US2224374A - Remote control system - Google Patents

Description

Dec. 1o, 1940. G W, BAUG'HMAN 2,224,374

REMOTE CONTROL SYSTEM Filed March 16, 1940 HIS All-TORNEY Patente-d Dec. 10, 1940 UNITE STATES REMOTE CONTROL SYSTEM George W.`Baughman, Pittsburgh, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of YPennsylvania Application March 16, 1940, Serial No. 324,369

12 Claims.

My invention relates to remote control systems, and more particularly to the communication or line circuits for such systems.

My invention is particularly adapted for, though in no manner limited to, centralized trafc control systems for railroads in which a train dispatchers oice or other suitable point of control is connected with a plurality of eld stations by line wires over which code signals are transmitted at times for controlling selected devices at the stations and at other times for indicating at the oice the condition of the station devices. The code signals may be of any suitable character, but in the illustrative embodiments which I have chosen, the code signals are formed by interrupting the current from a source located either at the ofce or at the. most distant eld station of the system.

One object of my invention is to provide additional communication channels over the line wires of an existing telephone or telegraph circuit, such additional channels being suitable for code signaling and for operating centralized traffic controlling apparatus, for example. vAnother object of my invention is to provide a constant check that such additional channel or channels are effective for communication purposes. A further object of my invention is to arrange the circuits and apparatus in such a manner as to avoid interference between the various communication channels so as not to impair the quality of the telephone communication or introduce distortion into the telegraph transmission. A still further object is to extend the length of territory which may be controlled by a centralized trac control system, this object being realized through the possibility of adding repeaters into the line, thus extending the length of line over which code impulses for centralizedtrafc control can be elTectively transmitted. Other objects, purposes and features of my invention will be pointed out as the description proceeds.

I accomplish the foregoing objects by superimposing upon the telephone and telegraph circuits carrier currents either at the omce end of the centralized traic control system or at both the oiice and a remote station of the system, the transmission being so arranged by means of suitable iiltering devices and other control appa- `ratus that the existing line between stations of the system carries all of the communication currents but the carrier currents are made to operate suitably responsive apparatus at the individual stations without interference with telephone and telegraph transmission.

I shall describe two forms of apparatus embodyingmy invention and shall then point out the novel featuresthereof in claims.

In the accompanying drawing, Fig. 1 shows an existing line circuit which may be a standard two-wire telephonecircuit or a composite circuit having, in addition, a duplex telegraph circuit (not shown) between each wire and ground. This line circuit has been modified in accordance `with my invention to provide two superimposed alternating current communication channels for centralized traffic control. Fig. 2 shows a modiiied form of the apparatus of Fig. 1, also embodying my invention, applied to existing circuits having four line wires and providing a single superimposed alternating current channel for centralized traflic control.

Similar referencecharacters refer to similar parts in each of the two views.

Referring to Fig. 1, the existing telephone circuit Y-Z, YI-ZI extends between the oice and a distant end station, through a suitable number of intermediate stations only one of which isv shown in the drawing. At the oioe and at each station location I have introduced a suitable low-pass lter PV which passes the band of Voice frequencies up to about 3500 cycles. I have found that satisfactory telephone communication between the telephone instruments 9 at each end of the line can be obtained even though the Voice v frequencies in excess of 3500 cycles per second are suppressed. For transmitting codes from the oice to the various field stations, I provide a source of current A having a suitable frequency suiiiciently removed from the voice frequency band to permit separation thereof by a suitable band pass lter PA located at the oce and at each station, which passes the frequency A along with minor variations of this frequency such as might occur at the source, but cuts off frequencies substantially below as well as above this 4irequency. A frequency of 5000 cycles per second is suitable for source A although, if preferred, a frequency of about cycles per second, below rather than above the voice frequency band, can also be used for the source A. In the latter case, a suitable change in the lters PV and PA would be required. I prefer the higher frequency because it permits a higher speed of transmission. At the distant field station, I provide a second source of frequency B for transmitting indication codes back to the office. A suitable frequency for source B is about'9000 cyclesper second, this frequency being suiciently far removed from the frequency of the control current source A to perl mit effective segregation of the two currents in their respective channels. High pass filters PB allow the passage of current from source B through the various intermediate station locations to the oflice, but cut off the voice frequency currents and the current from source A. A frequency such as 140 cycles per second, below the voice frequency range, can be used for source B, as in the case of source A, but the higher frequencies are preferred.

