Manual / Guide · 1957
AE Training Bulletin 821: Linefinder Switches
Linefinder switch operation and allotter.
Manual / Guide · 1957
Linefinder switch operation and allotter.
ALiTOMATjC ELECTRIC TRAINING S E R I E S ar This is one of the iielpflll booklets in the AVTOTdAT3C ELECTRIC 'I'KASBIP.!G SERIES Or! STROWGER AU'TOMATTC T E L E P H O f i E SYSTEMS E l e c t r i c a l P r i n c i p l e s of Telephony Pri.nciples of Telepholly F u n d a m e n t a l s of Apparatus and Trunking The P l u n g e r Lineswitch arid Associated Master-Switch R o t a r y Lineswitch The Connector The S e l e c t o r Pulse Repeaters m I runking P o w e r arid Supervisor:; Equipmelit Fa.rty-L_lirse Connec"Lor Tril,nk-BuntingConnectors Reverting-C all PJLe thods The Test and Verification Switch T r a i n 'TollSwitch T r a i n Switching Selector-Repeater ?. P r i v a t e Automatic Ea;changes with PABX 14ppexrai.x sc-~ m r r r u n i t yAutorzratic Exchanges fdanual S v i t c h b s a r d s Lifiefinder Switches $:lay w e send you others c"e3-+.dLnli:g i s equipment in -yourexchange? ;~ CONTENTS 1. Theory 1.1 Introduction . . . 1.2 The 100-line linefinder 1.3 100-line linefinder shelf 1.4 200-line linefinder shelf 1.5 Preferential hunting 2 . Physical arrangements . 2.1 Variations 2.2 Overflow shelves . 2.3 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Components 3.1 Line equipments [2-relay and f -relay circuits] . . . . . . . 3.2 Group relays . . . . . . . 3.3 Distributor . . . . . . . . 3.4 Linefinders . . . . . . . . 3.5 Fuse panel . . . . . . . . . 3.6 Meters . . . . . . . . 4 . Normal operation . . . . . 4.1 Marking the calling line . . . . . . . . . . 4.2 Starting . . . . . . . 4.3 Vertical stepping . . . . . . . . 4.4 Rotary stepping . 4.41 Need for rectifiers [message-rate service] . . . 4.5 Extending calling line to selector . . . 4.6 Stepping distributor to next idle finder . . . . . . . . . . 4.7 Releasing 5. Special operations 5.1 Linefinder does not find vertical marking 5.2 Linefinder does not find rotary marking . 5.3 All linefinders of one group a r e busy . . . 5.4 All linefinders of shelf busy 5.5 Linefinder fails to cut i n . . . . . . . . . . . . . 6 . Circuit features . . . . . . 6.1 Restricted service . . . . . 6.2 Conversation timing . . . . . 6.3 Class-of-service indication . . 6.4 Permanent timing and line lock-out 6.41 The three-relay lock-out line circuit 6.42 The mechanical lock-out line equipment 6.43 " 1 0 0 ~lock out'' . . . . . 6.431 Reverting calls . . . 6.5 Testing 6.51 Tracing a call o r a "permanent" . 6.52 Monitoring . . . . . . 6.53 "Cascading" test . . . . . . . . . . . 6.54 Test lines . 6.55 Timing-out and transfer test . . 6.56 Speed and ratio test . . . . . 6.6 Metering 6.61 Peg-count metering . . . . . 6.62All-trunks-busymetering . . . 6.63 Overflow metering . . . . . 6.7 Busy keys . . . . . . . . . . . . . 6.8 Supervisory alarms 6.9 Spark suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . Traffic-grading overflow shelves 7.1 Description . . . . 7.2 Application . . . . 7.3 Installation . . . . 7.31 Mounting . . . 7.32 Cabling . . . 7.4 Circuit operation . . . . . . . Complete circuits 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NDER SW Bulletin 821 1. THEORY 1.1 Introduction. At no one time will all. subscribers of a telephone exchange use their telephones simultaneously. Therefore, an automatic exchange need provide links to accommodate just a fraction of the subscribers a t any one t i ~ n e( a s in manual offices where the number of cord circuits is just a fraction of the number of lines). Since these are fewer links than lines, the links a r e placed in a pool, t o be used by the lines as needed. A device is required to connect a calling line to an idle link. It may be a r o t a r y lineswitch (putomatic Electric Company bulletin 806), a plunger lineswitch (bulletin 805), o r an "all-relay" linefinder (bulletin 403). However, the most common Jvay to connect a calling line to an idle link is with a "linefinder", a Strowger switch equipped with appropriate relays and bank contacts usually to serve 200 l i n e s . known as a "linefinder shelf9'. When a subscriber lifts his handset, his line equipment busies his line t o other callers, m a r k s his contact on all linefinder banks, and activates the group relays. The group r e l a y s cause the linefinder connected to them by the distributor to connect its wipers to the calling line's contacts. The linefinder switches the calling line through to the first selector (which returns dial tone to the calling party), causes the distributor switch to step t o the next idle linefinder, and releases the group relays which now a r e ready for another call. The distributor in allotting an idle link to each calling line, also equalizes the wear on the l i n e f i n d e r s . - Since the linefinder hunts for a marked line, rather than being positioned by dial pulses a s a r e connectors and s e l e c t o