US2490666A - Telegraph switching system - Google Patents

Description

2 Sheets-Sheet 1 Dec. 6, 1949 o. BLYHOLDER TELEGRAPH SWITCHING SYSTEM Original Filed Aug. 14, 1944 -35 hows Dec. 6, 1949 o. BLYHOLDER TELEGRAPH SWITCHING SYSTEM Original Filed Aug. 14, 1944 2 Sheets-Sheet 2 V MRER \Uh\s is! u Patented Dec. 6, 1 949 UNITED STATES PATEYNTOFFICE TELEGRAPH swrronme SYSTEM Orlando Blyholder, Chicago, Ill., assignor to Kellogg Switchboard and Supply Company, Chicago, Ill., a corporation of Illinois Original application August .14, 1944, Serial No. 549,339, now Patent No. 2,464,371, dated March 3 Claims.

GENERAL DESCRIPTION I The invention is illustrated as embodied in a telegraph switching system employing the socalled hub repeating arrangement wherein two or more lines may be interconnected through a common hub conductor having individual spoke conductors for the respective interconnected lines, and in which a two-way tele-. graph repeater is interposed between each individual line and its associated spoke conductor.

A specific object is to provide a telegraph switching system in which the lines have ordinary appearances at the switchboard and in which reliable provisions are made for signaling the switchboard operator over any line when connection or disconnection is desired.

Other objects and features of the invention, more or less incidental to the foregoing, will appear hereinafter.

THE DRAWING The accompanying drawings, comprising Figs. 1 to 3, show sufiicient of the apparatus incorporated in a system embodying the invention to enable the invention to be understood.

Fig. 1 shows one embodiment of the invention;

Fig. 2 shows a modification of the equipment associated with line LI of Fig. 1; and

Fig. 3 shows a second modification of such equipment.

Figure 1 Fig. 1 shows three subscriber lines of a system including any desired number of such lines; first and second lines LI and L2, and the last line LL. These lines serve stations SI, S2, and SL respectively, shown diagrammatically as teletypewriter stations. The illustrated lines are provided with similar two-way repeaters RI, R2, and RL respectively, repeater RI being shown in full. Spoke conductors SPI, SP2, and SPL connect the illustrated repeaters respectively with their switchboard jacks, J I, J2, and JL, with which the jacks J I J2, and JL', are respectively multipled, enabling patching cords such as PCI, PC2, and PC3 to be used to interconnect the spoke Divided and this 24, 1945, Serial No. 612,499

application August conductors in any desired arrangement of pairs' and hubs. 7

Each repeater contains a receiving relay, such as RR of. repeaterRI. The receiving relay RR may be of any desired form of polarized relay,

being illustrated as of the type in which the polarized armature passes axially through the winding space. Such a relay may be adjusted to assume a neutral position with no current flowing through any of its three windings. The armature of relay RR is normally held in marking position (against contact M) by the right-- hand or marking winding, energized in a circuit including the positive pole of the current supply and the associated regulating resistor. The right-handor marking winding may be eliminated if relay BB is biased to marking position, such as magnetically or by spring action. The circuit of line .LI includes the two conductors thereof in series withthe equipment at station SI. It may be considered as originating at the positive pole of the current-supply source, and the normal currenteflow thereover is controlled by the associated adjustable resistor. turn path for line current is through the lefthand or line winding of receiving relay RR, and

the plate of sending tube ST, to ground through the grounded cathode element thereof. Assuming that .the three windings of relay RR are,

equivalent, the line current is preferably regulated to be about double the current in the righthand or marking winding, and is in such a direc tion through the line winding as to assist the marking winding in holding the armature of the relay in marking condition, as is indicated by the fact that the vertical arrows on the windings in question are directed downwardly.

The magnetic efiect of the current through the left-hand or line winding of receiving relay RR is neutralized by the opposed current flow of subj ficial-line circuit, which includes the associated adjustable resistor connected between such winding and the positive pole of the supply source.

The return path for the artificial line is through the plate of the balance tube BT, to ground through the grounded cathode element thereof.

