CH672040A5 - - Google Patents

Description

DESCRIPTION The invention relates to an arrangement for signaling the status of at least one device via the telephone network. In particular, the invention relates to those systems in which the network is used in addition to monitor the conditions of one or more devices located in the premises of the subscriber.

The devices in question can be of different types. On the one hand, it can be a device for supplying an electrical alarm signal if an abnormal condition is detected in the subscriber's premises. Such an abnormal condition can include a fire, unauthorized entry, malfunction of any machine to be monitored, etc.

be. On the other hand, it may be a device that provides an electrical signal to indicate the status of any facility on the subscriber's premises, even if there is no abnormal condition. This can involve reading current measuring devices, meteorological measurements, operating data from processing plants, etc. Other types can also be considered.

The present invention is not the first to deal with such systems. In fact, a variety of such systems have been proposed. However, to the knowledge of the inventors, all of the systems proposed so far have had features which in one way or another have compromised their utility or convenience.

For example, some known systems used auxiliary signals that were audible to the subscriber when using the phone for ordinary conversation. This is not only annoying for the subscriber, but also conflicts with the technical standards of some telephone companies or government agencies.

Other such systems previously proposed have included the use of auxiliary signals that were above the audible frequency range. However, this required electrical filter devices within the system itself and possibly also within the normal telephone network, which are not only difficult to install, but are also expensive, and thus impaired the economy of the system.

Other suggestions simply sacrificed some desirable system functions to avoid the disadvantages listed above, such as reliable monitoring of facilities on the subscriber's premises or the like.

In contrast, it is a primary object of the present invention to provide a system which interacts with a conventional telephone network to provide additional functions, but in an improved manner compared to the systems previously proposed for the same general purpose.

It is another object of the present invention to provide such a system that does not have one or more of the disadvantages of the previously proposed systems.

It is a further object of the invention to provide such a system which does not use auxiliary signals which are audible to the subscriber during normal telephone operation.

It is another object of the invention to provide a system that does not use signals above the audible frequency range.

It is another object of the invention to provide a system which does not require special electrical filters within the ordinary telephone network.

It is a further object of the invention to provide a system which ensures a high degree of reliable monitoring of the devices in the premises of the subscriber.

These and other objects are achieved according to the present invention with the help of an arrangement according to claim 1.

At a location where a group of subscriber lines that are to be served by the system is easily accessible, a central control unit (hereinafter also referred to as an “interrogator”) is connected to all of these subscriber lines. On the premises of each participant, an individual electronic unit (hereinafter referred to as the "participant terminal" or "STU") is connected to the participant's line.

Various devices are connected to each STU, which are to be monitored by the system so that their output signals are available in electrical form in the STU.

s

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

672 040

In one embodiment, the interrogator interrogates the STUs at appropriate times using an audible signal within the audible range. The STUs respond by means of a similar signal to indicate the status of the devices on the premises of the corresponding subscriber.

This normal question and answer pattern is limited to times when the subscriber's phone is on-hook, i.e. when the subscriber is not using the phone for normal purposes, and is interrupted when the subscriber picks up the phone (i.e., a off-hook condition) is created).

A special signal with a frequency far below the audible range is also generated at the STU and transmitted to the interrogator via the telephone line, which of course also carries the other conventional telephone signals such as voice, dial tone, dial tone, busy signal, etc. This particular signal (hereinafter referred to as "low tone") is preferably present continuously, both when on-hook and off-hook, but only when all devices in the particular subscriber's premises are within their predetermined normal or safe operating range .

If one or more of these devices deviate from this range, i.e. assume a dangerous state or an alarm state, the generation of the low tone is interrupted within the STU. Such an interruption of the low tone is detected by the interrogator and results in an immediate inquiry to the STU in question, which is the origin of this phenomenon, even if the assigned telephone is removed, so that this STU normally does not answer (or do not answer) at this time. would. The response to such a request, which is then generated by the STU, provides information about the cause of the observed phenomenon, that is to say about the cause of the danger or the alarm.