r s , it i s known a s a non-numerical switch. Froni 100 lines 1.2 The 100-line linefinder. Under codrol of its relays, the 100-line linefinder can connect any of one hundred lines to one link. When the subscriber lifts his handset, the line equipment associated with his line (figure 2) o p e r a t e s , b u s i e s t h e calling l i n e to o t h e r callers, m a r k s the line's contact on the linefinder banks, and s t a r t s the linefinder. The linefinder lifts i t s wipers to the level that the line i s in, rotates i t s wipers to the calling line's contacts, and switches the calling line t h s a g h to the first selector (or connector) which returns dial tone notifying the calling party to dial. The only action upon the p a r t of the s u b s c r i b e r h a s been the lifting of t h e handset. 1.3 102-line linefinder shelf. More than one link is needed to handle the traffic from one hundred lines and s o a nurr-ber of linefinders a r e provided. A distributor switch, under control of relays common to the group, determines which linefinder will serve the next calling line. Ttle finders, banks, group relays, distributor, line equipment, and the frame on which they are mounted, a r e i r e 2 R e l a t i o n s h i p of 100 l l n e s and line e q u i p m e n t s and a l i n e f i n d e r . 1.4 200-line linefinder shelf. A shelf serving 200 lines r e q u i r e s m o r e linefinders than does a shelf serving 100 lines. However,-traffic tables show that 200 lines can have adequate service from fewer than twice the number of linefinders needed to serve 100 lines. Almost always, enough linefinders can be eliminated to more than compensate for the cost of doubling the number of bank contacts. T h u s , 200-line f i n d e r s a r e almost universally p r e f e r r e d and will be the subject of this bulletin. These finders operate just as the 100-line finders except that, after the wipers w e stepped up and around, there a r e two sets of wipers touching two sets of bank contacts. The linefinder relay q however, connect just the calling line's contacts to the first selector (or connector). BULLETIN 821 1.5 P r e f e r e n t i a l hunting. Any calling line can be reaci~edwith a maximum of twenty steps of the linefinder, ten steps up and ten steps across. The average hunting time is reduced by reversing the bank multiple (figure 3). Then a l l l i n e s a r e in the lower five levels of some linefinders' banks and can be reached in a maximum of fifteen steps, five up and ten across. The finders having e a c h hundred's lower-numbered line equipments connected to the banks' lower five levels a r e called group A finders. The finders having each hundred's higher-numbered line equipments connected to the banks' 2. PHYSICAL ARRANGEMENTS 2.1 Variations. A linefinder shelf consists of linef i n d e r s , group r e l a y s , distributors, line relays, associated equipment, and the frame on which they a r e mounted. Each shelf is assembled, wired, and shipped a s a complete unit, ready f o r mounting on an open frame. Linefinder shelves come in a variety of sizes and shapes depending upon the f r a m e s on which the shelves a r e to be mounted, the number of linefinders in the shelf, the type of line equipment (1-relay, 2-relay, j Distributor A 1 E 2 a 1 L o w e r bank I \ Line equipments (line a n d cut-off r e l a y s ) Group A l i n e f i n d e r s F i g u r e 3. Group B l i n e f i n d e r s Two-hundred-line l i n e f i n d e r s with bank r e v e r s a l f o r p r e f e r e n t i a l hunting. lower five levels a r e called group B finders. (Figures 3 and 12). With this arrangement, two distributors and two relay groups are necessary. A lower-numbered line equipment activates group A relays which start a preselected group A linefinder. A higher-numbered line equipment activates group B relays and a group B linefinder. This is known as "preferential hunting". A group A and a group B linefinder can be hunting simultaneously. If a low-numbered line equipment operates and all g ~ o u pA linefinders a r e busy, it will activate the group B relays which will cause a g r o w B linefinder to connect to the line. Group A linefindkrs will likegise assume the group B linefinders' lead when a l l group B linefinders a r e busy. This feature is valuable not only to give faster service but also permits routiniw the switches and group relays without denying s e r v i c e to any s u b s c r i b e r . or 3-relay), the type of metering, whether restricted service is furnished, and whether the shelf accommodates overflow finders. Figures 4A, 4B, and 5 show three typical linefinder shelves (in figures 4A and 5 all dust covers have been removed). The linefinder shelf is mounted on a l a r g e iron f r a m e . The frontispiece (figure 1) shows two linefinder frames, each mounting four linefinder shelves. The eight white cards at the lower right of each frame show the connector terminal or terminals connected to each line equipment. 