Manual / Guide · 1958

AE Training Bulletin 813: Party-Line Connectors and Trunk-Hunting Connectors

Connector variants for party lines and hunting groups.

Author: Automatic Electric Co.
Year: 1958
Type: Manual / Guide
Credit: Telephone Collectors International (TCI) Library, telephonecollectors.info (retrieved via Internet Archive Wayback Machine)

AuToMnTIc ELECTRIC TRAINING SERIES PARTY-L CONNECTORS and TRUNK-HUNT CONNECTORS O R I G I N A T O R S O F T H E D I A L T E L E P H O N E 813 This is one of the helpful booklets in the AUTOMATIC ELECTRIC TRAINING SERIES on STROWGER AUTOMATIC TELEPHONE SYSTEMS E l e c t r i c a l Principles of Telephony Mechanical Principles of Telephony Fundamentals of Apparatus and Trunking The Plunger Lineswitch and Associated Master-Switch Rotary Lineswitch The Connector The Selector Pulse Repeaters Trunking Power and Supervisory Equipment P a r t y - Line Connectors and T runk-Hunting Connectors Reverting-Call Methods The T e s t and Verification Switch T r a i n Toll Switch T r a i n Switching Selector-Repeater P r i v a t e Automatic Exchanges with PHBX Appendix Community Automatic Exchanges Manual Switchboards Linefinder Switches May we send you others pertaining to equipment in your exchange? PARTY-LINE CONNECTORS AND TRUNK-HUNTING CONNECTORS CONTENTS Page 1 . SCOPE O F BULLETIN 8 1 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 . PARTY-LINE CONNECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .......................................... 1 2 . 2 Seizure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 . 3 Dialing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.1 Types ............................ 1 ........................... 1 ........................... 1 ..................................... 2 2 . 4 Idle-called-line s e i z u r e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 . 4 1 Cutting off called-line line r e l a y . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 . 4 2 Pick-up (code ringing only) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 . 4 3 Ringing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 . 5 Called party answers; ringing cut off . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.31 2.32 2.33 2.34 F i r s t digit (tens selection) Second digit (line selection) T h i r d digit (party selection) Busy t e s t ...................................... 4 2 . 7 Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.6 3 . TRUNK-HUNTING CONNECTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ....................................... 5 3.2 Bank-marking patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.21 Night s e r v i c e . . . . . . . . . . . . . ; . . . . . . . . . . . . . . . . . . . . . 6 3.1 4 Conversation Introduction . OPERATION O F A TYPICAL TRUNK-HUNTING CONNECTOR . . . . . . . . . . . . . 9 ......................................... 9 4 . 2 Dialing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4 . 2 1 F i r s t digit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.22 Second digit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4 . 3 Called number idle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Seizure 4.4 Called line busy 4.5 Called line ( f i r s t line i n a group) is busy .. Subsequent line idle .. Individual line (or in a group. but not the f i r s t in the group) . . 9 . . . . . . . . . . 10 4 . 6 All lines in a P.B.X. group a r e busy . . . . . . . . . . . . . . . . . . . . . . . . . .11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 3 4 . 8 Called line a n s w e r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 4.9 Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3 5 . LEVEL-HUNTING CONNECTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 1 4 6 . GENERATOR CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 4.7 Ringing called party Figure 1. Typical terminal-per -line party-line connector. PARTY-LINE CONNECTORS 1. SCOPE O F BULLETIN 813 The connector is t h e only switch which can do n e a r l y e v e r y job i n e s t a b l i s h i n g a telephone connection. It r e s p o n d s t o the dial, r i n g s the called p a r t y , s u p p l i e s t r a n s m i t t e r c u r r e n t t o calling and called parties, and, when conversation is over, r e l e a s e s the connection. Thus i t is just about the most important switch in a switchtrain. Bulletin 807 described a simple 8-relay connect o r with the b a r e e s s e n t i a l s . This bulletin 813 describes connectors with additional relays, etc., to handle t e r m i n a l - p e r-line* p a r t y lines, and s u b s c r i b e r s with m o r e than one line. Automatic E l e c t r i c Company m a k e s many conn e c t o r s , all s l i g h t l y d i f f e r e n t . T h i s bulletin t r e a t s only 2 c i r c u i t s fully, but t h e s e a r e "typical" and i l l u s t r a t e principles adequately. 