Manual / Guide · 1937
BSP 550-550-110: 755A PBX, General Description (issue 1, 1937)
The 755A crossbar PBX: switch, numbering, power, circuit functions, and a local call relay by relay.
Manual / Guide · 1937
The 755A crossbar PBX: switch, numbering, power, circuit functions, and a local call relay by relay.
SECTION 550- 550-110 Issue 1, 10-28-37 AT&T Co Standard BELL SYSTEM PRACTICES Plant Series GENERAL DESCRIPTION 755A PBX Pac. CODteuta 1. GENERAL •••.•.•••••.•••.•••••••••••••••.•....•.. 2. EQUIPMENT DESCRIPTION ••••••••••••••••.••.....• Capacity . Numbering Plan . Station Equipment . Lamp Signals . Framework . Casings . Equipment Arrangements . Crossbar Switch . Power Equipment . Cabling and Wiring . . 3. OPERATING FEATURES •••••••••••••••••••......... General . Local Connections Between Key Stations . Calls Between Keyless Stations . Calls from Key Stations to Central Office . Calls from Central Office to Key Stations . Trunk Connections to Keyless Stations by Control Key . Local Conference Connections . Conference Connections on Trunks . Automatic Hunting on Station Lines . Emergency Key for Trunk Service . Restricted Service . Line Pickup Keys . 4. CIRCUIT FUNCTIONS •••••••••••••••••••••.••...... Station Circuit . Line, Line Switch and Can AUotter Circuit . Link and Link AJlotter Circuit . Central Office Trunk Circuit . Tone, Ringing, Alarm and Common Timing Circuit Charge and Discharge Circuit ........•.......... S. WIRING MODmCATIONS 1 1 1 1 2 2 2 3 3 3 4 4 5 5 5 5 5 6 6 6 6 7 7 7 7 7 7 7 7 7 7 8 . 8 General . Keyless Station Lines ................••.......... Control Key . Lockout, Non-Lockout or Restricted Senice . Line Hunting . Station Line Lamps . Line Pick-up Keys .. Connection of Trunk. to Wiring Plan•..... , . Vacant Terminals : . 8 8 6. OPERATING FUNCTIONS . General . Originating a Local Call . Outward Call to the Central Office .......••...•.. Incoming Call from the Central Office . 8 8 8 8 8 8 8 8 8 9 20 22 'I. MAINTENANCIt FaATUUS ••••••••••••••••....••... 24 General •......................................... Trouble Alarm . Link Cutoff Keys . Charge Control Relay . 24 24 24 24 8. CIRCUIT DESCRIPTIONS •••••••••••••••••••......... 24 L GENERAL This section describes the No. 'l55A PBX, which is • small dial telephone system employing crossbar switches. It provides dial intercommunication .ervice and out.ide trunk service without an attendant and can be used in either manual or dial central office area•. 1.01 The principal features of the No. 755A PBX ar~ summarized as follows: (a) Dial intercommunicating service. (b) A crossbar switch is used as the switching device. which eliminates all rotary moving parts. (c) All incoming central office calls can be answered and transferred at any key station. (d) Key stations may be arranged for anyone of three different classes of service in connection with central office calls as follows: (1) Lockout Service-This prevents a station from being connected to a busy trunk except when there is an unansVl<ered incoming call or hold condition on the trunk. (2) Non-Lockout Service-This permits a station to be connected to a busy trunk. This class of service is employed where trunk conference connections or "executive right-of-way" service is desired. (3) Restricted Service-This prevents a station from making outgoing trunk calls but does permit the station to answer incoming trunk calls or connect to a trunk where there is a..hold condition on the trunk. (e) Provision of a combined dial hand telephone set with the necessary key buttons mounted in the base for establishing local and trunk service. (f) A control key arrangement is available for use at key stations to provide means for connecting keyless type stations to central office trunks without limiting the use of the key station at which the control key is located. (g) One or more key stations may be connected as extensions of a main key station on any station line. (h) Arrangements are provided to permit line hunting on certain lines when required. (i) Local conference connections may be established by dialing. (j) Trunk lamps may be provided at the stations and are arranged to flash to indicate unanswered calls and to light steadily when their associated trunks are busy. (k) Lamp signals similar to those used for trunks may be provided for local lines. 