US2259960A - Polarized, two-wire, progressive, noninterfering signal system - Google Patents

Description

Oct. 21, 1941. M. w. MUEHTER 2,259,960

, PROGRESSIVE, NONINTERFERING SIGNAL SYSTEM POLARIZED, TWO-WIRE .Ml li I. m A w INVENTOR: iynsdW/WefiZei; BY I MA A1 TORNEY.

EDFUUBUD QZDOEV Patented Qct. 21, 1941 POLARIZED, TWO-WIRE, PROGRESSIVE, NONINTERFERING SIGNAL SYSTEM Manfred W. Muehter, Nutley, N. J., assignor to American District Telegraph Company, 1110., Jersey City, N. J.,' a corporation of New Jersey Application September 21, 1937, SerialNo. 164,887

26 Claims. (01. 177-361) This invention relates to electric signal systems and transmitters and receivers therefor, and more particularly to systems wherein a large number of transmitters on the same circuit are prevented from interfering with each other, and are successively operated when more than one are waiting at the same time to operate, though it is noted that in some of the claims the invention is not limited to systems of the non-interfering type.

One object of the invention is to provide an improved system of this kind in which the transmitters are provided with motors operated from the central source of current at the receiving station, and wherein the motor is operated by current impulses and operates means for varying the strength of these impulses for sending code Signals.

Other objects of the invention are to provide,

in a system of this kind, improved signal means for indicating grounds and line breaks.

Additional objects of the invention are to improve generally the simplicity and efficiency of such systems and apparatus and to provide an apparatus of this kind which is reliable in operation, and economical to manufacture and install.

Still other objects of the invention will appear as the description proceeds; and while herein details of the invention are described in the specification and some of the claims, the invention is not limited to these, since many and various changes may be made without departing from the scope of the invention as claimed in the broader claims.

The inventive features for the accomplishment tively from the first and second terminals of a current source, at a control station, and thence past a plurality of transmitters, the lines being connected by a resistance at the outer ends.

Each transmitter comprises a motor and a starting means for starting the motor, which is adapted to be connected through a rectifier between said lines. A code means is operated by the motor to connect said lines together through unequal resistances alternately; and means operated by said motor connects the lines and breaks the one line beyond the connection, thereby to change the rate of current flow in the lines at the control station. Said current change sets in operation a motor current supply means, and a reversing means for reversing the connection of said lines with the source to change the polarity. A marginal coding means is provided atthe control station in circuit with one of said lines.

The change in polarity prevents the starting means for other transmitters from receiving current for starting their motors, thus to prevent interference. The break beyond said connection established in the started transmitter shuts out more remote transmitters. Thus when a plurality of transmitters are waiting to start, the one nearest the current source can start first.

In the accompanying drawing showing, by way of example, one of many possible embodiments of the invention, the figure shows a diagram of the whole system, the normally disengaged contacts being shown in outline with uninked intra-marginal part, while the normally engaged contacts are shown in solidblack throughout.

1 The supervisory system The line circuit of the system is normally supervised for breaks and grounds. The supervisory circuitcan be traced from plus through relay winding RI throughthe left hand winding of break detector relay 13, contact 2 of relay U through line R and the various transmitters 5, 6, I connected to the line. The internal circuit through the :transmitter' 5 can'be traced from terminal Ra through conductor 8 back contact Mb of cam contact assembly M, back contact Ab, outgoing terminal Rb. At the end of the line a resistor REZ connects-the R. line to the L line. The supervisorycircuit extends through the L line back to the central station, then through contact 4 of relay U, right hand winding of relay B, the winding of relay R2, conductor ID to minus battery. The two windings of break detector relay B are arranged to add, holding this relay energized by the supervisory current. If a break in the line occurs, relay B .deenergizes and'gives an indication by closing its back contact to a buzzer or light (not shown).

