US2407640A - Telephone or like signaling system - Google Patents

Description

Sept. 17 1946. c. GILLINGS EFAL I 2,407,540

TELEPHONE 0R LIKE SIGNALING SYSTEM Filed Sept. 18, 1945 a Sheets-Sheet 1 JCK . AINVENTORS 'cHARLEs GILLINGS CHARLES EDMUND BEALE ATTORNEY 6 Sheets-Sheet 2 D58 D56 DSD 3&4 3&4 3&4

C. GILLINGS ET AL TELEPHONE O R LIKE SIGNALING SYSTEM Filed Sept. '18, 1943 Sept 17, 1946.

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INVENTORS Arrows! CHARLES GILLINGS CHARLES EDMUND BEALE Y cs2 Sept. 17, 1946. c. GILLINGS ET AL 2,407,640

TELEPHQNE OR LIKE SIGNALING SYSTEM I Filed Sept. 18, 1945 6 Sheets-Sheet 4 II c'm'i P [40 'ms. on

360 "ms. OFF

'INVIENTORS CHARLES GILLINGS CHARLES EWUND BEALE Sept.'17, 1946. IC.,GILLINGS ETAL 2,407,640

TELEPHONE OR LIKE SIGNALING SYSTEM Filed Sept. 18, 1943 6 Sheets-Sheet 5 vs GILLINGS CHARLES eouum BEALE ATTORNEY 'Sept. 17, 1946. c. GILLINGS EI'AL 2,407,640

TELEPHONE OR LIKE SIGNALING SYSTEM Filed Sept. 18. 1945 e Sheets-Sheet e EH 7 i mvemon;

CHARLES GILLINGS CHARLES EDMUND ems ATTORNEY Patented Sept. 17, 1946 TELEPHONE OR LIKE SIGNALING SYSTEM Charles Gillings and Charles Edmund Beale, Liverpool, England, assignors, by mesne assignments, to Automatic Electric Laboratories, Ihc., a corporation of Delaware Application September 18, 1943, Serial No. 502,885

In Great Britain October 30, 1942 11 Claims. 1

The present invention relates to telephone or like signaling systems and is more particularly concerned with the setting and control of automatic telephone switching apparatus over long trunk routes involving mixed systems of voice frequency and direct current dialing and supervision. From one point of view the invention may be considered as a development of that disclosed in our application Serial No. 502,884, filed Sept. 18, 1943, and finds advantageous application in circumstances where a particularly low limit is set to the power which may be transmitted over the trunk line for dialing and supervisory signals.

In the arrangement disclosed in the prior specification the numerical direct current impulses received at the sending end of the trunk were converted into Voice frequency coded signals which were transmitted over the trunk and redifficulty by suitable arrangement of the signal codes.

According to one feature of the invention it is arranged to retain the advantages of the prepare signal and the code method of signaling described in the prior case but an improved signal code is employed such that a maximum of tWo frequencies only are applied to the line at any instant for any particular code. This has been achieved by dividing the numerical digit codes into two groups representing the digits 1 to 5 and 6 to 0 and using a different prepare signal for each group to distinguish digits for which the code signals may be identical.

A typical code on this basis is set out below and this is assumed to be employed in the example which follows.

converted at the terminal end into numerical Digit Prepare Code Digit Prepare Code direct current impulses. Combinations of four different frequencies designated W, X, Y, and Z WY w wz W were used to characterise the code signals and WY X WZ X WY Y wz Z the arrangement was such that each voice frewY wx wz WX quency coded numerical signal was preceded by WY XY WZ XZ a prefix signal which was the same for all numerical codes and comprised a pulse of all four frequencies W, X, Y, Z, applied simultaneously. The prefix signal was in eliect a prepare signal which conditioned th V. F. receivers at the distant end of the trunk to receive the V. F. signals and also brought about the switching of any echo suppressor in the route into the correct direction of transmission.