1.02 2. EQUIPMENT DESCRIPTION Capacity 2.01 The capacity of this PBX is 20 station lines, 4 central office trunks and 3 link circuits. Where 10 or less station lines are equipped only one crossbar switch is provided; however, the other crossbar switch can be added later. The station capacity may be augmented by the use of k"y station extension stations and the trunk capacity by the use of pick-up keys. N.umbering Plan 2.02 The station line circuits are numbered from 20 to 39 with lines 20 to 29 being in the first group and lines 30 to 39 in the second group. The equipment is designed so that preliminary pulses are absorbed by the link circuits. If a number is dialed for which no station is assigned no connection will be completed and no tone will be heard. © American Telephone and Telegraph Company, 1937 Printed in U. S. A, Pag.l SECTION 550-550-110 Station Equipment 2.03 Four different types of telephone sets may be used at key stations and are as follows: (1) Combined hand telephone set. Note: On the earlier installations of the No. 7SSA PBX's the No. 205 type hand telephone set ",.ith an E type mounting and a subscriber set cont~ining a ringer, induction coil and condenser may be mstalled. (2) No. 213 (hang up) type hand telephone set. (3) No. 206 type hand telephone set. (4) No. 653 wall type subscriber set. 2.04 The combined hand telephone set is complete with ~e ringer, induction coil, condensers and keys mounted m the base of the set and is shown in Fig. 1. Fig. I-Combined Hand Telephone Set. The four key buttons used for ~aking connections !o trunks and the one used for makmg local calls remam he locked in the operated position after being ?epressed. remaining key button, which is used for ho.ldJ.ng connections on central office trunks, does not lock when It IS depressed. 2.10 The locking button furthest from the hold button is generally used for local calls and the other four locking buttons are used for trunks. 2.09 :r Lamp Signala 2.11 Lamp signals can be pro,:"ided at the stations to indicate local calls and iIfcommg trunk calls. When lamps are used for this purpose they are housed in one of the following types of indicators as required. (1) Beehive type indicators. l5A for one lamp. 17A for two lamps. l8A for three lamps. (2) Surface mounted indicators. 14A for three lamps. 20A for four lamps. (3) Flush mounted indicators. 16A for three lamps. 2.12 Fig. 3 shows some of these indicators. No.17A 2.05 The No. 213 type hand telephone set is for use at the side of a desk or any location where a hang-up type of instrument is desired. The No. 206 type set is for use where a set without keys in the base is desired. When these types of sets are used a separate subscriber set containing a ringer, induction coil and condensers is required. 2.06 When the No. 2~3 or No. 206 type hand telephone sets or the No. 653 type subscriber sets (wall type) are used for key stations the keys required for completing local or trunk calls are mounted external to the set and furnished as a unit known as the No. 602l-J3 key. This key is shown in Fig. 2. / No.20A Fig. 3-lndicatora. Hold Trunks Local Fi•• 2-No. 6021·J3 Key. 2.07 The No. 6021-J3 key for key stations consists of six push button type keys. Four of the key buttons are used for making connections to four central office trunks, one is used for making local calls between the P B.X stations and the other one is used for holding connections on central office trunks. 2.08 When anyone of the key buttons is depressed all other buttons are released. Page 2 2.13 B2 type lamps are used in all.indicators. Framework 2.14 The framework housing the PBX apparatus is constructed of steel members welded together and its design is similar to that of a single bay of floor supported relay rack arranged to mount 2" x 26" mounting plates in the 10Vlier portion and 2" x 20-1/2" mounting plates in the upper portion. Adjacent to the shorter mounting plates in the upper portion of the framework are details for holding the connecting cable terminal strips. The frame is approximately 6' high. It is provided with a wooden base to afford a means of insulating it from local ground where the storage battery is charged on a metallic return· basis. The lower part of the frame is arranged to house the power plant including the storage batteries and associated charge and discharge apparatus. The fuse panel is located just above the battery compartment. Space just above the fuse panel is available for two additional mounting plates for miscellaneous equipment. Above this are mounted the relay units for the common tone, ringing, alarm and link allotter circuits as well as the trunk and link circuits. Over this are mounted the two crossbar switches with the line relay units and the associated common control and cut-off unit between them. Figs. 5 and 6 show a front and rear view respectively of the frame with the apparatus in place. ISS Casing. 2.15 The framework 'is fitted front and rear with lightweight sheet steel casings designed to be lifted off the frame rather than hinged to it. Four separate casings are provided; one for the front and one for the rear of the hattery compartment and one for the front and one for the rear of the upper or switching compartment- The upper casings are equipped with locks which are operated by the same keys used for opening the keyshelves of manual PBX's. Fig. 4 shows the framework with the casings in place. 1 SECTION 550-550-110 Equipment Arrangemenb 2.16 The equipment is arranged on a unit basis to facilitate equipping varying numbers of line, link and trunk circuits as required for particular installations. For small installations, where the requirements do not exceed ten station lines, only one crossbar switch may be provided. When the requirements exceed ten station lines two crossbar switches are provided which will care for a maximum of twenty station lines. 