, but K remains' held to K2 until restoration of L2, when K holds to the permanent loop, and earth is applied to alarm lead PLA. Incoming calls. It is assumed that a line marker and penultimate selector (not shown) common to 1,000 lines co-operate with three LF/FS common control circuits, 200 lines served by which form part of another 1,000 line group. The marker, having determined which 400-line group contains the wanted line, tests in to lead TIC (Fig. 10) and if the circuit is idle, applies earth to start lead SIC to bring up IC. This relay disables A, AR and so on, and AGC to HGC, and prevents the marking of any outgoing call. The marker then marks the leads of HM (MKR) 1 to 4 (Figs. 5, 6), UM (MKR) 1 to 0 (Fig. 4), and DM (MKR) 1 to 0 (Fig. 8) relevant to the wanted line. The select magnets and group switching magnets in the primary and secondary switches are operated as before, as are the appropriate relays DA to DH (Figs. 8, 9) which disconnect battery from the corresponding test wires of the incoming trunks (Fig. 7), and ground leads GCA to GCH (Fig. 6) leading via the marker to the penultimate selector. On the operation of CO, ground is applied to start lead ST (MKR) (Fig. 10) and the penultimate selector tests the trunks by applying ground to their H wires (Fig. 7). A secondary switch bridge magnet, say 6BM (B), is thus operated, and at the same time ground applied by the marker to one of the leads PSA to PSH (MKR) (Fig. 6) to operate a primary bridge magnet, thus connecting the wanted line to the incoming trunk. The primary magnet holds over the H lead, e.g. 1H (AB), and the M wire to earth at the secondary cross-point. The marker and the common control equipment are released.