This method of signaling was based on a maxi- As in the prior arrangement the coded prepare pulse may have a duration of 160 in. s. followed by the coded digit pulse which has a duration of mum power limit to line of the order of 2 milli- Condition sigma watts and since when a compound signal was used, the power was divided in equal proportions seizing pulse 100 m, s, X, between the number of frequencies used in the {$8 gac(re compound signal, in the case of a prepare signal Subscriber answersignal peat ed until accomprising all four frequencies W, X, Y, Z, the Aknowle d gment 510ml v $32 5? power level of each individual frequency was 0.5 140 mIsIYf milliwatt. Where such signal codes have a dura- Called Subscriber clear signal ggg i-f tion of 2 or more seconds, there is a danger on operatowclarsicnal- {2seconds X. certain trunk routes of overloading the filters b 2 12 3 35 and other such apparatus if a maximum power {300111. 5. Y. level of 2 milliwatts is used and it may be neces- One of the advantages of the earlier arrangement was that the use of the WXYZ prepare signal exercised all tuned circuits so that provided the prepare signal was received satisfactorily, the following digit code signal comprising a lesser number of frequencies would also be received satisfactorily and hence the risk of setting up Wrong numbers was considerably reduced.

In order that this advantage shall be retained 3 in the improved signaling arrangement where the prepare signal no longer exercises all tuned circuits simultaneously, it is arranged according to another feature of the invention that when the prepare signal WY is used to characterise the numerical code group 1 to 5, the actual code signals in this group will be made up of a combination of W and Y frequencies plus the X frequency which has already been tested as regards the correct operation of the receiver by its reception as a seizing X pulse when the equipment is taken into use. Similarly when the WZ prepare signal is used to characterise digits in the group 6 to 0, the actual code signals for these digits will be made up from the W and Z frequencies plus the X frequency which has already been proved as regards correct reception durin the seizing of the equipment.

The invention will be better understood from the following description of one method of carrying it into eifect reference being had to the accompanying drawings comprising Figs. 1-6. Figs. 1-4 of these when arranged in the manner indicated in Fig. 4a show the circuits of an outgoing relay set, while Figs. 5 and 6 when arranged side by side with Fig. 5 on the left show the circuits of an incoming relay set, the arrangements being suitable for connection at opposite ends of an interexchange V. F. trunk line with operation on a sleeve-controlled basis.

It will be understood that these circuits are in many respects identical with those disclosed in the prior specification and therefore the description which follows will only deal in detail with the modifications which have been made to these circuits to adapt them to function with the revised code signaling arrangements which constitute the main feature of the present invention.

' Referring now to the outgoing relay set, Fig. 1, when the operator plugs into the jack shown, relays M and MM, Figs. 1 and 4, operate whereupon her supervisory lamp will flicker due to the connection of fiickerearth over lead 9. Relay MM in operating brings up relay SR, Fig, 4, which causes switch TS to advance to position I where relays CO and FXY are operated. Relay FXY in operating brings up relay CS and'causes switch TS to advance to position 3 whereupon relays FXY and CS release. During this time a 100 m. s. pulse of X voice frequency will be transmitted forward over the trunk line to the incoming relay set at the distant exchange to bring about the seizure of an associated first selector.

The operator now operates the dialing key, whereupon relay RR, Fig. 1, is operated and the supervisory lamp is extinguished. 'Relay'RR in operating brings up relay A and this in turn latter releasing relay RR. When the operator dials the required subscribers number, the impulses are received on relay A which repeats them via relay C, Fig 3, to the driving magnet DSAM of the first digit storing switch DSA which steps its wipers to a characteristic position. Relays B and C hold operated during the impulsing and the latter operates relay CS, Fig. 4, to prepare the V. F. signaling circuit. Relay C also energises the driving magnet DDM of the digit distributor switch DD but the wipers of this switch do not move until the magnet is de-energised.

At the end of the first series of impulses, relay A holds and relay C releases after its slow period brings up relays B, Fig. 3, and BR, Fig, 4, the

or less, relay CNA operates over its left-hand winding and as a result a short interdigital pause is provided between successive retransmitted digits as will appear subsequently. Relay CNA operates relay NS which at contacts nsl advances the sending control switch SC from its home position to contact I where relay ST is operated. Relay ST connects up its low resistance winding to self-drive the sending control switch to contact 4, maintains relay CS and at contacts st3 and std causes a prepare pulse of WY frequency to be connected to the trunk line to the distant exchange. When the sending control switch SC reaches contact 4, relay ST releases after its slow period to disconnect the WY prepare'pulse and operate relay S. The duration of the prepare pulse is measured by the time taken for switch SC to step to contact 4 plus the slow release period of relay ST which gives a total figure of 160 m. s.

Relay S holds relay CS and connects the digit code pulse to the line, this pulse being dependent for its composition on the setting of the digit switch DSA and is thus characteristic of the digit releases and releases relay CS and causes switch SC also to drive to its home position.