2.2 Overflow shelves. All working lines of an exchange should be distributed evenly between the different linefinder groups. Each group of lines should be a representative group s o that every linefinder gl*oup handles approximately equal traffic. Each linefinder shelf should be equipped with just enough linefinders to - BULLETIN 821 Figure 5. Two-hundred-line linefinder shelf with linefinders mounted below line and cut-offrelaye. Used w i t h type 11 equipment f r a m e s 9' tall. take care of its traffic. Sometimes the traffic originating in 200 l i n e s r e q u i r e s m o r e linefinders than can be mounted on the linefinder shelf. Then a shelf mounting nothing but linefinders and banks is utilized. These linefinders a r e connected to the group relays and the distributors of the 200 lines just a s a r e the regular finders. These linefinders operate just a s the regular linefinders. The bank multiple of this shelf i s reversed in the middle s o that the number of finders in group A and group B a r e kept about equal. This shelf is called an "overflow" shelf since the number of finders needed "overfl~w'~ the regular linefinder shelf. (Overflow shelves do not handle just "overflow" traffic. The linefinder shelf t r e a t s an overflow finder essentially the same a s a regular finder). For shelves of the type shown in figure 4, the overflow finders mount at the l e f t of the regular finders. F o r shelves of the type shown in figure 5 , the overflow finders mount either just below the regular finders o r on a non-adjacent frame. See also figure 10. 2.3 Connections. A linefinder shelf requires the following connections: two 305-conductor cables to connect the linefinders to the t, -, and C of the 200 lines; one cable to connect the linefinders to the first selectors; ground and battery cables for power; and a 12-conductor signal cable to supply busy tone, class-of-service tone, alarm leads, timing leads, and so forth. An overflow shelf requires one cable to connect the linefinders to the first selectors and battery and ground cables for power. Two 305-conductor cables connect the banks of a r e g u l a r shelf and i t s overflow shelf together. 3. COMPONENTS 3.1 Line equipments. One line equipment is associated with each working line. When a subscriber lifts his handset from the hookswitch to make a call, his line equipment (1)busies that line to other calls, (2) marks the position of the calling line in the linefinder banks, (3) signals the group relays to start a linefinder, and (4) when a linefinder has found him, clears the line of attachments. Also, when a subscriber is called, his line equipment clears his line of attachments. Line equipments a r e supplied in groups, assembled and wired on a steel plate which is bolted to a linefinder shelf. The line equipments a r e numbered from 00 to 199. (The line-equipment number bears no relationship to the directory number of the telephone to which it is connected. Through the distributing frame, any outside line may be connected to any line equipment.) a LINE F I r n E R SWITCHES A two-relay line circuit is shown in figure 6. When the s u b s c r i b e r lifts his handset to make a call, ground through springs on the cut-off relay (CO), the + line, the subscriber's telephone, the line, springs of relay CO, and the line relay (L) operates relay L. Relay L grounds connector control lead CN, busying the line to other calls, connects negative battery through relay CO to linefinder control lead C F to mark this line's contacts on the linefinder hanks, and grounds lead LEV. ST. to mark the level on which the calling line will be found and to signal the group relays to start a linefinder. The linefinder, connecting to this line, grounds lead C F operating relay CO. Relay CO locks, and removes relay L from a c r o s s the subscriber's line. Relay L releases, a d removes ground from lead LEV. ST. When - From connector banks line and start circuits linefinder. When the linefinder connects to this line, it grounds line-equipment lead CF (and thereby connector lead CN) to busy the line to other calls and to operate relay L fully. Relay L, operating completely, removes . ground from the level-start lead and removes relay L from across the line. Lock-out line equipments a r e d e s c r i b e d in 96.4. C l a s s - o f - s e r v i c e tone (usually dial tone) often is connected to lead C of the line equipment of a semipostpay paystation. When the paystation c a l l s an operator, a splash of tone warns the operator that the call is from a paystation; that is, that m e r e ticketing f o r t o l l s w i l l not suffice-tolls m u s t be collected. 