US2041873A - Switching system - Google Patents

Description

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11'. A. SPENCER 2,,MLW

' SWITCHING SYSTEM Filed Aug. 13, 1955 9 Sheets-Sheet l FIG.

FIG. 5 FIG 7 l/Vl/ENTOR By 7? A. SPENCER T. A. SPENCER SWITCHING SYSTEM Filed Aug. 15, 1955 S'Sheets-Sheet 2 ie m3 I04 I 7 INVENTOR 71A. SPENCER A TTOR/VEV R E C N E P S A T SWITCHING SYSTEM Filed Aug. 13, 1955 9 Sheets-Sheet 3 ATTORNEY May 26, 1936 T. A. SPENCER SWITCHING SYSTEM Filed Aug. 15, 1935 '9 s eets-sheet 4 J lNl/ENTOR y fl. SPENCER Ho T. A. SPENCER SWITCHING SYSTEM Filed Aug. 13, 1935 9 Sheets-Sheet 5 INII/ENTOR y 7; A. SPENCER ATTORNEY may 2% WM T. A. SPENCER SWITCHING SYSTEM Filed Alig. 13, 19 35 9 Sheets-Sfieet 6 FIG. 6 2/9 626 III m m In 5/? /3 7'0 SIP/9 SR 20 nvv TWLSPENCEP BY A TTOR/VEV T. A. SPENCER Emmm SWITCHING SYSTEM Filed Aug. 15, 1935 9 Sheets-Sheet 7' llvl/ENTOR By 7%. SPENCER May 26, 1936. T. A. SPENCER SWITCHING SYSTEM Filed Aug. 13, 1935 9 Sheets-$heet 9 I II'WTFJa/ 19/ a1 mi /@7 1 W 1 I Z l i .U l I i =E= lM/EA/YbR By TA. SPENCER A TTORA/EV Patented May 26, 1936 @NETED STATES PATENT OFFICE SWITCHING SYSTEM Application August 13, 1935, Serial No. 35,927

Claims.

This invention relates to switching arrangements for telephone systems and particularly to those using switches of the cross-bar type.

The objects of the invention are to increase 5 the accessibility of a group of lines, to economize in the amount of switching equipment necessary to serve the lines, and to improve the arrangement of lines in switching devices of the above mentioned character.

A feature of the invention is a switching system in which a plurality of trunks are accessible to groups of line-s which are assigned to the rows of contacts in a plurality of cross-bar switches in such a way that each one of the line groups appears in a vertical row of contacts in one switch, in a vertical row of contacts in a second switch and in a horizontal row of contacts in a third switch, and in which connections may be effected by establishing the connection direct- 1y between a particular line group in its verticalrow appearance in the first switch and a particular one of the trunks or by establishing the connection between said line group in the second switch and any one of the trunks by way 5 of a particular one of a number of link circuits, or by establishing the connection between said line group in the third mentioned switch and any one of the trunks by way of said link circuits, the particular link used depending on 30 which line in the group is involved.

Another feature of the invention relates to a system, as above described, in which connections not requiring link circuits are effected directly through the switches in which the lines appear,

35 whereas connections requiring link circuits are effected through other cross-bar switches in conjunction with the said switches in which the lines appear.

The foregoing and other features of the inven- 40 tion will be more fully discussed in the following detailed description, taken in connection with the annexed drawings, and will also be more fully understood from the appended claims.

In the drawings Figs. 1 to 8 when arranged 45 as shown in Fig. 10 illustrate the invention when applied to a line switch stage in a telephone system for connecting any one of a group of 200 lines through cross-bar line switches to an idle one of a group of trunks.

50 Figs. 1 and 2 show a number of subscribers lines in each of several line subgroups together with the line and cut-off relays and other control equipment.

Fig. 3 shows a cross-bar line switch in which 55 appears the group of 200 lines illustrated in Figs.

1 and 2 and which serves to extend said lines directly to the outgoing trunks.

Fig. 4 shows two cross-bar line switches in both of which said group of 200 lines appear. These switches serve as primary line switches 5 to extend the lines to link circuits which have access to the outgoing trunks through secondary cross-bar line switches. These secondary switches are shown in Fig. 8.

Figs. 5 and 6 show multi-contact switching re- 10 lays for controlling the completion of a call from any line group through the switches of Figs. 3,

4 and 8.