For ground supervision, relay G is provided connected to the center of battery I l and through contact 8,0f relay U to a ground. If a ground occurs on either of the'two line wires R or L, current will flow through the ground relay G and operate it, giving an audible or visible indication.

The transmitters Each transmitter consists of a" step by step device S operated by two individual magnets Sa, Sb, so arranged that the associated ratchet wheel I2 is advanced by the pawl I2 on the release of the spring retracted member l3 having a lost motion connection I4 with the armature I which is common to the two magnets. The ratchet wheel drives two control cams m and t and a code wheel 0. The armature in addition to operating the ratchet wheel operates armature contact A during its initial motion. The lost motion arrangement insures that the main load represented by the ratchet mechanism I2, I 2' is not picked up until after the operation of armature contact Ab, for reasons which will be apparent later.

The device that is supervised by each transmitter is arranged to operate a singlejp'ole, double throw switch D, shown in the normal condition. If an abnormal condition occurs, the transfer elerelay P and contact #3 of relay Q through conductor 23 to plus, thus energizing relay U after a half cycle of the operation of relays P and Q.

'the line circuit is open at this particular time causing the transmitter step device S to release its i armature and advance the ratchet wheel and asment of switch D is actuated to the other posiv tion, closing a circuit for the ep'emuontr the associated transmitter. This canbe traced as follows: from plus battery through relay-winding RI, left hand winding of relay B, contact 2 of relayU, line R, through intermediate transmitters if they are interposed between thecentral office and the transmitter under consideration, through resistance REI, a cold cathode vacuum tube I8, terminal SI, the normally open side dI of switch D, conductor 112, back contact Tb of cam contact assembly T, back contact Cb of the code wheel contact assembly C, left hand magnet coil Sb to line L from which minus polarity is obtained through contact 4 of relay U, right hand winding of relay B and the coil of relay R2. This provides the ignition circuit for the-cold cathode tube I8 which starts the current through the main gap thereof which is shunted around resistance REI.

'This current may or may not be of sufficient intensity to operate magnet S completely. How'- ever it is capableof causing contact Ab, A to be operated. The normally open contact A), now closed, shunts the .cold cathode tube, through conductor 8, contacts Mb, A,f,conductor I9, contact Cb, coil Sb, thereby eliminating the tube from the circuit. "Contact Ab, onopeni'ng, breaks the line L beyond the transmitter 5 preventing other transmitters 6, 'I" further on the line from starting. 7 a

Control station operation The increased'current caused by the connection of magnet S in the line circuit operates primary marginal relays'RI and R2 which close contacts of both relays RI and R2 is necessary to operate relay ST. This is toprevent a false start if an accidental ground should cause a current strong enough to operate one of these relays.

Relay ST operates, shunting out the coils of relays B, RI. and R2, and locks in" to' minus through its contact 'I andicontact 5 of relay U and contacts I of relays P and Q in multiple,'and also closes its contacts 2* and 3, thus shunting out the coils of relays B, RI and R2. Contact 4 of re lay ST closes a circuit from plus through a resistor ZI,ith'e winding ofmain pulsing relay P to negative, energizing'thisrelayJ When contact 3 of relay P opens auxiliary pulsing relay Q becomes not shunted and ,can likewise operate through resistor 22 and an obvious circuit. Re- "lay Q by closing its contact 2 shunts the coil of relay P causing it to deenergize after a short interval of time. As relay P closes its contact 4, reversing relay U is energized from minus through conductorlfl', the coil of relay U, contact 4- of sociated cam shaft 39 one step.