2. PARTY-LINE CONNECTOR 2.1 Types. Party-line connectors a r e available f o r code o r h a r m o n i c ringing. Some c i r c u i t s combine both party-selection and trunk-hunting a n d / o r both l o c a l s e r v i c e a n d t o l l s e r v i c e i n one connector switch. H a r m o n i c r i n g i n g ( f o r which e a c h p a r t y - l i n e station h a s a "tuned" r i n g e r responsive t o only one a - c frequency) probably is used somewhat m o r e widely t h a n code ringing. We u s e code ringing for the example in this bulletin, however, to introduce "pick up" (S2.42). A typical local, terminal-per-line, code-ringing, ground-connected-generator, party-line connect o r circuit is shown in figure 19. This connector is f o r a 5-code s y s t e m , f o r 5-party l i n e s with bridged ringing o r 10-party lines with divided ringing.** This connector differs from the simple *Automatic Electric Company engineering notes 2030 and 2037 cover terminal-per-STATION party lines. Engineering notes 2 030 is b a s e d on individual-line-type connectors s i m i l a r to the one described in bulletin 807. Engineering notes 2037 is based on a connector with 4-conductor banks (+, -,C, and EC for each station). **Actually, state commissions s e t r a t e s usually for only 4- o r 8-party lines. However, at the time a party's service is terminated you'll find i t a convenience that the equipment provides a 5th code o r frequency. Then you can add a new s u b s c r i b e r to the line a t once (keeping the line filled to 4 o r 8 parties) and yet not reassign the discontinued number. ... connector described in bulletin 807 i n that it is operated by three dialed digits and r e q u i r e s two additional relays and a minor switch. The minor switch, operated by the t h i r d digit, s e l e c t s the ringing-current code and applies i t to the proper side of the line. 2.2 Seizure. When a party-line c a l l e r lifts h i s handset, his hookswitch closes the loop circuit and s t a r t s a t t h e c e n t r a l office a l i n e f i n d e r which connects h i m t o a s e l e c t o r , f r o m which he receives dial tone.' He then dials into s e l e c tor(~) the f i r s t digits of the number of the called s t a t i o n . When h e h a s d i a l e d all b u t t h e l a s t t h r e e digits, t h e p r e c e d i n g s w i t c h s e i z e s the party-line connector. Connector relay A operates. Relay B operates and grounds lead C which m a r k s the connector busy to other select o r s and holds preceding switches i n the train, operates r e l a y C through i t s winding #2 (if not already operated on seizure by ground on lead C from the preciding switch), and operates relay M. 2.3 Dialing. During dialing, r e l a y A r e l e a s e s with each pulse, while slow-to-release r e l a y B remains operated. 2.31 F i r s t digit ( t e n s s e l e c t i o n ) . During the f i r s t digit, each time relay A releases, i t closes a circuit to the vertical magnet which s t e p s the connector wipers. At the s t a r t of the f i r s t step, vertical-off-normal s p r i n g s V.O.N. open slowbut pulses thru release relay C winding #2 winding #1 keep relay C operated. After the last p u l s e , r e l a y A r e m a i n s o p e r a t e d and k e e p s r e l a y C winding # 1 open s o long t h a t ( a f t e r a tenth of a second o r s o ) s l o w - r e l e a s e r e l a y C "gets tired", releases, and transfers the pulsing circuit to t h e r o t a r y magnet. .. .. . 2.32 Second digit (line selection). During the second digit, e a c h t i m e r e l a y A r e l e a s e s i t c l o s e s a c i r c u i t t o t h e r o t a r y m a g n e t , which s t e p s the connector w i p e r s . When A r e l e a s e s on the f i r s t pulse, r e l a y E o p e r a t e s and holds r e l a y M (winding #2). During the f i r s t r o t a r y step, rotary-off-normal springs R.0 .N. operate, and open r e l a y M winding # 1. Slow-to-release relay E does not r e l e a s e when relay-A operates between pulses, but when A r e s t o r e s a f t e r the last pulse of the series, about a tenth of a second l a t e r E r e l e a s e s , and r e l e a s e s r e l a y M, which t r a n s f e r s t h e pulsing path t o the minor-switch r o t a r y magnet. 2 . 3 3 T h i r d digit ( p a r t y selection). During the third digit, each time relay A r e l e a s e s i t closes. a c i r c u i t to the minor-switch r o t a r y magnet, stepping the minor-switch wipers. When relay A r e l e a s e s on the f i r s t pulse, r e l a y E operates quickly. During t h e f i r s t step, minor-switch off-normal springs M.S.O.N. a r e released, and s h o r t - c i r c u i t unoperated r e l a y F winding # l . 