2.17 The trunk and line units are designed on a two and four circuit basis respectively with arrangements for providing either a full or partially equipped unit. The link units are designed on a single circuit unit basis. Croubar Switch 2.18 The crossbar switch used in this PBX is equipped with 10 vertical units each having 10 sets of contacts with six pairs of make contact springs in each set. Each set of contacts has an actuating spring to control the closing of the contacts. Associated with each vertical unit is a holding magnet and holding bar which is attached to the armature of the magnet. There are five selecting bars which run horizontally across the switch unit and each bar is equipped with 10 selecting fingers, one for each vertical unit. Each selecting finger is a flexible wire about 3 inches long which extends from the horizontal bar to a position near the vertical holding bar between the actuating springs of two sets of contacts, which are located one above the other. Associated with each selecting bar are two selecting magnets; one of these magnets rotates the bar so as to move the selecting fingers upward and the other magnet rotates the bar in the reverse direction to move the selecting fingers downward. The actuating spring is moved horizontally to close the contact springs and this action is caused by the movement of the holding bar when the holding magnet is energized if a selecting finger is entrapped between the actuating spring and the holding bar before the magnet is energized. One of these crossbar switches is shown in Fig. 7. Fig. 4-Framework With Casing. in Place. SWitCh.~ l Crossba r for Lines 30·39 ~t:j9 28·3120-23 J 32.35~ 24.27~ Common-i Co~trol Crossbar Switch for Lines 20·29 Link 3 - { Link 2---------1[ 1--{ Link Trunk 3 - - - - - - - - { ( Tone T_ru_n_k_l_---{--(c Ringing-( (TrUnk 4 ~Trunk 2 \Link AIIotter Alarm Fig. S-Front View. Fig. 6-Rear View. Page 3 SECTION 550- 550-110 2.19 The operation of a selecting magnet causes its ass.ociated selecting bar to rotate ;lnd move all 10 of Its selecting fingers into traps between the vertical holding bars and the actuating springs so that any set of contact springs on the horizontal level of the operated selecting magnet can be closed when the associated holding magnet operates. Therefore, anyone of the 100 sets of -contact springs on a switch can be closed by first operating the proper selecting magnet and then operating the proper holding magnet. The contact springs which are closed by this procedure are those to the left of the holding magnet which has operated and above the horizontal bar which has operated if the selecting fingers on the bar are moved upward, or below the bar if the selecting fingers are moved downward. Fig. 8 shows an illustration of the selecting and operating elements of the crossbar switch. 220 After a set of contact springs has been closed as described in 2.19, the selecting magnet is released but the holding magnet continues to be energized to hold th~ contact Iprings closed. The pressure exerted by the holding bar through the selecting finger to the actuating spring holds the selecting finger in the selecting position after the selecting magnet is restored to normal. All other selecting fingers on the same bar which are not held by holding magnet armatures restore to normal when the selecting magnet is released. Each selecting finger is equipped with a damping spring which reduces its vibration when the selecting bar is operated or released so as to minimize the possibility of false operations of the switch contacts. 2.21 The release of a holding magnet permits the selecting finger to restore to its normal position. If a holding magnet should be operated when none of the selecting magnets are operated, none of the selecting fingers would become operated and none of the 10 sets of contact springs in the vertical unit would be closed. 2.22 Each voiding magnet is equipped with a set of offnormal springs which operate whenever the holding magnet is operated regardless of the operation of any of the selecting magnets. The selecting magnets are also equipped with off-normal springs. 2.23 The switch contacts which are closed by the holding magnets in combination with the selecting fingers and the selecting magnet off-normal springs are all of the make type. The holding magnet off-normal springs have break contacts as well as make contacts. 2.24 One of the springs of each pair in the sets of contacts operated from the selecting fingers is fixed while the other spring is moved by the holding magnet armature and thu.s the contact pressur:e is obtained by flexure of the moving sprIngs. The fixed springs for all 10 sets of contacts in a vertical unit are stamped from a single piece of metal so that thes~ contacts are per~a!1c:nt1y inte!"connected electrically. The movmg contacts are indIVIdual sprmgs. The movable springs are provided with twin electrical contacts and each of the contacts on each spring is of palladium capped nickel. 