The step device operates the master cam contact assembly Mto close normal open contact M),

establishing a permanent shunt through conductors 8' and 9 and contact C around the tube and opening the line R beyond the transmitter. Subsequent operation of armature contact A will have no result. a

After a short interval of time the shunt across the coil of relay'Q applied by contact 3 of relay P becomes effective, causingrelay Q to release. This relay opens its contact 2 allowing relay P to reenergize. This completes one cycle of the operation of relays P and Q which continue to operate in themanner described above for the duration of the signal transmitted. When relay P picks up subsequent to the operation of relay U, negative polarity is applied to the line R through contact 2 of relay P, right hand winding of relay ST through the coil of relay K and contact I of relay U again completing the circuit for the transmitter. It will be noticed, however, that because of the operation of contacts I and 3 of relay U, the currentfiowing is of opposite polarity. The purpose of this is to prevent from starting other transmitters located between the operating transmitter and the central ofiice; as current of opposite polarity will be prevented from flowing by the associated cold cathode tube I8'of any of said other transmitters which tube only passes current in one direction. Other types of rectifiers I such, for instance, as those of the copper oxide type or other suitable type could also be used for this purpose. V

' Transmitter operation I As impulses from relay Pare transmitted to the actuated transmitter, its cam shaft will be advanced step by step. The code is transmitted in accordance with the teeth of its code wheel which operate the transfer element of contact C when the associated pen spring is thrown up by a tooth. In that case, the right hand magnet coil Sa is cut in through conductor 3! instead of the left hand magnet coil Sb.- This magnet coil Sa is capable of causing the stepping of ratchet wheel I2, but has a much lower resistance and thereby increases the current fiow in the coding circuit to such a value that relay K is operated and closing its contact I operates the recording register 32 through an obvious circuit. The code as represented by the code wheel will thereby be recorded on the register.

It will be noticed that the locking circuit for the left hand winding of relay ST is open when both contacts I of relays P and Q are open, which occurs during the transmission of a line impulse. At this time, relay ST is dependent on the line current flowing through itsright hand winding and relay K for retaining its armature energized. In the illustrated transmitter, the code wheel 0 is provided with twosets of teeth,

each characteristic of the device controlling switch D, the second set having an additional tooth or group of teeth representing the restoration of the device to normal.

When the transmitter has completed its first signal, the cam m has moved a half rotation and the notch m2 allows the contact M to restore to normal position. The starting circuit through contact Tb is likewise open as contact T has been transferred by cam t to the abnormal side Ti during the transmission of the signal, con necting the transfer element of contact T with contact d3 of switch D. The line circuit within the transmitter is now restored to the normal condition. The next impulse transmitted over the line from relay P will only energize the endof-line resistance REZ. This current is too weak to maintain relay ST energized through its right hand winding and the relay releases opening its contact 4, deenergizing relays P and Q. Re-

lay U deenergizes when relay Q releases and opens its contact 3.

Release of relay U also restores the normal supervisory circuit. Other transmitters are now able to transmit their signal in a similar way.

When the device switch D is restored to contact d3 the normal condition, the associated transmitter is operated again but through the normally open side T of cam contact T. The operation is otherwise the same except that a slightly different code is transmitted by the corresponding side of the code wheel to indicate the restored condition.

It will be noted that the ground detector G is disconnected during transmission of a signal by the opening of contact 8 of relay U. This permits operation of the system with a ground on the line. A break on the line will affect transmitterslocated beyond the point of the break; but other transmitters will still be able to send signals.

The purpose of contacts 6 and I of relay U, contact 4 of relay P, and contact 3 of relay Q is to energize relay U after half a cycle of operation of relays? and Q so as to insure a proper starting impulse for the transmitter on the line before the current is interrupted. The second purpose is to have relay U release at the end of the signal at the moment when relays P and Q have completed their cycle so as to be sure that a new signal coming in immediately will start with the relays P and Q in their normal condition and that the time delay for the starting operation will be always the same.