At the office and at each station I provide a transmitting relay and one or more receiving relays. At the office, the transmitting relay is OT and the receiving relay is R. At the remaining stations, these relays are designated T and R, respectively. At intermediate stations, I also provide a master relay M and an auxiliary receiving relay BR. The designations of the transmitting relays, the master relay and the receiving relays are the same as those in the copending United States application Serial No. 600,786, filed March 23, 1932 by LloydV. Lewis, for Remote control systems, which corresponds with British Patent No. 415,787issued September 3,1934.

The transmitting relays transmit codes from the office to the various stations and from the stations back to the office. These codes operate the receiving relays which translate them into suitable control or indication functions at the re- .ceiving location. The manner in which the individual transmitting relays OT and T, master relays M, and the receiving apparatus associated with the receiving relays R are controlled forms nopart of my invention and this control is accordingly not shown in the drawing. A complete description of this portion of the apparatus is given in the aforesaid Lewis application and patent. For the purposes of the present application it is sufficientto state that the master relay M at an intermediate station must first be energized before the transmitting relay T at that sta- '.tion can become energized to transmit code '1mpulses to the lne.` Also', the receiving relay R at that station must also besteadily energized before transmission can start, otherwise it will cause the circuit for relay M to .be interrupted, thereby vpreventing energization of relay M and preventing transmission from that station.

As shown in Fig. 1, the telephone circuit extends from the telephone 9, over the low pass filter PV, line Wires Y-Z', additional low pass filters PV at any intermediate stations, line wires YI-ZI, and the last filter PV, to the telephone 9 at the other end of the circuit. The outgoing control line circuit extends from the source A over vback contacts I and 2 of the transmitter relay OT at the control office, through the band pass filter PA to the line wires Y and Z. At each intermediate station, the control circuit is shunted around the voicev frequency filter PV and passes over the wires 3 and 4, filter PA, line windings of the relay transformer RTA, and back contacts 5 and 6, tothe line wires YI and ZI. At the end station, the control circuit is completed over the line wires 'I and 8, filter PA, line windings of the relay transformer RTA, and the resistor I which-'terminates the line. This resistor should preferably have a value equal to the characteristic impedance of the line in order to eliminate reflection.

The incoming indication circuit extends from the source B, over back contacts II and I2 of the ktransmitter relay T and the high pass filter PB, to the line wires .YI and ZI. From thence, the indication circuit by-'passes the voice filter PV at the intermediate location and passes over wires I3 and I4, filter PB, line windings of relay transformer RTB, and back contacts I5 and I6 of transmitter relay TB, to the line Wires Y-Z. At the office, the indication circuit comes in over Wires I'I and I8, filter PB, and line windings of relay transformer RTB, being terminated by the resistor Ill, as before.

Control codes each comprising a series of time spaced impulses and intervals are transmitted from the office over the outgoing circuit just traced, by intermittently energizing the office transmitter OT. These codes are received by the line relays R which control the selective apparatus at each station, the apparatus at each station being selective to a different code. Indication codes are transmitted from an intermediate station by energizing the corresponding relay M at the transmitting station for the duration of the code which permits energization of the transmitter T and which deenergizes the portion of the line circuit extending to stations more remote from the office. The portion of the line extending to the office is completed over the compensating resistor I0 which, as stated before, has a value approximately equal to the characteristic impedance of the line. Relay T is then operated intermittently to transmit a code which` is characteristic of the station and the particular function whose operation is being indicated, this code being received by the line relay R at the oflice to operate suitable indicating apparatus. At the end station, since the line is permanently closed over the resistor I0, relay M is not necessary and transmission takes place simply by code operation of the transmitter relay T. The apparatus is so arranged that a code may be initiated by the ofce or by any station at any time when the line is free, and is further so arranged that no interference occurs in the event that two or more transmitters begin to operate simultaneously, the several codes being transmitted one at a time in a predetermined order under this condition, as fully explained in the Lewis patent. If a given station is transmitting, all of the R relays from the oice up to that station will be following code and so will not be steadily energized. Accordingly, no other field station can take command of the line at such time.