2.34 B u s y t e s t . M i n o r - s w i t c h o f f - n o r m a l s p r i n g s M.S.O.N. connect wiper C to relay H windings #2 and #1 in s e r i e s . If the called line is idle, r e l a y H does not operate. If the called line is busy, at the time of the busy test (before slow-to-release relay E r e s t o r e s ) , w i p e r C will be r e s t i n g on a grounded control lead, and 2-step relay H will operate halfway, closing i t s contacts "X". When r e l a y E releases, i t connects ground from called-line lead C thru relay K contacts 9-10 and 2-3 to relay H winding # l . Relay H o p e r a t e s fully, opens the pulsing path, and connects b u s y tone back to the calling line. An idle called line has on lead C negative battery f r o m the cut-off relay. 2.4 Idle-called-line s e i z u r e . About a tenth of a s e c o n d a f t e r the l a s t m i n o r - s w i t c h pulse, r e l a y E r e s t o r e s , r e m o v e s the s h o r t c i r c u i t f r o m relay F winding #1, and connects relay CO to r e l a y L winding # l . Relay F operates, opens t h e pulsing path, and, in our example (figure 19), if the last digit i s an even* number, operates relay M. If operated, relay M would transfer relay K to t h e +wiper, and, m o r e important, would swing t h e ringing-generator lead toward t h e -wiper. 2.41 Cutting off called-line line relay. Lineequipment cut-off relay CO operates immediktely, followed by slow-to-operate r e l a y L. Cut-off relay CO cuts off the line relay s o the line relay will neither interfere with ringing nor operate when t h e called p a r t y a n s w e r s . (That is why r e l a y L is m a d e slow to operate: s o the line r e l a y w i l l h a v e b e e n c u t off b e f o r e ringing current is applied to the line.) * T h e p u r p o s e of thus alternating the p a r t i e s (odd=+ and e v e n = - ) is to equalize the capacitanceto-ground on the 2 s i d e s of a d i v i d e d ringing p a r t y line when p l a n t - a s s i g n - ment c l e r k s asslgn p a r t y telephone n u m b e r s i n l o g i c a l -1, -2, - 3 , -4 o r d e r . Figure 2. Standard codes for 10-party divided ringing. When relay L operates, it locks, short-circuits i t s own winding #1 to ground wiper C directly, grounds lead R.M. START to s t a r t the ringing machine, connects r e l a y K to the called line, and, in o u r example ( c ~ d e ringing**), connects relay C t o the PICK-UP lead. 2.42 P i c k - u p ( c o d e ringing only). F o r o u r study-example, l e t u s a s s u m e the d i r e c t o r y n u m b e r f i n a l digit w a s "3". A c c o r d i n g to f i g u r e 2 a n d f i g u r e 3, t h i s c a l l s f o r 4 s h o r t r i n g s . P r o b a b l y "our" c a l l is not the only call going thru the c e n t r a l office at this time; m o s t l i k e l y o t h e r c a l l s a r e i n p r o g r e s s and already will have s t a r t e d the ringing machine. It is quite possible, for example, that the moment "our" connector relay L operated, the ringing i n t e r r u p t e r w a s a t that point in i t s cycle where i t had just sent out the f i r s t 2 of t h e 4 s h o r t r i n g s f o r code 3. If we w e r e to connect the ringing machine to o u r c a l l e d line r i g h t a w a y , only the remaining 2 short rings would go out during the f i r s t ringing cycle, and [instead of o u r party " 3 ' ' ] p a r t y "2" would think he was being rung. Whether he answered by mistake o r n o t , i t c e r t a i n l y would be confusing and annoying s o we obviate this with "pick up". .. . The next time i t is about to s t a r t a new s e r i e s of complete rings, the ringing interrupter sends for about second a ground pulse on the PICKUP lead (figure 3). a - T + 1 - L ... **If we h a d h a r m o n i c r i n g i n g w h i c h , by t h e way, is more usual t h e P I C K - U P l e a d would b e grounded p e r m a n e n t l y , and r e l a y C would o p e r a t e i m m e d i a t e l y . Although h a r m o n i c ringing is m o r e usual, we h a v e chosen to w r l t e the party-line section of thls booklet a r o u n d coded nnging, because then we can teach t h e "pick-up'' principle, and m a k e t h e booklet m o r e educational. ... f '-- 2 . 4 3 Ringing. C o n n e c t o r r e l a y C o p e r a t e s , locks. dis c o n n e c t s i t s e l f f r o m the PICK-UP lead, s e n d s ring-back tone to the c a l l e r , and c o n n e c t s t h e r i n g i n g m a c h i n e to t h e c a l l e d line. - The heavy line in figure 3, a n example based on "divided' ' (earth- return) ringing, shows how (after r e l a y C h a s o p e r a t e d ) ringing-machine i n t e r r u p t e r c a m - l o b e s c o n n e c t 'the r i n g i n g g e n e r a t o r to t h e called line according t o the required code. (250S2 r e s i s t o r A limits current s o that if the connector wipers were on a faulty [grounded] line, the generator wouldn't be shortc i r c u i t e d n o r o v e r h e a t e d . ) Ringing c u r r e n t c o m p l e t e s i t s path f r o m the r i n g e r , t h r u the e a r t h , b a c k to t h e c e n t r a l - o f f i c e g e n e r a t o r . If the ringer and i t s capacitor were "bridged" (=connected a c r o s s the two w i r e s of the line, without earth return, a s in figure 17), the ringer circuit would be completed thru relay K and the central-office battery. It is f o r this c a s e that relay K is made slow to operate . to prevent it f r o m being operated by half-cycles of a l t e r nating (ringing) current. Meanwhile, the ringer capacitor keeps d i r e c t (battery) c u r r e n t f r o m flowing thru relay K. .. 