'If in the meantime the operator has dialled "i6 second digit, say 4, on to switch DSB (not shown), relay CNR will be maintained operated in series with relay CNB (not shown) which reoperates relay NS on the release of relay CNA.

Accordingly, when the switch SC comes into its home position, it will again self-drive to contact l where relay ST will re-operate to send out the prepare pulse of WY frequency followed by the code pulse KY for digit 5 which is picked up from the setting of switch DSB.

If, as assumed, the first digit dialled was 5 or less, the interdigital pause between the two code digits transmitted to line will be measured by 20 self-driving steps of the switch DSA, say 400 m. s., plus 16 self-driving steps of switch S0, say 320 m. s., giving a total of 720 m. s. During this period five machine-generated-pulses are delivered at the distant exchange to the incoming selector after which hunting takes place to select an idle trunk in the level selected. While the selector is hunting, the incomin de-coding relays are released and re-set to receive the second prepare and code signals which will require a time period of 160 plus m. s. giving a total of 260 m. s. which added to the 720 m. 5. already mentioned gives a total of 980 m. s. for the complete interdigital pause.

If the first digit dialled had been 6 to 0, a

longer interdigital pause would be required to 7 enable the incoming selector to perform its sequence of operations and this will be described in relation to the change of the prepare signal which must also take place to characterise a digit in the second group. Under these conditions relay CNA will be operated over its right-hand Winding in series with relay CNR and at contacts cna3 brings about the operation of relay HS which looks independently of wiper SC2 via its contacts he I. Relay N8 is inoperative in these circumstances owing to the shunting effect of the low resistance common relay CNR. Relay HS at contacts hsl, Fig. 2, changes over the connections of the prepare pulse circuit so that when relay ST operates on the first step of switch SC, a WZ pulse is connectedto line. This is followed by the coded digit pulse and when relay Z operates to terminate the pulse, switch DSAdrives to its home position thus measuring off part of the interdigital pause. Relay HS is then disconnected at wiper and bank DSAl but holds for a short period because of its slug and at contacts hs2 causes switch .DSA to make another halfrevolution. During this latter operation relay HS is released but relay Z .remains held and thus brings about a substantial increase in the interdigital pause, For example, if the digit 0 has been dialled the interdigital-pause would be made up as follows: 14 steps of the switch DSA plus an additional half-revolution making 39 steps in all gives a total period of 7 80 m. 5. To thismust be added 16 steps of switch SC, say 320 m. s., plus a further260 m. s. representing the transmission time or the prepare and code pulses for the second. digit giving a total intercligital pause of 1,360 m. s. The purpose of this extra pause is to ensure that the distant selector has sufficient time to perform its level-selecting function plus its trunk hunting before the next decoded digit is transmitted. 7

When all stored digits have been transmitted in code relay NS or HS finally releases so that no further kick-on circuit is completed for the sending control switch SC. All the relays CNA-CND and relay CNR will also be restored to normal.

Although four digit switches DSADSD have been provided, it is possible in practice that a lesser number may be found satisfactory dependin upon the speed at which the storing and retransmitting in code can be accomplished. The digit storing switches DSA to DSD are taken into use in cyclic repetition, that is to say, when the fourth digit has been stored on DSD the switch DSA which will be normal by this time will be again taken into use to store the fifth digit and so on for as many digits as are concerned. Slight circuit modification, such as is described in the prior-specification are desirable to ensure that the cyclic function is operative under all conditions.

When the operator restores her dialling key at the end of dialling, the current in the sleeve circuit is increased and the battery applied to the tip and ring conductors is removed shortly after- Wards. Relay KR, Fig. 1, thereupon operates and releases relays A, B and BR in turn. Relay B in releasing homes the digit distributor switch DD (Fig. 3) to its mid-position preparatory to the operation of relay AA at a later period, while relay BR in releasing connects the low resistance lower Winding of relay RR into the sleeve circuit, relay RR being non-operative, but causing the operators supervisory lamp to glow. Relays M, MM, KR, 00, FC and BS remain operated and the line condition is now such that the operator may listen to tones and, if necessary, speak without the necessity for a called party answer signal, but no supervision is given.