3.2 Group relays. Operation of the linefinders of each group is controlled by a group-relays assembly. As soon a s a line equipment grounds lead LEV.ST., the associated group relays cause a preselected linefinder to hunt for and connect with the calling line, After switching the calling line through to the first selector, the linefinder steps the distributor switch to the next idle linefinder. The group r e l a y s then r e l e a s e and a r e available to operate another linefinder when another line calls. ,To linefind~r+banks-, I Figure 6. Usual 2-relay line circuit. the subscriber receives a call, the connector grounds lead CN to operate the cut-off relay. The cut-off relay "cuts off" o r removes relay L from. the line. Less usual is a one-relay line circuit (figure 7) which consists of a two-step relay, s o called since only i t s contacts marked "X" close when a circuit is closed through i t s t h r e e windings i n s e r i e s . All contacts operate fully when 48 volts a r e connected to winding 1 alone. When the snbscriber lifts his handset, ground through winding 2, springs of relay L, the + line, the subscriber's telephone, the line, springs of relay E, winding 3, and winding 1 magnetizes relay L. Contacts "X" connect relay L winding 1to the linefinders' control contact (CF) to mark the line on the linefinder banks, and ground the level-start lead to signal the group relays to start a - L Two-step relay i From Level connector banks start Figure 7 . One-relay line circuit, using "2-etep" relay, -I Figure 8. Croup relays BULLETIN 821 A typical group-relays assembly consists of thirteen r e l a y s , f i v e r e s i s t o r s , t h r e e capacitors, and two rectifiers, mounted on a base plate (figure 8). A metal cover (not shown) p r o t e c t s the assembly from dust and physical injury. An extension frame at the bottom of the base plate mounts (from left to right) a busy key, a number-plate holder, and a test jack. Operation of the busy key causes the other gronp's linefinders to find this group's calling lines. The number-plate holder mounts a card showing the number of the finder and also the location and number of i t s associated selector. The purposes of the test jacks a r e described in $6.5. The g r o u p r e l a y s can be jacked in and out rapidly and conveniently. Two pairs of ~ h e l f ~ j a c k ontacts close when the group relays a r e removed from the linefinder shelf, making i t possible f o r that group's lines to be served by the other group's finders. Figure 9, Distributor. *Applicable also to 10-point distributor swltches Figure 10. Distributor bank-contact assignment. Figure 1 1 . Two-hundred-line linefinder switch. LINE FINDER SWITCHES I 3 . 3 Distributor. The distributor switch (figure 9) determines which linefinder will operate next. When activated by the line equipment, the group relays send a "finder start" signal, through the distributor switch, switched to a preselected linefinder. After the finder the calling line through to the f i r s t selector, the group relays step the distributor to the next idle linefinder. If a switchman busies the linefinder which a distributor has pre-selected, the distributor automatically steps to the next idle finder (see 86.7). @ The distributor consists of a 2-level 25-point rotary switch or a 2-level 10-point rotary switch. The group relays step it one step after each call is found; thereafter it steps self-interrupted, seeking absence of ground on the GUARD lead. A pointer and an indicator wheel show on which contacts the wipers are resting. Figure 10 shows to which linefinder each wiper position is connected. Level 3.4 Linefinders. The linefinder (figure 11) connects the calling line to the associated idle selector. Every line is connected to the contact banks of all thelinefinders on its shelf. Each line has a + wire, a - wire, and, within the central office, a control wire. Figure 12 shows their location on the group A and group B linefinder banks.(In the control bank each number represents a metallic contact; each horizontal line, a n insulator.) The line contacts for line equipments 00 to 99 a r e in the lower line bank and the line contacts for line equipments 100 to 199 are in the upper line. bank. The control contacts of the 300 lines a r e in the control bank. Those lines appearing on the lower levels of group A linefinder banks appear on the upper levels of group B iinefinder banks (figures 3 and 12) because of the reverse multipli~~g between the groups. The linefinder's vertical wiper (figure 11) touches a different vertical contact after each vertical step. The bottom vertical contact keeps tne vertical-bank wiper properly aligned when at its normal position. GROUP "A" GROUP "B" LlNEFlNDER BANKS LINEFINDER BANKS 200 -POINT 200 -POINT CONTROL BANK CONTROL BANK 0 m Level a ( Level 5 161 151 141 200 -POINT 200-POINT LINE BANK LINE BANK 150 160 1 70 180 Level 5 h Level 1 18 a a m a a 0 c Level 5 (Level 1 11 12 200-POINT ZOO -POINT LINE BANK LINE BANK 13 08 Figure 12. Bank numbering for 200-line linefinders. 