2.5 Called party answers; ringing cut off. When the called party answers, his hookswitch (figure 3) d o e s away with the r i n g e r - c a p a c i t o r path (which w a s f o r a.c. only), and t h e hookswitch closes a d i r e c t - c u r r e n t circuit f r o m -battery thru r e l a y K winding # I ; t h e c a l l e d line; the called-telephone t r a n s m i t t e r , induction coil, and hookswitch; t h e o t h e r s i d e of t h e called line; and thru the code cam springs, either thru the generator and thence to ground (+battery), o r directly to ground (+battery). T h i s d i r e c t c u r r e n t t h r u r e l a y K winding # 1 closes relay K contacts X. Relay K winding #2 operates relay K completely, removes ground from the R.M. START lead to stop the ringing machine, releases relay C, disconnects ringing c u r r e n t f r o m t h e c a l l e d line, and s w i t c h e s the called line t h r u t o b a t t e r y - f e e d r e l a y D. R e s i s t o r B m a k e s r e l a y C quick to r e l e a s e , and, during r e l e a s e of C, limits c u r r e n t (from ground thm relay B contacts 8-9 and thru relay C momentarily "bunched" contacts 8-9-10) fed back on the PICK-UP lead to l e s s than the nonoperate current of any waiting pick-up relays of o t h e r c o n n e c t o r s . Relay C r e s t o r e s , and disconnects ring-back tone. Figwe 3. Pick up (for code ringing only). The next time it i s about to start a new complete series of code rings, the ringing interrupter sends for about %second a ground pulse on the PICK-UP lead. Heavy line shows ringing circuit. Relay D supplies t r a n s m i t t e r current t o the called p a r t y ; o p e r a t e s ; i f the connector h a s u s u a l l a s t - p a r t y - r e l e a s e wiring, connects an additional ground to hold r e l a y s K and L; and r e v e r s e s the polarity* of the transmitter current fed f r o m r e l a y A toward t h e caller. 2.6 Conversation. Relay A supplies transmitter current to the caller, and relay D supplies transmitter current to the called party. Relay A holds r e l a y B, which keeps ground on control lead C to hold: The c a l l e r ' s line-equipment cut-off r e l a y Linefinder Relay L k e e p s ground on w i p e r C to hold t h e called l i n e ' s cut-off relay. 2.7 Release. T h e connector m a y be a r r a n g e d f o r e i t h e r of two types of r e l e a s e . With r e l a y D s p r i n g 5 connected a s shown i n f i g u r e 19, the connector is held until both the c a l l e r and t h e called p a r t y hang up. T h u s we s a y t h e connector is a r r a n g e d f o r "last-party r e l e a s e " . T h i s is usual. If r e l a y D s p r i n g 5 is d i s c o n n e c t e d , t h e c a l l e d p a r t y c a n d o n o t h i n g t o hold the connector, the connector releases when the c a l l e r hangs up, and we s a y i t i s ' a r r a n g e d f o r "calling-party release". Calling-party r e l e a s e is l e s s u s u a l . It is found only - (1) Where all lines have "lock-out" bulletin 82 11, [see (2 ) Where trunks f r o m nearby towns come directly into incoming connectors [no selectors].** Assuming last -party release, figure 19 indicates the s e q u e n c e of o p e r a t i o n and r e l e a s e of the relays and off -normal springs of the connector when the caller is the first to hang up. (Etiquette recommends that the called party should hang up a f t e r t h e caller.) When the c a l l e r h a n g s up, r e l a y A r e l e a s e s , and operates relay E. When r e l a y B r e l e a s e s , it o p e n s t h e c i r c u i t of s l o w - r e l e a s e r e l a y E , and removes ground from lead C. The preceding switches release. Slow-to-release relay E remains operated a moment. Then i t r e l e a s e s , and connects ground from relay L contacts 3T-4T thru relay E contacts 5-4, relay H contacts 9 -10 and 2-1, and relay K 125D winding, to lead C to m a r k the connector a s "busy" - - not able yet to accept another call. Next, t h e c a l l e d p a r t y h a n g s up, r e l e a s i n g relay D. Relay D contacts 5-4 r e l e a s e r e l a y M (if o p e r a t e d ) and r e l a y s K a n d L . R e l a y D contacts 2-1 operate the minor-switch r e l e a s e magnet and the Strowger-switch release magnet. R e s t o r i n g , t h e m i n o r s w i t c h o p e n s i t s own r e l e a s e - m a g n e t c i r c u i t , and a l s o r e l e a s e s relay F. When the Strowger-switch shaft drops to i t s n o r m a l position, it o p e r a t e s the v e r t i c a l off-normal s p r i n g s V.O.N.: s p r i n g s 1-2 open the r e l e a s e - m a g n e t c i r c u i t , a n d s p r i n g s 3-4 