DE731796C - Circuit arrangement for telephone systems - Google Patents

Description

Circuit arrangement for telephone systems The invention relates to a circuit arrangement for telephone systems with preferred and non-preferred Participant positions. In such systems, preferred locations must be opposed to the usual participant positions have the opportunity to rely on an occupied one To connect the line.

There are now systems in which the ordinary participants also help the call-back button can bring about the intrusion status in an unauthorized manner. It must therefore in this case. Provision is made that these participant locations, if they are also able to bring about the connection status, at least not with the line occupied by them can enter into speech communication.

One solution to this problem is for a DC powered one Telephone system already known. This goes to the extent that the usual subscriber sites in speech communication with the line on which you have intruded, thereby be prevented from using your intercom during the time you press the override button Press, is short-circuited. After releasing the override button, the speech communication is for such subscriber stations prevented by removing the intrusion potential the connection is always canceled by the line. These Solution cannot now be used for connection lines sealed off on both sides, since it is not possible here, as is the case with non-sealed lines for ringing serving potential is connected to the line for the entire duration of the call circuit allow. That would be the case with cordoned off lines during the call circuit Activation criterion, which in this case is generated from alternating current or inductively If there are direct current surges, that would be continuously applied to the line Conversation also between the legitimately intruding body and the desired subscriber station are disturbed. For this reason, the well-known Arrangements in which alternating current is continuously activated while the activation button is pressed or a series of alternating current surges through the cordoned off connection line and after releasing the button the ringing is canceled again, not suitable for solving the problem at hand. Neither are others for it known arrangements useful in which when the override button is pressed for To bring about the connection state, an alternating current surge is sent over the line, however, the switching status can be maintained after releasing the activation button will, since in this case the unauthorized body after the termination of the The switching measure carried out for the purpose of activation would be activated.

In contrast, the invention solves the problem in that when initiating the switching measure used to switch the call to a busy line from the outgoing transmission of a two-wire connection line sealed off on both sides from a duration over or under an election surge, in the incoming Transmission of the current impulse causing the connection state (half-length or short) and when this switching measure is canceled, a current surge equal to the duration of a selective current surge is transmitted, which cancels the override state in the incoming transmission.

The accompanying figures represent exemplary embodiments of the invention represent.

In Figs. I and a, an embodiment of the invention is shown, at which the preferred subscriber station Tb over the ordinary subscribers Tn has the opportunity to intrude and during the call switching with your To speak to those called. It is activated by pressing the AT (Fig. i) a current surge from the outgoing transformer Uei via the connecting line VZ to the incoming transmitter Ue. (Fig. 2), which differs from the rest Electric shock differs by its length. Since this is the transfer inductive current impulses is, in the context of this patent under j current impulse a pair of impulses consisting of one positive and one negative induction pulse, separated by a shorter or longer pause, under impulse only a single positive or negative induction shock. The activation pulse is longer than the normal occupancy and elective current surges and shorter than the long trip current surge; it will therefore be referred to as a half-length current surge in the course of the description. Such arrangements are known per se, only the invention now consists in that as soon as the preferred position releases the override button again, automatically from the outgoing transformer Uei the transmission of another current surge is normal Length takes place, which in the incoming transformer Ue. The preferred point Tb places the ground potential that initiates the ringing circuit Choke coils Dr to the speech wires, while the ordinary participants Tii through an actuation of their inquiry button RT aimed at the ringing circuit as well apply the switching potential to the speech wires, but short-circuit your station, so that the non-preferred position is finite while the override button is pressed cannot speak to a busy subscriber. If the ordinary participant further by improperly closing his or her number switch The handset fork specifies the activation criteria for the outgoing transformer, for which purpose the possibility. exists, it can be as a result of the arrangement according to the invention do not communicate with a busy subscriber, there, nevertheless the override status is brought about, his speech loop ineffective during these switching measures is and after completion of this switching measure, the activation status is canceled again will.

