Improvements in and relating to telephone systems

Intercom systems have been developed to provide relatively small groups of subscribers with telephone connections between them. Such systems can be of s, k. house type, provided only for connection between the telephones within the system, or they can take the form of private extension systems by ensuring at least some of the subscribers connection via one or more external lines to the public telephone network and / or to other subscriber exchanges (PBX). Some intercom systems are characterized in that the required control equipment, which is needed for the system's function, is distributed on the system's various telephone stations, so that the amount of common control equipment is reduced to a minimum. In this way, the initial cost of such systems can often be determined by the number of connected devices.

However, it has been found that with previous systems there is a lack of flexibility in terms of scope and versatility in connectable accessories, such as conference capability for internal calls, transfer of external calls between landlines and the like, which is economically justifiable.

The object of the present invention according to the main requirement is to create the possibility of better arrangements for intercom systems of the above-mentioned type, where a large number of different aids can be connected in a simple and economical way.

7313832-3 ₂

In order to clarify the invention's relationship to prior art, it may be appropriate to begin with a brief history of developments in the field.

For a long series of years, the connection method was used exclusively with a pair of wires for each connected telephone or station, so that each new connected station in an existing system required a new pair of wires up to the exchange.

«* I

Attempts have been made to set up a single wire pair loop in the subscriber telephone system, where each ongoing^- Although a lot of money has been spent on creating a subscriber telephone system according to this principle, which is practically useful, no one seems to have succeeded in doing so so far. One can understand this mct background, ay that it is a matter of arranging stations in multiplex operating in the form of ordinary telephone sets, which, in contrast to what happens in exchanges, are often handled quite carelessly, dropped on the floor, etc., why the problems have actually been Another point of view is that such a tld multiplex system, where the speech signals are to be laid on time channels, leads to a deterioration of the sound, which is poorly accepted by users of local telephone systems.

As far as we know, the first proposal to have fewer voice transmission lines than the number of connected devices was made by British Patent Specification 1,083,312 (publ. 1967). The same idea is presented in French Pat.

The inventive advantages of our invention arise in that the call lines are material but are coordinated with each of a number of data channels in time multiplexes, which are led in a special line to all stations. You can thus get a very simple solution, with a variety of possible additional benefits, e.g. connection through a special code to an external line, conference calls, transfer of external calls between different local telephones, possibility of question calls during ongoing calls and even the possibility of connecting a question call via an external line.

In contrast to what applies to the system according to the mentioned French patent specification, it is not the exchange but the calling station itself that responds to calls and connects to a line ordered via the TDM line. A lot of other simplifications in the circuits are possible precisely through the original organization of local telephone systems according to the invention,

7313832-3

The above-mentioned and other objects and advantages are achieved according to the invention by a subscriber telephone system, which exhibits the features stated in claim 1.

Advantageously, all electronic means, coordinated with the individual subscriber telephones in such a system, can be accommodated in a single integrated circuit, designed as a unit separate from the telephone set, which can, however, be connected to suitable connection pins in the device via wires from the integrated circuit.

The various telephone sets in the system can then be considered to consist of three substantially different parts, namely the telephone set itself, preferably of a type with a built-in buzzer or the like instead of the usual bell, partly a suitable switch means and partly a logic unit in the form of an integrated circuit. The three parts can be placed separately from each other, or they can be brought together inside the phone cover, depending on the number of call channels, manufacturing technical reasons, etc. The electronics part can also be located in the housing, at the same time as the switch means is located in a special switch drawer unit. In the latter case, a switch drawer unit can contain several switches, each of them coordinated with one of the units in an apparatus group. to fit a number of different installation configurations. When the electronic logic means and the switch means are located together with the corresponding telephone, e.g. the wires, which lead call channels, data main channel, power supply, etc., are connected in a series loop from the device to. apparatus. When, on the other hand, the switch means, which are coordinated with resp. telephones in a group, are placed separately in a common box, this can function as a distribution point, to which, respectively, telephones with associated electronics can be connected in star form.

If a number of such star-shaped groups exist, the wires forming call channels, data main channel, etc., can be connected in series loop from the central control unit via the various switch boxes, or also in star form from the central control unit to each of the switch boxes.

The central control unit suitably contains a unit, which contains means for. to generate signals for data channel synchronization.

For house-type telephone installations, all operating stations consist of telephones or the like, while in subscriber exchange-type installations, some of the operating stations consist of terminating circuits or the like on incoming telephone lines, and these are suitably also located at the central control unit. j

In a suitable embodiment of the invention, each of

7313832-3 ₄ The multiplexed channel channels are divided in time to create a number of tracks for binary digits within each time channel period. A group of these time traces is reserved for the transmission of binary codes, which represent digital address information. The remaining time tracks are reserved for signal transmission^-, and control data. The function is such that a data channel is used for a call, and in order to allow each device affected by a call to recognize the data channel assigned to it, the control logic in each device contains means for registering which number it itself has. , and also to register the identity of the data channel used for the call. By arranging the control logic in each device so that it continuously counts the data channels, as they occur in each data channel group, and also continuously reads the data words coming in via the data main line, the recognition of a relevant data channel can be achieved either by detecting coincidence between the the registered spelling address and the address obtained in a data channel, or by detecting the behavior of the data channel used for the call. The recognition method used will initially be determined by whether the station is calling or being called. Once a station in the system has answered a call by recognizing its own address / identity, it will also register the identity of the data channel used for the call, and because the call channels in. the system is mutually coordinated with the data channels, by registering the identity of a data channel also necessary information is obtained for setting the switch means to the call channel to be used for the call.

When the telephone system also provides access to one or more lines or connections to private subscriber exchanges, these lines end in resp. line circuits, and to each of them a separate call channel can be assigned. For each such line, there can thus be one of the time-multiplexed data channels which exclusively belong to this line.

In addition to the digital address data, for which the address part of a data time channel is intended, as mentioned above, control information is also indicated as to whether the corresponding call channel is busy or free, as well as the same information on the condition of the subscriber or subscribers being called. This indication occurs through the presence or absence of a one signal in the resp. tracks reserved for the transmission of control information. In this way, data channels assigned to incoming and outgoing lines can be distinguished from those assigned to serve internal calls, by reserving a control track in each data channel to advance a binary digit. denoting the line type. Then, from 7313832-3, the presence of a signal in this time track means that the time channel serves an internal line, for example, while the presence of a binary digit in this time track means a time channel for a line from the outside. Correspondingly, other time tracks in the control part can be assigned to present other information, e.g. called station busy, the signals go to the called station that rings, called station answers (picks up the handset), conditions, etc.