Fig. '7 shows control equipment and circuits.

Fig. 9 is a schematic of the line switching stage 15 shown in Figs. 1 to 8.

For the purpose of simplifying the disclosure, only one switch contact is shown at each crosspoint in the sleeve or control lead for each line, whereas the switch in actual practice would 20 carry three or more contacts at each cross-point, two contacts being required for the talking leads shown at the top of Figs. 1 and 2. The arrangement of the switches is such that any contact set is capable of operation upon conjoint displacement in definite order, of the two bars, not shown, which intersect in the locality of this set of contacts. The contact sets are maintained actuated by the displacement of the last actuated one of the two bars, and are restored to normal by their own resiliency on the release of the last actuating bar. The first bar to be actuated in operating any given contact set is referred to as the horizontal bar and is actuated by a selecting magnet shown immediately to the left of each row of contact sets in each switch, and the second bar to be actuated is referred to as the vertical bar and is actuated by a holding magnet shown directly below the vertical row of contact sets in each switch unit. The cross-bar 40 switches shown in Figs. 3, 4, 8 and 9 may be of any well-known type, such as the one described in detail in the pending application of J. N. Reynolds, Serial No. 702,453, filed December 15, 1933,

granted as Patent 2,021,329, Nov. 19, 1935. Also certain portions of the cross-bar switches have been omitted, as well as much of the control equipment, to simplify the disclosure, only so much being shown as is necessary to a clear understanding of the invention.

The line and cut-01f relays of the first 100 lines of the entire 200 lines are shown in part in Fig. 1,

in which the first vertical row of relays represent lines 1 to 10, with a line relay Ll, L2 or LI!) and a cut-off relay, I, 2 or H) for each line, intermediate relays being indicated at taps I30. The second vertical row of relays represent lines 11 to 20, and a third row represent lines 91 to 100, with intermediate rows of relays represented at taps I3I. In a similar manner the line and cut-off relays for the second group of one hundred lines, lines 101 to 200 are shown in part in Fig. 2 with intermediate relays indicated at taps 2I2 and intermediate rows of relays at taps 2I3. Two control relays, such as Al and BI, are associated with each of the 10 horizontal rows of line relays. These relays in conjunction with another group of control relays, such as CI and EI, associated with each vertical row of line and cut-01f relays, identify the number of a line that is calling so that the trunk to which the line has access may be tested for busy as will later be described. The control relays referred to above are further controlled or directed by relays 29!, 202, 203, 204 and 223, located in the lower right corner of Fig. 2, to prevent more than one line at a time from being assigned to an idle outgoing trunk as well as to direct the selection of a trunk.

The system also includes 20 sets of switching relays, such as SRI and SRIZ, shown in Figs. 5 and 6 respectively, there being one set of relays for each vertical row of 10 line and cut-oil relays. As will later be explained,'each subscribers line is given three successive points of access to the outgoing trunk circuits, so that the purpose of the various sets of switching relays is to direct the completion of a call first through the crossbar switch LS, shown in Fig. 3, or, if the particular trunk to which the subscriber has direct access in this switch is busy, then the switching relays will direct the call to crossbar switch PLSI in the upper part of Fig. 4, and, if the link to which the subscriber has access in this switch is busy, they will direct the call to cross-bar switch PLS2, shown in the lower half of Fig. 4. To effect these accesses, the subscribers lines have multiplied appearances in cross-bar switches LS, PLSI and PLSZ.

A group of 20 outgoing trunks TI, T2 appearing in Fig. 8 is provided with 20 test relays, one per trunk. Six of these are shown in Fig. 7. These relays act as busy test relays, causing a call being directed over one of the vertical links from crossbar switches PLSI or PLS2 of Fig. 4 to be directed to the first idle trunk in the two groups of 10 outgoing trunks TI and T2. The two cross-bar switches designated SLSI and SLSZ represent the secondary element of a primarysecondary switch arrangement with the links MEI to 420, inclusive, shown vertically in Figs. 4 and 8 and interconnecting the primary switches PLSI and PLS2 and the secondary switches SLSI and SLSZ. The outgoing trunks 30! to 32!] are shown connected horizontally to the cross-bar switch contacts, thus permitting any link to connect with any idle trunk.