As stated above, the polling and answering takes place within the audible frequency range and is therefore overlaid on any other use of the phone during the off-hook condition, for example a conversation by the subscriber. However, this only occurs if an alarm or a danger is displayed. It is therefore not an impairment and is not prohibited by the telephone standards. It actually has the desirable side effect of simultaneously making the telephone subscriber aware of the situation.

If no such phenomenon as described above occurs, the interrogator normally interrogates the individual STUs in a predetermined cyclic sequence. However, other deviations from the sequence are also advantageously provided. For example, if a particular STU is activated as part of the system first, it is preferably queried immediately, even if it is outside the normal sequence, to determine the state of its associated devices without delay. Likewise, whenever a subscriber hangs up, the STU is preferably queried immediately at this point, even if a low tone does not indicate any danger or alarm status.

In another embodiment, the low tone signal is the origin of the information regarding the states of the devices at a STU. Instead of starting to transmit an interrogation signal and receiving the responses, the interrogator monitors the presence or absence of the low tone signal both when the telephone is off-hook and on-hook. Only when the low tone signal is absent, indicating a change in the state of at least one of the devices, does the interrogator ask the appropriate STU to determine the reason for the low tone signal being suspended.

For further details, reference is made to the following description with reference to the accompanying drawings. Show it:

1 is a block diagram of the overall system of an embodiment of the invention;

Figure 2 is a block diagram of an interrogator which forms part of the embodiment of Figure 1; and

3 is a block diagram of a subscriber terminal (or STU) that forms part of the embodiment of FIG. 1.

The same reference numerals designate similar elements in the different figures.

Figure 1 shows a telephone network 10 which is conventional in every respect. In fact, it is one of the features of the present invention that it can be carried out using a conventional telephone network without in the least interfering with its operation.

This network 10, which is shown in a very simplified form in FIG. 1, comprises a plurality of subscriber telephone handsets which are designated by the corresponding reference numerals 11, 12 and 13. Each of these handsets is connected to its own local telephone line or ring line. These are designated by the corresponding reference numerals 14, 15 and 16. These local lines in turn all lead to a switching system 17 which is normally arranged in the telephone exchange. All of these elements of the system can be of substantially any conventional form. The subscriber handsets 11, 12, 13 can be dialing or push-button devices. The switch of the central switching point 17 can be a rotary switch or a crossbar switch or a completely electronically operating switch. The local lines 14, 15, 16 may also be of a suitable conventional form, for example amplifier lines, equalizer lines, etc., provided that they are able to pass on low-tone signals.

In accordance with the present invention, the system of Figure 1 further includes an interrogator 18. This interrogator is preferably located at a location where it can be conveniently connected to the various subscriber lines. This can be the center at which the switching device 17 is also arranged. 1 shows connections 19, 20 and 21 between the interrogation device 18 and the corresponding lines 14, 15 and 16. Furthermore, according to the present invention, separate subscriber stations, abbreviated as STU, are provided at each individual subscriber station, which are designated by reference numerals 22, 23 and 24.

Attention is drawn to the fact that only one device for three participants is shown in FIG. However, it goes without saying that this is only for the sake of simplicity of illustration. The invention can be used for a larger number of subscribers, the number of handsets, connections and STUs also being increased accordingly. This is represented symbolically in FIG. 1 by spaces between the aforementioned elements of the entire system which are bridged by means of dashed lines.

FIG. 2 shows the “interior” of the interrogation device 18, namely a first, a second and a third multiplexer 25, 26 and 27. These are time-division multiplexers, which in the usual way connect the signals from the different connections 19, 20 and 21 Select different times and transmit the selected signals to their corresponding outputs. In addition, the multiplexer 25 is a two-way device, which is also capable of optionally distributing signals supplied to the different connections 19, 20 and 21 at the other “ends” of the multiplexer 25 at different times. Thus, each end of the multiplex 25 can serve as input and output depending on the purpose. The multiplexer 25 is connected to a transmit / receive switch 28 (also abbreviated as T / R switch).

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

672 040

4th

The part of the T / R switch 28 which operates in the receive mode in turn supplies signals from the multiplexer 25 to the demodulator 29 for signals which are frequency-modulated (abbreviated to FSK). The part of the T / R switch 28 which operates in the transmission mode receives its (to be transmitted) input signals from a modulator 30 for FSK signals.