It will be seen from the foregoing that when an intermediate station is transmitting, the master relay M at that station sectionalizes the line for the code signaling channel, but the voice frequency channel remains continuous through the low pass filters PV at each station.

The receiving relays R and BR are energized from suitable line transformers which have equal portions of their primary windings in each of the line wires. This is for the purpose of balancing vthe characteristics of the two line wires as closely as practicable.

It Wi1l-be noted that the alternating current code channels which include sources A and B are both normally closed whether a code is or is not being transmitted. That is, outgoing energy from source A maintains the receiving relays R at each intermediate station and at the end station normally energized, whereas incoming energy from source B'maintains the BR relays at each intermediate station and the R relay at the office normally energized. At each intermediate station, front contact of relay BR is included in Vthe energizing circuit of the station receiver relay R, so that the latter relay can never become energized unless both alternating current code channels `are effective,thus providing a check on the availability of these channels. Should either source'A or source B become disconnected for any reason, the relay R at each intermediate station will become deenergized.

When codes are being transmitted from the office, the transmitting relay OT opens and closes the line for long or short periods in accordance with the selected code. The relays R at each station will respond to this code but the desired function will be operated only at that station which has beenassigned theparticular code being transmitted.

Since transmission from the source A-is always in the outgoing direction, and from source B always in the incoming direction, unidirectional amplifiers or repeaters may be placed in either of these channels at any intermediate field station, thus making possible an extension of the system to lines of very considerable length without undue diiculty.

It will be noted that contact 40 of transmitter relay T at the end station is included in the energizing circuit for the receiving relay R. This provision eliminates the necessity for an auxiliary receiving relay BR at the end station. When T is transmitting, relay R at the end station will follow the code lloeing transmitted, by virtue of the operation of contact 40, so that the cooperation between relays T and R which is necessary in order that transmission may continue, is eifective. If, on the other hand, at the time T began to transmit its indication code, relay R was not free but was following a control code transmitted from the ofce by OT, then the end station could not at such time take command of the line, but would await completion of the control code. The circuits involved in preventing transmission by relay T when relay R is not free to follow such transmission are not essential for an understanding of my invention and so are not shown but are shown and described in the aforementioned Lewis patent.

Similarly, at the oiiice, contact |9 eliminates the necessity for an additional receiving relay responsive to control codes transmitted by OT, since relay R at the oce is capable of cooperating with transmitter OT only if a station has not previously taken command of the line by causing relay R to respond to an indication code. Once the response of relay R to the indication code is completed, this relay is free to follow a control code and OT may then take command of the line.

Whenany intermediate station is sending, the relay M at that station opens the line for all more remote stations as far as the control chan'- nel of frequency A is concerned, while` at the same time intermittently opening and closing the line in accordance with code insofar as the indication channel of frequency B is concerned.

It will be apparent from the foregoing that the apparatus of Fig. 1 enables centralized traffic control functions to be properly and safely performed over existing telephone and telegraph circuits, through the provision of independent channels, one for each direction of transmission, so inter-linked with one another that no station can transmit unless both channels are available to that station at the particular time. Moreover, the unidirectional character of the transmission channels makes possible the use of repeaters for extending the line.

In Fig. 2, the carrier frequency channel is applied to four existing line wires which can carry a number of telephone 'and telegraph circuits at `the same time that the centralizedtraic control apparatus is functioning. Since the four wires compriseonlya single code signaling. channel in which the code signals go out on one pair of wires and. return on the other pair, only one source of carrier current, that is-a single carrier frequency A, is necessary. As in Fig. 1, the frequency of this carrier is preferably selected above the voice frequency range and can be 5000 cycles per second, for example. Ther low pass filters PV pass the voice frequencies but exclude the higher frequency of the source A. Similarly, the filters PA (which in this case can be of the high pass type, rather than of band pass design) pass frequency A but exclude the voice frequencies. As in Fig. 1, the office and each station are equipped with a transmitter relay and a receiver relay, with a master relay M at each intermediate station for initiating transmission and removing all more distant stations ,from the line when that particular station is transmitting;

The centralized traffic control channel extends from source A, over back contacts 2| and 22 of transmitter OT and filter PA at the ofce, to the line Wires Y-Z. Thence over wires 42 and 23, through back contacts 24 and 25 of transmitter T and back contacts 26 and 21 of master relay M at each intermediate eld station, wires 28 and 29, line wires Y|Z|, filter PA at the end station, back contacts 30 and 3| of transmitter relay T and line windings of the relay transformer RTA at this station, wires 43 and 44, line Wires W |-X|, wires 32 and 33, back contacts S4 and 35 of relay M, filter PA and line windings of the relay transformer RTA at the intermediate eld station, line wires W-X, and Wires 36 and 31, lter PA, line windings of the relay transformer RTA, and line terminating resistor I0 at the oice.