connect -battery thru r e l a y C 500Q winding # 2 to l e a d C to t e l l b a t t e r y - s e a r c h i n g s e l e c t o r s this connector is ready to handle another call. *For most calls, this battery reversal is of no significance. It has a useful function in these cases: (1) (2) If the call i s from a paystation, it results in collection of the charge for the call. In a measured-service o r message-rate exchange, it causes the completed call to be counted. (3) If the call is from an operator, it gives her ''off-hook'' (dark) supervision. **Altho P-A-X's do not use t h e s e frequency-selecting party-line connectors, a P-A-X of under 100 lines [=no selectors] using fan-tail plunger lineswitches needs callingparty release too. SERVICE TO PRIVATE BRANCH EXCHANGES Inset: How radio-type 1200Q 1-watt resistors (for figures 9, 14, 20, etc.), supported by their leads, mount on the bank terminal block. Figwe 4. Right-hand end of a shelf of trunk-hunting connectors for night-line service. The local connectors at the left have 11 relays each, as mentioned on page 8. The connectors at the center, with 13 relays each, are combination local-and-toll trunk-hunting connectors. At the right is the bank terminal block, at which all strapping (figure 9 i n this case) i s done. 3. TRUNKrHUNTING CONNECTORS 3.1 I n t r o d u c t i o n . T h e p r e c e d i n g c h a p t e r d i s c u s s e d c o n n e c t o r s to handle s u b s c r i b e r s who s h a r e a line. T h e s e next 3 chapters disc u s s c o n n e c t o r s which ' s e r v e s u b s c r i b e r s , each of whom has the exclusive use of s e v e r a l line s . E a c h m u l t i - l i n e s u b s c r i b e r usually* h a s a switchboard of i t s own, to which a r e brought the "outside" l i n e s o r t r u n k s , and a l s o t h e station lines of the internal organization. Thus the s u b s c r i b e r has a private branch exchange (abbreviated P.B.X.), the trunks and stations of which can b e interconnected a s r e q u i r e d . The equipment of the operating telephone company should not c o m p e l a calling s u b s c r i b e r to t r y dialing a s e r i e s of numbers, a l l serving the s a m e multi-line subscriber, in an effort to find an idle trunk o r line. The directory should list but one telephone number for the multi-line s u b s c r i b e r , and the c e n t r a l office must be so equipped that i f t h i s directory-listed number * S o m e t i m e s , as f o r e x a m p l e at a s m a l l r e t a i l s t o r e , there may merely be a couple of order-taking telephones. is dialed, and is busy, the connector will take the initiative to hunt f o r an idle line amongst the remaining circuits t o the called establishment. This requires, of course, that the several lines must be connected t o adjoining o r consecutive bank-contact positions. Circuit variations a r i s e f r o m four questions: How s h a l l the consecutive-line group be marked in the connector banks ? (See S3.2 .) Shall the connectors hunt during the hours the P.B.X. switchboard is not attended, and "night connections" are set up? (See S3.21.) Does any P.B.X. subscriber have more than the t.en t r u n k l i n e s which r e p r e s e n t the normal capacity of one level of a connectorswitch bank? (See 45.) Does the operating telephone company have enough multi-line s u b s c r i b e r s torwarrant assigning a whole special connector group to t h e m ? ** **If not, Automatic Electric Company can supply connectors which COMBINE both the party-line and the trunk-hunting functions i n every connect0 r . Figure 5. Five consecutive line numbers (43, 44, 45, 46, and 47) i n the banks of a connector. 3.2 Bank-marking patterns. A P .B.X. group bank-marking circuit pattern developed in 191'8 was so well conceived that it continues in use today in many Strowger trunk-hunting conne c tors. A fourth lead, the "extra control" lead, abbreviated EC, i s required for each line in the trunk-hunting connector banks. Control lead C and e x t r a control l e a d EC a r e strapped on all lines i n a P.B.X. t r u n k group, except t h e l a s t line in the group. F o r the five lines in figure 5, this is shown schematically in figure 6. The t a s k of s t r a p p i n g l e a d s C and EC i s actually simpler than the schematic of figure 6 suggests. Both leads a r e multipled throughout all banks on a shelf, and s t r a p p i n g is done a t the shelf bank t e r m i n a l block (figure 4 ) only once f o r each line. The trunk-hunting feature of these connectors usually r e q u i r e s two additional relays in each connector: if the r e g u l a r c o n n e c t o r s in the c e n t r a l office have eight r e l a y s , the t r u n k hunting connectors will have ten relays. If any.,-line (such a s 43, f i r s t in the group in figure 5); is dialed and is idle, the connector s e i z e s the line in the usual way, namely by grounding lead C to operate the cut-off r e l a y of the