The demodulator 29 in turn supplies its output signals to the system controller 31, and the modulator 30 receives its input signals from the same controller 31.

The multiplexers 26 and 27 pass their corresponding output signals to the different filters 32, 33. The filter 32 in turn supplies its output signals to the detector 34, while the filter 33 supplies its output signals to the detector 35. Both detectors finally feed their output signals to the same system controller 31 mentioned above.

Because of the interrelated relationship between the operations of the various elements of interrogator 18 shown in Figure 2 and the elements of each STU 22, 23 and 24 (see Figure 1), the latter will be described in more detail before proceeding to the complete description of the Ways of working of both is addressed. In this regard, reference is now made to FIG. 3, which shows the “interior” of the STU 22. Of course, all STUs are essentially identical in construction, so that the description of STU 22 applies equally to STUs 23 and 24 and to any additional STU that is symbolized by the dashed lines between STU 23 and STU 24 in FIG. 1 .

The STU 22 (FIG. 3) is connected to the line 14, which also connects the subscriber handset 11 to the power switch 17 and the interrogator 18.

The signals received by line 14 are fed to a filter 36 within STU 22, the output signal of which is fed to a demodulator 37 for frequency-modulated FSK signals.

Signals from line 14 are supplied by a modulator 38 for frequency-modulated FSK signals.

The line 14 is also supplied with signals from a signal generator 39.

The output signals from the demodulator 37 are in turn fed to the signal processing circuit 40.

The signal sources 41, 42 and 43 in FIG. 3 represent electrical signal outputs from corresponding devices (not shown) which are arranged in the same premises as the STU 22 and the handset 11 and which are to be monitored by the system which represents the present invention .

For example, the signal source 41 can be a relay that closes and thereby creates an electrical signal path when a fire alarm device detects an alarm condition in the premises of the subscriber. At the same level, signal source 42 may be a relay that operates similarly when a burglar alarm is activated, and signal source 43 may act similarly in the event of interruptions in a temperature control system.

The modes of operation of the components described above will be described below, it being pointed out first that the interrogation device 18 and each STU 22, 23 and 24 are connected to one another in the form of a relationship to be referred to as a closed control loop. That is, the interrogator generates signals that are transmitted to the STU, which responds to it and sends signals back to the interrogator, etc. back and forth, in a "closed loop".

The “closed control loop” is therefore first described below, starting with the system controller 31, which is connected to the FSK modulator 30 (FIG. 2). This system control is designed so that it repeats at the output

FSK-modulated signals are generated in a predetermined pattern. This pattern is such that the STUs 22, 23, 24, which form part of the overall system, respond to it in turn to produce certain output signals, as described below. The FSK modulator 30 responds by generating FSK-modulated signals corresponding to the signals from the controller 31. The two carrier frequencies used are preferably around 2,700 and 2,900 Hz, that is, near the upper end of the frequency band that passes through a typical local telephone line.

These FSK-modulated signals from the modulator 30 then pass through the T / R switch 28 to the multiplexer 25. There, control signals from the system controller 31 are also supplied in such a time assignment to the signals from the FSK modulator 30 that the latter to one of connections 19, 20 and 21 are routed depending on which of STUs 22, 23 or 24 is the intended destination of these signals from FSK modulator 30 at that time.

If the intended destination for a particular signal sequence is STU 22, multiplexer 25 provides that sequence to link 19 when it reaches STU 22 over telephone line 14. It should be noted that the same signal sequence also reaches handset 11 via line 14. More about this will be discussed later.

After reaching the STU 22, now with reference to FIG. 3, the signal sequence described above is demodulated in the FSK demodulator 37 after passing through the filter 36, which is preferably a bandpass filter and is constructed in such a way that it is selectively controlled by the frequency band occupied for both FSK carrier frequencies is transmitted (for example the band from 2,700 to 2,900 Hz). At the output of the demodulator 37, the modulated signals originally supplied by the controller 31 (FIG. 3) are therefore reconstructed. These reconstructed signals are fed to the signal processing unit 40 in the STU 22.