2.25 One verti.cal ~nit of a crossbar switch is provided for each statton line. Therefore each station line can be connected to anyone of 10 different points through the 10 sets of contacts in the vertical unit. Four of these 10 sets of con~ tacts are arranged for connections to four central office trunk circuits. The other six sets of contacts are provided for the three link circuits, there being one set for the originating end of each link and another set for the terminating end of each link. One crossbar switch is sufficient for 10 station lines. The No. 755A PBX is designed for two crossbar switches therefore having a capacity for 20 station lines. 2.26 Since the selecting magnets and associated selecting fingers are common to all vertical units on a crossbar switch, only one connection through a switch can be completed at a time although after one connection has been completed. others can be completed in turn until a total of 10 holding magnets have been operated in succession on different connections on one switch. The time required to complete a connection by the operations of the selecting magnets and holding magnets is less than one second. If two calls should originate in the same one-second period, one of the calls would be delayed until the other was completed and this is accomplished by the call allotter circuit. Power Equipment The power plant consists of 5 small two cell storage batteries (15 ampere hour capacity) with associated s;harge and discharge equipment. The storage battery pro'rides a voltage range of 18 to 25 volts .and may be charged over cable pairs from a 48-volt central office battery or by a copper oxide rectifier connected to a 1I0-volt 8lI cycle commercial power source. 2Z1 Cabliq &lid Wirin. 2.28 The wiring between the apparatus on each equipment unit is done on a surface wiring basis. Only three different colors of wire insulation are employed l1r-ith this type of wiring. Ground supply wires are black, battery supply wires are red and all other wires are green. Wires between terminals on the same unit can be identified by referring to the running sheets which are provided instead of wiring diagrams. On these running sheets each wire is numbered individually and the apparatus terminals to which it is connected are definitely indicated. 2.29 Wiring between the various equipment 1Jnits is pro-vided in local cables and these wires are connected directly to the apparatus terminals as no terminal strips for this purpose are provided on the units. The surface wiring to each terminal, to which a local cable skinner is to be attached is wound around the terminal so that the skinner can b~ fastened through the regular hole at the end of the terminal. These local cables are of the conventional type using wires having various combinations of colored insulation. 2.30 The outside cable can be brought into the PBX either . through a hole in the top of the framework adjacent to the nght hand corner or through a hole in the left hand side of the framework just above the ftoor. The outside cable may be connected either directly to the "terminal strips on which the PBX. circuits a:e te~inated or to separate terminal strips, for whIch space IS proVIded, from which cross-connections are extended to the terminals of the PBX circuits. Selecllng Magnet HoIdi"l 8IIr - - - _ fIxed Contacts on Vetticll MullipIe Slrip Cross Point Selecting' Armatur. Selecti"l fincer Coil-----I""'_ ActuIli"l Sprinas-------------' Fig. 7-Croaabar Switch. Page 4 ISS 3. OPERATING FEATURES General 3.01 Local connections between PBX stations are established by the link circuits. A link circuit is automatically connected to a station line when the hand set is removed from its mounting after the local push button key is depressed. 3.02 A link allotter circuit selects the particular link which is connected to the station line. The allotter is arranged to allot successive links on successive calls thereby providing an equal distribution of traffic among the links and also causing different links to be used on successive calls from the same station during periods of light traffic. 3.03 The links contain the apparatus for receiving the dial pulses and for establishing the connections to the station lines that are called and for ringing and tripping. 3.04 If none of the push button keys at a key station are in an operated position when the hand set is removed from its mounting, no connection is established and normally no tone is heard in the receiver. However, tone will be heard under this condition if the PBX busy tone interrupter happens to be started at the same moment by anyone of the following conditions: (1) by an attempt to originate a local call from any other station when all links are busy or (2) by an attempt to originate a trunk call from a.' restricted station or (3) by an attempt to originate a trunk call from any other station when all trunks are busy. Local Connection. Between Key Station. 