19 09 10 20 30 40 50 80 90 00 ,,Z Level 1 j BULLETIN 821 I F r o m connector banks m - + I + " F r o m lines ' i LINE FINDER SWITCHES The wiring between the vertical contacts of group A and group B linefinders is also reversed s o that when group A l e v e l 3 is marked, group B l e v e l 8 also is marked (figures 13). A typical 200-line linefinder consists of a Strowger switch and t h r e e r e l a y s mounted on a b a s e plate (figure 11). A metal cover (not shown) protects the assembly from dust and physical injury. The Strowger switch mounts, in addition to those contacts already mentioned, vertical-off-normal contacts (which release mechanically when the shaft and wipers a r e lifted from their normal position), rotary cam contacts (operating when the shaft and wipers take the eleventh rotary step), and normal-post springs (operating when the linefinder shaft and wipers rise to a level whose lug (figure 14) has been bent*, 46.1, 06.2, and 06.3). The pattern of bent lugs A linefinder seldom need take more than five vertical steps to find a calling line, but when all finders having the calling line on t h e i r lower five l e v e l s a r e busy, a linefinder of the other group will s t e p to its upper levels to connect to the line. When all finders a r e busy, no linefinder finds the calling line, and no dial tone is returned to the subscriber; he must wait until he hears it b e f o r e he dials. 3.5 Fuse panel. The shelf fuse and lamp panel mounts 3-ampere fuses; 1 for each 20 line circuits, 1 for each 5 linefinders, 1 for each relay group and associated d i s t r i b u t o r , 1. f o r m e t e r s , and 1 f o r miscellaneous circuits. The red FA (fuse alarm) lamp will light when any of these fuses blows and remains lighted until the blown fuse is removed. The white ST SIG (start signal) lamp lights, after 4 to 4 minutes, when one o r more calling l i n e s a r e not being s e r v e d . A white FDR BLOCKED (finder blocked) lamp lights when its associated r e l a y group becomes inoperative and causes the other group's finders t o find its lines. The FDR BLOCKED lamp will remain lit until the corresponding non-locking GROUP RESET push button is operated. The white PERM (permanent) lamp lights* when a subscriber whose line equipment is equipped for lock-out fails t o dial within two to four minutes after lifting his handset.** The light remains lighted until the lock-out is released. The white EB (lower bank) lamp lights if one presses together the lower test-jack s p r i n g s of a linefinder connected to a line in its lower bank. The green RLS (release) lamp lights when the circuit of a linefinder release magnet has been closed several seconds wit'lout the finder releasing. 3.6 Meters. Linefinder shelves may mount up to four c o u n t e r s (figure 5 s h o w s two). A BCA (peg count, group A) meter counts tine calls made through group A linefinders; a PCB meter, through group B linefinders. An ATB (all trunks busy) meter counts the times that all linefinders on the shelf a r e busy. An O F (overflow) meter counts the number of calls that a r e not completed because all finders on the shelf a r e busy. Figure 14. Normal-post springs. Arrows show cam lugs bent 90° to operate left normal-post springs when wiper-shaft is opposite or cut in on level 2 o r 3. on group A and group I3 linefinders is reversed since, due to reverse multipling, the subscriber lines appearing on the lower levels of group A linefinder banks appear on the upper levels of group B linefinder banks. At the bottom the switch h a s (left to right) a busy key (06.7), a number plate, and a test jack (56.51 and 06.52). The number plate shows to which selector the linefinder is connected. All connections between the linefinder and t h e linefinder shelf a r e made through a jack-in connector called the shelf. jack. When the linefinder is removed from the shelf, one pair of shelf jack contacts, c l o s e . T h i s g r o u n d s t h e control lead t o m a r k that linefinder unavailable f o r s e r v i c e . 4, NORMAL OPERATION T h e following d e s c r i b e s t h e n o r m a l operation of a typical linefinder. 4.1 Marking the calling line. Lifting the subscriber's handset operates his line relay (figure 6, ground through relay CO contacts 6-5, + line, subscriber's telephone, - line, and relay CO contacts 3-4). Relay L grounds connector l e a d C N t o busy t h e l i n e t o other calls, connects the cut-off relay to linefinder bank contact C F to m a r k the line, and grounds the appropriate level- - *Often the PERM lamps throughout the office are commoned to a toggle switch or the like, SO that the PERM signals can be cut off except when the maintenance personnel wish to check permanent signals. **Also, in an office 1M)%equipped with lock-out, and wlth reverting-call *Cam adjuster H-47202-A i s used to bend the cam. switches wired not to supply talking battery, the P E R M signal lead 1s grounded throughout a reverting call. BULLETIN 821 Pre-selected linefinder Group relays F P C N K J I I I I I Figure 15. Group relays start the preselected linefinder. Figure 1 6 . Vertical stepping. t LINE FINDER SWITCHES marking and start lead. Ground on the level-marking and start lead marks the vertical contact corresponding to the calling line's leva1 in each linefinder. (Because of reverse multipling (figure 13), different levels a r e m a r k e d on group A and group B linefinders. For example, line 34 marks level 3 of group A linefinders and level 8 of group B linefinders.) The ground on the level-marking and start lead also operates (through a 2000f2 r e s i s t o r ) relay C of the group relays. The 2000f2 r e s i s t o r s (mounted on the r e a r of the linefinder shelf) r e s t r i c t direct* ground to the vertical contacts of the calling line's level. 