I claim as my invention:

1. In combination, a source of direct current; a series of transmitters; first and second lines extending from the first and second terminals of said source past said transmitters; marginal relays .having their coils in the respective lines near said source; each transmitter including a one-way rectifier, a motor operatedby impulses, means to connect said lines through the rectifier to start the motor, motor operated means to change current flow in the lines to operate the marginal relays, and a codemeans operated by the motor to vary the current in said lines; starting means operated by the marginal relays;

pulsing means set in operation by the starting relay; a coding relay conductor having therein a marginal codingrelay; a reversing relay energized by the operation of the pulsing means and having means to disconnect the first line from first terminal and connect it to one endofthe coding relay conductor, and disconnecting the second line from second terminal and connect it to the first terminal; an interrupter operated by the pulsing means for intermittently connecting the other end of the coding relay conductor to the second terminal to cause the motor to intermittently release its armature.

2. In a signaling system, a loop circuit including a transmitter and a source of potential, a pair of cooperating relays connected to said source for supplying driving pulses to said transmitter, a ground detecting relay connected between said source and ground whereby a ground in said loop energizes said ground detecting relay, means including a switch for starting said transmitter and means responsive to operation of said switch and one of the pulsing relays for opening the ground connection to said ground detecting relay.

3. Ina signaling system, a transmission line including a transmitter and a source of potential, a pair of cooperating relays connected to said source for supplying driving pulses to said transmitter, means connected between said source and ground and responsive to ground conditions in said transmission line, means connected in circuit with said transmitter for starting the same and means responsive to operation of said transmitter starting means and one of the pulsing relays for removing the ground connection to said ground responsive means whereby transmission of signals is not interfered with by a ground in the system. 7

4. In a signaling system, a loop circuit including a transmitter and a source of potential, a ground detecting relay connected between said source and ground whereby a ground in said loop energizes said ground detecting relay and means for controlling said transmitter, said controlling means including a pair of primary starting relays connected one in each side of said loop for completing a starting circuit for the transmitter whereby a ground in one side of said loop effective to energize only one of said starting relays does not cause false transmitter operation.

5. In a signaling system, a transmission line including a transmitter and a source of potential, means connected between said source and ground for detecting ground conditions in said transmission line, means for starting said transmitter in response to a change in the current in said line andmeans connected in circuit with said transmission line for preventing false operation of said starting means upon the occurrence of a ground condition in said transmission line.

6. In a signaling system, a transmission line including a transmitter and a source of potential, means connected between said source and ground for detecting groundconditions in said transmission line, means for starting said transmitter, relay means connected in circuit with said transmission line for preventing said transmitter from starting upon the occurrence of a ground condition in said transmission line, and means responsive to said starting means for removing the ground connection to said ground detecting means upon the occurrence of a signaling condition on said transmission line.

7. In a signaling system, a two-wire loop circuit including a transmitter and a source of potential, a break detecting relay connected in series in said loop circuit whereby a break in said loop circuit causes deenergization of said break detector and a break signal and switch means for reversing the polarity of the loop circuit and of transmission of a signal. 7

8. In a signaling system, a loop circuit having at least. two transmitters therein, a central sta' tion including a recorder and a source of potential connected to said loop, a pair of normally unoperated marginal relays connected at said central station, one in each side of said loop, a rectifier tube connected to one side ofeach transmitter, a magnetically operated switch connected to the other side of each transmitter, means including a. stepping magnet for operating said switch and transmitter, a switch connected between said rectifier and said magnet for establishing a circuit through said rectifier to operate .said magneticallyroperated switch and shunt the extended portions of said loop beyond the operatedtransmitter whereby the current in. the connected portion of the loop increases,- contacts operated by the stepping magnet for shunting said rectifier and further increasing the loop current, a start circuit including a main starting relay and the contacts of said marginal relays and energized by the increased current resulting from actuation of said magnet, a switching magnet actuated by said startingrelay'for changing the polarity of the potential connected to said loop whereby every rectifier at said transmitters ceases to function and prevents connection of other transmitters; a pulse transmitter connected to said loop by said switching magnet for driving said transmitter step by step through said stepping magnet, a second magnet at said transmitter having a lower resistance than the stepping magnet, a code wheel at said transmitter, cam operated switch means for connecting'the contacts operated by said wheel and said second magnet in series and across said loop whereby impulses of greater than normal value are transmitted to said central oflice, a marginal coding relay in said loop responsive to said code impulses for energizing said recorder in accordance with the transmitted code.

