324,982. Standard Telephones & Cables, Ltd., and Wright, E. P. G. Nov. 8, 1928. Automatic exchange systems. - In a system using single-motion switches associated with common control circuits, the control circuit of a second (or later) stage is seized and operated over a bye-pass from the control circuit of the preceding stage. The bye-pass persists until the control circuit of the second stage has connected itself to the switch to be controlled, whereupon it is cut out and replaced by a direct connection to the conversation trunk. A switch in the control circuit acts first as a numerical marker. controlling the hunting of a second switch to find an appropriate outlet, and then hunts to connect the control circuit to the conversation switch, which is then hunted to the same outlet. Three group-selecting stages and a final stage are shown in Figs. 1 - - 4. The 100 point no-normal conversation switches are represented by the magnets S and wipers S-, S+. &c. The common control circuits, shown below the chain line, each comprise two switches, one represented by the magnet R1 and wipers Y, the other by magnet R2 and wipers Z. The switches R1 are 50-point while the switches R2 are 100-point with two sets of wipers. The operation of the second selector, Fig. 2, is typical and is briefly as follows.-It is tested from the preceding control circuit over wire T, and is not available although idle itself if the associated control circuit is busy. The switch R1 is set from normal by a digit arriving over the bye-pass from the preceding control circuit. The switch R2 hunts to an idle outlet in a group marked by R1 and extends a bye-pass to the next stage, and R1 hunts to connect the control circuit to the conversation trunk. The incoming test circuit is opened, to initiate the release of the preceding control circuit and the conversation switch S is hunted to the outlet seized by R2. The control circuit is released when the control circuit of the next stage has connected itself to the trunk on which S has set itself. It will be noticed that the switch S includes only one stepping magnet and one relay. Microphone, ringing, and busy-tone currents are supplied from the third selector, Fig. 3. The final stage, Fig. 4, is controlled by two digits, the switch R1 acting as a register-marker for each in turn, and has means for serving PBX groups. The first stage, Fig. 1, is designed to absorb a digit of particular value. Specifications 317,521, 324,983, and 324,984 are referred to. First selector, Fig. 1. This is tested over wire T in a circuit including -wipers Y6, ZC in their home positions and if it is available wire K is grounded at a preceding non-numerical switch to energize KS. Relay A is energized over the calling loop and energizes B which locks KS and energizes M. The digit is repeated through C to R1 which steps the Y wipers. At the first step M is cut out and closes a self-stepping circuit for R2, which steps the Z wipers, and the self-stepping circuit is maintained over ZN1 until wiper ZC reaches the terminal marked by wiper Y6, when M is re-energized, stops R2, and energizes E, which locks itself, opens M to restart R2, and connects the upper and lower bank test relays P, Q to wipers ZT1, ZT2. When an available outlet is found P (or Q) is energized, stops R2, and energizes H (or J followed by H) and the loop is bye-passed to the next stage over Z - 1, Z + 1 (or Z - 2, Z + 2). (If no outlet is available in the group, M is energized over ZC in the end position of the group and energizes G to send back tone from BT). Relays A, B release but E, KS are kept locked by ground returned over ZT1 or ZT2. Meanwhile E has closed a self-stepping circuit for R1 to associate the control circuit with the conversation trunk, of which the wire K is marked by ground from the control circuit. When this trunk is found, T is energized and stops R1, and if or when H is energized a self-stepping circuit for S is closed over Y4 to step the conversation switch. Wiper SM is grounded over Y6, so that, when it reaches the outlet pre-selected by the switch R2, relay M is energized over ZM1 or ZM2 and stops S. Relays G, HS are energized and the calling loop is switched through. Relay HS grounds the wiper SH to mark the selected conversation trunk so that it may be identified by the control circuit of the next stage. Release of control circuit. When, as described later, the control circuit of the next stage disconnects ground from ZT1 or ZT2, relays E, KS, T, M, H fall back and homing circuits are closed for the switches R1, R2. Absorbing first digit 6. If R1 is set to its 6th position, W is energized over Y7, locks itself, and closes a circuit to drive R1 to its 11th position. The next digit sets R1 from this position and unlocks W. Subsequent operations are as before. the second digit only being effective for marking, which is controlled over the 12th and subsequent terminals Y6. Failure to dial. If successive pulses arrive