4.3 Vertical stepping. Linefinder relay B also (figure 16) operates group relay A. Group relay A operates the vertical magnet of the preselected linefinder. The vertical magnet lifts the shaft and wipers one step and opens the vertical-interrupter contacts. The verticali n t e r r u p t e r contacts r e l e a s e group r e l a y A which r e l e a s e s the vertical magnet. Upon releasing, the vertical magnet closes the vertical-interrupter contacts which operate relay A . This cycle repeats until the wipers are lifted opposite the level of contacts containing the calling line. The vertical wiper then touches the marked vertical contact. 4.2 Starting. The operation of a group's relay C indicates that a linefinder of that group must find the calling line. Relay C operates relay F (figure 15) and connects relay N in s e r i e s with the distributor motor magnet. Relay N operates (but the distributor motor magnet does not). Relay N closes a circuit through relay K winding 2 and relay J winding 2. Relays K and J do not operate but, being pre-energized, will operate quickly when needed. Also, through the FST (finder start) wiper of the distributor, relay N operates relay B of t h e p r e s e l e c t e d linefinder. Linefinder relay B operates associated first-selector relays A and C and connects the preselected linefinder's vertical and rotary magnets to the group relays. Since only the preselected linefinder's vertical and rotary magnets a r e connected to the group relays, the preselected linefinder is the only one that can be stepped by the group relays. As the vertical a r m a t u r e completes i t s stroke, the vertical-interrupter springs release relay A, removing a short circuit from relay M which operates in series with the vertical magnet (ground through line-relay contacts 7-6, the vertical wiper, linefinder relay B contacts 6B-7B, relay P contacts 4T-3T, relay M, relay P contacts 9T-8T, linefinder relay B contacts 6T-7T, and the vertical magnet). The vertical magnet holds* operated in series with relay M. Ground through r e l a y N contacts 6T-7T, relay P contacts 4B-5B, relay M contacts 3-4, relay M winding 1, and relay P operates relay P. Relay P locks, releases relay M by opening both its windings, r e l e a s e s the vertical magnet, and transfers the pulsing path from vertical to rotary magnet. The vertical magnet, releasing, operates relay A. *Tne "feedback" ground through 400051 on all other vertical-bank contacts i s insufficient t o stop the finder. I Group relays F A M P N K - *If the vertical magnet fails to hold operated in series with relay M, relay A will reoperate, immedmtely. However, relay M contacts 1-2 will prevent ground from relay F contacts 3-4 through relay A contacts 1-2 from operating either the vertical or the rotary magnet. In this case, relay M on restoring comptetes the circult of the rotary magnet. Linefinder I I J Figure 17. Rotary stepping. BULLETIN 821 4.4 Rotary stepping. Upon operating, relay A operates the r o t a r y magnet of the linefinder (figure 17). The r o t a r y magnet rotates the shaft and wipers one step and opens the rotary-interrupter contacts. (When the bank-contact wipers rotate, the vertical wiper swings away from the vertical bank.) The rotary-interrupter contacts r e l e a s e group relay A which r e l e a s e s the r o t a r y magnet. Upon releasing, the rotary magnet closes the rotary-interrupter contacts which operate relay A. This cycle is repeated until the control wipers touch the marked control bank contact. If the highernumbered line is calling, i t s line relay is operated and i t s cut-off relay operates in series with relay K. If the lower-numbered line is calling, its line relay is operated a n d its r e l a y CO o p e r a t e s in s e r i e s with relay J. Relay J o r K opens the circuit to the rotary magnet to prevent further rotary stepping. Relay CO locks and removes the line relay from a c r o s s the subscriber's line. Upon releasing, relay L removes ground from i t s level-marking and start lead (figure 131, 4.41 Need for rectifiers. In an exchange where billings a r e based on the number of calls made, provision must be made so that completed calls (only) a r e recorded. This is done with meters which will not