9. In a signaling system, a loop circuit having at least two transmitters therein, a central ofiice including'a recorder and a source of potential connected to said loop, a unidirectional device connected to one side of each transmitter, switching means at each transmitter for establishing a circuit through said device and shunting the outer portion of said loop. beyond an operated transmitter whereby the current in the connected portion of said loop increases to start the transmitter, means operated: bythe transmitter to furtherincrease the loop current, a marginal starting circuit at saidcentral ofiice responsive to the change of current in said line, means to change the polarity of the potential across said loop in response to actuation of said starting circuit whereby every unidirectional device at the transmitters in the connected portion of said loop ceases to function, a pulse transmitter connected to said loop in response to the closure of said starting circuit for driving the operating transmitter, meansat said transmitter for changing the current value in said loop, a code wheel in said transmitter for controlling said means to transmit code impulses and amarginal means responsive to code impulses for operating said recorder.

10. In a signaling system, a transmission line having at least two transmitters connected therein, a central office including a source of potential connected to said line, means for initiating operaremoving said' break detector from the operating circuit of said. transmitter upon the occurrence tion of a transmitter in response to-signaling condition, means responsive to initiation of operation for disconnecting the outer portion of said transmission line, unidirectional devices connected between the other portion of the transmission line and each of the transmitters therein, means at the central ofiice for changing the polarity of said transmission linein response to operation of said transmitter whereby the transmitters in said other portion of the transmission line are rendered inoperative by said unidirectional devices and means for shunting the rectifier at the operated transmitting station whereby it ismaintained operative.

11. In a signaling system, a transmission line having at least two transmitters therein, a source of potential connected to said transmission line, means for initiating operation of a transmitter in response to a predetermined signal condition, means responsive to initiation of operation of said transmitter for disconnecting the outer portion of said transmission line whereby transmitters in said outer portion are disabled, and a unidirectional means associated with each transmitter for disabling idle transmitters in the other portion of said transmission line whereby interference between transmitters is eliminated.

12. In a signaling system, a two conductor line having at least two transmitters therein, a source of potential connected to said conductors, a unidirectional means associated with each transmitter, means including said unidirectional means for initiating operation of a transmitter in response I to a predetermined signal condition, means responsive toinitiation of operation of said transmitter for interrupting the outer portion of said two conductor line beyond the operating transmitter whereby transmitters in said outer portion are disabled, and means responsive to initiation of operation of said transmitter for reversing the connection of said line to said source to reverse the polarity of said two conductor line whereby the unidirectional devices become effective to disable the remaining idle transmitters in said line to eliminate interference between transmitters. 1

13.. In a signaling system, a-transmission line including a transmitter therein, a central oflice associated with said transmission line, means to initiate operation of. said transmitter, means responsive thereto for changing the current in said transmission line-to createa control'condition,means at said central office responsive to said control condition to start operation of said transmitter, means responsive to said starting operation for sending code by current conditions differing from the control condition and means at said central oflice responsive to the different current conditions for receiving code signals.

14. In a signaling system, a transmission line having a transmitter therein, a central office including a recorder and a source of potential connected to said loop, means in each of said transmitters for setting up a control condition in said transmission line, a pair of normally unoperated marginal relays connected at said central station, one in'seriesrelation with each side of said transmission line, a marginal codingrelay, a starting circuit at said central station including a main starting relay connected to the contacts of said pair of marginal relays and across said source of potentialand responsive tov said control condition created at a transmitter, a pulse transmitter at said central station, means for initiating the operation of said pulse transmitter in response to operation of said starting circuit, means including said pulse transmitter for establishing a circuit through said main starting relay, said line, said source and said marginal coding relay and for establishing a change of polarity in said trans-- mission line, and means at an operated transmitter for changing the current condition in said transmission line in accordance with code signals for controlling said marginal coding relay to cause said recorder to receive signals.