When the called party answers, the incoming relay set transmits back 140 m. s. pulses of Y frequency at spacings of 360 m. s., and this signal is repeated until acknowledged by the transmission of an X signal from the outgoing relay set. Upon reception of the Y frequency, relay Y in the V. F. receiver VFR, Fig. 2, operates andopens the circuit for relay BS. On the release of relay BS, relays MS andMT are operated in .turn .and hold for the remainder of the Y pulse. When the Y pulse is finished, relay BS re-operates and further opens the circuit for relay MS. If the interval between received pulses is of the correct duration, relay MS will release but not relay MT, so that on reception of the next Y pulse, relay SA will be operated and at the end of this pulse relay SB operates in series with relay SA. Relay SB operates relay AA, Fig. 3, and relay AA re-operates relays CS, Fig. 4, and at contacts aal, Fig. 2, applies X frequency to the outgoing trunk. The duration of the X frequency acknowledgment signal is measured by twelve steps of the switch DD plus the release time of relay AA which releases when the DD switch reaches the home position, and which in turn releases relay CS. When the X pulse is received at the distant incoming relay set, transmission of the Y pulse ceases and relay BS remains operated, while relay MT releases. Relay SA in operating gives local and through supervision at the outgoing operators position.

Conversation now proceeds, and when the called party clears, the incoming relay set transmits Y pulses of the same type as for the called party answer signal. At the outgoing end relays M, MM, KR, CO, FC, BS, SA and SB are held operated so that the first m. s. of Y frequency releases relay BS and operates relays MS and MT as already described. The following 360 m. s. break period operates relay BS which releases relay MS and when the next Y pulse is received relay BS again releases and during the slow release period of relay MT relay SY now operates. Relay SY locks up to the Y pulse, operates relay SZ and releases relay SA which gives the necessary supervision at the outgoing operators position.

When the operator clears by removing the plug from the jack, relays M, M and KR release in turn. Switch TS is thereupon stepped from position 6 to position 1, whereupon relay BR operates in series with relay SR. Relay CS now re-operates whereupon X frequency is applied to the trunk line. With relay SB operated the switch TS self-drives from position I to position -15, and then steps by means of interrupted earth on lead E3 to position l9. Relay FXY is then operated to disconnect the X frequency after a period of application of two seconds, while it also connects up Y frequency to the trunk line. On the same switch bank TSA, relays SR and BR are caused to release after their slow periods, and on the release of relay BR, earth is extended over bank TS3 to step the switch from position 19 so that relay FXY releases, and after its slow release period terminates the Y frequency after a period of application of 300 milli-seconds.

Switch TS then steps from position 2K! to position 23 via bank TSB and then slow step-s over contacts 23 and 24 via bank TS5 to reach its home position where relays CO, FC and SB release. During the application of Y frequency relay BS will have released and relays MS and MT operated but without useful effect.

The circuit operations under special conditions where the operator clears before receiving the answering signal, or in the face of any tone or on a non-metered call, or Where the outgoing equipment is taken into use from a selector level or where the operator inadvertently withdraws the plug during conversational conditions, are

describedin the previously mentioned specification.

' Considering now the operations in the incoming relay set, Figs. and 6, when the 100m. s. X seizing pulse is received over the trunk, relay IX (not shown but assumed to be located in the Valve 7 receiver IVFR) operates and brings up relay XR. Relay XR at contacts :rr3 operates relay K which looks and operates relay PY and extends earth forwardly over the P conductor to the incoming selector to busy this switch. At the end of the X seizing pulse, relay IX releases and drops relay XR which operates relay NN, Relay NN further earths the P conductor to the selector, operates relay IN over the negative line in series with the A relay in the selector, and also operates relay GX. Relay GX at contacts gx l causes a small current to flow through the various relay contacts in the line circuit which are thus wetted. to prevent coherer trouble. The selector is now ready to receive the train of impulses.

When the outgoing relay set transmits the 160 m.'s. prepare pulse of WY or WZ frequency as described, relays IW and IY or IW and IZ operate in the V. F. receiver in the incoming relay set and bring up their relief relays WR and YR or V WR-and ZR. Depending upon which of the two prepare signals is transmitted relay CN or relay CH is operated, either of which is effective to convert the normal line termination into a loss pad with considerable attenuation which prevents the passage of signals beyond the incoming relay set so that the line is blocked at this point. The main purpose of this arrangement is to pre- Vent a double transmission of impulses which might take place in connection with tandem calls via a number of exchanges in series as is fully described in the prior specification.