This processing unit 40 is constructed to respond to the above-mentioned signals and to generate a sequence of modulated signals which in turn are fed to the FSK modulator 38, where they generate a corresponding sequence of FSK-modulated signals, preferably at the same carrier frequencies like the signals received by the interrogator 18. This sequence of FSK-modulated signals returns via line 14 and connection 19 to the interrogator. Moreover, it is pointed out that this response signal sequence naturally also reaches the handset 11.

At the interrogation device, the multiplexer 25 is activated by the controller 31 in time association with the expected arrival of this response signal sequence from the STU 22, so that signals on the connection 19 reach the FSK demodulator 29 via the receiving part of the T / R switch 28. The demodulator 29 recovers the signals that were used to modulate the FSK signals at the STU 22 and supplies them to the system controller 31.

If one now looks again at the STU 22 (FIG. 3), the modulated signals generated by the signal processing circuit 40 provide information about the state of the signal sources 41, 42 and 43. If, for example, the STU 22 does not have any relay closures to indicate the alarm states of the various monitored devices take place, the signal processing circuit 40 generates a predetermined pattern of modulated signals. When one or more relays close, different patterns are generated accordingly, and the (demodulated) signals that ultimately reach controller 31 from STU 22 are also different. The system controller 31 responds to the particular pattern of the received signals in a particular way. For example, if a signal pattern is received that indicates

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

5

672 040

that an alarm state prevails at the signal source 41 of the STU 22 (FIG. 3), the system controller 31 supplies a corresponding alarm signal for any desired display. This can be, for example, a notification to the fire department, which is responsible for the premises of the subscriber in which the STU 22 is arranged. Such applications of the signal patterns received by each STU are common per se and can take one of a variety of forms without affecting the characteristics of the overall system underlying the present invention. The facilities for the implementation of these applications are therefore not further described or shown here.

It will now be discussed again that, as already mentioned, the FSK signals which go back and forth between the interrogator 18 and any given STU also reach the telephone handset to which the STU is assigned. This is not a problem as long as the telephone is on-hook, since these FSK signals remain unnoticed by the subscriber when the handset is on-hook. However, this does not apply while the handset is off-hook. The FSK signals are within the audible frequency range. You will therefore be heard by the subscriber when he tries to use the phone for a normal conversation. This is very annoying, especially since these FSK signals intentionally return at short intervals to indicate the relatively current status of the devices to be monitored. Even if the sound of these FSK signals were acceptable to the subscriber, in many cases they would still violate the regulations of the telephone companies and / or state regulations which prohibit the introduction of such audible external signals into the network while the system is removed.

According to the present invention, this problem is solved as follows:

The generator 39, which is provided on each STU (FIG. 3), is constructed in such a way that it generates signals with a frequency substantially below the audible frequency range. For example, this generator 39 may be able to generate signals with a frequency of approximately 25 Hz. Because of this mode of operation at such a low frequency, generator 39 is referred to as a "low tone" generator.

The generator 39 does not work continuously. Rather, it is controlled by the signal processing circuit 40 so that it is in one of the two states. As long as signal sources 41, 42, and 43 are all in their non-alarm condition, signal processing circuit 40 causes low tone generator 39 to generate its low frequency output signal, which then reaches interrogator 18 via telephone line 14 and link 19. In contrast, whenever one or more of the signal sources 41, 42, or 43 is in an alarm condition, the signal processing circuit 40 causes the low tone generator 39 to stop generating the low frequency output signal. This low-frequency signal therefore does not reach the interrogation device 18. As already mentioned, each STU is constructed essentially in a similar manner to the STU 22, which is shown in detail in FIG. 3, and operates essentially in the same way,

At the interrogator 18, the multiplexers 26 and 27 are excited by the system controller 31 so that they sequentially switch the connections 19, 20; or select 21 signals present.

A signal, which in the periodic sequence represents the on-hook state of the different handsets 11, 12 and 13, is derived from the multiplexer 26 via the filter 32 and the on-hook detector 34 and fed to the system controller 31.

A signal is derived from the multiplexer 27, which also represents the presence or absence of the low-frequency signal from the various low-tone generators of the STUs 22, 23 and 24 in a periodic sequence, is derived via the filter 33 and fed to the system controller 31.

It should now be observed what happens during each of four possible situations that prevail at any given STU, for example STU 22 of FIG. 3.