Each of the tubes ST and ET is indicated as a tetrode, having its control and screen grids in Each screen grid the usual relative positions. is supplied with, positive potential from the resistor. The control'grids of tubes ST and BT The reare both connected to the marking contact M of receiving relay RR, and are normally supplied with ground potential through the associated grid resistor GR, permitting a normal flow of current through both. A tube such as the commercially available 6L6 beam-power tube 'is satisfactory at ST and BT, using a positive supplypotential'of 250 volts, for example.

Lines Ll to LL are provided with line circuits" L! to LCL respectively, of which the-line'cir cuit LCI is shown in full. Line circuit LC! is controlled over the associated spoke-conducton SPI through the grid of linetubeLT. Tube;'LT,' as well as the other triodes shown herein; may" be of the commercially availablex6051typetusing the same positive supply potential asthe sending and balance tubes.

The purpose of line circuit LCI, is to light call' lamp Cl at the switchboard responsive to-:a prolonged opening of line Ll (as by break BKI) in excess of one second, for example, and to light the disconnect lamp Dfresponsive to a break-in theassociated line in excess of.five seconds, for

example, at a time when aconnection with the line exists through the switchboardi. Line circuit LCl includesline relay-{normally operated through the plate-cathode circuit of line tube LT by half-wave-rectified cur-rentfromrelay terminal R, supplied with alternating; current of commercial frequency and suitable voltage Relay 2 is arranged to controlthe potential of the grid of timing tube TT by. normally. maintaining a cutoff'bias on such-grid,.acro;-is condenser 6, and by discharging this biasing potentialthrough resistor I for a one-second delay,.andthrough re-' sistors l and 8"for a five-second-de-lay. As will hereinafter appear, timingtubefTT controls one w or another of the call and disconnect relays: and4, accordin to the operated-or restoredtcondition of transfer relay '3-,-.controlled.1from either ofthe associated jacks Jl andiJ-l'. Relays3 to 5 are supplied-with current from the associated relay terminals R.- Each such relayiis preferably a shaded-pole relay which remains operated steadily when energized -by-alternatingf pulsating,

or direct current.

The switchboard operator is supplied witha telegraph station si also illustra ted as a teletypewriter station, controlled overthe operator's lineLU. This line-terminates inthe operator's repeater R0, exactly similar to repeater RI. Re-- peater R0 is connected with the operators switchboard jack'Jfl by'the spokecon'ductor SPO.-

In Figure 2, the equipment ofaline Ll oflFig. 1. ismodified by replacing linetube- LT and-relay 2 of line circuit-LCI by relay 2'-A' of line-circuit LCl-A. Line circuit LCl"Af-isarranged torespond only to breaks occurring'in the associated line and is independent of 'control'over'th'e common hub by way of the associated spoke conductor SP1, as will appear hereinafter. For this purpose, relay 2A isa double-wound differential relay, having. one'winding in series with line LI and the other winding inlseries'lwith the artifi'cial or balance line of the repeater RI. In the arrangement of Fig. 2, the transmission of -a disconnect signal from a connected line causes the lighting of the disconnect lamp associated with that line only, whereas in Fig. 1. the disconnect lampslight at all interconnected lines. responsive toa disconnect break at anyone of them.

Figure 3 Fig. 3 shows a further modification of the equipment of line Li (Fig. 1), illustrating what may be termed a uniform-current system. The arrangement in Fig. 3 is such that current flows alternately in thesubscriber'line and in the associated artificial or balance line, .thereby simplifying the power-supply requirements, in addition to halving the simultaneous or demand current for the several natural and. artificial lines, as will be explained more indetail hereinafter. Fig. 3 cmploys the'additional tube PI, used as a phase-inverter triode effective to transmit a potential of the opposite sign to the control grid of balance tube BT'when a'potential of a given sign is impressed on thecontrol grid of the sending tube ST.

DETAILED DESCRIPTION The disclosure havingbeen described generally,..a detailed'description of the operationoi the system willnow be given.

A. FIGURE 1 The system of "Fig." 1* will be considered first.

A1. Calling the operator Assuming that the-subscriber at'substatlon Sl (Fig. 1) desires a: connection,- he opens the associated line Li, as by break key BKI, for an interval somewhat" in excessof'one second, and then reclosestheline. W'henathe' line is opened, the flow'of line current ceases, deenergizing the left-hand orline winding of'receiving relay RR.