Discrimination regarding the service desired when a call is initiated can be indicated by giving a specified first digit, which first digit is prohibited as the first digit in an internal station number. In this way, a subscriber who wishes to make a call via an external line can dial the number 9, for example, in order for his station control equipment to scan the time channels in the order in which they occur and seize the first free external line, ie someone in which a a signal is found in the line type track, while there is no one in the track busy / free.

If there is no free line, this can be detected after a complete period of the data channel and a busy signal is output to the caller's station. For an internal call, only the digits of the searched station's numbers are dialed and in such a case the calling station's control equipment searches for a free internal line data channel, ie one in which there are no binary digits • in either the exchange or busy / free-time track.

To make a call to an external line, they can dial. lines identifying the desired subscriber are transmitted between the calling station and the connection circuit to the appropriate outgoing line, one digit at a time, during different appearances of the corresponding output line data channel, so that the digits are transmitted in binary coded bytes in appropriate time tracks in the time channel.

The above and other desirable and possible features of the invention appear from the following embodiment description, in which Fig. 1 illustrates an example of a subscriber exchange telephone system, Fig. 2 shows a possible distribution of the time tracks in a data channel in the system and Fig. 3 in block diagram form shows the embodiment of a telephone exchange with associated electronic logic means, as well as coordinated switch means.

The intercom system, schematically shown in Fig. 1, contains a central control unit CN, an alarm clock BR ​​for calls incoming from the line, and a coordinated logic element BL, a number of telephone stations, designated station 1, station 2 ... station N, and a multi-pair cable, through which the stations, the central control unit and the BR clock are connected. The depicted system has the various stations of the system and the central control unit connected in series with the multi-pair cable routed in a loop from station to station and to the central control unit. However, other coupling methods can be off

7313832-3 star type is conceivable, so that the multi-pair <cable is branched, and each branch 'can then supply one or more stations in the system, without the need to change the station or the central unit.

As will be appreciated from the following description, the number of stations that can be installed is determined only by the quality of the service desired, and by the scope of the number system used for station selection.¹ For the sake of clarity, a number system is adopted, which gives a maximum number of

100 number. However, since the system requires the first three digits (eg digits 8, 9 and 0) to be used for call discrimination, in this case the maximum number of stations using two digits is limited to 70.

The telephone stations themselves comprise devices, which can be of any known shape, and which in addition to the usual call transmission circuits STC are designed to accommodate an electronic station control circuit, ESC in Fig. 1. The device can also advantageously be equipped with a keypad for dialing, hereinafter referred to as keypad (KF) instead of the finger dial mechanism, and a TR3 dial instead of the standard telephone clock. The apparatus further optionally has an additional push button switch (not shown in the figure), to allow a line holding condition, as will be described in the following. Each apparatus also has a line switching switch LCS, which may advantageously consist of a group of multi-contact miniature relays, which are connected in the well-known matrix form, as seen in Fig. 1 (groups SCP1, SCP2 ... SCPN). As will be described in detail below, these switches can connect the station wires T and R to any of a number of wire pairs contained in the multi-pair cable and extending between the respective wires. stations and the central control unit CN.

For installations that require a relatively small number of connecting wire pairs »eg, 'up to eight pairs, the relays that form the line connection switch can also be built into resp. telephones. For larger installations, these relays are placed separately, which will also be described in more detail.

The exemplary embodiment in Fig. 1 has eight connecting wire pairs, as seen. Three of these are intended to serve only local calls between the system's stations and are designated resp. local link 1, local link 2 and local link 3. The remaining five link pairs serve exclusively as mediators of outgoing and incoming external calls and are called exchange link 1 r 5. This distribution is of course completely arbitrarily chosen and can be chosen differently, without the need to change station control circuits. More or fewer connection links he built-in, depending entirely on the size of the anticipated telephone traffic. In addition to the link wire pair are the stations and the exchange

7313832-3 connected via a data main channel DAH, which advantageously consists of an additional pair of wires to prevent crosstalk difficulties, and three power supply lines, of which one year common ground return line and the other two supply, one 50 V DC for the relays' power supply and the other 3Q V direct current for power supply to the stations' electronics. The latter also has a superimposed AC component of a fixed frequency, which serves as a time standard for the system stations.

A relay switch that can connect any of sixteen pair of link wires can be made to fit in the phone cover.

The central control unit CN contains the equipment that is common to the system's stations. It also contains the power sources DCP, which supply 50 V and 30 V DC, a control oscillator OSC, which generates the time marking frequency, and a generator of the synchronization pattern SPG. The output of the latter is connected to the data main line DAH and serves to maintain the alignment of the stations in the correct time division multiplex, in the manner described below. The central control unit can also contain a monitor for the system, called the system monitor SYM. This equipment can be designed as needed. The length of the calls and the identity of the stations concerned can e.g. need to be registered and memory and timing equipment for this is then placed in the monitor part of the central control unit. In addition, the monitor may contain a contact field for jumpering, so that undeveloped stations in a facility provide a busy signal during calls. Busy signal can also be generated in the calling station, if no confirmation is received. A contact field for connection of various management devices (eg preference), and the like can be inserted if necessary. Finally, the central control unit contains the switching circuits ELC, which form intermediate stages between the switching links, partly the internal links, partly the lines to the public telephone network or the connection lines to other subscriber exchanges (PBX). These switching circuits are designed as replaceable circuit boards, as in Fig. 1. External lines and corresponding switching links end on contact pins (not drawn), corresponding to resp. circuit board. As will also be described in more detail, the connection is such that the insertion of a circuit board for line in a circuit board contact in the switching circuit part of the central control unit causes the corresponding internal link to serve only external calls, using this particular switching circuit and switching link.

In the same way, the removal of such a circuit board restores the corresponding link to serve internal calls. As shown in Fig. 1, in each line switching circuit there are means for detecting incoming calls from the line, call and pulse transmitter means for establishing outgoing line calls, holding means for holding a call on

7313832-3 β line, e.g. to enable an interrogation call, as well as logic means for providing appropriate signals to the data main channel DAH.

The data main channel DAH serves to transmit both control and address data to the central control unit and to the stations of the system, this data being multiplexed on a time division basis. In the exemplary embodiment, this takes place via a cyclically recurring data channel group consisting of 17 "data time channels, each divided into 12 data time tracks. Of the 17 data time channels, one is used to transmit a synchronization pattern of data bits transmitted by the generator SPG in the 17th data time channel. The remaining sixteen data channels each respond to the call pairs, which unite the central control unit and all the stations in the system. In this way, the system can suffice for a maximum number of sixteen call pairs. Fig. 1 shows only eight call pairs, and when there are less than sixteen connected or in use, the redundant ones are blocked by the monitor in the manner described below. It can be pointed out that since the data transmission in time data multiplex works in relation to the call channels of the system and not in relation to the stations themselves, the number of stations that can be connected is in no way limited by the data transmission system.