3.05 To originate a local call at a key station the hand set is lifted from the mounting after the local key is depressed. Through the operation of the call allotter, common control and link allotter the calling station line is connected to a link. Then dial tone is heard, except that if all links are busy, busy tone is heard instead of dial tone. After dial tone is heard the number of the desired station line is dialed. After the first digit has been dialed the dial tone is disconnected. After the second digit has heen dialed audible ringing tone is heard if the called station line is not busy. However, if the called station line is busy, busy tone is heard. If the number dialed corresponds with a line to which no station apparatus is connected, no tone is heard. The call is answered at the called station by removing the hand set from its mounting after depressing the local key. When the called party answers, the ringing stops, the audible ringing tone is disconnected and the connection is completed from the calling station line through the link circuit to the called station line. 3.06 A diagram illustrating the progre~s of a .typical 10c~1 call between station 36 and station 22 IS shown 111 Fig. 9. If a trunk caU should occur simultaneously with a local call the local call must be answered first. If an attempt is made 'to answer the trunk. call first or if under any condition the hand set is removed With a trunk key depressed 3.07 ~End~End- Unes20·]9 91 9 I:D I~o LInk 3 , , Link 2 .f! L ~ J::., TrIl4 TrIl3 TrIl2 Trill tr-lSI ~lil 3.08 Machine ringing is tripped immediately after answering a local call in the proper manner at a key station regartltess of whether the answering occurs during a ringing or silent period. 3.09 The hold key is effective only in connection with trunk calls and its operation while the local key is depressed will not establish a hold condition on the link but will cause a disconnection of the station from the link. Call. Between Keyless Station. 3.10 Local connections between keyless stations are accomplished in the same way as for key stations except that no local key is used and also the tripping of machine ringing may not always occur Immediately after answering if the answering occurs during a ringing period. However, ringing will always be tripped immediately during silent periods. Call. from Key Station. to Central Office 3.11 An outgoing trunk call to a central office can be made from any key station, except those arranged for restricted service, by operating the trunk push button key associated with the particular trunk over which the call is to be made and removing the hand set from its mounting. If the station is arranged for lockout service and the trunk selected is busy, busy tone will be heard. Successive trunk buttons should then be depressed until a connection to a trunk is completed. However, if the station is arranged for non-lockout service and trunk lamps are not provided it is necessary to listen for conversation or challenge in order to determine whether the trunk is busy as no busy tone is returned to these stations. If trunk lamps are provided, these lamps can be observed to determine which of the trunks are free for use, a free trunk being indicated by an extinguished lamp. A diagram illustrating the progress of a typical outgoing trunk call from station 25 is shown in Fig. 10. 3.12 If a key station arranged for restricted service attempts to make an outgoing trunk call the occurrence of such an attempt will cause busy tone to be heard unless an unanswered incoming call or hold condition is on the trunk in which case the station is connected to the trunk. If any key station is unint~ntionally connected to a trunk when an unanswered incoming call is waiting on the trunk, the incoming call should be completed in the normal manner from this station because the flashing of the trunk lamps is stopped and the bells are silenced by this connection and a disconnection by this station without completing the call, might prevent the call from being completed and cause the calling party to be charged, 3.13 \ 91 Unk3 Unk2 Unk 1 1 :z: t ..f ~~'- . . . . . . . . . ......,...J°LL~ . . . ~---J . Lint - St& 22 c.Ied ••••• Key (l) SECTION 550- 550-110 after the terminating end of a link circuit has been connected to the called station line, the talking connection through the link is not completed and audible ringing tone is heard by the called party the same as that heard by the calling party. When the local key is depressed the connection through the link is completed in the normal manner. I j LInk I 1 Central Office ==~'=ttatata3 To Other Switch 30· 39 When Req. TrIl43 TrIl TrIl2 . St& "I ~ Une .00 •• _________..&!!I..!lIJ!lM.Q!P!l_-!.