The diagram of Fig. 9 shows the cross-bar switch arrangement in schematic form and brings out more clearly the manner in which the subscriber lines are given access to outgoing trunk circuits. Cross-bar switch LS, shown at the upper left of this figure is the same as shown in detail in Fig. 3 and provides for a subscriber line multiple of 200 lines arranged in such a way that each sub-group of 10 successive lines appears in a vertical row and has direct access to one of the outgoing trunks 3IJI to 329. The same 200 lines have a second appearance in the cross-bar switch PLSI, shown in detail at top of Fig. 4 and sche- 75 matically at the upper right of Fig. 9, and have the same multiple arangement as in the case of the cross-bar switch LS, namely, each of 10 successive lines appears in a vertical row. This appearance of the lines, however, instead of having direct ac cess to the outgoing trunks 31H to 320 has access to these trunks through link circuits 4M to 420. A third appearance of the same 200 lines is shown in the cross-bar switch PLSZ in detail at bottom of Fig. 4 and schematically just below the switch PLSI of Fig. 9, these lines having access to the same group of link circuits, but with the links for the first and second hundred lines interchanged. In addition to this, the sub-groups of 10 lines appear in horizontal rows rather than vertical rows. Arranging the line multiple in the above manner provides a high degree of trunk accessibility, considering the limited number of line appearances, because in any subgroup of 10 lines one particular trunk is directly available as a first choice, any one of all the trunks over a particular link circuit is available as a second choice or any one of all the trunks over a diiierent link circuit is available as a third choice. Furthermore, if 10 incoming calls should be centered in one subgroup of 10 lines, for example, all 10 calls would have access to the outgoing trunks over separate links and no call would be blocked as is the case in ordinary subgroup multiple arrangements.

While the invention is illustrated in a system 5 in which the switches act as line switches, it is obvious that it applies to other switching arrangements, such as the intermediate and final switching stages of a telephone system.

A detailed description will now be given of the manner in which an incoming call is connected directly to an idle trunk, or if said trunk is busy the manner in which the call may reach any idle trunk over a particular link circuit, or if said link is busy the manner in which the call may reach any idle trunk over a dilierent link circuit. When the usual receiver is removed at a subscriber station, such as A-I, a bridge is placed across the line conductors I32 and I33. This causes line relay LI to operate, ground being connected to lead I25 at outside break contact of cut-01f relay I, thence to conductor I32 of line A-I. Battery through winding relay LI is connected to lead I36 at inside break contact cutoil relay I, thence to conductor I33 of line AI. Upon operating, relay LI at its right inner contact prepares a path for operating relay CI by connecting lead I23 to lead I26 as later will be described. Relay LI also closes a circuit for operating relay AI of Fig. 2 over a path traced from ground at right outside break contact relay EI, lead IZI, right outer contact relay LI, lead I22 of top break contact relay 223 of battery through winding relay AI. Relay AI at its front contact connects lead 296 to left winding relay BI. Lead 266 being a test lead running through the back contacts on other relays A2 to AIII will be open at the left back contact relay 283 if another call in the grOup of 200 lines is already in the process of selecting a trunk. Assuming, however, that relay 223 is not operated, then ground from the top break contacts of relays 20! and 282 will be connected through break contact relay 203, lead 206, through the break contact of relays A2 to AIEI, make contact on relay AI and through left winding relay BI to battery. Had it been assumed that simultaneous calls had originated on subscriber lines AI and A-2 or AI and A-IIJ, then relays B2 or BIG respectively would have operated in the path previously traced. in preferenceto relay BI. Since a call from line AI, is being traced, relay BI operates to close a circuit from battery through the right Winding relay BI and front contact to another test lead. 201, through the armatures and break contacts on other B relays to ground through the winding relay 203. Relay 203 operating, then opens lead 206 as mentioned above and. prevents later calls from interfering with the handling of the call on line AI. Test leads 206 and 201 therefore control the vertical preference in the ZOO-line group and permit the lowest of the calling lines in any group of 10 to search for a trunk and lock out all other lines until an idle trunk is found or until the call is timed out, when another attempt is to be made to select a trunk.