on wires TP1, TP2 while R1, R2 are still at home, R1 is driven to find the conversation trunk, the switech S of this trunk is driven to the terminals marked by R2 in its home position, relay HS is energized and locks over the calling loop, and the control circuit is released and R1 restored. Second selector, Fig. 2. The general sequence of operations is the same as in the preceding stage. Relay A is energized over the bye-pass loop and energizes B, which returns ground over the bye-pass test circuit. Relay TT, which is energized when R1 finds the conversation trunk, connects A (or the outgoing bye-pass if H has already been energized) direct to the calling loop, opens the incoming test circuit to release the control circuit of the preceding stage (the byepass of that stage being no longer required) and grounds the incoming third wire of the conversation trunk to provide an interim holding circuit for relay HS of the preceding stage. The control circuit of Fig. 2 is released when its outgoing test circuit is opened later by the relay TT of the control circuit of the next stage. Third selector, Fig. 3. The operations are similar to those in the preceding stages, but the control circuit is not released until the final selector has found and tested the wanted line, and ringing (or the return of busy signal) is then effected from the third selector as described later. Final selector, Fig. 4. This is tested over wire T in a circuit including wipers ZC, Y9 in their home positions, and if it is available A, B, M are energized. The tens, digit is repeated to C and to R1 which steps the wipers Y. At the first step M is cut out and closes a self-stepping circuit over Y8 for R2, which steps the wipers Z until ZC reaches the tens terminal marked by Y9, when M is re-energized. Wiper ZC has only one stopping-position for each pair of terminals Y9, a relay J being energized in alternate positions of Y1 to bring into play one or other set of wipers Z1 or Z2. Relay M closes a circuit over Y7 for G which disconnects J (at the same time locking it if energized) and closes a self-stepping circuit to drive R1 to its 12th position. Relay M is de-energized at the first step, so that the circuit for R1 is first over Y7 and then over Y8. The units digit steps R1 from its 12th position, and when C falls back a selfstepping circuit is closed over Y8 for R2 until ZC reaches the units terminal marked Y9. Relay M is then again energized and energizes E, which locks itself and unlocks G, and a fleeting test circuit is closed through P to ZT1 or ZT2. If the line is tree, relays P, H become energized, ZT1 or ZT2 is grounded, and R1 is driven until it finds the conversation trunk, when T is energized and diverts the driving circuit over Y4 to S. Wiper SM is connected over Y9 to M, which is therefore energized and opens the driving circuit when S reaches the terminal marked by ZM1 or ZM2. Relay HS is then energized, opens the incoming test circuit, locks itself to the third wire (grounded at this moment by ZT1 or ZT2) and switches this wire back to the preceding stage. Supply of ringing current from third selector. The opening of the bye-pass test circuit in the final selector unlocks relay E, Fig. 3, and relays BS, GS are energizel over Y5, Y6 by ground returned over SH. Relays T, TT, H &c. are released and the control circuit is restored. Relay A of the control circuit of the final stage is disconnected and this circuit also is restored. Relay AS is switched across the calling loop and ringing current is supplied to the called loop through FS. On reply, FS switches through to the feed and reversing relay DS. Release of conversation switches. Relay GS, Fig. 3, which grounds the incoming third wire to hold the relays HS of preceding stages, is locked by the calling feed relay AS, while BS, which groundss the outgoing third wire to hold the final selector, is locked by either feed relay AS (via GS) or DS. The test circuit of the third selector is controlled by both the relays BS, GS while the test circuits of the other selectors are controlled by relays HS. The third and final selectors are therefore released only when both-parties hang up, but the preceding selectors as soon as the calling party hangs up. If the called party releases first, an alarm circuit CSH is closed until the calling party also releases. Wanted line busy. In this case relays P, H in the final control circuit are not energized. When the switch R1 finds the conversation trunk the incoming test circuit is opened by T to release the control circuit of the previous stage. The relapse of E, Fig. 3, closes a resistance circuit over Y5 for BS, GS but only GS is energized (since BS is marginal and there is now no ground on SH) and busy tone is returned from BT. Calls to PBX. The first line of a group is connected to battery and the last line to ground in the bank ZX1 or ZX2. If the first line is busy, relay V is energized directly after E, opens the locking