Tliereupon, relay RR operatesits armature fromengagement with its marking contact M into engagement with its'spacing contact S; by virtue of "the fact that the more powerful current in-the -middle or balance winding then overcomes the current in the "right-hand or marking winding. Spacing current from the negative biasing terminal isthereby'impressedon the associated spoke conductor SPI.

5 Since spoke conductorSPFis normally disconnected at the jacks J I and J l", the placing of the negative spacing'potential thereon has no effect except in the line circuit LCl, where it blocks current flow through line tube LT. The normally operated line relay 2 thereupon' restores, disconnecting the grid of timer tube TT (and the upper terminal-of timingcon'denser 6) from the source of biasing potential and connects it through the adjustable one-second -resistor l, and contacts 3 of transfer relay =3,- to ground;- At the end-of about one second, the normal negative charge of grid condenser 6 sufficiently dissipated through resistor l to renderthe control grid of timing tube TT sufficiently positive to enable plate current of relay-operating value to flow-therethrough. Call relay 5 thereupon operates in a circuit from ground, through the. cathode and plate of tube -TT,' armature l of transfer relay 3 and its back contact, and thence through the winding, of call relay 5 'to relay terminal R, supplied With alternating potential. The current which flows through relay 5 at this time is pulsatingbecause of the half-Wave rectifying action of tube TT; Upon operating, relay 5 closes alocal self-locking circuit-at its contacts I, to ground through the back contact of armature 2 of transfer re1ay3; This self-locking circuit may include the illustrated current-limiting resistor. At its contacts 2', relay 5' closes a lighting circuit for call lamp Cl.

When the circuit of line Ll is reclosed, the resumption of current flow through the line winding of receiving relay RR. again neutralizes the effect of current flow through the balance winding of the relay, permitting the right-hand or bias winding of the relay to return the armature from spacing position to marking position. When this occurs, positive (ground) potential, obtained through grid resistor GR in the repeater RI is again applied to the grid of line tube LT in line circuit LCI, whereupon current flow is resumed through tube LT and line relay 2. Relay 2 thereupon reoperates and disconnects the grid of timing tube TT from ground (cathode) potential by way of resistor 1 and transfers it to negative biasing potential. When this occurs, condenser 6 again charges to a negative value and the grid of tube TT becomes negative, terminating the flow of plate current therethrough. The resistor included in the lead extending to the front contact of relay 2 is to prevent momentary lowering of the biasing potential on the common supply lead by a sudden inrush of charging current to condenser 6. The cessation of the flow of plate current through tube TT does not cause call relay 5 to restore because of the above-noted selflocking circuit of relay 5 through its contacts I, Only alternating current now flows through the winding of relay 5, but this relay is able to remain operated steadily when supplied only with alternating current, as previously noted.

A2. Answering Upon noting the lighted condition of call lamp Cl, the operator may insert one plug of an idle patching cord, such as PC3, into the jack J l (or the jack J l) and insert the other plug of such patching cord into the operators jack J 0. By this operation, spoke conductor SP! is connected electrically with spoke conductor SPO, the concerned single-conductor patching cord serving as the common hub between the two spokes. With the repeaters RI and R both in marking condition, both spokes are at ground potential, wherefore there is no immediate interchange of current over the interconnection.

As a result of the insertion of a connecting plug into either of the jacks J I or J l', the upper contacts of such jack connect ground to the lower terminal of transfer relay 3, operating relay 3 in an obvious circuit. Armature I of relay 3 disconnects the plate of tube 'I'T from call relay 5 and transfers it to relay 4. Armature 2 of relay 3 disconnects ground from the self-locking circuit of call relay 5 and prepares a self-locking circuit for disconnect relay 4. Call relay 5 now restores, thereby extinguishing call lamp C'l. Contacts 3 of transfer relay 3 remove the shunt normally existing around resistor 8 so as to increase the effective discharge time of grid condenser 6 from about one second to about five seconds, thereby insuring that no likely combination of telegraph signals nor a break-in signal will result in a premature operation of disconnect relay 4.