Before a search is made for an idle outgoing trunk for the call on line AI, it is necessary to determine in which vertical group of 10 lines the calling line A-I is located. This is determined by the common relay 204 in lower right corner Fig. 2 and the relays CI to C20, inclusive, together with lead I25. Therefore immediately after the vertical preference is determined, described in the preceding paragraph, the B5 relay closes a circuit for operating relay CI over a path traced from ground at the top break contact relays 2DI and 202, through lower contact and armature relay 204, thence over multiple lead 208, contact 209 on relay BI, multiple lead I24, through operated inner contact on line relay LI, multiple lead I23, right inner contact relay EI and through bottom winding relay CI to battery. Relay CI in ope-rating locks to ground through its upper winding and. front contact to lead I25 traced through all the top break contacts and armatures of relays C2 to C20 thence through winding of relay 204 and ground at top contacts of relays 20I and 202. Relay 204 in operating opens the bottom contact which breaks the operating circuit previously traced for the lower winding of relay CI thus preventing later calls that might originate in other groups of lines from interfering with the handling of the call on line A--I. The identity or location of .the calling line is thus established by virtue of the fact that only one of the B and one of the C relays, namely, BI and CI remain operated. Before proceeding, however, to show how this line is actually connected with an idle one of the outgoing trunks, it is thought that the function of the circuits in Figs. 1 and 2 will be more clearly understood if a description is given of several other cases where more than one line originates a call at the same time.

Assuming, for example, that subscribers on lines AI and A9I lift their receivers about the same time, line relay LI and corresponding BI and CI relays will operate as previously described. Line relay L9I will alsouoperate and connect ground that is on lead I26 to lead I22. Since both relays LI and LDI connect ground to lead I22, which path is further traced through relay AI of Fig. 2, it is evident that only the BI relay will operate. The vertical preference is thus determined by the BI relay being operated, both lines being the topmost one in their respective sub-groups of ID. When, therefore, ground is placed on lead I24 at contact 209 by relay Bi, a circuit is established for operating both the CI and CID relays traced over lead I24 to right inner contact relay L9I, lead I21, right inner contact relay EID to battery through lower winding relay CID, also traced over lead I24 through inner contact relay LI ,lead I23, right inner contact relay EI, to battery through lower winding CI. In this case the locking circuit through the upper winding relay CID is completed back over the series lead I25 to the winding of relay 204 to ground, while the locking circuit through the upper winding of relay CI is held open at the top break contact on relay CID. Relay 204 operating, thus opens at its bottom break contact the circuit for energizing the lower windings of relays CI and CID. Relay Cl therefore releases and relay C I remains operated. In this manner, line ASI is identified as the calling line to be handled first and search for an idle outgoing trunk will be directed by the combination of the BI and CIO relays remaining operated as will later be described.

Another example is assumed where subscriber lines AI, A--9I and AIZD lift their receivers nearly simultaneously, in which case relay AI operates over lead I22 through contact on relays LI and LDI as previously traced. Line relay LIZD also operating connects ground that is on lead 2I0 to lead I28 thence through top inner contact on relay 223 and through winding relay AID to battery. The operation of relay AIO causes relay BID to operate from ground on lead 206 and at the same time the right break contact opens the circuit to the armature of relay AI and prevents relay BI from operating. The failure of relay BI to connect ground tolead I24 prevents relays CI and CID from operating. On the other hand relay BID at contact 2 It connects ground to lead I23 further traced through inner contacts relay LIZD and EI2 to battery through lower winding relay C I 2. In this manner line AI 20 is identified as the calling line to be handled first and search for an idle outgoing trunk is determined by the combination of the BID and CI 2 relays remaining operated as will later be described.

Going back to the condition ensuing forthwith upon the identity of calling line AI being determined by relays BI and Cl remaining in an operated condition, there will be now by describing the manner in which a search is made for an idle trunk in the two groups of trunks designated TI and T2 in Fig. 8. Relays 203 and 204 it will be remembered are also in an operated condition at this time. A calling line as previously stated has three possible channels for reaching an idle trunk, so that it will first be assumed that the first choice trunk 301 of Figs. 3, 7, and 8 is idle. Relay I of Fig. 5, therefore, is normal, because if the trunk 3DI were busy relay 5 would be operated as will presently appear.

Relay 203 when it first operated toopen test lead 206 also connected ground from the top break contacts of relays 20I and 202 to multiple lead 2I5, thence through contact 2 IE on relay BI to lead 2I'I, through inner make contact relay 5I2, which is operated at this time, to top armature 5I3 of relay 5I I, thence over lead 5M of Figs. 5, 6, '7 and 3 to battery through winding of selecting magnet 322 of switch LS. In the path just traced relay 5 I 2 of Fig. 5 is operated because relay C I being operated, connects ground at its bottom inner armature to lead I38, thence to battery through winding relay 5 I2. Having operated the selecting magnet 322 in cross-bar switch LS of Fig. 3 corresponding tothe horizontal row in which the multiple of line A--I appears, the circuit will now be described that operates the corresponding holding magnet.