When the oice is transmitting, contacts 2| and 22 of the office transmitter OT interrupt the carrier in accordance with the particular station code and the control function which it is desired to operate. All of the R relays throughout the territory willrespond to this code but the control function will be performed only at the selected station.

When an intermediate field station sends an indication code, the M relay at that station will pick up, thus closing the line over wires 38 and 39 and resistor l0 (divided into two parts for purposes of keeping the line balanced) and dropping out all of the R relays beyond the sending station. As in Fig. 1, the transmitter relay T cannot become operative unless relay M is first picked up. Moreover, relay M cannot be picked up unless the receiving relay R at the trans-' mitting location is steadily energized. The control of relay M by the associated relay R and of relay T by its relay M forms no part of my invention and is accordingly not shown in the drawing. A complete description of one form which such apparatus may assume is given in the Lewis patent referred to herein.

It will be apparent from the foregoing that I have provided a system in which an existing two wire or four wires telephone line may be adapted for satisfactory and reliable transmission of centralized traiic control and indication codes with a minimum of auxiliary apparatus required at the various stations of the system. By employing two simultaneously effective frequency channels over a single pair of line wires, with sources at opposite ends of the line, locking out of unwanted stations when a .given station is transmitting is performed in a simple, effective, and reliable manner. By employing a single frequency channel over a four wire line, locking out can similarly be performed by any station which first begins to take command of the line by transmitting. All of the control and indication functions of the centralized traiiic control channels are performed without interfering in an appreciable manner with the quality of the telephone communication or the effectiveness of the telegraph transmission which may take place simultaneously with transmission over the additional code channels.

Although I have herein shown and described only two forms of apparatus embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In a remote control system, an cnice and a plurality of stations connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code signaling circuit including said line wires and a first source of periodic current having a frequency substantially removed from the voice frequency band of said. voice communication circuit, a receiving relay at each of said stations, means for repeatedly interrupting said outgoing circuit to operate said receiving relays in accordance with different control codes, an incoming code signaling circuit including said line wires and a second source of periodic current having a frequency substantially removed from the frequency of said first source and from said voice frequency band, an auxiliary receiving relay at each intermediate station and at said cnice, means for repeatedly interrupting said incoming circuit to operate said auxiliary receiving relays in accordance with different indication codes, means comprising electric lters for preventing interference between said outgoing and incoming circuits and said voice communication circuitl and means at each intermediate station governed by the associated auxiliary receiving relay for controlling the receiving relay at that station.

2. In a remote control system, an oce and a station connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code signaling circuit including said line wires and a source of periodic current having a frequency substantially removed from the voice frequency band of said voice communication circuit, a receiving relay at said station, means for repeatedly interrupting said outgoing circuit to operate said receiving relay in accordance with different control codes, an incoming code signaling circuit including said line wires, a transmitter at said station for repeatedly interrupting said incoming circuit to operate receiving apparatus at said ofce in accordance with different indication codes, means comprising electric filters for preventing interference between said code signaling circuits and said voice communication circuit, and means governed by said station transmitter for controlling the energizing circuit of said station receiving relay to cause said receiving relay to follow the indication code delivered by said transmitter, thereby enabling said transmitter to complete its indication code once said station takes command of said code signaling circuit.

3. In a remote control system, an oflice and a station connected by two line wires, a voice frequency communication circuit including said'line wires, and outgoing code signaling circuit including said line wires and a source of periodic current having a frequency substantially removed from the voice frequency band of said voice communication circuit, a receiving relay at said station, a transmitter at said oflice for repeatedly interrupting said outgoing circuit to operate said receiving relay in accordance with different control codes, an incoming code signaling circuit including said line wires, a receiving relay at said office, a transmitter at said station for repeatedly interrupting said incoming circuit to operate said cfiice receiving relay in accordance with different indication codes, means comprising electric filters for preventing interference between said code signaling circuits and said voice communication circuit, and means controlled by a transmitter when transmitting code and including the receiving relay at the other end of the line for locking out the transmitter at said other end for the period of a code, thereby preventing the remote transmitter from taking command of said code signaling circuit until the transmitting station has completed its code.