called line. Because leads C and EC of line 43 a r e strapped, in grounding lead C the connector grounds also lead EC a t every bank multiple appearance of line 43. If, now, a n o t h e r s u b s c r i b e r d i a l s line 43, during dialing of the l a s t digit 3, t h e second connector t e s t s lead C of line 43 and finds i t busy. Lead EC being grounded, relayinterrupted rotary stepping takes place. During this automatic r o t a r y stepping, t h e connector, seeking an idle line, looks for absence of ground on lead C, and each grounded contact EC causes another rotary step to be taken. If all lines in the group a r e busy, the wipers step to the last line in t h e g r o u p . T h e r e l e a d E C i s open s o no f u r t h e r s t e p s are1taken, and ground on lead C causes busy tone t o b e returned to the calling party. 3.21 Night s e r v i c e . L e t us now a s s u m e five o'clock h a s c o m e , t h e "night connections" shown in f i g u r e 8 have been put up, and t h e P.B .X. o p e r a t o r h a s gone home. A call to 43 will ring the night watchman, who cgn inform c a l l e r s that the establishment opens a t suchand-such time the next morning, can give out: home numbers for urgent calls, etc. If; however, two calls a r e made to 43 in close succession, the f i r s t w i l l busy 43, the connector f o r t h e second call will hunt past 43 and ring 44, and perhaps no one will answer. Again, perhaps the manager is working a f t e r hours, and has told CONNECTOR NO. I EC- c- CONNECTOR N 0 . 2 -- BRIDGE CUT-OFF RELAYS -------u - z z CONSECUTIVE LINES - -- -- - - - - - -I MULTIPLE OF 5 CONSECUTIVE LINES Figure 6. The mast common pattern for 100-line connectors. Leads C and EC of all lines in the group, except the last, are strapped. CONNECTOR NO. I CONNECTOR NO. 2 500 I C- - 1 TTT 7 L - - ---------+- - - - - - - - - - ' 5 CONSECUTNE LINES I -- T T I T T L - - - - BRIDGE CUT-OFF RELAYS -- -- - - - - - - - n- MULTIPLE OF 5 CONSECUTIVE LINES Figure 7. The pattern for 200-line connectors. No special marking on any line used for telephone conversations, but a line i s wasted. ? DIRECTORY-LISTED NUMBER I NIGHT NO. 43(OR 21) I I MANAGER NlGHT N 0 . 4 4 (OR 36) SHIPPING CLERK NIGHT NO. 4 5 (OR 14) I 0 1TRUNK JACKS 1NIGHT CONNECTIONS 0 / PUBLIC EXCHANGE PRIVATE BRANCH EXCHANGE DIRECTORY-LISTED NUMBER3'43* F i g w e 8. Night connections at a private branch exchange. people he c a n be r e a c h e d during the evening on 44. If meanwhile he calls out, busying 44, a c a l l to 44 finds the line busy, the connector hunts past 44, and 45 will be rung. The station on l i n e 45 probably is in a n o t h e r p a r t of t h e building and goes unanswered, o r is answered by the shipping c l e r k who must a s k the caller (who m a y b e a t a paystation! ) to hang up and call again l a t e r . Inasmuch as night calls a r e infrequent, little difficulty arises, and f o r years most operating telephone companies have found the s y s t e m of f i g u r e 6 e n t i r e l y s a t i s f a c t o r y . If the problem locally is serious, however, two solutions a r e possible. Simplest is the c a s e of j u s t o n e i n d i v i d u a l who w o r k s o v e r t i m e frequently, and wishes h i s c a l l e r s to get busy tone when h i s telephone is in use; he should be plugged into the l a s t line in the P.B.X. trunk group, 47 in figure 8. But p e r h a p s s e v e r a l people work a f t e r hours in widely separated departments. In this case, assign an additional connector terminal to each line requiring night service. The second s e t of numbers cam be assigned at random. Disconnect the line and cut -off relays of the second terminal numbers. If the second numbers a r e on a trunkhunting connector shelf, none of t h e i r l e a d s C and E C a r e s t r a p p e d . T h u s a l l t h e s e c o n d CONNECTOR NO. I EC--.C~--- 1 numbers a p p e a r in effect a s the l a s t line of a one-line g r o u p , a n d , i f b u s y , c a n n o t c a u s e hunting, but will return busy tone. Again using figure 8, if the manager dials 21 from his home, he is s u r e e i t h e r t o ring t h e night watchman o r t o h e a r b u s y tone. If t h e m a n a g e r t e l l s people to call him on 36 at night, they will get him o r busy tone; they will never get the shipping clerk. However, the telephone company must give up two s e t s of numbers, and each P.B.X. station u s e r must ask his most frequent callers to r e m e m b e r two d i s s i m i l a r n u m b e r s , o r have a separate night listing i n the directory. The l i m i t e d s u i t a b i l i t y of f i g u r e 6 to night s e r v i c e led to the introduction i n 1932 of the bank-marking circuit pattern of figure 9. This, too, uses a fourth bank contact for extra control lead EC, for each P.B.X. trunk line. The first line in a trunk group has leads C and EC strapped.