15. In a signaling system, a transmission line having a transmitter therein for generating code impulses having certain characteristics, a central oifice including a recorder and a source of potential connected to said transmission line, means in each side of the transmission line at said central ofiice responsive to predetermined current conditions, a starting circuit including said means and responsive to a line-current control condition created at said transmitters, means controlled by said starting circuit for reversing the polarity of said transmission line and initiating transmission of coded signals from said transmitter, and means responsive to the signals for controlling 'said recorder in accordance therewith.

16. In a transmitter, a motor comprising a pair of operating coils, one having less resistance than the other, a stepping mechanism, an armature engaging the stepping mechanism and having lost motion with respect thereto and driven by said coils, a pair of contacts adapted to be operated by said armature in predetermined time relation to operation of said stepping mechanism, a first cam and a first pair of cooperating contacts operated by said stepping mechanism, a second cam and a second pair of cooperating contacts operated by said stepping mechanism, a code wheel having teeth thereon and a third pair of contacts associated therewith and operated by said stepping mechanism, a normal line circuit extending through one contact associated with said first cam and one contact associated with said armature whereby the line is normally closed around said transmitter, an initial starting circuit completed only under predetermined conditions including a rectifier, a starting switch, the normally closed contact associated with said second cam, the normally closed contact of said code wheel and one of said operating coils whereby the contact associated with said armature is opened to break the line circuit, the connection of the coil varies the current in the circuit to cause a control signal, and said rectifier is shunted by a contact associated with and operated by said armature whereby operating current of opposite polarity can flow and a transmitting circuit which is closed through a contact associated with said code wheel, a contact associated with said first cam and the second operating coil whereby the code is transmitted at a different current value when the stepping mechanism is actuated.

17. In a transmitting system, a line circuit, a first resistance, a second resistance having a different value, means including a unidirectional device responsive only to a predetermined condition in the line circuit for establishing a circuit through said first resistance and the line circuit to accomplish a control condition of the system and means responsive to said control condition for shunting said device and establishing a circuit including the line circuit and said second resistance for generating a signal in the line circuit.

18. In a transmitter, a motor comprising a pair ofoperating coils, one having less resistance than the other, a' stepping mechanism controlled by said motorpswitching means controlled by said stepping mechanism for inserting one of said coils in an operating circuit to create a distinctive current condition for accomplishing a control function, a second switching means controlled by said stepping mechanism in response to said control condition'for connecting a second coil in an operating circuit to create a second current condition, and acoding means operated by 'said' stepping 'mechanism for making and breaking the circuit through said second coil to utilize said second current condition for the purpose of transmitting coded signals.

' 19. In a transmitter, a motor comprising a pair of operating coils, one having less resistance than the other,a stepping mechanism controlled by said coils, a switch controlled by said stepping mechanism, means including a rectifier in circuit withsaidswitch adapted to conduct current of predetermined polarity whereby said transmitter can operate. onlyjunder predetermined conditions of polarity, a second switch controlled by said stepping mechanism for connecting one of said operating coils in circuit to create a control condition when the predetermined polarity condition exists,.means responsive to said control condition for shunting said rectifier and connecting a second operating coil in an operating circuit tocreate a distinctive current condition, and a coding device operatedby said stepping mechanism for making and breaking the circuit through said second operating coil to transmit a coded signal by means of the distinctive current conditions created by said operating coil.