__ Fig. IO-Typical Outgoing Trunk Call. Page 5 SECTION 550·550·110 If any key station is unintentionally connected to a trunk on which a call is being held the hold key should be depressed at this station before the station is disconnected from this trunk in order to avoid a cut-off on the call beinif held. 3.14 Calls from Central Office to Key Station. 3.15 An incoming call on a trunk from the central office is indicated by either an audible or flashing visual signal or by both, depending On the facilities provided. This type of call can be answered at any key station which has a push button key associated with th-e particular trunk on which the call appears. The incoming trunk call is answered by removing the hand set from its mounting after depressing the proper trunk key button. This causes the audible signals to be silenced and the visual signals, if provided, to light steadily. 3.16 If the incoming call is for a person located at another key station the call is transferred to this other station by first depressing the hold key to hold the call on the central office trunk. The local key is then depressed and the desired station is dialed as on a regular local call. When this local call is answered, the person answering is advised that a call is waiting en a particular central office trunk. This person then depresses the trunk key button which is associated with that trunk and the incoming call is thereby completed. The person who originally answered the trunk call performs no further functions in connection with the call after the desired party has been advised of the call over the local connection other than to restore his hand set to its mounting. A trunk call can be transferred from the second key station to another key station in the same manner as it was transferred from the first key station. Trunk Connections to Keyless Stations by Control Key 3.17 Facilities for connecting a keyless station to the central office trunks may be provided by means of a control key located at a key station which is known as a control station. 3.18 other key station, it is transferred to the control station. The hold key is deprt:ssed at the control station to hold the call on the trunk and then the keyless station Iscalied over a link circuit in the normal manner. The person -answering at the keyless station is advised of the incoming call and requested to remain on the line. The person at the control station then returns to the trunk connection by operating the trunk key button in the normal manner and then operates the control key associated with the keyless station. The operation of the control key connects the trunk to the keyless station and the control key is released when the completion of the connection is indicated by a characteristic click and change in sidetone. The person at the comrol station advises the others that the connection is completed and after hearing the conversation restores his hand set to the mounting. This disconnects the control station from the trunk and thereafter it is free for use on other calls in the normal manne·r. The connection of the keyless station to the central office trunk remains until the hand set is restored to its mounting at the keyless station. This automatically releases the connection. Local Conference Connection. 3.20 A local conference connection can be originated by any key or keyless station. To originate a conference call, station "A" calls station "B" in the normal manner. Station "B" answers and then after being advised of the conference call replaces his hand set on its mounting. Station "C" is then dialed at station "A" without the hand set being restored at station "A." Dial tone is not heard at station "A." After the first digit of the number for station "c" has been dialed the bell at station "B" will start to ring in accordance with the regular machine ringing cycle. After the second digit of the number for station "C" has been dialed the bell at station "C' will ring in synchronism with that at station "B" and audible ringing tone will be heard at station "A" if station "C' is not busy. 3.21 When the hand set is removed at either station "B" or "C" the ringing ceases at both stations. Normally the person at station "B" would wait until ringing stopped before removing his hand set so that he could be assured that station "C" had answered. The answering of stations "B" and "c" connects these stations in multiple to the link and this completes the conf~rence connection. 3.22 If station "C" is busy the completion of the dialing of the second digit from station "A" will cause busy tone to be heard in the receiver at station "A" and the ringing to cease at station "B." The discontinuance of ringing is the signal for station "B" to come back in on the connection. When station "B" returns to the connection busy tone is rem?v.ed and stations "A" and "B" are connected together as ongmally. 