* Subsequent lines in the group have lead EC open, except the l a s t line i n the group which has a 1,200fi resistor permanently connected between leads C and EC . *For some circuits, first-line bank contact EC is connected permanently to ground. CONNECTOR NO. 2 1200w --, 5 CONSECUTIVE LINES --.El--, MULTIPLE OF 5 CONSECUTIVE LINES BRIDGE CUT-OFF RELAYS 1 - 7 Figure 9. A pattern for 100-line connectors w i t h night-line service and for level-hunting connectors. The bank wiring of figure 9 usually r e q u i r e s three additional relays in the connector: if the r e g u l a r connectors in the c e n t r a l office have eight relays, the trunk-hunting connectors will have e l e v e n r e l a y s . If any line ( s u c h a s 43, f i r s t i n the group in figure 5) is dialed, and is idle, the connector s e i z e s the line in the usual way, namely by grounding lead C to operate the cut -off relay of the called line. If now another s u b s c r i b e r dials line 43, during dialing of the last digit, 3, the second connector t e s t s lead C of line 43, finds i t busy, and then t e s t s lead EC. Lead E C being grounded, r e l a y - i n t e r r u p t e d rotary stepping takes place. During this automatic r o t a r y stepping, the connector operates on the battery-searching. principle, looking for negative b a t t e r y t h r u the unoperated cut-off relay of a n idle line. If all lines in the group a r e busy, the wipers step to the last line in the group. T h e r e , ground (from lead C) thru high resistance (1,200a) m a r k s lead E C to prevent further steps, and the ground on lead C causes busy tone t o be r e t u r n e d to the calling party. Comparing figure 6 and figure 9, we notice that in f i g u r e 6 l i n e s in the middle of a group a r e m a r k e d the s a m e a s the f i r s t line of a group, and cause hunting, whereas in figure 9 lines in the middle of a group lack marking, just a s do lines not in a group, and cannot cause hunting. Thus, if any line in the group except the first is dialed, the connector wipers have no opportunity ".., to come t o r e s t on the f i r s t line, and the con$ nector does not s e t itself for hunting. If a caller dials directly any line (other than the f i r s t ) i n a group, he will e i t h e r ring the line number he dialed, o r will h e a r busy tone; the connector will not a t t e m p t to c o m p l e t e h i s c a l l o v e r another line in the group. This s o r t of connector is recommended where special attention to night service is desired, a s all terminals of the group except the f i r s t one may be used f o r night listing. It is sometimes r e f e r r e d to a s a "night-service" o r "pilotn u m b e r " c o n n e c t o r . If the c a l l e r dials the r e g u l a r d i r e c t o r y - l i s t e d "pilot" d a y t i m e number, and it is busy, the connector sets itself to hunt; i f any o t h e r n u m b e r than the f i r s t in the group is dialed, as a t night, the connector will not hunt. Such connectors make unnecessary the assignment of the e x t r a t e r m i n a l s shown dotted in figure 8. It is such a connector we'll study in $4. TYPICAL TRUNK-HUNTING CONNECTOR , 4. OPERATION O F A T Y P I C A L TRUNKHUNTING CONNECTOR r e l a y E w i l l r e m a i n o p e r a t e d d u r i n g dialcontrolled r o t a r y stepping. Figure 20 is the circuit of a typical trunk-hunting connector with a chart illustrating the sequence of operation of the r e l a y s when a call is made to the f i r s t o r "pilot" number. The t i m e chart i l l u s t r a t e s a u t o m a t i c t r u n k hunting when the f i r s t 3 l i n e s of a g r o u p a r e b u s y . F i g u r e 9 shows the bank-marking a r r a n g e m e n t a n d the wiring of a l e v e l containing a trunk-hunting group of this connector. When the pilot number is dialed and found busy, t h e connector will hunt until it connects to an idle line. When a number of t h e g r o u p o t h e r t h a n t h e pilot n u m b e r is dialed and found busy, the connector d o e s not hunt but r e t u r n s busy tone to t h e calling party. Thus any number o t h e r than the pilot n u m b e r m a y s e r v e a s a night number. At the end of t h e second digit, the wipers a r e on the dialed line. The connector encounters one of four s e t s of conditions: 4.1 Seizure. When a c a l l e r lifts his handset, his hookswitch closes the loop circuit, and s t a r t s at the central office a linefinder which connects 3him to a selector, f r o m which he r e c e i v e s dial tone. He then d i a l s . When h e h a s dialed a l l but the l a s t two d i g i t s , t h e p r e c e d i n g s w i t c h seizes the connector. Connector relay A operates. Relay B operates and grounds lead C which marks the connector busy to other select o r s and holds the preceding switch t r a i n , and (if C w a s not a l r e a d y o p e r a t e d on s e i z u r e by ground f r o m the preceding switch) o p e r a t e s relay C 4.3 C a l l e d n u m b e r i d l e . If t h e d i a l e d line is i d l e , wiper C encounters -battery; r e l a y Z will n o t o p e r a t e . When A r e o p e r a t e s a f t e r the l a s t p u l s e , i t o p e n s t h e c i r c u i t s of t h e r o t a r y magnet and of slow-to-release r e l a y E. Relay E , r e l e a s i n g , connects - b a t t e r y f r o m the called-line cut-off r e l a y and via wiper C to r e l a y J. . . 