Furthermore, the recurring frequency of the data channels can be quite low, of the order of 1000 channels per second, so that, with a reasonably moderate length of the connections, the system can be synchronized from the central control unit, without cumbersome devices to compensate for time delay in the data main channel. Thus, the multi-pair cable can be installed in a building in a single stretch from which the stations are connected T-shaped, either individually or in groups, and the central control unit is set up in a suitable place anywhere in the wiring, taking into account the maximum allowable distance, about 2000 meters, between the central control unit and the one from this most remote station. Furthermore, the multi-pair cable can be connected to sockets, where necessary, along the cable route and each telephone set can then be provided with a wall socket socket, which can be connected to any socket along the cable route. Each device has its own digital address, which is independent of its location. Thus, the location of the telephones can be changed without the need to make any changes to the system, and the telephones retain their old number. Each telephone can be arranged so that it stores its own address and plug-in devices are conceivable, e.g. in the form of plugs or cards, which are inserted into the outside of the device cover to set the device for a specific address, so that the number of the device can be changed simply by_jto replace the plug-in device.

7313832-3

Fig. 2 shows which data bit combinations can occur in the twelve data time tracks in a data time channel during the establishment of local and external calls. The time tracks are numbered 1-12, in the order in which they occur. Time tracks 1-5 are reserved for control data bits, while time tracks 6-12 carry address data bits. Each of the control data bits has a special meaning as follows:

Time track 1. Line marking. The presence of a one-signal in this time track j. Any channel indicates that the corresponding wire pair is leased for a line via a line exchange card in the central control unit (see Fig. 1). The one signal is maintained at all times by the logic element in the gear circuit. The presence of this one signal prevents this time channel from being used for an internal call, but, as described in detail below, this data channel and its assigned wire pairs are used in internal interrogation calls, whereby a retention condition is applied to the corresponding outer line.

Time track 2. Answer bit. The presence of a one-signal in this time track in a data channel indicates that a called subscriber has lifted the microphone to answer. When this one signal occurs, all tone signals at the calling station cease and all address data in the address portion of the time channel is eliminated.

Time track 3. Confirmation / call. In the case of internal calls, the occurrence of a one-signal in this track means that a called station is idle and on call. As will be described later, a tone generator built into the telephone is activated when a station senses that it is being called, and at the same time it sets the dial bit in this track in the data channel. If the system is provided with a common signal clock to indicate incoming calls from the line, the presence of a single signal in this track in the corresponding line time channel causes the common clock to ring. The system is arranged in such a way that any station can answer the incoming call, which is done by pressing the number 0 in the keypad.

Time track 4. Channel busy. When a microphone is raised, the dial tone is generated within the station. Depending on the value of the key first pressed, the control equipment of the station determines whether a local connection or a line is desired. The first suitable free time channel is held and busy is marked by inserting a one signal in this time track.

Time track 5. Line retention. Each station has a push-button switch for holding, so that a station connected in conversation with a line can let a holding condition be exercised on the lij via assigned switching circuit. In this way, by pressing the hold button, a single signal is inserted during this time.^rtrace, and detection of this one signal by the logic of the shift circuit causes a hold relay to operate in the shift circuit, which interrupts the call path and applies a hold condition to the line. As explained in more detail below, the system is arranged so that a further depressing of the hold button and the concomitant occurrence of a single signal in this time track causes the line to be recovered.

. As already described, the address data bits are located in time tracks a 6 -.12. These are divided into two groups of three and four pieces respectively. The three bits in time tracks 6, 7 and 8 are coded to represent the first digit of an internal station address. Only three bits are needed, because, as mentioned, the digits 0, 8 and 9 are reserved as discriminatory digits and thus can not appear as first digits in station numbers. The remaining four bits, which are in time tracks 9, 10, 11 and 12, are coded to represent the second station address number.

In this way, the two digits in a station address appear together in successive appearances of the same data time channel.

In the case of a number for calls on the line, time slots 9-12 serve to transmit the number, one digit at a time, in successive occurrences of the respective data time channel, the numbers being registered in the switching unit and repeated as pulse trains, signals at speech frequency (300- 3OOO Hz) or in the manner required in the public telephone network switchboard.

The pulse sequences shown in Fig. 2 illustrate the manner in which data is presented to the control equipment in stations participating in a call. Pulse sequences a - f are those that occur in local calls, while pulse sequences g - s are those that occur during calls on the line.

Let us e.g. assume that station 17 calls station 28. In the manner described later, lifting the handset to station 17 causes a dial tone to be generated in the station, and the caller dials the first digit of the desired number. Station 17's control equipment registers the entered number and finds from it that a local call is requested. Station 17's control equipment now tests the various data time channels each time they occur and maintains the first free time channel for local calls, ie. such that no one signal is present in either time track 1 or 4 (pulse sequence a). If, after searching an entire data channel group, no idle time channel for local calls has been found, station 17's control equipment will generate a barring tone, to notify the caller that there is no idle inline line. However, if an idle internal time channel is encountered, it will be held for the call, and a single signal is inserted in tracks (see pulse sequence b), in this and all subsequent occurrences of this time channel, as long as the call is in progress, to block it from intruding on others. calling. At the same time, and only this first time that the time channel appears, the calling station 17 inserts its own identity, in binary code, in the intended time tracks 6 - 12, according to the dotted representation in pulse sequence b. This information does not matter in the establishment of the call but is added to enable call recording in the central control unit, if needed. As will be explained in more detail later, station 17 also registers the identity of the time channel, in relation to its ordinal number in the data channel group of the time data multiplex, in order to enable the appropriate data time channel to be recognized when it subsequently appears in the data channel groups. This task also provides the necessary information for setting the station's switch to the two-wire link to be used for the call. The caller now presses number two in the desired number, which is also registered within the station's control equipment, so that when the retained time channel next appears, the two digits in the desired number in binary code can be entered in appropriate time tracks 6-12, according to pulse sequence in Fig. 2. The control equipment of the station 17 inserts this number signal into only five consecutive occurrences of this time channel, and during this time the station awaits confirmation from the called station 28.

Each station in the system is arranged so that it continuously scans the data time channels, as they come, and responds to each data time channel, the time traces of which in the address part give the station's own numerical identity. Thus, when station 28 detects the presence of its own numerical identity in the time channel used for this call, it will lock itself to this time channel by registering its number. This also provides the necessary information to set station 28's switch to the corresponding two-wire call link. If station 28 is idle, its control equipment responds by generating a buzzer signal to provide an audible signal, to notify the incoming call, and by inserting a one signal in time track 3 as confirmation, the next time the data time channel appears. If station 28 is already busy in another call, it will refrain from transmitting such a one signal, and no buzzer signal is generated.