operate on the exchange voltage (48 volts) but will operate on 98 volts. A booster battery of 50 volts and the exchange battery I Group relays N F K J Rectifier o f f e r s very high resistance to booster-battery polarity; Linefinder of 48 volts in s e r i e s a r e used (figure 18). When the called party answers, his battery-feed relay operates and reverses* the polarity on the calling line. This battery reversal triggers a relay group which momentarily replaces the + ground on the C lead with + booster battery. The calling line's meter is operated. It i s possible that a linefinder control wiper, as it hunts across a level of control contacts, will touch one with booster battery on it (figure 18). Then, if nothing had been done to prevent i t , c u r r e n t would flow from + 50 volts on linefinder control contact, linefinder control wiper and relay B contacts, group relay J or K and group relay F contacts to ground, and would operate relay J or K falsely and cause the linefinder to switch through into the conversation of another line. The rectifiers in the relays J and K circuits prevent this by blocking current attemplng to flow from + booster battery to ground. The linefinder will then rotate to the calling line's contacts. The rectifiers allow current to flow in the other direction (from ground through relay F contacts, a rectifier, relay K o r J, linefinder relay B contacts, a linefinder control wiper, a line's control bank contact, the calling line's relay L contacts, the cut-off relay, to -48 volts) and operate relay J o r K. *If called party does not answer there will be no reversal. I I Line equipment of a line other than the one being sought by linefinder I I I Another I Succeeding (linefinder1 equipment t I t I I Exchange battery ;r -48 volts I I - Booster battery .C f t50 volts. I t The "succeeding equipment" may be a trunk circuit between the linefinder and the first selector, o r it may be part of the connector. Figure 18. Use of and need for rectifiers. LINE FINDER SWITCHES i Group relays Linefinder From calling line Figure 19. Linefinder extends caller through to pre-seized f i r s t selector. (Upper-bank line shown a s example. ) 4.5 Extending calling line to selector. If the highern u m b e r e d line is calling, group r e l a y K operates, operating finder relay A. Relay A locks and extends t h e calling line to the f i r s t s e l e c t o r . If the lowern u m b e r e d line is calling, g r o u p r e l a y J operates, locks and extends the operating finder D' lower-numbered line to the f i r s t selector. (In case both lines a r e marked when the two control wipers step onto them. both relays J and K will operate. Relay K will ope; the circu-it t o linefinder r e l a y ' D s o only Line equipment CO I I I relay A cart operate (figure 19). The linefinder will therefore extend the higher-numbered line to the first selector, and the next selected linefinder will serve the lo;ver-numbered line.) id tone from tfae first selector notifies the subscriber to commence d i a l i x (figure 20). Ground from,selector relay B now holds the linefinder A Or Operated. This ground is also on lead CN to keep the calling line marked busy at the connector banks. F i r s t selector Linefinder F A A B or D I Dial tone nd ground U .* E ,%. i? k connector banks i I Figure 2 0 . F i r s t selector holds the cut-off relay and the linefinder, and aupplies dial tone. BULLETIN 821 off-normal contacts to open the release magnet, and to remove ground from this linefinder's lead G so the group r e l a y s can s e i z e the linefinder again when needed. 4.6 Stepping distributor to next idle finder. The operation of relay A or D (figure 21) places ground on the linefinder G (guard) lead, operates the mator magnet of the distributor and short-circuits relay N, making N slow to release. Upon releasing, relay N releases group relays A and F (figure 17) and linefinder relay B, and steps the distributor by opening the circuit of its motor magnet. Relay F releases relays P and J or K. The distributor wipers now a r e on the contacts for the next linefinder. If this linefinder is busy, ground on its lead G through group relay N contacts 2B-lB, group relay B contacts 8-9, distributor interrupter springs, and motor magnet operating the motor magnet which operates the interrupter springs and opens its own circuit, and the distributor steps. This cycle will continue until the distributor rests on contacts leading to an idle linefinder. When another line calls, this newly assigned linefinder will find it and extend it to a first selector. Should a line call while the distributor is searching for an idle finder, group relay C will be operated but group relay N cannot operate until the distributor finds an idle linefinder. 