The circuits of this system are organized to avoid interference by preventing the holding magnet circuit from being closed until a particular selecting magnet has operated. Thus the operation of the selecting magnet 322 connects ground at its lower armature and front contact to multiple lead 323 of Figs. 3, and 2, to battery through relay 223. Operation of relay 223 opens the circuit for relay AI and closes ground to multiple lead 220 which is further traced through top operated contact on relay 284, to multiple lead 22I of Figs. 2 and 1, bottom front contact of operated relay C I, thence over lead I39 to battery through winding relay 5I6. The latter relay operates and prepares the 10 leads indicated at 522 for operating one of the holding magnets. Ground placed on lead 220 by relay 223 of Fig. 2 also operates slow release relay 28I over lead 22! which in turn operates slow operate relay 202. The latter relay provides a time interval in which to insure operating the holding magnet before ground is removed from the controlling circuits involved in selecting a trunk for line A-I. Relay 20I provides another time interval before restoring ground to the controlling circuits and preparing the system for another call. Although relay 5I6 closes the circuit for 10 holding magnets only one holding magnet circuit will be grounded as determined by which one of the BI to BIO relays of Fig. 2 is in an operated position. The circuit for holding magnet 32I as determined by relay BI may be traced from ground through operated make contact relay 2E3, multiple lead 2I5, contact 2I8 on relay BI to lead 2I9 of Figs. 2, 6 and 5, through contact 5I5 on operated relay 5I6, thence through armature 5! I, relay 5| I and back contact over lead 5I8 of Figs. 5, 6, 7 and 3 to battery through holding magnet 32I. The latter operates in this circuit causing contacts at the cross-point for line A-I to close, thus connecting line AI directly with trunk '30I over a path represented by lead I34 of Figs. 1, 2 and 3, lead 323 of Fig. 3 and conductor 3M of Figs. 3, 7 and 8.

Upon connecting with an idle trunk, ground is returned over the sleeve or control wire, just traced, for holding the connecting switches until the subscriber restores his receiver to the switchhook, all in a manner well understood in the art. Therefore soon after contacts I in the switch LS of Fig. 3 close, ground may be traced from the trunk of Fig. 8 back over lead 30I, thence over lead IGI, through winding relay 5| I, lead 5M to left winding of holding magnet 32I, to the midpoint of which windings direct operating ground was traced in the previous paragraph. Thus when the timing relays 2!]I and 262, previously referred to, remove ground from the lead connected to the mid-point of holding magnet 32 I, the magnet continues to: remain operated on the trunk holding ground through both windings in series. Furthermore, since the winding of relay 5i I is included in the path just traced, relay 5| I also operates and remains operated as long as the connection is held by the subscriber. At this time cut-off relay I of Fig. 1 also operates over lead I3I from ground on the sleeve wire I34, thus releasing line relay LI, this action taking place before ground is removed by the timing relays.