Having thus interconnected operator line L0 with the calling subscriber line Ll, the operator may now type out the usual challenge at the operator station S0 to inform the subscriber at station SI that he is in connection with the operator, the word operator, for example. By this operation, line L6 is opened and closed, as required. Each time the line is opened, the cessation of current in the line winding of repeating relay RRU causes the armature of such relay to move from its illustrated marking position into engagement with its spacing contact, thereby impressing negative potential on spoke conducto r SPO Each time line current is resumed in the line L0, the armature of relay RRU is reoperated from spacing position to marking position, thereby disconnecting the negative spacing potential and reconnecting conductor SP0 to the grids of tubes STE} and BTU. At the end of the instant sending operation, relay RRll comes to rest in its illustrated marking position, in readiness for the receipt of instruction from the calling subscriber.

Each time negative spacing potential is placed on conductor SP6 as above noted, such potential is transmitted through jack J 0 and the patching cord in use (P03), to spoke conductor SPI, by wayof jack J l or J I. With the armature of relay RR in marking condition, each application of spacing current to conductor SPI results in the application of a negative biasing potential to the grids of tubes ST and BT, causing a cessation of current through both tubes and consequently through the associated subscriber line and the artificial line. When this occurs, the armature of relay RR is unaffected because it is still held in marking position by current flow through its right-hand or biasing winding. The resulting cessation of current flow over the subscriber line causes the usual response at the calling station SI, whereat the operators challenge message is typed.

The subscriber at substation sl may now type out the desired instructions to the operator. It may be assumed that the calling subscriber desires that his line be interconnected with lines L2 and LL. In this event, the operator may telegraphically interconnect line L2 with line L! through jacks JI and J2, and a cord such as PCI, and may then employ a patching cord such as PCZ to interconnect line L2 with line LL through jacks J2 and JL. The concerned patching cords now comprise the hub of the interconnected system, the spokes being SPI, SP2, SPL, and SP0. It will be understood of course that as many, or as few, lines as desired may be interconnected with the calling line by using the above procedure.

Having made the interconnection, the operator may signal the subscriber at substation SE to proceed, as by typing a word, such as start," at station S0, resulting in the transmission of the corresponding combination of spacing and marking signals over spoke conductor SP0, and thence by way of patching cord PCS, to spoke conductor SPI. At this time spoke conductors SP2 and SPL are obviously energized by the spacing Signals, giving the same response at all interconnected subscriber lines. The response at repeater RI is as described hereinbefore, the current flow being stopped in each of the tubes ST and ET for the duration of each negative spacing signal.

Having transmitted the proceed signal, the operator may withdraw from the interconnection by removing the plugs of patching cord PCS from jacks J! and J0.

A4. Intercommunz'cation The interconnected stations S l, S2, and SL may now intercommunicate as desired. On transmission from station 8! receiving relay BR operates to spacing position on each interruption of line LI and returns to marking position on each reclosure of the line, thereby transmitting negative spacing signals over spoke conductor SPI and thence over the: interconnecting hub (includingtliepatcliinga cords)" to the interconnected spoke conductors SPZ'andxSPL. It will be apparent that a very large number of lines may be: thus interconnected? foreffective: intercommunication; without overloading 'thespacing contact 1 of; any of: th relays such as RR, because the only load imposed on such spacing contact is that represented by the, current fiow through the grid resistors of the repeaters such as Ri and RL,,corresp onding to. grid resistor GR of the. repeater R1. grid resistor maybe on theorder of 100,000.0hms, or higher, up to a substantial fraction of a megohm, as it needs only to pass sufficient current: to insure the quick resumption of positive potential on the control grids of the associated tubes such as ST and BT upon disconnection of negative spacing. potential from the interconnected spoke and. hub conductors.

During message transmission, line tube LT in each line circuit responds by terminating the current flow through the associated line relay Z'for each spacing signal; and by reestabli'shing current flow therethrough for each marking, signal. For the most part, the spacing signals are too short.

for a slow-restoring relay of the nature of'relay 2 to respond.

During transmission, the subscriber at any station may break'in onthe transmittingstation' by opening the break key thereat, such'as BKI, for about one or two seconds. that station SIv is the transmitting station and that station S2 is the break-in station. Opening of line L2 causes the repeater R2 to transmit spacing potential over SP2 to the repeaters interconnected therewith. This transmission is begun immediately if the break occurs during marking condition. If the break occurs during spacing condition, the receiving relay in'repeater R2 does not respond until the existing spacing condition is terminated.