3.23 If the person at station "C" does not answer the person at station "B" returns to the connecti~n before ril!g!ng ceases and is thereby connected to station "A" as ongmaIly, except that the line circuit of station "C" continues to test busy until the link is released by the disconnection of stations "A" and "B." 3.24 :When a confere1"!ce connection is completed the link IS held busy untIl all three of the stations are disc?nnected. Station "A" (which is the calling station) can be dIsconnected .from the link before stations "B" and "C" by merely restormg the hand set to its mounting. If stations "B" an~ "c" (which are the ~aIled stations) are of the keyless type, neLther of these two statIons can be disconnected from the link until the receivers are restored at both stations. If stations "B" and "C" are of the key type neither of these two called stations can be disconnected from the link by the depression of a trunk push button key unless the station receiver is restored at the other of these two called stations. To originate an outgoing central office trunk call from a keyless station the line circuit of which has been equipped with a control key, the key station at which the control key is located is first called in the normal manner from the keyless station and the person who answers is requested to connect the keyless station to a central office trunk; and if the trunk terminates in a dial central office the person at the control station is also advised of the central office number desired in order that the dialing of the number can be performed from the control station. After receiving instructions from the keyless station the person at the control station observes the trunk lamps to see if an idle trunk is available and if all trunks are busy advises the person at the keyless station. If an idle trunk is available the person at the control station connects his telephone to a central office trunk in the normal manner. After the control station is connected ~o a tr!lnk circuit and the desired number is dialed, if dialing IS reqUired, the control key is operated to its position associated with the keyless station desiring the trunk connection. The ope~ation of the control key automatically connects the keyless statIon to the central office trunk in multiple with the control station. The control key is held operated until the connection is completed which will generally occur within an interval of about one s~cond; then the key is released. The person at the control statIOn can tell when the connection is completed by a characteristic click and change in sidetone. The party at the keyless station remains at the telephone during the above proc~dure and becomes advised of the completion of the connectIon when he hears the audible ringing signal or the answer of the called operator or subscriber. The hand set at the ~ontrol station is :est~red to its mounting after the connection IS completed. ThIS dIsconnects the control station from the trunk and thereafter this control station is free for use on Conference Connections on Trunk. other ca!ls in the normal manner. The connection of the key3.25 S.everal stations can be simultaneously connected to a les~ statIon to the central office trunk remains until the hand smgle trunk and thus be arranged in a conference set is r~stored to its mounting at the keyless station. This conne~tion if all of the stations or all but one of the stations automatically releases the connection. are WIred for non-lockout service in connection with the trunk. 3.19 To complete an incoming central office trunk call to a 3.26 A conference connection on a trunk with ,,11 but one . keyless station, the. call is answered at the key station of the stations wired for non-lockout service can be at whIch the control key IS located or if answered at some established by the following procedure: First a station with Page 6 ISS non-lockout service establishes the connection to the trunk to be used for the conference. Then the hold key is depressed at this station. Following this the station which is arranged for lockout or restricted service is called over a link in the normal manner and the person at this station is advised to dept'ess the proper trunk button for the confere~ce. A~ter this station is thus connected to the trunk, the stattons with nonlockout service are called and they connect to the trunk in the normal manner by the operation of their trunk keys, and then the conference connection is completed, since all stations are connected to the same trunk. 