Having described a call from subscriber line A-I on the assumption that the outgoing trunk 30I was idle, this call will now be described on the assumption that trunk Sill is busy and that the next trunk choice is available. In this case it will be observed that the second choice appearance of line A-I is in switch PLSI at top of Fig. 4, and to reach an idle trunk of group TI or T2 it must be assumed that the link MI is idle, which means all cross-bar switch contacts associated with this link are open and that relay I of Fig. 8 and holding magnet HI are unoperated. It will be remembered that the BI relay and the CI relay have operated to mark the location of the calling line and together with relays 203 and 204 remain operated until the line is connected to a trunk. When relay 203 first operated, ground was connected to lead 2 I I as previously described, which is now further traced through inner make contact relay 5I2 to top armature 5I3 and operated contact of relay 5| I, to armature 523 and back contact on relay 520, thence over lead 524 to battery through selecting magnet 42I of line switch PLSl. Relay EU is operated at this time because the first choice trunk 30I is assumed to be busy. Relay 5I2 is operated by relay CI and relay 520 is unoperated because the link busy relay Bill of Fig. 8 is unoperated. Operating selecting magnet 42I at its lower contact prepares a path for operating relay 223 and at its inner armature and contact closes ground to multiple lead 422, which connects to a series circuit running through the right-hand armatures and front and back contacts of the 20 test relays associated with the 20 outgoing trunks 3DI to 320. Lead 422 connects first to the armature of relay I2I, but, if the associated trunk 30I is busy, ground on the sleeve lead holds it operated and lead 422 is thereby extended to the right armature of relay I22. If associated trunk 302 is busy, the latter relay will be operated and lead 422 will thereby be extended to the next test relay. Therefore, any selecting magnet in the switch PLSI or PLSZ which operates and connects ground to the multiple lead 422 will automatically operate the selecting magnet associated with the first trunk in group TI available to take the call, or if all TI trunks are busy it automatically in like manner selects the first idle trunk in trunk group T2 or if all T2 trunks are busy, ground on lead 422 will operate relay I23 and unlock all the test relays such as relay I2 I that have operated. Since these relays provide for uniformly distributing the traffic over all the trunks, some relays will continue to be held operated, although the trunk itself may be idle. Also those relays associated with trunks that are busy will remain operated even though their locking winding is opened by relay 123. When all the trunks have had a call, relay 723 therefore makes all trunks that are not busy available to receive another call. Assuming now the trunks 30I to 309 are busy and the trunk 3I0 is the first one that is idle, relay 124 will be the first normal relay in the series starting from the top of Fig. '7, so that lead 422 may then be traced through right back contact relay I24 to battery through winding of selecting magnet 802. Selecting magnet-862 operating connects ground to multiple lead 803, further traced through bottom make contact on operating selecting magnet 42I, thence over lead 323, to battery through winding relay 223. The latter relay as previously described supplies ground at its bottom make contact to lead 220 and traced further through top make contact relay 204, lead 22I, lower contact on CI relay over lead I39 to battery through relay 5I6.

The operation of relay 5I6 closes the 10 holding magnet leads indicated at 522, one of which will be grounded for operating the holding magnet HI associated with the idle link 40I of Figs. 4 and 8. This path may be traced from ground on lead 2I9 previously described, through operated contact 5I5 of relay 5H5, armature 5I'I and front operated contact relay 5! I, armature 525 and back contact relay 525, thence over lead 525, right inner armature and back contact relay 6M to the midpoint of the two windings of holding magnet I-II, to battery through the right-hand winding. Operation of holding magnet HI on cross-bar switch SLS2 connects ground through the windings in series of the holding magnets that are associated with the same link circuit. In this manner line contacts I in cross-bar switch bank PLSI are held operated by holding magnet 42% and trunk contacts 804 are held operated by holding magnet 805, thereby extending the calling line AI traced over lead I55 to lead 425, contacts I on cross-bar switch PLSI, link lfll, contacts 805 of cross-bar switch SLSI, and finally to trunk 3H]. Referring to the series circuit through the holding magnets referred to above it will be observed that the holding magnet 521 of cross-bar switch PLS2 will also be operated although no selecting magnet in this switch was energized. The only effect of this is to prevent any contacts at the cross-points in this switch associated with link MI from being operated as long as this magnet is operated. Similarly the holding magnet HI of cross-bar switch SLSZ prevents any of the cross-points for link lIlI from being operated.

A call originated by subscriber line A-I will now be described on the assumption that trunk 35H is busy, that link AM is busy, and that the next choice link is idle. The link in this case is M I, because it is the only other onethat can connect the multiple appearance of line A-I in cross-bar switch PLSZ with one of the idle trunks of groups TI or T2. Since link MI is assumed to be idle, relay 856 and holding magnet HI I are unoperated. When relay 2613 first operated, ground was connected to lead 2I'I as previously described, which now may be further traced through inner make contact relay 5I2, top armature 5I3 of relay 5II, armature 523 and front contact relay 525, thence .over lead 521 to battery through selecting magnet 328. Operation of the latter magnet at its bottom outer contact prepares a path for operating relay 223 and at its bottom inner contact connects ground to multiple lead 422, which connects to the series sets of contacts on the test relays shown in Fig. '7 previously described. For this call it will be assumed that the first l trunks 3M to 3m are busy or are being held busy so that relay 725 is the first in the series that is unoperated due to the associated trunk being idle. Therefore, ground on lead 422 may be traced through the armatures and front contacts of relays 'I2I, I22 and I24, to right armature and back contact relay I25, to battery through selecting magnet 851 of cross-bar switch SLS2. This magnet operates and closes ground to multiple lead 853 for operating relay 223 through the operated contact on selecting magnet 426, thence over lead 423 and lead 323 to battery through relay 22. Operation of relay 223 closes ground for operating the holding magnet associated with link II I over a path traced on multiple lead 220, top contact relay 2M, lead 22I, bottom front contact relay CI, lead I39 to battery through relay I6. The operation of relay 5S6 prepares the circuits for all ten holding magnets, .one of which will be grounded for operating holding magnet I-III associated with the idle link III of Figs. 4 and 8. This path may be traced from ground on lead 2l9 previously described,