In the repeater Bi, no current'can flow through.

either the subscriber line or. the'artificial. line during the time the negative spacing potential resulting from the break-in operation is impressed on the control grides of tubes ST and BT, wherefore. the apparatus at the transmitting'stationsl temporarily ceases to function; therebyinforming the subscriber of the break. Heis expected to'cease transmission to permit his line L! to" remain closed for the receipt of a message from the break-in station;

When the break key BKZ at thebreak-in station S2 is subsequentlyreclosed, the consequent resumption of current flow over'line L2' causes the repeater R2 toremove spacing potential from the interconnected hub system, enablingtransmission to proceed'thereover from any station;

During transmission of the break-in. spacing signal over the hub system, line relays2may'be restored in all line circuits, and start the timing;

operation of each of the associated" timer tubes TT, but this does not cause disconnectsignaling of the operator, for in the connected'condition'oi any line, the associated transfer'relay 3 is in op erated condition to increase the length of the timing operation to. about five secondsby including the associated resistor 8. in circuit with timing resistor l, as previously noted.

A5. Disconnect signaling The subscriber at any interconnected'station can signal the operator at any time to request that the entire connection be taken down or that his line be disconnected f'romthe otherline, or

In practice; this It may be assumed scriberandartificials It maybenotedthahsinces.

sumed herein to be fiveseconds;

.scriber line.

g?! lines, ofithe' interconnected group. Disconnect signaling is accomplished by opening-= the-brealc= keysuch as BK! atstation Si for an intervalexceeding the disconnect timing interval, as- Opening ofthe line causes spacing potential to be placed on the associated spoke conductor, whence it passes through the-hub system to the other spoke con-- ductors, causing'a response at the linecircuit= of all connected lines. In the line circuit LCI, the disconnect spacing potential on conductor SPlcauses line tube LT to restore line relay 2; Relay Z'disconnectsthe upper-termina1 of timing condensert and the grid of timing tube TT fromnegative charging potential and transfers them to ground through resistors I and 8 in series- (contacts 2 of transfer relay 3 are open at thistime becauserelay 3 is operated through theupper contacts of J l or J I At the end of about five seconds; condenser 6- isdischarged sufli ciently to permit an operative flow of current through timing tube TT, whereupon relay 4 oper-- ates'in a circuit from the associated alternating current terminal R, the winding of relay ll, front contact of armature i of relay 3 andthe said armature, plate of tube TT, and thence to groundthrough the grounded cathode of such' tube. At its contacts 2, relay closes a lighting circuit fordisconnect lamp DI, while at its contacts I-it l closes a seli-locking circuit to ground through the front contact of armature 2 of transfer relay" 3. Accordingly, relay 5 remains'operated following reclosure ofthe break key at the signaling station andthe consequent reoperation of line. relay 2 and cessation of current flow through" timing tube TT.

Since. the same line-circuit operation occurs at each interconnectedline (each of the concerned disconnect lamps such as D1, D2, and DL becomes lighted at about the same time), itmay,

be necessary for the operator to inquire which station requires disconnection, or Whether all stations require disconnection. This may be accompiished by interconnecting the operators equipment with the interconnected hub system. as by Way of patching cord P03 and jacks JL" and Jll; Upon. learning. what is required, the operator may temporariiy take down the entire interconnection to restore the line circuits, and then reestablish such portion of it as may be desired.

Upon removal of the plug from jack J l trans fer relay 3.restores, disconnecting and restoring; disconnect relay 4 to extinguish lamp Dl. Similar results occur at th other line circuits.

B. FIGURE 2 As previously noted, Fig. 2 shows a modifica-- tion'of the equipment associated With line L] of Fig; l, in that diiierential line relay 2A replaces line tube LT'and line relay 2. One winding of relay 2A is in series with the associated subscriberiine Li, and one winding is in series with the artificial line. Normally, relay 2A is-not operated because the opposed windings are of substantially equal strength, but it is operated (by cur-rent flow through its lower winding alone) whencurrent flows in the artificial line ata time when no currentis flowing in the associated sub- This condition obtains only when the associated station Si is transmitting. It does not obtain-when station Si is receiving, for at' that time tubes ST'and BT are both locked, to: prevent current him through both lines, sub