In the exemplary embodiment according to FIG. 3, the ringing is effected by a short current surge which is less than the normal dialing current surges in terms of duration and the override state is canceled by a current surge of normal length which is automatically emitted when the override button is returned. By the dial or listener fork directed towards the ringing circuit Herbeifübrung the short current pulses generated by non bevprzugter participants are necessarily always brought to the normal 1 Läng e anal current surges, thus preventing a ringer circuit in advance. This is achieved in that different receiving relays in the outgoing transformer at -the selection and release of the part of the calling subscriber affected, The operation of the arrangement of Figure I and 2 in detail is the following. When the preferred subscriber Tb a connection via the connecting line VL wants to establish, so he picks up his receiver and thereby causes the start of the preselector VW assigned to him. If the preselector has found a free first group selector I. GW, the supply bridge relays A and B respond via the closed subscriber loop. At the same time the occupancy relay C is energized. The first group selector is then set to a free transmitter Uel by dialing a number. If the first group selector has found such a transmitter, the occupancy relay Cl responds in the test circuit: - Earth, test relay of the first group selector (not shown), switching arm c of the first group selector, winding of relay Cl, battery, earth. The relay Cl causes, in a manner not of interest here, the transmission of an occupancy current surge via the line ITL to the incoming transformer. The occupancy current surge consists of an induction surge in a positive and negative direction and is generated by means of the transformer Tr. In the incoming transmitter Ue. this brings about the occupancy.

When dialing numbers, relays A and B of the first group dialer commute due to the loop interruptions at the calling subscriber. The current impulses are given by the first group selector to the current impulse receiving relay Al of the transformer Uel in the following way: earth, contacts ia, 2 c, switching arm a of the first group selector, contact 6x, winding of the relay Al, battery, earth. The delay relay V is excited via Tontakt iial, which is maintained during a series of impulses; Relay X does not respond via contact 4a1 during the short number current impulses. When relay Al responds, a direct current circuit is closed via winding I of transformer Tr: earth, contact 1-2a1, winding I of transformer Tr, battery, earth. This induces a pulse in winding II of the transformer, which is directed positively and is transmitted to line VZ in the following way: winding II of transformer Tr, contacts 14a1, 15 v, primary windings of the transformer, contact 17 cl, winding II of the transformer Tr. After energizing the relay V, the following circuit is closed to transmit the current impulses: winding II of transformer Tr, contact i6v, primary windings of the transformer, contact 17 cl, winding II of transformer Tr. When relay Al drops out, contact i2a, the DC circuit opened through winding I of transformer Tr. This in turn induces a pulse, this time in the negative direction, in winding II of this transformer, which is transmitted to the connection line VL via the circuit already described.

At the incoming end of the line, Ue. the polarized relay P is switched to the working position by the positive pulse. This energizes relay J via contact i8 p. Relay T switches on the -1, "delay relay S via contact igi. Relay S remains active during a series of impulses. Contact i also closes the following circuit for relay E: earth, contact 25 i, winding of relay F_, battery, earth Relay E passes the current impulses on via line I_ in a manner which is not of further interest here. When relay E responds, a circuit for relay U is closed: earth, contacts 21 e, 22 g, winding of relay U, battery, earth The relay U is designed as a delayed response relay. As a result, it does not respond when dialing the number despite this circuit being closed, because the relay P is returned to the rest position after a short time by the incoming negative pulse T and E fall again before relay U could respond: During the fall time of relay F_, the following circuit is established for relay G: earth, contacts 24i, 23 e, winding of relay G, battery, earth. Relay G stays on earth, contacts 28s, 27g, winding of relay G, battery, earth during a series of impulses. As a result, the circuit for relay U is kept open at contact 22g while dialing the number.

If the desired line is found to be busy after all the bursts have been sent out, the calling station Tb receives a busy signal in the usual way. She now presses the activation button AT in order to connect herself and thereby applies earth to both speech wires via the choke coils Dr. This causes relay B to drop out in the first group selector. Relay A, on the other hand, is kept energized. The override button is kept pressed from the preferred location. Regardless of the length of time the button is pressed, a half-long current surge is transmitted from the transformer Uel via the line. After relay B in the first group selector drops out, relay A, in transformer L% e, is energized: earth, contact 3 b, switching arm a of the first group selector, contact 6: t, winding of relay Al, battery, earth. Re-3 lais Al gives a positive impulse via the connection line V L by closing its contact r2 a1. Since relay A1 is energized for a longer time when switched on than when dialing a number, relay X, which responds with a delay and is not energized by the dialing current surges, now speaks to: earth, contacts 4a1, 7x, winding II of relay X, battery, earth. Relay X first opens the short-circuit across its winding I through contact 7x. This makes the relay more sensitive. The relay ends the current surge by opening the contact 6x and closing the contact 5x. The relay <-4l is switched off by the speech wire and drops out. The circuit across winding I of transformer Tr is interrupted, a negative pulse goes over the line. In this case, the time span between positive and negative impulse depends on the response time of relay X. This response time is measured in such a way that the time span between positive and negative impulse is about twice as long as that of a normal electrostatic surge, i.e. about 80 to 60 milliseconds . After the relay A1 in the transformer Uel drops out, the relay X is kept energized in the following way: earth, contact 3 b, switching arm a of the first group selector, contact 5 x, windings I and II of relay k, battery, earth.