20. In a signaling system, a transmission line including a plurality of transmitters and a source of potential therein, each transmitter including an initiating switch, a rectifier and a motor in series with said source whereby operation of said switch energizes'said motor, a switch operated by said motor for disconnecting said motor, a switch operated by said motor for disconnecting the outer portion of said line from said transmitter, switch means responsive to energization of said motor, a pair of parallel connected pulsing relays connected across said source by the switch means and having contacts in said line and other contacts for alternately shunting each other whereby they operate in alternation, a transfer relay for reversing the connection of said source to the line whereby said rectifier in each transmitter ceases to function, a contact closed by the release of the first one of said pulsing relays and a second contact closed when the second pulsing relay is energized for energizing the transfer relay after one half an operating cycle of the pair of pulsing relays and a second contact opened by said first pulsing relay for opening said line circuit to operate said motor and shunt the rectifier at the operative transmitter to start the transmitter.

21. In a signaling system, a transmission line, including a transmitter and a source of potential, initiating means for transmitting a starting impulse from said transmitter, means including an interrupter responsive to said starting pulse for driving said transmitter, and means whereby the initiating means does not operate unless the line is closed.

22. In a signaling system, a transmission line including a plurality of transmitters and a source of potential, means for initiating operation of each transmitter, means responsive to the initiating means for disconnecting the outer-portions of saidtransmission line andconnectin'g the operat'- ing transmitter across said line, means connect= ed to said line in response to said connection for operating a connected transmitter and means responsive to said last=mentioned means fordis= abli'ng' the initiating means whereby only the op erating transmitter can operate. I,

23. -In a signaling system, a twoewire trans= mission line including a plurality of transmitters and a source of energy, initiating means at each transmitter including a switch; a rectifier and a motor for connecting an operating transmitter to said line and disconnecting remote transmitters, starting means responsive to operation of the initiating means, a pulse transmitter 'responsive to the startingmean's for driving said motor, switching means responsive to the pulse transmitter for reversing the polarity of said line with respect to said source of energy whereby the initiating circuits of said transmitters are disabled, code transmitting means operated by said motor, cam operated means driven by said motor for connecting said code 'means'to the line for transmitting an alarm signal, switch means operated by saidcarn for controlling the starting means to restore the entire circuit to normal after the transmission of an alarm signa anq a 'second'cam means operated the motor for preparing another initiating circuit whereby restoration of the first-mentioned switch at the operated transmitter to its normal position will start the transmission of 'a restoration signal.

24. In a signaling system, a transmission line,

a plurality of transmitters and a source of current connected thereto, means including an initiating circuit for each transmitter for transmitting a starting signal over said line, means connected to said line in response to said starting signal for operating the active transmitter to send analarm signal and means for conditioning said transmitter whereby it will send a restoassess-o ration signal When-said initiating circuit is restored to normal.

'25. in a signaling system, a twoconductor transmission line, a plurality of'transrnitter's and a source of current therein, meansvincluding an initiating circuit in each transmitter for'di'scon necti'ng'one of said conductors and connecting the transmitter across the line whereby transmitters in said one conductor beyond an active transmitter are disabled, polarity responsive means in each initiating circuit and means responsive to connection of a transmitter to reverse the con -ne ction of said source to said line whereby the polarity responsive means in idle transmitters connected to the other of said conductors becomes effective to disable said last-mentioned idletra'nsmitters.

26. In a signaling system, 'a two-conductor transmission line, a plurality of transmitters and a source of current therein, means including an initiating circuit in each transmitter for disconnecting'one of said conductors and, connecting the transmitter across the line whereby transmitters in said one conductor beyond an active transmitter are disabled, polarity responsive means in each initiating circuit, means responsive to connection of a transmitter to reverse the connection of said'source to said line whereby the polarity responsive means in idle transmitters connected to the'other of said conductors'be- "comes e'fiective to disable said last-mentioned idle transmitters, means for driving each transmitter until a complete alarm signal is transmitted, means responsive to said driving means for restoring the normal connections of said transmitters to, said conductors whereby said line is again rendered completely operative and means responsive to said restoration for restoring the normal connection of said source whereby the li'n'e'is available to all transmitters.

MANFRED 'MUEHT'ERL

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