If at the dialing station 17, during the five consecutive occurrences of this data time channel, no single signal is detected in time track 3? will, as shown in pulse sequence d, the control equipment in

7313832-3 ' 12 ^rstation 17 to clear all information from the time channel, so that it becomes free for other calls, and at the same time it initiates a busy signal, which remains until the microphone in station 17 is turned on. However, if the called station 28 is idle, during any of the five consecutive occurrences of the data time channel, one signal time track 3 will be detected. When station 17 detects a signal in time track 3, its control equipment will generate a ringtone to indicate that the called station has been called.

When the microphone, in station 28, is raised, its buzzer signal ends, and the next time the data time channel appears, station 28's control device inserts a single signal in time track 2. See pulse sequence f. , to be used for the call.

At station 17, the detection of one signal in time tracks in the data channel causes the ring signal to cease, and the switch to connect the two-wire link to be used for the call, as well as remove all address information from the address portion of the data time channel, and conversation may follow. The dialing station holds the one signal in time track 4, and the called station holds the one signals in the acknowledgment and response time tracks 3 and 3, respectively. 2. Both stations must now find confirmation and busy signals in the respective data time tracks. If any of these are not detected, the station control circuit is reset, the connection is lost and a dial tone is generated. In this way, the call can be disconnected from any side by restarting the microphone.

At any time, after the conversation has started, it is possible to set up a conference call via the same data time channel and link that was used for the original two-participant call. Suppose, for example, that the calling station 17 wants to call in a third station. To do so, the discriminating key 8 is pressed at the calling station 17. This is detected by station 17s control apparatus, which responds by resetting its own address number registers. The number digits of the desired station are now entered, the corresponding binary code being inserted in the time slots 6-12 in the data time channel, the next time it appears. In the manner just described, this address identity is recognized by the third party station, and a buzzer signal is initiated thereon. However, since the calling station 17 is already connected to the called station 28, no ring tones or busy tones corresponding to the called third-party station are obtained. When the third-party station microphone is picked up, its switch is set to the two-way link used for the call, so that it

7313832-3

Γ the third subscriber can answer orally. If no oral answer is received, or if a fourth station is desired to be called, the procedure described above is repeated, pressing the discriminating key 8 resets station 17's address register and completes it to receive the fourth station number. This procedure can be repeated to connect additional stations, as needed.

In the present time data multiplex system (TDM), the time channels intended for connection to a line are marked with a one signal in data time track 1. To establish an outgoing call from a station within the system, the key 9 is pressed to prepare the station for an outgoing calls, so that the control circuit detects a free data time channel for line calls, ie. a channel free from data except the one signal in time track 1. If, after traversing an entire data channel group, no free time channel for line calls is detected, a special busy signal is generated in the caller's station, to indicate that all lines are busy. However, if a free data time channel for line calls is detected, it is maintained by inserting a busy signal into the time slot 4 of the channel, together with the identity of the calling station, which is later inserted only once, as in the local call case. See pulse sequence h. The switch is also set to the corresponding gear link, and call status is applied to the line. The call now continues under the control of the public telephone network switchboard. Thus, when the switching tone of the exchange is obtained, the desired number is pressed digit by digit, the digits, one after the other, being transmitted in binary code via time tracks 6 - 12. Each such code signal is given only in a single data time channel, as previously described. See pulse sequence in, which shows an example with the number 6 inserted in binary code. In the central control unit, the register transmitter element in the corresponding switching circuit registers the digital code corresponding to the desired number and transmits the digits to the general switching in this suitable manner, e.g. in the form of impulse trains, multi-frequency signals or the like. The call now continues under the control of the general exchange, while the relevant data time channel is marked occupied by continued insertion of the one signal in the time track 4 from the station's control equipment.

This one signal disappears at the end of the call, when the caller's microphone is hung up again.

At any time during a line call, the caller can press the hold button in his station to establish a hold condition on the line via the corresponding line exchange unit in the central control unit. Thus, the depression of holding 7313832-3 causes ι4- - -.

Γ button, that the station control equipment removes the one signal for recording from time track * + and instead inserts a one signal for holding in time track 5 in the data time channel used, which is shown in waveform k. The control unit also generates a special dial tone to indicate that the caller now can make the required answer call, and if required, a warning light on the device can also be lit. In the switching circuit of the central control unit, detection of the one signal in time track 5 causes the switching of a bistable element, which, when switched, activates a holding relay in the switching circuit. Switching on the holding relay interrupts the speech path to the line and places a suitable line holding end on the line.

The caller can establish an internal query call, using the same data time channel used for the original line call, or he can establish a new line call by using a new data time channel for line calls. In case of an internal question call, e.g. to station 28, the caller dials the desired number, and at the next occurrence of the data time channel for line calls, the binary codes corresponding to these digits are inserted in time slots 6-12, as well as in pulse housing 1. As previously described for local calls, the digital codes are maintained during five appearances of the time channel, while it. the calling station waits for the occurrence of a one-signal in time track 3 · If no such one-signal is detected during this time,. digital data is cleared from the address portion of the time channel, and a busy tone is generated within the calling station. See pulse sequence m. If a signal is detected in the time track 3 »as in pulse sequence n, a ring tone is generated in the caller's apparatus to indicate that the called interrogation station is being called. Lifting the microphone at station 28 causes one signal to be inserted in time track 2 in the time channel, which is shown in pulse sequence p, and station 28's switch connects the exchange link, which prepares the call path for the interrogation call. At the calling station, upon detection of a signal in time track 2, the digital address data is deleted from the address part of the time channel, and the query conversation can now follow.

When the interrogation is completed, the caller can now return to the original line call, or he can transfer the line call to the interrogation station. In the former case the interrogation station is disconnected, while in the latter case the calling station is disconnected.

If the called interrogation station is disconnected, the calling station can retrieve the line call by pressing the station's hold button. The control device of the station then removes the one signal for holding from the time track J and reinserts the one signal for recording in time track 4 in the data time channel according to pulse sequence q.

7313832-3 ^rSwitching circuit for line, the change from time track 5 to time track 4 of the one signal will cause the bistable element in it to be reset, whereby the holding relay is disengaged to reset the call path and remove the holding condition from the line.

For interrogation using a line, the caller proceeds exactly as described above for the first line call, ie. he dials the prefix digit 9, to occupy a second time channel for line calls, over which he dials the number of the subscriber he wishes to request.