' relay 5I6 of group SRI.

through contact 5I5 of relay 5I6, armature 5I'I and. front operated contact relay 5i I, armature 525 and front operated contact relay 52E], thence over lead 529, right inner armature and back contact relay 8% to the mid-point of the two windings of holding magnet HI I, to battery through the right-hand winding. Operation of holding magnet HIE on cross-bar switch SLSZ connects ground through the windings in series of the holding magnets associated With link M I. In this manner contacts I in cross-bar switch bank PLS2 are held operated by holding magnet 428, and trunk contacts 828 of switch SLS2 are held operated by holding magnet HI I, thereby extending the calling line A-I to trunk 3| I. Holding magnet 529 and magnet 8515 also being operated at this time merely serve to prevent any other cross-bar switch contacts associated with link M I from being operated by the selecting magnets as previously explained.

Let it be assumed now that both lines AI and A9I attempt to originate a call at the same time. As previously described only line A-9I will be identified by the control relays of Figs. 1 and 2 wherein relays BI and CIEI remain operated until a connection is established between line 9! and an idle outgoing trunk. This line, as in the case of line I, has three different channels for reaching an idle trunk so that it will only be necessary to describe one channel which, for the purpose, may be assumed to be the second choice one in which the multiple of line A--9I appears in switch bank PLSI of Fig. 4. The associated link lIil under this assumption is therefore free at this time which means that test relay M5 and holding magnet HIU are unoperated. Since it is assumed the first choice trunk 3Iil for this line is busy, it is evident that holding magnet 325 of switch LS will be operated together with its associated switching relay (not shown) in switching relay group SRIIl corresponding to relay 5II previously described for relay group SRI Selecting magnet l2i of switch PLSI will then be operated from ground through contact 2I5 on control relay BI over lead 2I'I and lead 533 through operated contacts on the switching relays in group SRIB corresponding to relays 552 and 51! I, and back contact on relay corresponding to relay 525, thence over lead 53!] to lead 525 to battery through selecting magnet 52i Switching relays in group SREB rather than in SRI function in the above path due to relay CI 0 being operated instead of relay CI. Selecting magnet dZI then connects ground to lead 422 which connects to the series contacts running through relays 'I2I, I22, etc. It is now assumed that all trunks 351! to 3H are busy by way of example so that relay 7125 is the first one that is unoperated. Ground on lead 422 may therefore continue to be traced to battery through selecting magnet 3I2. Operation of this magnet connects ground to lead 853 for operating common relay 223 as previously described, which in turn connects ground to lead 225 for operating the switching relay in group SRIE corresponding to This relay closes the circuits of all associated holding magnets so that ground connected by relayBI to lead 2I9 may then be further traced over lead 52H and through operated contacts on the switching relays corresponding to relays 5H5 and 5i I and back contact on relay corresponding to relay 520, thence over lead 532, through inner back contact of relay 8II1 to mid-point of windings on holding magnet HI 0. Line 9| appearing in switch bank PLSI is therefore connected over link 4H] and trunk contacts 8| 4 to trunk 3 I 2 in a similar manner as previously described for line A-l.