The prepare pulse of 160 ms. is followed by the code digit pulse of 100 m. s. duration. In the example in question the first digit is 5 so that relay CN is operated in response to prepare pulse WY andprepares a circuit for relay CB which is at present short-circuited. When the prepare pulse is replaced by the code XY for digit 5, relay CN is energised via contacts chl 0111, and :crZ and yrZ in parallel, in series with relay CB to arth via contacts s23, ir5, s04 and nnZ. Relay CB in operating locksrelaysXR and YR and these relays over contacts :cr5, 2M and W5 of a suitable pyramid chain of contacts connect earth to the sixth contact in the bank SS3 of the sending switch SS in order to terminate sending after five impulses have been transmitted to the incoming selector as will appear subsequently.

It will be noted that the contact pyramid of the code relays WRZR connects markings to the 'two banks SS3 and SS4 of the sender switch to control the sending of digits 1 to 5 and 6 to 0 respectively. The particular bank which is to be effective at any time is determined by the selective operation of the relays CN and CH which are responsive respectively to the two types of prepare signals which are used to prefix the digit signals. Relay CN which is operated at this time at contacts cnfi operates relays C and P and the latter locks up. Relay C operates relay CC which prepares a circuit for relay IG which operates when the impulse springs 66% M next open. Relay IG drops relay IN and transfer the forward holding loop extending to the selector to the impulse springs 33% M which are closed at this time since they are out of phase with the magnet impulse springs 66% M which have just opened to bring 8 1 about the operation of relay IG. Accordingly, the sender switch SS now steps its wipers until they encounter the marking earth on contactfi of bank SS3 over contacts of the coding relays XR and YR; while at the same time the impulse springs 33% M count out a corresponding number of impulses to the incoming selector; When the appropriate sender-switch wiper, SS3 in this in stance, picks up themarking, relay SZ- operates to short-circuit the 33% M springs to terminate the impulses to the selector and also release relays CN and CB. Relay CB drops relay IG which causes the sending switch SS to home while the holding circuit for the selector is again transferred to relay IN. Relay CB also drops relays XR and YR and relay C releases after-itsslow period and this is followed by the release of relay CC also after its slow period. Relays K, PY, NN, IN, GK and P. remain held.

In order'to guard against an inductive kick via the repeating coil RPCZ to the incoming side of the circuit when relay IN is transferred to the incoming selector on the release of relay 1G, it is arranged that relay C holds for a short period because of its slug and maintains the loss pad in circuit to absorb this inductive surge and prevent it interfering with the V. F. receivers on the incoming side of the circuit. I v 1 After a pause long enough to permitthe incoming selector to hunt over the selected level, which cause is measured in the outgoing relay set as already described, the next combination of prepare and code pulses is transmitted and the operations described are repeated until all the impulsetrains have been dealt with. l

The connection is thus fully set up and the called subscriber isrung. When the called party answers, battery is reversed'over the forward trunk so that relay IN releases and relay I operates. Relay I brings up relay IR which alters the connections to the loss pad to cut out the line termination and give a straight-through connection. Relay IR also releases relay K which opens the circuit to relay PY and during the slow release period of the latter, relay MA is operated and locked. Relay MA connects relay FY to interrupted earth on lead I 4 having a periodicity of m. s. on and 360 m. s. 01f. Relay FY therefore pulses to extend backward over the trunk a repeated 140 m. s. Y pulse with 360 m. s. spacing.

When this signal is accepted at the outgoing relay set, the latter returns a 440 m. s. X acknowledgement signal which brings up relays X and KR in the incoming relay set. Relay XR opens the circuit for relay GX and operates relay 'GY which disconnects relay FY to terminate the answer signal. Relay GX releases after its slow period and drops relay MA whichconnects the line through for conversation. At the conclusion of the acknowledgment signal, relay XR releases, relay GX re-operates and relay GY releases.

The remaining features of operation such as .normal clear down, clearing in face of tones,

clearing in face of a short-circuited trunk, nonmetered calls and tandem connections via a number of exchanges in series are fully described in the prior specification and will therefore not be dealt with as they are identical in allrmaterial respects. 7

A possible alternative code to the one utilised in the foregoing description is as follows:

The use of the two frequency code for each digit minimises the possibility of wrong numbers which might occur in cases where mixed single and double frequency codes were used and if the source of one of the frequencies failed during the sending out of a double frequency coded digit. Its use Will of course involve rearrangement of the bank wiring of the digit switches DSA-DSD and of the contact pyramid extending to the banks SS3 and SS4. Moreover it does not pres rve the feature previously mentioned, that the numerical code includes only those frequencies which have already been used either for the seizing signal or the prepare signal. In some circumstances however the advantages of the standardised two frequency code may be held to outweigh this slight disadvantage.