line relay 2-A is normally restored instead of normally operated, the negative biasing potential is connected to the back contact of the relay armature, and the connection to timing resistors 1 and 8 is brought to the front contact of such armature. In the arrangement of Fig. 2, when one station of an interconnected pair orgroup transmits a disconnect signal (a prolonged line break) such signal operates on no line circuit LCIA except the associated one, and consequently results in the lighting of the disconnect lamp, such as DI, only at the station transmitting the disconnect signal. Except as noted, the line circuit LCl-A is exactly similar to the line circuit LCI (Fig. 1) and operation and restoration of relay 2-A produces results as hereinbefore described for restoration and operation of line relay 2. When a disconnect signal is received, it suffices for the operator to disconnect the line over which it was received, it being unnecessary to disturb the connections to the remaining lines.

C. FIGURE 3 As previously noted, Fig. 3 shows a further modification of the equipment of line I (Fig. 1). Line circuit LCIB is exactly similar to line circuit LCIA, except that the two windings of line relay 2-3 are connected in aiding relationship rather than in opposed relationship, because operating current flows through only one line (subscriber or artificial) at a time in Fig. 3.

Repeater RIB is generally similar to repeater RI of Figs. 1 and 2, except that phase-inverter tube PI has been added, and the grid circuit of balance tube ET is controlled from the plate circuit of tube PI instead of in parallel with the control grid of sending tube ST. More in particular, the control grid of tube ST is connected to marking contact M of receiving relay RR, and is supplied with ground potential through grid resistor GR, as in Figs. 1 and 2. The position occupied in Figs. 1 and 2 by the control grid of balance tube BT is occupied in Fig. 3 by the grid of phase-inverter PI. Accordingly, current is normally flowing in the platecathode circuit of tube ST and in the platecathod circuit of tube PI. Balance tube BTin Fig. 3 has its control grid normally supplied with ne ative biasing potential through grid resistor GR, wherefore current flow is normally blocked through the plate-cathode circuit of balance tube BT to provide a condition of no current in the associated artificial line (including the middle winding of relay RR, the lower winding of line relay 2B. and the associated adjustable resistor). It will be noted that the connecting resistor OR is interconnected between the plate of phase inverter PI and the control grid of balance tube B'I'. Accordin ly, current flows continuously from the positive plate of tube PI to the negative control grid of balance tube BT. With no signal being received over conductor SPI at the repeater RIB. however, the flow of current through resistor OR is very small because the positive voltage maintained on the right-hand terminal of resistor GR is very low with tube PI in its normal conducting condition. Under this condition, the grid resistor GR is of such value that it accepts all of the current passing through control resistor CR and still maintains the potential of the control grid of tube BT sufficiently negative to substantially block the flow of currielnt through tube BT and associated artificial l e.

When transmission occurs from station S I (either to signal the switchboard operator or to transmit signals to one or more interconnected stations) the opening of line Ll terminates the flow of current therein and through the left-hand winding of relay RR. Since current is normally not flowing in the artificial line, the only current flow remaining through the winding of relay RR is the normal current flow through the right-hand or-bias winding thereof. The current flow through this winding is in such a direction as to cause the armature of relay RR to move from marking position to spacing position. At this time. the current flow through tube PI is not affected, wherefore tube BT remains unaifected.

When the associated line Ll is reclosed, the resultant resumption of current flow thereover andv through the left-hand winding of receiving relay RR causes the armature of such relay to return from its spacing contact to its marking contact, thereby terminating the transmission of spacing potential over the associated spoke,

conductor SP l.

With receiving relay RR in its illustrated marking position, receipt of spacing potential inverter triode PI to become negative, therebyterminating the flow of plate current in both tubes. Tube ST directly terminates the current flow in the associated line Ll, as a spacing signal to station SI. rent flow through the plate-cathode circuit of phase-inverter PI, the plate element thereof tends to assume the full positive potential of the supply source, thereby increasing the flow of current from this positive source through control resistor' OR to the grid of balance tube BT. As a result, the potentialdrop across grid resistor GR is increased sufiiciently that the grid terminal thereof' assumes arelatively positive (or cathode) potential, rendering .the plate-cathode circuit of bal-' ing of line. relay 2- -B. The cessation of current flow in the plate circuit of tube ST and the starting of the flow of current through the plate circuit of tube BT occur substantially simultaneously, wherefore there is no open interval, when current is flowing through neither. Since the current normally flowing through the left-hand winding of relay RR is replaced by a substantially equal current flow through the middle winding thereof, and in the same effective direction, the armature of relay RR remains in engagement with its marking contact throughout the received marking signal.