4. In a remote control system, an oiiice and a station connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code signalingr circuit including said line wires and a source of periodic current having a frequency substantially removed from the voice frequency band of said voice communication circuit, a receiving relay at said station, a transmitter at said office for repeatedly interrupting said outgoing circuit to operate said receiving relay in accordance with different control codes, an incoming code signaling circuit including said line wires, a receiving-relay at said office, a transmitter at said station for repeatedly interrupting said incoming circuit to operate said oiice receiving relay in accordance with different indication codes, means comprising electric filters for preventing interference between said code signaling circuits and said voice communication circuit, and a contact controlled by one of said transmitters and included in the energizing circuit for the receiving relay at the transmitting location for causing the receiving relay to follow the code being delivered to the line by said one transmitter, thereby enabling said one transmitter to complete its code once the transmitter takes command of said code signaling circuit.

5. In a remote control system, an oflie and a plurality of stations connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code signaling circuit including said line Wires and a first source of periodic current having a frequency substantially removed from the voice frequency band of said voice communication circuit, a receiving relay at each of said stations, a transmitter at said office for repeatedly interrupting said outgoing circuit to operate all of said receiving relays in accordance with different control codes, an incoming code signaling circuit including said line wires and a second source of periodic current having a frequency substantially removed from the frequency of said rst source and from said voice frequency band, an auxiliary receiving relay ai each intermediate station and at the oflice, a transmitter at each station for repeatedly interrupting said incoming Vcircuit to operate all of the auxiliary receiving relays between the office and the transmitting station in accordance with different indication codes, means comprising electric filters for preventing interference between said code signaling circuits and said voice communication circuit, and means controlled by a station-'transmitter and including the auxiliary relays at intervening stations between the transmitting location and the oiiice for locking out the-transmitter at each said intervening station for the duration of the indication code once said station transnfitter takes command of said code signaling circuit.v

6. In a remote control system, an ofce and a plurality of stations connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code signaling circuit including said line wires and a source of periodic current having a frequency substantially'removed fromthe voice frequency band of said voice communication circuit, a receiving relay at each of -said stations, a voice frequency filter included in the line at each of said stations; a path around said voice frequency filter at each intermediate station including the line windings of a transformer for said receiving relay at such station, an electric filter for excluding voice frequency current but passing current from said source, and back contacts of the transmission control relay at said intermediate station; means for repeatedly interrupting said outgoing circuit to operate said receiving relays in accordance with different control codes, and a line shunt at each intermediate station closed over the front contacts of said transmission control relay at the associated station and effective during transmission from the intermediate station to the oiiice for rendering ineffective that portion of the code signaling -circuit which extends beyond said intermediate station.

'7. In a remote control system, an oflice and a plurality of stations connected by two line wires, a voice frequency communication circuit including said line wires, an outgoing code -signaling circuit including said line wires and a first source of periodic current having a frequency substantially removed from the voice frequency band of said voice communication circuit, a main receiving relay at each of said stations, means for repeatedly interrupting said outgoing circuit to operate said main receiving relays in accordance with different control codes, an incoming code signaling circuit including said line wires and a second source of periodic current having a frequency substantially removed from the frequency of said rst source and from said voice frequency band, an auxiliary receiving relay at each intermediate station and at said officameans for repeatedly interrupting said incoming circuit to operate said auxiliary receiving relays in accordance with different indication codes, a voice frequency filter included in the line at each of said stations; a first path around said voice frequency filter at each intermediate station including the line windings -of a transformer for said main receiving relay at such station, an electric lter for current from said first source, and back contacts of the transmission control relay at said intermediate station; a second path around said voice frequency lter including the lin'e windings of a transformer for said auxiliary receiving relay at the intermediate station, an electric fil-ter for current from said second source, and back contacts of the transmitting relay at the intermediate station; and a line shunt at each intermediate station closed over the front contacts of said transmission control relay at the associated station and effective during transmission from the intermediate fstationftol the ofiice for renden,

ing ineifective that portion of the Vcode signaling circuit-which extends beyond said intermediate stati'on.