A possible situation is that the telephone handset 11 assigned to the STU 22 is on-hook and there is no alarm state on the STU 22. The applied state is detected at the interrogator 18 and the output of the FSK modulator 30 is periodically fed to the STU 22, which responds with a non-alarm output from its FSK modulator 38. This is detected by means of the FSK demodulator 29 and the system controller 31 does not act as an alarm, but simply follows the periodic scanning of the connection to this STU for both the continuous hang-up signal and the continuous non-alarm FSK signals.

A second possible situation is that the telephone handset 11 remains on-hook, but that an alarm condition occurs at the STU 22. The applied state is also detected at the interrogation device 18 and the output from the FSK modulator 30 is still fed to the STU 22 periodically. However, the STU now responds with an alarm signal from its FSK modulator 38. This is detected by the FSK demodulator 29 and the system controller 31 takes the appropriate alarm action as described above.

A third situation is that the telephone handset 11 is off-hook and that there is no alarm condition on the STU 22. This removed state is now determined on the interrogator 18, which then does not supply any output signals from the FSK modulator 30 to the STU 22 for the duration of this third state, provided that the interrogator 18 also detects a low-tone signal which it receives from the same STU. It should be remembered that the generator 39 produces a low tone when there is no alarm condition.

The fourth situation is that the telephone handset 11 is again picked up, but there is at least one alarm condition on the STU 22. The removed state at the STU 22 is again determined on the interrogator 18, but no low-tone signal is determined by the same STU. Under these circumstances, the interrogator 18 provides output signals of the same type from the FSK modulator 30 to the STU 22 as it would supply if the handset 11 was in the on-hook instead of the off-hook state. This naturally results in a response from the STU 22 in the form of FSK-modulated signals from its modulator 38, which in turn are detected at the interrogator 18 and used to generate the desired alarm response.

It can now be seen that as long as there is no alarm condition at the STU, there are no signals in the audible range during operation of the present invention while the associated telephone handset is being used by the subscriber. This fulfills the wishes of the participants and complies with the regulations. On the other hand, when there is an alarm condition, there are audible signals of this type, even when the telephone handset is in use, in the form of FSK-modulated signals that travel back and forth between the interrogator and the subscriber's premises. During the alarm conditions, such audible signals are not only permitted by the regulations, but are also highly desirable from the subscriber's point of view because they immediately warn the subscriber that an alarm condition is present.

It is also clear that the multiplexers 25, 26 and 27 in the interrogation device 18 (FIG. 2) guarantee the same treatment for the STUs 23, 24 in periodic order as is given with regard to the STU 22. That is, first

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

672 040

6

if the interrogator 18 is connected to cooperate with the STU 22 in the manner described above, then it is connected to the STU 23 for the same purpose, then to the STU 24, etc. in periodic order.

If there are more than three STUs - which is normally the case with any practical application - they are all treated periodically in the manner described above for the single STU 22.

A number of additional considerations regarding the present invention are appropriate.

A particularly "happy" combination of the interacting features can be seen in the fact that primarily the high-frequency part of the audible range is used for a warning when the telephone set is in the on-hook state, while for an initial warning when the telephone set is removed, it is below the audible range lying signal is used.

The use of the high frequency part enables a relatively high data rate, which means that a relatively detailed information about the devices to be monitored can be obtained and that this can also be repeated frequently and that a relatively large number of STUs can be recorded by the system. Using the frequency below the audible range makes it possible to speak without interference during the (presumably predominant) times when there is no alarm. However, by shifting the frequency from the audible range to the high frequency, the desired detailed information is obtained when it is needed, that is, when there is an alarm condition, even during the times when the handset is off-hook Condition.

It is also noteworthy that the low tone provides redundancy for the alarm function of the system during the on-hook condition. During this state, the low tone continues to be fed from each STU to the interrogator as long as there is no alarm at the STU. Any low-tone interruption during the on-hook condition is also detected by the interrogator and provides a so-called "rough" security alarm display for the more detailed display that is normally generated by the FSK-modulated response from the STU.