When the negative spacing signal on the associated spoke conductor SPI is terminated, current flow through tubes PI and ST is resumed. Current flow through the plate-cathode circuit of tube ET is terminated as a result of the lowering of the positive plate potential at tube PI and the consequent lessening of current flow (in a positive sense) through the resistor CR to the control grid of tube BT. Upon the above-noted resumption of normal marking conditions in the repeater RIB, the armature of relay RR is held in marking position by current flow through the left-hand winding thereof instead of by current through the middle Winding thereof.

In the line circuit LCl-B, line relay 2--B is normally held in operated condition by current As a result of cessation of curhow through the upper winding thereof, in series with line 1. The front contact of the-armature of relay 2-B maintains anegative potential on-the grid of the'associated'tube TT and maintains timing condenser'li in charged condition. During the reception of a spacing signal at the repeater -R|'B, the cessation of current flow in the upper winding of line relay 2B accompanied by the establishment of the current flow through thelower winding thereof, in circuit with tube BT and the middle winding of relay RR. Consequently, line relay 2B remainsaoperated during the reception of spacing signals atthe repeater Ri B and their transmission over line L1 to station Si. On the other hand. with the repeater 'Rl'B in its'normal marking condition, the opening of line Li at station SI pursuant to signal transmission causes a cessation of current flow through the upper winding of relay 2-B with no compensating current flow in the lower winding thereof, under which condition relay -2-B is currentless for the duration of the transmitted spacing "signal. Berngof the same type of slow-restoring relay as noted for relays 2 and 2-A (Figs. 1 and 2) ,relay 2 'B may not respond tonormal transmission. It does however respond to the comparatively prolonged line breaks at hreak key B'K'l of station 181, :as when the associated --line is opened fora period somewhat in excess of one second to signal the operator lay a lighting of the associated call lampCL-or isopened *for an interval slightly in excess of five secondsto signal the operator by a lightingoif the disconnect lamp Di ata ftimewhen :a-p'lug is in either of the jacks Ji-andill.

:1. Ina telegraph :system wherein a switchboard is-wprovided for interconnecting "and disconnecting two-way telegraph lines terminating thereat, line circuits individual respectively to said lines, means, effiective when two lines are interconnected at said switchboard for mutualtwo-way transmission, for controlling either eoncerned line circuit from-its. individual' li-ne to transmit a signal to the iswitchboard indicative of such ind-inidual dine, and means for preventing the other line circuit from responsively transmitting a signal to the switchboard; thereby preventing the .12 control exercised over one line from causing; signal to be transmitted to the switchboard indicative .of the other line.

2. In a telegraph system wherein a switchboard is provided for interconnecting and disconnecting two-way telegraph lines terminating thereat formutua'l two-way transmission of signals between interconnected lines, line circuits individual respectively to said lines, means, effective when two lines are interconnected .at said switchboard, 'for controlling either concerned line circuit from its individual line to transmit a supervisory signal to the switchboard responsive to a telegraph signal from its individual line and subject to such signal being of prolonged (duration, and means for preventing the other "line circuit Jfrom transmitting a similar supervisory signal responsive to the'prolonged signal consequently transmitted over the other line.

'3. In a telegraph system wherein a switch board is provided for interconnecting and disconnecting more than two two-way telegraph lines for mutual two-way transmission of signals between any line and reach of two or more lines so interconnected therewith, line circuits individualrespectively to said lines, means, effective when lines are interconnected at said switchboard, for controlling any concerned line circuit from its individual line to transmit a supervisory signal to the switchboard responsive to a telegraph signal from its individual line and subject to' such Signal being of prolonged duration, and means for preventing any other line circuit from transmitting a similar supervisory signal responsive to the prolonged signal consequently transmitted over its individual line.

ORLANDO BLYHOLDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,206,663 Cramer et a1 July 2 1940 2252,6129 Hanley Aug. 12, 1941 2362,1941 Dodd Nov. 7, 1944 2;3fi9;578 Kinkead Feb. 13, 1945 2,387,246 Demonet Oct. 23, 1945

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