At any time during the interrogation conversation, by pressing the station's hold button, a hold condition is applied to the second line, in exactly the same way as described for the first line, and as in this case the caller receives the locally generated, special dial tone, after which he can connect a local interrogation call, using the time channel to line number two, in exactly the same way as described above for the first internal interrogation call. If the called internal interrogation station is disconnected, the caller again receives the special dial tone, and he can then connect an internal interrogation call number two, and he can repeat this as many times as he wants. Whether the caller connects to an internal call or not, he can retrieve the first line by pressing the station's hold button. It can be noted that the previous pressing of the hold button led to the caller receiving a special dial tone, and that a residence condition was applied to line number two by exchanging the data bits for busy and hold with the respective binary value of 0 resp. 1. By re-using the hold button, the busy bit and the hold bit are exchanged again, and they get resp. values ​​1 and 0. In this position it can be arranged so that the first held line is found in the above-mentioned manner. However, if the caller wishes to find line number two, he can repeat the sequence of operations on the hold button. Ie. the hold button is depressed to reapply the hold condition on line number one, and then the hold button is pressed again to locate the hold line number two. When the desired line is found, the conversation can continue, as previously described, and when this call is terminated, the caller must reset the microphone to disconnect the public telephone network equipment used for the call. A feature of the proposed system, however, is that if the caller resets the microphone, either out of danger or out of necessity, while still maintaining a line, the buzzer of the device will automatically start, to give an audible indication of the situation. When the station's microphone is raised again, the caller again receives the special dial tone, and by pressing the station's hold button again, the held line is regained. At the end of this call, the microphone is reset, which restores both the general telephone exchange and the system equipment to normal condition.

For incoming calls to the system, a common bell can be arranged, the system being arranged so that any station in the system can answer the call by pressing the number 0. If one call is received via one of the lines in Fig. 1, this will is detected by the call detector in the line circuit unit corresponding to this line, whereby a single signal is inserted in time track 3 in the data time channel coordinated with the line, which serves as a call time track in line calls. See pulse sequence r. The common bell is connected to the data main line via a logic element, which responds to the combination 1,0,1,0,0 (according to pulse sequence r) in the control part of data time channels, and then it gives rise to the common bell calling. Pressing the number “0 in any station causes the control equipment of that station to look up the time channel, which has. this combination in the control part, and when it is detected, the control equipment of the station is locked on this time channel, as previously described, and it places a busy one-signal in its time track 4. This is detected by the logic element in the corresponding line circuit unit, which responds by removing the call the one signal from time track 3 », whereby the common bell stops ringing, and when the answering machine's microphone is picked up, this station is connected to the calling line. See pulse sequence p. The answering station can in the future make transfer or interrogation calls by pressing the station's hold button, as previously described.

Fig. 3 shows in block diagram form, sufficiently detailed for the operation of the system to be clearly understood, the control equipment in a station. Incoming from the left in the diagram, the supply lines SL1, SL2, SL3 and the data main channel DAH are shown. As previously mentioned, the line SL1 is a common line for the DC power supplies, the SL2 is the 50 V power line for the relays that form the link switch LCS, and the conductor SL3 transmits the DC power supply (30 V) to the electronic control equipment and a superimposed control. , fr sequence.

As already described with reference to Fig. 1, these supply lines are routed from sources located within the central control unit. The 30 V DC supply is separated from the conductor SL3

7313832-3 ^rand the AC time signal by means of the filter element FE, from the output of which the voltage supply of 30 V DC is taken, and the capacitor CA is applied to a locked oscillator PLO. In this way, the oscillators of the different stations will oscillate in step with each other and also within tight tolerances in phase with each other and with the main oscillator in the central control unit. The output from the oscillator is applied to a clock pulse generator CPG, which delivers timing pulses with the bit or data time track frequency in the time data multiplex system. The time pulses CK1 fall in the center of each data time track, while the time pulses CK2 mark the beginning of each data time channel.

These time pulses are used in station electronics to control the reading and writing of data in the main data channel DAH, in a well known manner. The time pulses CK2 also drive two counting chains CN1 and CN2 in tandem. The counting chain CN1 contains a number of counting steps, which is equal to the number of time tracks in a data time channel in the TDM system, ie 12 steps in the present exemplary embodiment. Each time the counter CN1 assumes its twelfth position, it delivers an output clock pulse CK.3, which marks the beginning of the twelfth time track in each time channel. Likewise, each time it switches from its twelfth to its first position, it gives an output driving pulse to step the counter CN2, so that this counter is stepped at the beginning of time track 1 in each time channel. The calculation chain CN2 has a number of calculation positions, which are equal to. the number of time channels in a data channel group in the TDM system, ie 17 steps in the present case. Time channels 1-16 are data time channels in the TDM system, and as these are counted, the counter delivers code markers in a group of four outputs CML, to mark in binary code the numerical identity of the Output channels, as they are counted, in relation to the data channel group. The seventeenth time channel in each data channel group is a carrier of a synchronization meadow pattern, which is transmitted from the central control unit via the data main line in the manner previously described. In the station control apparatus, the synchronization pattern is detected by a synchronization pattern detector element SPD, whose input is connected to the data main line DAH, and whose output signal is a pulse, which occurs at the end of the seventeenth time channel, and which is applied to reset the counters CN1 and CN2. Thus, it is ensured that the counters CN1 and CN2 can never be out of step with corresponding counting chains in other stations in the system for longer than the time of a data channel group in the TDM system.

The data main line DAH is also connected to the input of a receiving access register SR1 and to the output of a sending shift register SR2. During each time channel period, data bits are read in

7313832-3 ’

'the receiving shift register SR1, in order ,, under control of the bit sensor clock pulses CK1. They are read out in parallel during the time of time track 12 in the time channel under the control of the clock pulses CK3. During the time of the twelfth time track in each channel, the control data bits in the time channel will thus come out in outputs 1-5, while the station address data bits will come out in outputs 6-12 in the receiving shift register. The control data bits pass via the buffer amplifiers BUA1 to a logic element LEM. This logic element consists of simple electronic switch and discrimination means, which can take various known forms and are therefore not described in detail. However, its functions are clarified by the following description. The address data bits pass via the buffer amplifiers BUA2 to one end of a comparator element COMP1, at the other end of which the own number register ONS is applied. This own number register stores the address data bits which identify the station, and it is arranged in such a way that a station identity can be connected by means of a jumper or the like, e.g. by using a plug-in printed circuit board or by screwing in special connection plugs, or in another way, so that the station number can be established or changed from the outside of the device. The comparator COMP1 gives a yes output signal to the logic element LEM via the conductor LTI, as soon as it detects coincidence between a received data address and the address of its own number. The outputs from the own mimmftrrflgl straw ONS are also connected to the respective inputs on a coincidence set consisting of two gates, marked with the rectangle GSA. The second input of these gates is supplied via the common output line L01 from the logic element LEM.