Relay X also closes a circuit via its contact ro: r for the delay relay Y, which is responsible for the Power surge prepared to cancel the activation.

In the incoming transformer Ue_, the relays P, J, E and S are in turn energized by the positive pulse when switching on. Since the time until the negative impulse arrives is longer in this case, the relay U now also responds. As soon as the negative impulse arrives, the relays P, J, E, S fall again in the order of their energization. After the relay J has dropped out, relay G is again energized during the drop-out time of relay E. At the same time, relay D is now also speaking in the following circuit: earth, contacts 24 i, 267c, winding I of relay D, battery, earth. Relay D closes its contacts ao d, 29d and 31d . Relay G, which remains until relay S drops out, interrupts the circuit of relay U at contact 27g. Relay U drops out and interrupts the circuit of winding 1 of relay D. Relay D remains in 5 the following circuit: earth, contact 30e, winding 1I of relay D, contact 29d, upper wire of line L, winding of relay F, battery, earth. In this circuit, the Aufschaltelais F is energized, which switches through the speech wires to the occupied line via its contacts. For reasons of symmetry, a choke coil Sii is connected to the lower wire of the line L via contact 31d. The preferred subscriber Tb can now speak to your desired subscriber as long as he keeps the override button pressed.

If the subscriber Tb releases the override button, the relay B responds again via the subscriber loop. By opening the contact 3 b in the first group selector, the earth is switched off from the upper speech wire. As a result, relay I in the transformer Uel drops out. Relay .1 interrupts the circuit of relay Y at contact rox. During the release time of relay Y, the following circuit for relay Al is closed: earth, contacts 9y, 8 x, winding of relay Al, battery, earth. The relay A1 responds and gives a positive pulse via the line VL by closing its contacts 12a and 13a1. The circuit of relay Al is interrupted after relay Y has dropped out; Relay Al drops_off. As a result, a negative pulse is generated in the transformer Tr, which goes via the line VL. The release time of the relay Y is dimensioned so that the time span between positive and negative pulse corresponds to the time span of a normal selective current surge.

In the incoming transformer Ue, the relays P, J, E, S are in turn excited by the positive pulse. In this case, when relay J responds, relay G is also excited immediately: earth, contacts 25 i, 2o d, winding of relay G, battery, earth. By opening the contact 30e, the holding circuit for the winding II of the relay D and the switch relay F is interrupted. The two relays come to waste. The _'- # activation is canceled again. When the relay G is energized, it opens the contact 22 g and thus prevents the relay U from being accidentally re-energized: After the relay D has dropped out, the relay G remains in the following circuit: earth, contacts 28 s, 27 g, winding of relay G, Battery, earth.

After the negative pulse arrives, the relays P, J, E, S, G drop. The final release occurs when the calling subscriber hangs up the phone. The switching processes triggered in the transformers as a result are of no interest to the invention. If the non-preferred subscriber Tn, who can also make connections to the transmitter Uel and the connecting line VZ via preselector VW 'and the first group selector I. GW, tries, for. B. to intrude by pressing his inquiry button Rt, the relay A is also held in the first group selector occupied by him and the relay B is brought to waste. In doing so, earth is applied to the upper speech artery via contact 3b. The relay Al in the transformer Uel, in cooperation with the relay X, gives a half-long current surge via the line VL to the incoming transformer, in which the connection is thereby effected. However, since his station is short-circuited via the RT button, the subscriber Tn cannot speak to the desired subscriber despite the intrusion. After releasing the £ inquiry button, relay B in the first group selector and relay X in transformer Uel drops out. Relay Al in the transformer Uel responds briefly again and sends a normal current surge via the connection line, which cancels the connection.