2 During dialing r e l a y A r e l e a s e s with e a c h pulse, but s l o w - t o - r e l e a s e r e l a y B r e m a i n s operated. 4 4.21 F i r s t digit. Each t i m e A r e l e a s e s during the f i r s t digit, it o p e r a t e s the v e r t i c a l magnet, causing the connector wipers to make a vertical step. During the f i r s t step, t h e V.O.N. springs r e l e a s e , and open r e l a y C winding # 2 . P u l s e s f r o m r e l a y A t h r u winding #1 k e e p s l o w - t o r e l e a s e r e l a y C operated. After the las$ pulse of t h e s e r i e s , r e l a y A s t a y s o p e r a t e d , a n d r e l a y C r e l e a s e ? , and t r a n s f e r s t h e pulsing circuit t o t h e r o t a r y magnet. 4.22 Second digit. Each time A releases during ',the second digit, it operates the r o t a r y magnet, /causing the connector wipers to make a r o t a r y s t e p . Relay E o p e r a t e s on the first p u l s e , connecting wiper C to relay Z. Slow-to-release NUMBER DLALED DIALED LINE Individual o r pilot number Idle Individual number Busy - Pilot number Busy A line is idle 54.5 Pilot number Busy All lines busy 54.6 OTHER I.,INES SEE IN GROUP Relay J o p e r a t e s , locks, grounds wiper C to operate the called-line cut-off relay and to mark the called line busy, and operates relay H. Relay H g r o u n d s t h e R.M. START l e a d to s t a r t the ringing machine, connects ringing current (thru r e l a y F) t o the called line, and connects ring-back tone to the calling line. 4.4 Called line busy -- Individual line ( o r in a group, but not the f i r s t in t h e group). The characteristics of this series of operations are: Relay Z operates for a short time, and then restores. Relay K does not operate at all. Relay G locks thru winding #2, only. If a n individual line is dialed and found busy, wiper C encounters ground, and relay Z operates. When A reoperates after the last pulse, it opens the circuits of the rotary magnet and of slow-torelease relay E. Relay E, releasing, disconnects w i p e r C f r o m s l o w - t o - r e l e a s e r e l a y Z, and o p e r a t e s r e l a y G . Relay G locks (winding # 2 only). Relay Z releases and connects busy tone to the calling line. K (2-step relay) Figure 10. Wipers have just been dialed to first line in group. It is busy. 2-step relay K operates contacts IT-2T innediately. When slow-release relay E restores, relay K will operate completely, and lock. 4.5 Called line (first line i n a group) is busy -Subsequent line idle. The characteristics of this series of operations are: Relays Z and K operate; remain operated until idle line i s found. Relay G operates and releases (rotary-interrupter relay). Figure 11. Relay G and the rotary magnet step the wipers, relay-interrupted, across other busy lines. When wi er C finds -battery of first idle line, relay J wi& operate and stap the rotary stepping. - If t h e p i l o t n u m b e r w a s d i a l e d a n d is b u s y , wipers C and EC r e s t on grounded bank contacts (figure 10). Ground f r o m wiper C operates r e l a y 2 . Ground f r o m w i p e r E C t h r u r e l a y K hig.h- r e s i s t a n c e n o n - i n d u c t i v e winding # 2 magnetizes r e l a y K winding #1 feebly; r e l a y K c l o s e s i t s c o n t a c t s "X" only. S l o w - r e l e a s e r e l a y E r e l e a s e s , o p e r a t e s r e l a y G , connects to relay K winding #1 a direct ground which l e t s relay K operate completely and lock, and (figure 20) connects relay J to wiper C. Altho figure 10 shows r e l a y E opening the c i r c u i t of r e l a y Z, r e l a y Z is slow t o r e l e a s e , a n d operation of r e l a y K ( f i g u r e 11) k e e p s r e l a y Z o p e r a t e d . With r e l a y s K, Z, and G operated (figure ll), t h e r o t a r y m a g n e t g e t s e n e r g i z e d and s t e p s the wipers away f r o m the busy "pilotJ' number. Relay Z, i n p a r a l l e l with the r o t a r y magnet, is kept operated. A s the wipers come t o r e s t on the bank contacts of the second line in the , I ' 7 All lines o f group o r e b u s y -1 I I I Z I L I I I - Another - - - - -connector - - - - - -J' Figwe 12. All lines in the group are busy. Ground on bank-contact E C , holding relay G, revents rotary magnet from stepping the wipers beyond this group. Slow-release relay Z final1 will fall ack, release the rotary magnet, and connect busy tone. &lay G will be held thru both its windings. 1 group, rotary-magnet i n t e r r u p t e r s p r i n g s 1 - 2 open t h e c i r c u i t of s l o w - r e l e a s e r e l a y Z, and rot