The sending shift register SR2 receives data in parallel form during time track 1 in a time channel and reads out the same data in the channel time track 2-12, under control of the clock pulses CK2. It may be recalled that any one signal in time track 1 is the line mark which is inserted when the central control unit is required, and consequently the station control equipment never has to put any one signal in time track

1. The control data bits 2-5 are applied to the corresponding inputs on the shift register SR2 via the line group LCD .. The address data bits 6-12 are applied to the corresponding inputs from the gate data GSA, via the lines LCA, or from the keypad in the manner described below.

The station's numeric keypad is denoted in Fig. 3 by KP. When a key is pressed, the keypad delivers a binary code in parallel form via the four code lines DML. The code markings pass via a restraining element ABC, which may be of a known type, to the logic element LEM, which discriminates with respect to the provided 7313832-3^rtake number, which is entered. The digital code also goes to a keypad memory KPS, which stores the two digits in a station number, after which they are dialed, and it also has code outputs, which represent the two digits over the seven outgoing conductors SDL. These leaders are arranged in groups of resp. three and four, to present the respective codes of the first and second digits. The outputs from the keypad memory are also applied to the respective inputs on a further set of coincidence gates GSB, each with a second input connected via the conductor L02 to the logic element LEM.

The logic element further comprises a ring for creating a ring signal, the output of which is applied via the conductor L03 to start the station's bell TR3, if necessary, and further there are means for generating other tone signals, such as dial tone, busy tone, etc., and these are applied via leader LQ^!+ to the transmission tansformer CTR to switch the tones to the station wires T and R, tj11 to which the voice transmission circuit TTE is connected, so that the user hears these tones in the telephone in the microphone. The station also has a standard hook contact HS, which closes, when the handset is picked up, to apply a ground signal to the logic element via conductor L12, and a push button switch PB, which when pressed, applies another ground signal to the logic element via conductor L13 ·

The numerical channel identities, which are led from the channel counter CN2 via the code marking conductors CML, are applied to the channel number memories, three of which, designated CNS1, CNS2 and CNS3, are plotted in Fig. 3. These codes are also applied to a comparator-relevant C0MP2. A logic switch element LSE is coordinated with the channel number memories and with the comparator C0MP2 and operates under the control of signals, which are passed via the conductors SL1, SL2 and SL3 from the logic element LEM, to coordinate the comparator with a specific channel number memory and also to command this memory to read in, the station being locked to the channel identity which is just passing at the time when the signal is obtained from the logic element LEM. In this way, at each subsequent occurrence of this time channel, the comparator will respond with an output yes signal to the logic element LEM, in that the comparator finds the coincidence between the channel identity which it receives from the channel counter CN2 and the one stored therein. coordinated channel number memory. The outputs of the channel number ranges are also connected to a set of coincidence gates GSC, the second inputs of which are all connected via a signal conductor L06 to the logic element LEM, and whose outputs are transmitted to the link switching switch LCS, which is able to connect the station wires T and R to the

7313832-3 ^Pthe link pairs ELI - LL3 and ELI - ELJ, which correspond to the relevant data time channel. The general mode of operation of the station control devices depicted in Fig. 3 will now be described. First, assume that the station is being used to establish a call to another of the system's stations. Elevating the microphone closes the hook switch contacts HS, which applies a call signal via the conductor LI2 to the logic element LEM. The reception of this signal causes the logic element to generate its own dial tone, which it applies to the device via conductor L04, and when the caller hears this, he dials the two digits of the desired number. Corresponding binary codes are successively applied from the keypad via the code lines DML and the rebound element ABC on the logic element LEM and the keypad memory KPS, which stores the entered digits. The logical element LEM examines the value of the first digit to check that it is not one of the prefix digits 8, 9 or 0, and finds that an internal call is requested. As a result, the logic element is set to scan the control data bits in each channel in the TDM system, after which they are read out from the receiving shift register SR1 via the buffer amplifiers BUA1, and respond for the first free time channel for local calls, ie. a channel, whose five control bits all have the value 0. If after reviewing a complete data channel group no such channel is found, the logic unit generates a signal that the system is busy, via conductor L04, and at the same time a reset signal is supplied via conductor LOJ to keypad memory KPS. However, if a free data channel for internal calls is found, the logic element responds by outputting a signal via the conductor SL1 to command the first channel number memory CNS1 to register the identity of this time channel and connect the comparator C0MP2 to this memory. As mentioned, the data bits from one of the time channels in the system are read from the receiving shift register SR1 during time track 12 in the data channel, while the channel counter CN2 is stepped during time track 1 in the subsequent time channel, so that detection and registration of a free time channel must be performed during the twelfth time year. in the relevant data channel. However, since the various elements included in a control apparatus of a station form a single fast integrated circuit, these requirements for speed can be satisfied as required.

Here, the next used channel appears, the comparator C0MP2 will find the coincidence between the identity stored in the channel number memory CNS1 and the one supplied from the output of the counter CN2. It then responds in the first time track in the time channel period

7313832-3 by sending a yes signal to the logic element LEM via the conductor LÄ ·. In the sequence of operations which the logic element is now set up to follow, the reception of the yes signal from the C0MP2 causes the logic element to send a control signal via the conductor L01, during the time of a single time track, to derive the station's own identity, which is stored in the own number register ONS to the inputs 6-12 in the corresponding shift register SR2. At the same time, the logic element emits a one-signal for busy to input 4 · in the sending shift register, and this one-signal is kept during the duration of the call. The information thus recorded in parallel form in the transmitting shift register is read out sequentially under the control of the clock pulse CK2 to the data main line DAH, in the time tracks 2-12 in the data channel used.

Upon receiving the yes signal from C0MP2, when this time channel next occurs, a signal from the logic element IEM is applied via the conductor L02 to open the gates GSB, to release the identity code of the called station from the keypad memory KPS to the inputs of the transmitting shift register 6-12, and this data, together with the busy signal, which is again inserted in input 4, is read out to the data main line. As mentioned, these data are repeated at the four subsequent occurrences of the data channel, or until the logic element detects a confirmation bit in the time slot 3 of the control part, when this is read in from the receiving shift register SR1. If such a signal in time track 3 is not detected during the four subsequent appearances of the data channel, the logic element generates a busy tone, which is transmitted via the conductor L04- to the station apparatus. At the same time, the logic element disconnects all data memories, including the channel number memory CNS1, so that the data channel is freed for other calls. When the microphone is switched on and the hook contact HS is broken, the generation of the busy tone stops. However, if the acknowledgment signal is detected before the end of the time for the next four data channel groups, the logic element will generate a local ringtone and apply it via conductor L04- to indicate to the caller that the called station is on call, as previously described.