If the subscriber Tn also tries, for example, artificially to slow down the sequence of his dial or, by pressing the handset fork, to bring the relays A and B of the first group selector to fall just long enough that the relay Al of the transformer Uel is energized long enough to use of relay X one. To send a half-long connection current surge via the Leiturig, the participant cannot be heard in this case despite the connection being initiated, since his speech loop is interrupted or short-circuited during these switching measures. Furthermore, since the termination of this illegal switching measure, as well as termination of the connection on the part of the preferred subscriber station, a current impulse is automatically given by the transformer Uel to the transformer Ue, which cancels the connection, the non-preferred subscriber must communicate with an occupied subscriber and listen in not possible.

Fig. 3 shows at the outgoing end of the line VZ a bidirectional Transformer Ue. As on the arriving. End of the line VL a completely identical one Transformer is located, it is not shown.

The arrangement according to FIG. 3 works in detail as follows: If a preferred subscriber Tb has established a connection to the transformer Ue via the preselector and the first group selector, the occupancy relay C responds via the test wire from the group selector. At the same time, the following circuit is closed via the loop of the calling subscriber: earth, windings II of relays E and A, switching arm b of group selector I. GW and preselector VW, subscriber loop. Switching arm a of preselector VW and group selector I. GW, contact 5o d, windings I of relay A and E, battery, earth. In this circuit, relay A responds, but not differential relay E. Relay C closes the following circuit for relay B: earth, contacts ic, q. d, winding of relay B, battery, earth. The relay B responds and closes the following circuit via the winding I of the transformer Tr: earth, contacts za s, 8 b, winding I of the transformer Tr, battery, earth. Relay B also closes the following circuit for relay D: earth, contacts 5c, 7b, winding of relay D, battery, earth. The relay D remains in the above circuit via its contact 6 d after the relay B has dropped out; the circuit for relay B is namely at contact q. d open. When relay B responds, a pulse is induced in winding II of transformer Tr, which goes through contact 2i c, winding II of transformer Tr, contacts gb, i2 v and the carrier windings via the connecting line. This impulse has a positive direction. Since relay V is excited via contact 43 b , the circuit running via winding II of transformer Tr is maintained after relay V has responded via contact iiv.

After the relay B drops out, the circuit for the winding I of the transformer Tr is open. This creates a negative impulse via the connecting line VL.

In the incoming transformer, the polarized relay P is switched over by the positive pulse. The following circuit is closed for relay J: earth, contact 23 p, winding I of relay J, battery, earth. The following circuit is closed for relay G: earth, resistance Wi, winding of delay relay G, contact 26 i, battery, earth. Relay G remains independent of relay J via contact a5 g. Relay S is simultaneously on via contact 249 as long as relay J is energized and contact 2g i is open. Relay G connects earth to the c wire to the outgoing group selector GW via contact 52g. Furthermore, the two speech wires are switched through by the contacts 53g and 5q.g, so that after the relays J and S drop out, the choke coil Ds is in a bridge to the speech wires, via which the line relays of the group selector are energized. Due to the incoming negative pulse, the relay P puts its armature back into the rest position; the relays J and S come to waste.

When the subscriber Tb dials the number, the loop is alternately opened and closed. As a result, the relay A oscillates in the outgoing transformer Ue . When the relay A drops out, the contact 47a, via which the relay A was on the a-wire after the contact 5o d was opened, is opened. When relay A drops out, the following circuit for relay B is closed: earth, contacts 2 a, 3d, winding of relay B, battery, earth. The relay B switches on the relay IV via contact 43 b. Furthermore, a direct current circuit is again closed via the winding I of the transformer Tr via contact 8b. A positive pulse is induced in the winding II of the transformer, which is transmitted via the line VL. Relay I 'closes the following circuit for relay TV: earth, contact 46v, winding II of relay W, battery, earth. Relay L3 'switches relay A back on to the aAder via contact 48 @cr. The relay A can now be energized again via the subscriber loop. The pause between the fall of the surge receiving relay A and its excitation via the subscriber loop and thus the time span between the positive and negative pulses is inevitably brought to a certain minimum value by the relays B, f 'and W. After the relay.-1 has responded via the subscriber loop, the relays B, L- and II; ' to waste; Relay = 1 remains independent of relay TV via contact 47a. When relay B drops out , a negative pulse is sent over the connection line. The other current surges are given in the same way.