If, as previously discussed, lines Al, A9 I and Al20 attempt to originate calls simultaneously, then the control relays of Figs. 1 and 2 will identify line A-|2fl as the line next to be served and relays Bi 0 and Cl2 will remain operated. Since relay C|2 controls the switching relays 62l and 622 in group SRI2, the selecting and holding magnet circuits will be connected through this group of switching relays in preference to other groups. Assuming now that the first and second choice trunks for line A--l20 are busy, relays 623 and 624 will be operated in a manner similar to that previously described for relays 5| l and 520. The circuit for selecting magnet 432 may then be traced from ground through operated contact on relay 263 to lead 2l5, operated contact relay BID to lead 222, then over lead 625, operated contacts on relays 62 l, 623, 624, lead 626 to battery through selecting magnet 432. The selecting magnet connects ground to lead 422 as previously described, in order to operate the selecting magnet associated with the first idle outgoing trunk. In this case it is assumed all trunks are busy except the last one, namely trunk 326. The corresponding test relay 12! is therefore normal and the ground on lead 422 thereby extended to battery though selecting magnet 813. Now that both selecting magnets are operated ground is returned to lead 803 for operatingrelay 223 over a path extending through lower make contact on selecting magnet 432 to multiple lead 423, thence over lead 323 to battery through winding relay 223. The latter relay operated then connectsground to lead 220, traced further through operated upper contact relay 204, lead 22| operated lower contact relay CI2, lead 224 to battery through winding of relay 622. Relay 622 operated prepares the circuits for the 10 associated holding magnets. Holding magnet HID, however, is the only one that will operate due to the BIO relay, which connects ground to lead 225, thence over lead 621 through operated contact 628-on relay 622, operated contact 629 on relay 623, operated contact 630 on relay 624, thence over lead 63! to lead 532, right inner break contact test relay 8 ID to mid-point of holding magnet I-Ilfl. Line I20 appearing in switch bank PLSZ is therefore connected over link 4| I] and trunk contacts 8| 5 to trunk 320 in a similar manner to that previously described for line AI.

From the foregoing description it is obvious that a multiple arrangement of subscriber lines is provided which increases the accessibility to a group of outgoing trunk circuits. It will further be obvious that the particular circuit arrangement disclosed provides means for connecting calling lines one at a time over any one of three connecting channels in a predetermined order to a group of outgoing trunk circuits but it will readily be understood that the circuit arrangement may be varied to suit the particular unit of a telephone system and that other ways of accomplishing the same results might be employed.

What is claimed is:

1. In combination, switches, each having rows of separately operable contact sets, a group of lines appearing in a row of contacts in each of a plurality of said switches, trunks, links, means including a particular one of said trunks for effecting a connection in said group of lines by way of one of said switches, means including any of said trunks and a particular one of said links for eifecting a connection in said group of lines by way of a second one of said switches, and means including any one of said trunks and any one of said links for effecting a connection in said group of lines by way of another one of said switches.

2. In combination, switches, each having rows of separately operable contact sets, a plurality of groups of lines, each group of lines occupying a row of contacts in each of said switches, a group of trunks, a plurality of links, means including a particular one of said trunks for establishing a direct connection with a given group of lines by way of a particular one of said switches, means including any one of said trunks and a particular one of said links for establishing a connection with said given group of lines by way of a second one of said switches, and means including any one of said trunks and any one of said links for establishing a connection with said given group of lines by way of a third one of said switches, the particular link used depending upon the line involved.

3. In combination, three switches, each having vertical and horizontal rows of contacts, a group of lines appearing in a vertical row in one of said switches, appearing in a vertical row in the second one of said switches and appearing in a horizontal row in the third switch, a plurality of trunks, one

of which appears directly in said vertical row of 35 contacts in said first mentioned switch, and a plurality of link circuits connectible to said trunk lines, one of which appears in said vertical row of contacts in said second switch and all of which appear in the horizontal row of contacts in said third switch.

4. In combination, primary switches, secondary switches, a group of lines appearing in each of said primary switches, trunks appearing in a particular one of said primary switches and in said secondary switches, links appearing in certain of said primary switches and said secondary switches, and means for operating said switches to extend a calling line in said group either through said particular primary switch directly to one of said trunks, or through one of said primary switches and one of said secondary switches by way of one of said links, or through another one of said primary switches and one of said secondary switches by way of another of said links.

5. In combination, first, second and third cross-bar switches, each having vertical and horizontal rows of contacts, groups of lines, each group appearing in a vertical row of contacts in the first and second switches and in a horizontal row of contacts in the third switch, a plurality of trunks appearing respectively in the vertical rows of contacts-of said first switch, a plurality of links appearing respectively in the vertical rows of contacts of said second and third switches, and other cross-bar switches in which both said trunks and links appear.

THOMAS A. SPENCER.

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