When the handset in the called station is intercepted, to answer the call, an answer one signal appears in time track 2 in the control part of the data channel, and when this one signal is detected, the logic element emits a signal via the conductor L06 to open the gates GSC, passes to the link switching switch LCS, which in the manner previously described connects the station lines T and R to that of the wire pair LL1-LL3, which corresponds to the

7313832-3 ^rdata channel, used for the call. At the same time, a signal is applied over the conductor W5 to reset the keypad memory KPS, so that all the address data is removed from the address part of the time channel, and the call can now start. During the call, the calling station continues to insert the busy one-signal into time slot 4 in the time channel, while the called station continues to insert one-signals for acknowledgment and response in the respective time slots 3 and 2. Continued maintenance of this time channel depends on the maintenance of these three one-bits in both the dialing and dialing stations. Thus, if one of the stations is disconnected by turning on the microphone and the consequent opening of the clock contact HS, the logic element in that station will cease to insert its one / its signal (s), which will no longer be detected in the other station, which also will disconnect, releasing the data channel and recovering the local dial tone signal, until the microphone in this station is also reset.

At any time during the call, the caller may wish to call another station to make a conference call, and as mentioned before, he does so by first pressing the prefix digit 8 and then pressing the number digits of the desired station number two. When the logic detects the prefix digit 8, a signal is applied to the conductor LOJ to reset the keypad memory KPS, which is thus prepared to receive the number digits when they are entered on the station keypad. At the next occurrence of the time channel used for the call, the logic element inserts in the previously described manner a control signal via the conductor L02 to open the gates GSB, whereby address data in the keypad memory KPS is entered in the transmitting shift register SR1, and this data is read in the data main conductor DAH time tracks 6-12. Since the answer confirmation and busy one signals are also inserted in time slots 2-4 of the data channel, in order to maintain the original call, the confirmation and answer one signals, 'transmitted from station number two, are ineffective, and the caller must await oral response from called station number two. As also previously described, the described operation sequence can be repeated to summon additional stations to the conference call, and all the stations are then connected to the same two-wire link, which has been used for the original call, so that they can all participate. Stations called for a conference call can be disconnected from the call by placing the handset in the cradle.

The original call is not affected but can be maintained until all the dialed stations are disconnected, or until

7313832-3 ^rthe original caller disconnects.

As previously described, the receiving shift register constantly reads data from each time channel in the TDM system. If the station is called, the station's own address will be read out from the receiving shift register in the time channel used by the calling station. It will be applied via the buffer amplifiers BUA2 on the comparator COMP1, to which the station's own address is also applied from its own number register ONS. In response to the coincident addresses being applied, the comparator COMP1 will apply a yes signal to the logic element LEM, which responds by applying an output signal via one of the conductors SL1-SL3, depending on whether the station is already connected to another calls or other calls, to allow the channel identity received via the channel counter CN2 to be recorded in one of the vacant memories CNS1-CNS3. The memory output is connected to the comparator's C0MP2 output. At the same time, the logic element LEM is set to operate in call state, so that when the comparator C0MP2 responds to the next occurrence of this time channel, its output yes signal will command the logic element LEM to via the one of the conductors LCD, which is connected to step 3 in the transmitting shift register, apply a confirmation "" signal. At the same time, a tone ring signal starts, which via the conductor LO3 starts the station's bell TR3. During the reading of the transmitting shift register, the acknowledgment one signal is inserted in time track 3 in the time channel used for the call, the called station responding in the manner previously described. When the microphone of the called station is intercepted, the clock contact HS is closed to apply a signal via the conductor LI2 to the logic element LEM, which, if in call mode, responds by stopping the generation of the ring signal at the next occurrence of this time channel and entering a response signal. one bit in that of the conductors LCD, which leads to step 4 in the sending shift register SR2. Furthermore, a signal is applied via the conductor L06 to open the gates GSC to transmit the time channel code to the link switching switch LCS. In this case, a link pair is connected, which corresponds to the time channel used. In the manner described, the answer-one signal is inserted in time channel 4 in the time channel via the stepping function in the transmitting shift register to notify the calling station that the called station has answered, and the call can begin. If the called station was busy at the call, only the identity of the time channel over which it was called will be recorded. The clock signal and the answer one-bit are absent, and no further action is taken in response to the call until the calling station disconnects due to the busy tone which

7313832-3 is created in this in the manner previously described. This removes all address and control data from this time channel. Consequently, the channel number memory and thus coordinated logic, which has been used for the call at this station, is restored.

In the case of line calls, the control equipment functions from the beginning in much the same way as for internal calls, except that for a line call the first dialed digit is the prefix digit 9, in which the receiving logic element LEM is set in a line call state, where the time channels are searched to find a free time channel for line calls. One of these is characterized in that the control data bits in the first five time tracks have the configuration 00001. The one signal in time track 1 is the line marking, as previously described. If no free time channel for line calls is found during an entire data channel group, a busy signal is generated, which is transmitted to the device via the conductor L04. However, if such a channel is found, the identity of that channel is registered by the channel number memory CNS1, and the next time the channel appears, a busy one-signal is inserted in time track 4, and the station's own identity is signaled during a single occurrence of the channel. Likewise, a signal is transmitted via the LOJ conductor to reset the keypad memory KPS. A signal is passed through the conductor L06 to open the gates GSC, which transmit the channel identity code to the link switching switch LCS, which is then set to connect the station wires T and R to the switching pair (any of ELI - ELJ) corresponding to the used time channel. As previously explained, upon switching the busy signal in time channel 4, the switching circuit of the central control unit will connect the switching link to the relevant line, so that the public telephone network switch responds to the paging state transmitted from the calling station and delivers switching tones in the usual manner. When the caller hears the dial tone, he dials the desired subscriber number via the keypad memory KPS. At each occurrence of the data channel, which follows the reset of the keypad memory, the logic element LEM, upon receipt of the yes signal from the comparator,

C0MP2 via the conductor L14, to apply a signal via the conductor L02 to the gates GBS. Thus, when a digit is entered into the keypad KPS, it is read out in the subsequent time channel via the gates GSB to steps 9-12 in the transmitting shift register SR2, which inserts these data bits in the manner previously described, together with the busy one-signal in time track 4, · i corresponding time traces in the data channel, which then appear in the data main line DAH. As previously described, the digital code in time tracks 9-12 will be registered by the corresponding switching circuit in the central control unit and retransmitted, for example as an impulse train, via the line

7313832-3 'to the public telephone exchange. The caller now hears all tones coming from the PBX, and when the called subscriber answers, conversation can follow.