5. SPECIAL OPERATIONS 5.1 Linefinder does not find vertical marking. If the calling party hangs up just after the linefinder starts, the calling line's vertical contact will not be marked. If there are idle finders in both groups* and the linefinder is hunting a line in its lower levels, it will step to the fifth level where ground on the fifth vertical contact (from its group's relay F, figure 22) will stop its vertical step?ing. This ground also prevents the group relay C from restoring when the line relay L is restored. The finder will rotate to the eleventh rotary step and release ($5.2). 5.2 Linefinder does not find rotary marking. If the calling party hangs up either before the linefinder finds his level (95.1) or just after the linefinder cuts in on the calling line's level, no calling-line bank control contact on the level is marked with negative stopping battery. The linefinder then s t e p s around to the eleventh rotary step where i t s cam springs operate 4.7 Releasing. Upon hearing dial tone, the subscriber dials to establish his connection. After the conversation is concluded, connector (or inter-office pulse repeater) relay B releases, removing ground from lead C which releases the selector(s), finder relay A or D (figure 20), and the calling line's cut-off relay, and removes ground from line lead CN to allow the line to receive incoming calls. The cut-off relay reconnects the line relay a c r o s s the line. Finder relay A or D operates the linefinder release magnet and group relay G in series (figure 48). The shaft and wipers rotate leftward and d r o p to their normal position causing the vertical I Group reiays F P B *In the event that one group i s busy relay B in restoring will prevent ground, from contacts of group relay F of the not-busy group, appearing on the fifth vertical contact of the not-busy group. In this case, when line relay L restores, group relay C restores and restores group relays F and N which restores linefinder relay B. Linefinder relay B in restoring grounds lead G and steps the distributor. N 1 1 I Distributor I I I I Figure 21. Stepping distributor to next idle finder. Linefinder which just found a calling line B A D -- LINE FINDER SWITCHES Group B linefinclers Group A linefinders . -- --.--- Group B relays Group A relays Figure 22. Call abandoned before. linefinder reached level containing caller's line relay K (figure 23) which operates linefinder relay A (figure 19) and opens the rotary magnet's circuit to prevent further rotary stepping. Relay A operates the distributor's motor magnet (figure 21) which steps the distributor to the next linefinder. The distributor's interrupter contacts release relay N which releases group r e l a y s F and P and finder relay B, Because there is no calling-line loop to hold selector relay A, the r e l e a s e of finder r e l a y B (figure 20) releases s e l e c t o r relay A. First-selector relay B releases finder relay A which operates group relay G (figure 48) and the linefinder release magnet in series. The shaft and wipers, upon returning to normal, open the vertical off-normal contacts which r e l e a s e relay G and the r e l e a s e magnet. (If the calling line's control lead w a s unmarked due to a circuit fault, the next idle linefinder will hunt for the calling line.) I I Linefinder A ~ r o u prelays Test 1 I I Cam springs operate on 11th rotary step Figure 2 3 . Call abandoned before linefinder reached rotary stopping. caller's line - - vertical stopping. 5.3 All linefinders of one group a r e busy. A group's relay B (figure 24) is held operated by ground through relay A contacts of the group's idle finders (and, if a finder is hunting, by ground through group relay N contacts). After the last idle finder of a group becomes busy, group relay N restores, steps the distributor, and removes the last ground holding group relay B. Group relay B releases, disconnects i t s distributor's motor magnet (figure 21) from the distributor guard wiper so the distributor won't hunt an idle finder when there is none, breaks (figure 22) the path by which the other group relays might ground the other group's level 5, and (figure 24) transfers the START A lead from the busy group's relay C to the not-busy group's relay C. Then any calling line of the busy group will operate the other group's relay C causing a linefinder of the other group to find the line in its upper levels. 5.4 All linefinders of shelf busy. When all finders in both groups a r e busy, both relays B a r e released and neither relay C may be operated to start a linefinder. I£ meanwhile a subscriber attempts to call, upon hearing no dial tone, he will either wait until he receives it or hang up. A s soon a s any finder becomes idle and restores, it will cause its group relay B to operate, and if a call is waiting will immediately s e a r c h for it. 5.5 Linefinder fails to cut in. Operating during a linefinder's hunting action, group relay F grounds the s t a r t lead of a timer circuit (usually located on the powerboard or the power shelf). If the linefinder wipers a r e badly out of alignment, they may jam into the