When the first positive pulse arrives in the incoming transformer, the relay P is again switched. Relay P energizes relay J. By opening contact 29i, the short circuit via relay S is opened. Relay S responds via contact 24g. It is temporarily short-circuited via contacts 30 i and 27 s, but without falling during a series of numbered current pulses. Relay S closes the following circuit for relay W: earth, contact 32s, winding I of relay 1 ', battery, earth. The relay IJ 'responds before the negative impulse, which marks the end of each current impulse, and prevents by opening its contact 33 z ", an excitation of the Aufschaltelais H during the dialing. Relay S causes a current impulse loop over the speech wires via contact 57s The choke coil Ds is switched off. The current surges are passed on as loop strokes by alternately opening the (closing of the contact 55i).

If the desired line is busy, the calling subscriber Tb receives a busy signal. If he wants to intrude, he presses the intrusion button <-1t on his station, whereby earth is applied to both speech wires via the choke coil Di. In addition to the relay A, the differential relay E also responds in the transformer Ue. Relay E simultaneously closes a circuit for winding I of the transformer Ti and for the secondary winding II of the transformer. The first circuit is as follows: earth, contacts 223, 20f, winding I of transformer Tr, battery.

Earth. The circuit for the secondary winding of the transformer runs as follows: Transformer windings, contact 21 c, winding II of the transformer Tr, contacts 15 e. 13 f, transformer windings. As a result, a positive pulse is transmitted via line 1'L. Relay E also closes the following circuit for delay relay V: Earth. Contacts 39f, 41 e, winding of relay h, battery, earth. Furthermore, the following circuit for relay F is closed by relay E: earth, contact 37 e, windings I and II of relay F, battery, earth. Contact 20 f is opened and this causes an interruption of the direct current flow through the winding I of the transformer Tr, which results in a negative pulse. The secondary winding of the transformer Ti is still connected to the transformer winding via contact rzv, even if contact 13f has already opened. By opening the contact 39f , the circuit of the relay V is interrupted. The response time of the relay F is very short, for example about 20 milliseconds. This brings the time span between positive and negative impulse when switching on to a value below the normal dialing current surges. The preferred position is activated. The relays E and F in the transformer Ue are energized.

In the incoming transformer, the polarized relay P responds due to the positive impulse when switching on. Relay P energizes relay J. After relay J has responded, relay S is energized. Relay S switches on relay W via contact 32 s . Since the negative impulse has already arrived before the relay ITT has responded, the following circuit for the relay H is closed after the relay P has dropped out: Earth, contacts 35p, 36i, winding I1 of relay J, winding I of the relay II, contacts 33w, 3ig, battery, earth. To ensure that this circuit is safely closed, relay J is held in this circuit until relay H has responded. After relay W responds, relay H remains in the following circuit: earth, contacts 35p 34h. Winding II of relay H, contact 31 g, battery, earth. The relay J now comes to waste; This means that the relays S and W also drop out. Relay H applies ground via its contact 56h and a throttle Dr each to both speech wires of the outgoing line as an activation criterion.

The preferred point can now be with the desired participant speak.

If subscriber Tb puts the activation button back, the differential relay E 'in the transformer Ue drops out. Relay F, whose winding I is short-circuited via contact 38f, is delayed when contact 37e is opened. During its fall time, the following circuit is closed for the primary winding I of the transformer Tr: earth, contacts 22 s, ig f, @ i7 e, winding I of the transformer Tr, battery, earth. The secondary side of the transformer Tr is connected to the transformer windings via contacts 14f, 16e, winding II of the transformer Tr, contact 21c. A positive pulse is sent over the connection line. During the release time of relay F, the following circuit is closed for relay V: earth, contacts 40 f, 42 e, winding of relay Tue, battery, earth. When the contact 37e opens, the relay F drops out. Relay F interrupts the circuit via the winding I of the transformer Tr at the contact ig f. The secondary side of the transformer Tr is still connected to the line during the release time of the relay V via contact iiv when contact 14f has opened. As a result, a negative pulse goes over the line VL. The time span between positive and negative pulse is determined by the release time of relay F. Since winding I of relay F is short-circuited via contact 38f, this is of the order of magnitude of a normal selective current surge and is about 60 milliseconds.