When a call is received via a line, the switching circuit belonging to the line responds to the call condition by inserting a dialing bit in time channel 3 in the relevant time channel. The control part of this time channel will then have the binary pattern 00101, when it appears in the data main line DAH, and the logic of the common bell reacts, whereby the common bell is started. If e.g. the station shown in FIG. 3j wants to answer the incoming call, the microphone is intercepted, whereby the hook switch contacts close. A signal is then output via the conductor LI2 to prepare the logic element EEM to receive the first digit, which is dialed on the keypad KP.

In this case, the discriminating digit 0 is depressed. Receiving this logic condition causes the logic element to transmit the data channels to search for it, the control part of which has the above-mentioned binary pattern. When this is detected, the logic element locks onto the channel by storing its identity in the channel number memory CNS1. The next time this time channel occurs, the logic element responds by transmitting its own number once and inserting a busy one-signal into time track 4 in the time channel. This is done via the outgoing shift register as above. At the same time, the logic element LEM emits a signal via the conductor L06 to open the gates GSC to connect the station to the exchange link to which the time channel corresponds. When the switching circuit in the central control unit detects the busy one-bit in time track 4- in its data channel, it removes the calling one from time track 3. can follow.

When this station is busy in a line call, either incoming or outgoing, the user can apply a hold condition to the line by pressing the hold key PB, as described above. When the hold button is now pressed, a signal is transmitted to the logic element via conductor LT3. thus in the code transmitted in the time channel. At the same time, a special switching tone is generated and applied via the conductor LOh ·. The user can now establish an internal query call to another of the system stations, as described above. The data time channel, whose identity is already used, was used

7313832-3 is stored in the channel number memory CNS1, and also the exchange link, to which the station is already connected via the link switching switch. The user can also choose to establish a question call via another of the lines available to the system. In that case, when he hears the special dial tone, he depresses the prefix number 9, the logic element finding and using the first free time channel for line calls, as described above. However, when in this case such a free data channel appears, the logic element via the conductor SL2 applies a signal to have this channel identity stored in the data channel memory CNS2, the logic switching element LSS coupling the comparator COMP'2 to both outputs of these memories. The inputs to the gates GSC are switched from the memory CNS1 to the memory CNS2, so that the link switching switch is first reset and then set on the gear link corresponding to the channel identity stored in the channel number memory CNS2. The call now proceeds in an analogous manner as for the original line call. Each time the data line of the first line, whose identity lies in the memory CNS1, passes, the comparator outputs a signal via the conductor L14 to the logic element LEM, the logic element responding by inserting a hold one signal in time track 5 in this time channel. Similarly, each time the second line data channel passes, a comparator signal will be supplied to the logic element LEM via the conductor LIJ, the logic element responding by continuing to insert a busy one-signal into time slot 4 in this second data channel.

If in the case of an internal interrogation call as above, the called station wants to transfer the original line call to the called interrogation station, this is done by shutting down the calling station's microphone, without the caller having to do anything more.

When the microphone is switched on, the splice signal is removed from the conductor LI2 by opening the contacts in the switch HS, whereby the station returns to idle mode with concomitant loss of the hold one signal from time track J in the time channel. In the called interrogation station, which already stores all the data for the call as above, the absence of the hold bit will be detected by the logical element, and this is perceived as a message that the called station has been disconnected and that the called station must now take over the line call. , which is done by inserting a busy bit in the time slot 4 of the time channel. In the switching circuit of the central control unit for the line, the deletion, ay of the hold set signal and the presence of the busy set signal in the corresponding time channel will cause removal of the hold condition, and the continuous call path between the line and the gear link is restored. The call can then continue between the called interrogation station and the subscriber on the line. However, if the called interrogation station did not want to receive the line call, this station would have been disconnected by resetting the microphone, so that the answer and acknowledgment bits, to which the station contributed, disappeared. The calling interrogation station is then informed by the absence of these bits that the called interrogation station has been disconnected, and the calling interrogation station returns to the state before the interrogation call, whereby the special dial tone is generated as above. If no more interrogation is required, the station can retrieve the line call by pressing the hold button PB again. A new signal then goes via the conductor LI3 to the logic element LEM, which responds by removing the hold data bit from time track J and reinserting the busy one signal in time track 4 in the time channel of the line, so that the line is switched via the switching circuit, as described above.

For interrogation calls via a line, the caller can apply a retention condition on this line in exactly the same way as described above for the original line call. He can then establish an internal interrogation using the time channel for this second line, perform transmission or return to the interrogation on the line, just as described for the internal interrogation using the first time channel. If the caller returns to the line query call, this call can be terminated by resetting the microphone. In this case, when another line is still held by the station, the reset of the microphone and the subsequent removal of the clock signal on the conductor LI2 causes the logic element LEM to remove its output signal via the conductor ££, 2, which resets the channel number memory CNS2 and the link switch LCS. The transmission of the busy one-bit in time track 4 · in this channel ceases.

Then the corresponding switching circuit in the central control unit returns to idle mode and disconnects the line and the equipment used for the call in the general telephone exchange. However, the station is not completely disconnected but generates the tone signal, which it applies to the station's bell TR3 to remind the caller that another line is still being held and needs to be addressed. When the caller hears the ring signal, he again picks up the microphone, which causes the splice signal to return to the conductor LI2. When this is detected, the ring signal stops, and the logic element applies signals to the conductors SLI and L06. The signal in the conductor SLI reconnects the gates GSC to channel 7313832-3 *

the memory CNS1, while the signal in the conductor L06 Opens the gates GSC to transfer the channel identity in the memory CNS1 to the link switching switch LCS, this switch switching on the link to which the channel identity corresponds. In addition, the special connection tone is generated and connected to the device via the conductor 10¹¼. If no more interrogation calls are required, the station's pushbutton PB is now pressed to retrieve the line call, as previously described.

Many different variations are conceivable within the scope of the invention. Some examples of such variations, which seem promising, are given below.

The number of time tracks in the time channels can be increased from 12 to 13 or 1¹¼. One of them can then be used for explicit busy marking from a called station. This avoids the need for described negative method, where a station is assumed to be busy, if it does not respond to five consecutive data channel groups, where it is called. The two second-new time slots can be used to allow the digits 8 and 0 to be used as first digits in the number scheme.

The tone signals (busy tone, barring tone, ring tone, etc.) can be generated centrally and distributed by superimposing on the power supply lines. All interrupted tones can be distributed in succession via a power supply line, with the station selecting the required one. An advantage of such central tone generation is that the electronics in the stations can be simplified. The system for conference calls can be improved by deleting the confirmation one-signal when a call is answered, so that an ongoing call is maintained only by the presence of the busy one-signal and the one-answer signal. This leaves the acknowledgment bit free to use when a third station is called for an ongoing call, so that tones indicating the state of the third station can be generated. Likewise, in that case, the identity of the third station can be removed from the time channel, when it answers, so that several stations can be called into the conference call.