3.27 No circuit arrangements are provided to improve transmission for conference connections. Automatic Hunting on Station Lines 3.28 Optional wiring arrangements are provided in the link circuits to permit line hunting on lines Nos. 26 and 27 and on lines Nos. 36 and 37. When the hunting arrangement is provided, a connection will be completed to the higher numbered line of the pair if the lower numbered line is busy when a calling station dials the lower numbered line. Busy tone will not be heard at the calling station after dialing the lower number of the pair unless both lines of the pair are busy. However, dialing the higher number of the pair when it is busy will not cause the connection to be completed to the lower numbered line if it is idle. Emergeacy Key for Trunk Service 3.29 To insure central office service under all trouble conditions an emergency key can be provided at one of the key stations and connected so that the operation of this key connects the key station directly to· one of the central office trunks. Restricted Service 3.30 Key stations which are arranged for restricted service cannot connect to trunk circuits to originate outgoing calls but can be connected to trunks to answer incoming or transferred calls or to originate outgoing calls after the hold condition has been established on the trunk at a non-restricted station. The restricted service feature can be provided in connection with any or all of the trunk circuits at any key station. All keyless stations have restricted service. Line Pickup Key. Geaeral 3.31 Any station may be arranged to be connected through pickup keys to more. than one local line or to one or more central office lines in addition to those terminated in the P.B.X. trunk circuits. Pickup keys for this purpose may consist of one of the following arrangements. 1. Spare trunk push buttons at key stations. 2. Non-locking keys external to the key stations. 3. Wiring plan keys at keyless stations. These arrangements permit one station to be arranged to answer calls for another station, permit two or more stations to make common usage of their local lines, and permit the connection of a station to additional direct lines to the central office. 3.32 The pickup arrangements for key stations do not include facilities for holding calls. The operation of the hold push button key during a connection through one of these arrangements will not produce a holding condition but will release the connection. Key Station Buttona Used as Line Pic:kup Keys If a key station does not have access to a maximum of four trunks the spare trunk push button keys may be used as line pickup keys. Each available key may be used to connect the station to the local line of another station or to a direct line to the central office which is not otherwise connected to the PBX 3.33 3.34 A call is originated or answered with this arrangement by removing the receiver after operating the trunk push button key used for pickup service. 1 SECTION 550-550-110 Separate Non-Loc:king Pic:kup Keys at Key Statlona 3.35 At key stations where all of the regular push button keys are employed for regular service, separate nonlocking pickup keys can be installed external to the station. These keys are associated with the regular local push button key at each of the key stations where they may be used. 3.36 A call is originated or answered with this arrangement by operating first the local push button key removing the receiver and then operating the external key. Wiring Plan Keys 3.37 Keyless stations may be arranged for pickup service by employing various wiring plans for this service. Each line from a wiring plan may be connect~d to an individual central office line, a keyless station line circuit or to a trunk circuit of the 755A PBX 3.38 A call is originated or answered with this arrangement by removing the receiver after operating the wiring plan key. 4. CIRCUIT FUNCTIONS Station Circuit 4.01 The station circuit drawing shows the station equipment required for key stations the cmitrol key for use at control key stations and the keys required for emergency service. No circuit drawing is provided for keyless stations since the apparatus employed is of the common type used for regular two-wire dial PBX extensions except that high impedance ringers are required. Line, Line Switch and CaD Allotter Circuit 4.02 The line, line switch and call allottercircuit provides means for connecting a key or keyless station to a link circuit and a key station to a central office trunk. It also prOVides means for alJottingcaUs in succession so that not more than one line may be associated with the crossbar switch selecting magnets at one time thereby preventing double connections. Lid and LiDk ADotter Circuit 4.03 The link circuit is used in establishing local connections between PBX stations, for controlling dial and busy tone, for receiving dial pulses from the calling station, recording the pulses, causing the operation of the proper switch magnets, ringing the bells of the called station and tripping the ringing when the called station answers. Each link circuit requires two horizontal paths on the crossbar switch, one for the originating end and one for the terminating end of the link. 4.04 The link allotter rotates the traffic over the links, automatically makes a link busy if its fuse should operate and controls the connection of busy tone to a calling line when all links are busy. eeDtral Office Trunk Circuit 4.05 The trunk circuit provides a means for connecting a central office trunk to the crossbar switch and includes the relays which hold central o