At the incoming line end, the Ue in the transformer is positive Pulse in turn energizes relays P, J, S and W. By opening contact 35p the hold circuit of relay H is interrupted. Relay H drops out and takes it through Opening of 'contact 56h the activation criterion of the two speech cores. Of the negative impulse takes a long time to arrive, so no circuit can override winding II of relay J and winding I of relay H through "contact 35p be closed because the relay W has already responded in the meantime and had interrupted the circuit for this relay at contact 33w.

The triggering of the transformers at both ends takes place when the device is placed of the calling subscriber in a way that is not of further interest here Emission of a long tripping current.

If a non-preferred participant, e.g. B. the participant Tzz, after establishing a connection when the desired connection line is busy to intrude, be it that he press the inquiry key or the number switch or the receiver cradle is pressed briefly, these switching measures are based on the following Way made ineffective: When you press the inquiry button, there is indeed dadurcby that the differential relay E responds in the transformer Ue, the call circuit is established; only as long as the inquiry button is pressed at the subscriber station, is none Communication with the occupied participant possible. since the station Tn is short-circuited is.

If the subscriber tries to generate short current surges by means of his dial or by means of his handset fork, these are inevitably brought to the length of normal elective current surges, since the relay A of the transformer Ue, which drops out when the loop is interrupted, as a result of the interconnection of the relays B, T ', W is only reapplied to the line after a certain time. A call circuit in the incoming transmitter cannot be caused by such a current surge.

Claims (1)

PATENT CLAIMS: i. Circuit arrangement for telephone systems, at which both preferred and non-preferred subscriber sites the transmission of the criterion used for ringing on a busy line is possible, however, the utilization of this switching state by the during his Prevents existing actuated switching means at the non-preferred subscriber stations is, characterized in that when initiating the call switching to a busy one Line serving switching measure from the outgoing transmission (Uel or Ue) of a two-wire connection cable (VZ) sealed off on both sides or under an elective surge, in the incoming transmission (Ue, resp. Ue) the current impulse causing the connection state (half-length or short) and at Cancellation of this shift inaccessibility a current surge equal to the duration a polling surge is transmitted, which in the incoming transmission the Switching state cancels again. a. Circuit arrangement according to Claim r, characterized in that that the surge receiving relay (Al) in the outgoing transmission (Uel), which the received direct current surges as a pair of inductively generated direct current surges in the opposite direction via the connecting line when putting on of the connection potential (earth) to the line through the connection point Auxiliary relay (.x) which does not respond to the short selective current surges is excited, which after a certain time, the surge receiving relay drops and through its response time is the pause between your ringing pair of Induction surges determined (Fig. A). 3. Circuit arrangement according to claim 2, characterized characterized in that the auxiliary relay (X) excited during the call circuit is the receiving relay (Al) turns off the excitation circuit running over a speech artery and turns itself off in its place via this speech artery and the switching potential. q .. circuit arrangement according to claim 2 and 3, characterized in that the auxiliary relay (X) at its Excitation a delayed releasing relay (Y) switches on, which a local Excitation circuit for the receiving relay (Al). prepared. 5. Circuit arrangement Mach claim d., Characterized in that the local excitation circuit for the receiving relay (Al) through the auxiliary relay that drops out when the call switching is terminated (X) is closed until the delay relay (Y) dependent on the auxiliary relay came to waste. 6. Circuit arrangement according to claim 5, characterized in that that the release time of the delay relay (Y) is the pause between the cancellation the pair of induction surges positive and negative direction. 7. Circuit arrangement according to claim r, characterized in that that the number current surges and the activation criterion through different receiving relays (A or E) recorded in the outgoing transmission and inductively from these Generated current pulse pairs separated by pauses of different lengths in opposite directions Direction are implemented (Fig. 3). Ä. Circuit arrangement according to claim 7, characterized characterized in that during the connection of the switching potential to the lines on the part of a subscriber station in the outgoing transmission (Ue) as a receiving relay a differential relay (E) is actuated, which by means of a fast responding and delayed dropping auxiliary relay (F) when energized by a short one Break separate pair of induction shocks of opposite direction, at his Fall is an election surge, separated by a long pause Pair of induction currents in opposite directions across the connecting line transmits. Circuit arrangement according to Claim 7, characterized in that a on the part of a non-preferred subscriber station to bring about the connection status current surge generated by temporary loop interruption by the number current surges receiving relay (A) is received, which this current surge through a chain of auxiliary relays (B, Ü, 9 ') inevitably in a number current surge corresponding pair of induction shocks, which the ringing circuit does not able to bring about.