What we claim as new and desire to secure by Letters Patent is:

1. In a telephone system having two offices connected by a trunk line, a plurality of sources of alternating current of different frequences at one of said offices, means at said one office for successively transmitting over the trunk line a plurality of alternating current impulses, each composed of one or more of said frequencies, to

transmit a digit, each impulse having a part in determining the digit value, and means at the other ofiice responsive to said impulses for effecting a numerical switching operation in accordance with the digit value of said impulses.

2. A telephone system as claimed in claim 1 in which the digit values are divided into groups and in which the first of said plurality of impulses is characteristic of one of said groups.

3. In a telephone system having two offices connected by a trunk line, a plurality of sources of alternating current of different frequencies at one of said ofiices, means at said one ofiice for successively transmitting over the trunk line two alternating current impulses each composed of not more than two of said frequencies, said two impulses jointly characterizing a digit value, and means at the other office jointly controlled by said two impulses for effecting a numerical switching operation in accordance with the digit value of said impulses.

4. A telephone system as claimed in claim 1 in which the nature of the first of said plurality of impulses varies in accordance with the digit value,

and said means at the other office being responsive to the first of said plurality of impulses to produce a switching effect which is the same for all digit values.

5. A telephone systemas claimed in claim 1 in which the digit values are divided into groups, the first of said plurality of impulses being characteristic of one of said groups, and said means at the other ofiice being responsive to the first of said plurality of impulses to produce a switching effect which is the same for all roups.

6. The method of controlling automatic telephone switching equipment over a trunk line which consists in transmitting a seizure signal composed of a single frequency over the line, subsequently transmitting signals representing digit values over the line, each of said subsequent signals consisting of two impulses, the first of which characterizes a group of digit values and is composed of one or more frequencies, and the second of which characterizes the digit value in a group and is composed of one or more of the frequencies used for seizure and for characterizing the group, and in utilizing the received signals to produce numerical switching operations corresponding to the digit values of said subsequent signals.

'7. In a signaling system including two stations and a line connected therebetween, a source of a series of impulses at one of said stations, translating means controlled thereby adapted to transmit successively two composite alternating current signals over the line in response to each se ries of impulses, said two alternating current signals jointly characterizing the number of impulses in a series and the composition of the first signal depending upon which one of a plurality of groups the number of impulses in said series corresponds to, and means at the other station responsive to said two alternating current signals for effecting a numerical switching operation cor responding to the number of impulses in said series.

8. A signaling system as claimed in claim 7 in which said translating means is adapted to provide a variable pause between successive groups of signals in accordance with which one of said plurality of groups the number of impulses in the previous series corresponds to.

9. In a telephone system including two exchanges and a line connected therebetween, means at one of said exchanges for transmitting successively two composite alternating current impulses over the line, means at the other of said exchanges responsive to the first of said impulses for preparing to effect a switching operation in one of a plurality of numerical groups, said last means also being responsive to the sec- 0nd of said impulses for effecting a switching operation in the numerical group prepared by the first impulse.

10. In an automatic telephone exchange including impulse generating means and automatic switches controlled thereby, an incoming line over which groups of composite alternating current impulses are at times received, means associated with the line for selecting One of a plurality of groups of marking circuits in response to the first impulse of a group, and means response to a second impulse in the group for selecting a marking circuit in the previously selected group and for causing said impulsing means to deliver a series of impulses to the automatic switches in the exchange, the number of impulses in said series being determined by the selected marking circuit.

11. In a telephone system, a trunk line, transmitting equipment associated with one end of the trunk line and receiving equipment associated with the other end thereof, means in the transmitting equipment for sending coded signals corresponding to digits in two numerical groups over the line, said signals each comprising a first impulse varying in accordance with the numerical group to which the digit belongs and a second impulse varying in accordance with the value of the particular digit in the group, and means in said receiving equipment responsive to said coded signals for efiecting a switching operation corresponding to the digit value thereof.

CHARLES GILLINGS. CHARLES EDMUND BEALE.

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