8.In a remote control system, an oflice and a pluralityV of stations connected by two line wires, a voice frequency communication circuit including said-line wires, an outgoing code signaling circuit including-said line wires and a rst source of periodic current having a frequency substantially removed from the voice frequency-band of said voice communication ycircuit, means for transmitting control code impulses from the ofiice over` said outgoing circuit, an incoming code signaling circuit including said line wires and a second source of periodic current having a frequency substantially removed from. the frequency of said first source and from said voice frequency band, means for transmitting indication code impulses from a station to the office over said incoming circuit, a voice frequency filter includedin the line at each station, a first path around said voice frequency filter at each intermediate Astation including an electric lter for current from said rst source, a main receiving relay at each intermediate station energized by current in said first path, a second path around said voice frequency filter at eachintermediate station including an electric filter for current from said second source, and an aux-` iliary receiving relay at each intermediate station energized by current in said second path.

9. In combination with a dispatchers telephone line circuit connecting an ofce and a plurality of stations, a normally closed outgoing code signaling circuit including the line wires of said telephone circuit and a first source of periodic current having a frequency substantially remo-ved from the voice frequency band of the telephone circuit, a normally closed incoming code signaling circuit including said line wires and a second source of periodic current having a frequency substantially removed from the frequency of said rst source and from said voice frequency band, means comprising electric filters for preventing interference between said outgoing and incoming circuits and said telephone circuit, and receiving apparatus at each intermediate station for checking that both said outgoing and said incoming circuits are effective.

10. In combination with a first and a second dispatchers telephone line circuit connecting an office and a plurality of stations, a code signaling circuit extending from the oflice to the end station over said first telephone line circuit and returning from the end station to the office over said second telephone circuit, a source of periodic current for said code signaling circuit having a -frequency substantially removed from the voice frequency band of said telephone circuits, means comprising electric filters for preventing interference between said code signaling circuit and said telephone circuits, a receiving relay at each of said stations and at the ofce, means for repeatedly interrupting said code signaling circuit to operate said receiving relays inv accordance with different codes, and a transmission control relay at each intermediate station effective when the intermediate station is transmitting for disconnecting the portion of the code signaling circuit which extends beyond said intermediate station to thereby prevent -transmission from stations beyond said intermediate station.

11. In combination with a first and a second dispatchers Itelephone line `circuit connecting an oflice and a plurality of stations, a, code signaling circuit extending from the oice to the end station over said first telephone line circuit and returning from the end station Ato the oice over said second telephone circuit, a source ofperiodic current for said code signaling circuit having a frequency substantially removed from the voice frequency band of said telephone circuits, a receiving relay at each of said stations and at said oiiice, a voice frequency filter included in each of said telephone line circuits at each station, a first normally closed path around the voice frequency filter at an intermediate station in said first telephone line circuit including normally closed contacts of a transmission control relay, a second normally closed path around the voice frequency filter at said intermediate station in said second telephone line circuit including normally closed contacts of said transmission control relay and an electric filter for current from said source, means for energizing the receiving relay at said intermediate station from current in said second normally closed path, and means controlled by said ltransmission control relay and effective during transmissionfrom said intermediate station for disconnecting the portion of the signaling circuit which extends beyond said intermediate station to thereby prevent transmission from stations beyond said intermediate station.

12. In combination with a rst and a second dispatchers telephone line circuit connecting an office and a plurality of stations, a code signaling circuit extending from the oce to the end station over said first telephone line circuit and returning from the end station to the oce over said second telephone circuit, a single source of periodic current for transmitting control code impulses from the oflice to said stations and for transmitting indication code impulses from a station to said office, the frequency of said source being substantially removed from the Voice frequency band of said telephone circuit, means comprising electric filters for preventing interference between said code signaling circuit and said telephone circuit, a receiving relay at each station and at the office responsive to said control code impulses and to the indication code impulses, and a transmission control relay at each intermediate station effective when the intermediate station is transmitting for disconnecting the portion of the code signaling circuit which extends beyond said intermediate `station to thereby preven-t transmission from stations beyond said intermediate station.

GEORGE W. BAUGHMAN.

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