The alarm indicator during times when the handset is off is also fail-safe because the absence of the low tone signal indicates an alarm condition. Thus, a failure in the components of the STU causes a pseudo alarm display, which is followed by an immediate correction.

In a preferred embodiment of the invention, in addition to those described above, there is a further case when a certain STU receives the FSK-modulated signal sequence from the interrogator 18, even if this STU is actually not the turn in the periodic sequence.

This case is the return from the off-hook state to the on-hook state of the associated telephone handset.

As stated above, the status of each telephone handset for each STU is sensed at interrogator 18. If a change from the off-hook to the on-hook state is detected in this way, the STU is preferably switched next in the premises of this subscriber in order to receive the FSK-modulated signal from the interrogator and is therefore also the next one to receive its FSK -modulated response returns. The reason for this is that only non-detailed information about the state of the devices at this location has been obtained during the previous period with the handset removed and it appears desirable to update this information without delay when the handset's removed state is no longer present .

Another very remarkable feature of the invention is that the individual parts of the overall system can be part of a number of known embodiments.

Thus, all elements of the interrogation device 18 (FIG. 2) as well as all parts of the STU 22 (FIG. 3) have a known shape.

If one first looks at the interrogation device 18, it can be seen that the multiplexers 25, 26 and 27 have a known construction. The filters 32 and 33 can have a known construction which is suitable for selecting the specific frequencies which are passed to the detector 34 for the state of the telephone handset or the low-tone detector 35. These detectors themselves are also known, the telephone handset condition detector 34 being a conventional part of the telephone system and the low tone detector 35 being a detector that detects the presence or absence of the 25 Hz low tone signal. T / R switch 28 is of known design and system controller 31 may include any known circuitry capable of performing the directional query and response control functions described. Thus, system controller 31 may include a conventional source of clock signals from which the sequential work signals for multiplexers 25, 26 and 27 are derived. The sources, such as shift registers of the modulated signal sequences for the FSK modulator 30, can also be driven by signals derived from the clock signals. AND gate circuits can be used to detect the simultaneous presence of the handset off-hook and low tone signals. Such detection is achieved by conventional circuits for preventing signals on the FSK modulator 30. Conversely, the non-detection of these simultaneous signals causes such signals to be transmitted to the FSK modulator.

In the following, the STU 22, the FSK demodulator 37 and the FSK modulator 38, which in turn can have a known construction, as well as the filter 36, which lies in front of the demodulator 37, will be considered. The low tone generator can also be of a known shape capable of generating the desired 25 Hz signals when energized. The signal sources can be simple relay circuits, which are actuated by corresponding alarms, and the signal processing device can be a known digital logic circuit, or can be designed as a microprocessor circuit, which is programmed in the usual way to emit the appropriate, time-synchronized signals, as already described.

The telephone system itself is not affected by the invention. No additional filter is required in the telephone system; in particular, the low-tone signals are not interspersed from one side of the mains switch 17 to the other, since the mains switch has suitable filter devices for other reasons, namely in order to separate the different levels of the direct current on the opposite sides of the switch.

When the system of the present invention is not used to signal an alarm, but to communicate other states of the devices in a subscriber's premises, e.g. Reading readings, meteorological measurements, etc., the low tone can be used to indicate that all measuring instruments are working, even while the telephone handset is in the off-hook condition. During the times when the handset is on-hook, the signals from any given STU can then provide the desired, more detailed data about the devices.

The system shown in Figures 1, 2 and 3 can be operated in a second way. Instead of monitoring through the transmission of query signals and reception

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

7

672 040

of responses, the interrogator 18 can detect the presence or absence of the low tone signals and only query that STU after the low tone signal of a particular STU disappears to determine the reason for the low tone signal disappearing. In this mode of operation, the low tone signals provide the initial indications of the states of the devices. As long as the low tone signal is received by the interrogator from a particular STU, the interrogator does not interrogate that STU because the presence of the low tone signal is already an indication that this STU need not be interrogated (i.e. all devices connected to this STU are located in a "non-alarm" state). After the disappearance of the low-tone signal, the interrogator detects a change and transmits interrogation signals to the STU concerned in order to determine the reason for the disappearance of the low-tone signal, which is indicated by the response signals transmitted by the STU.

Of course, various modifications are obvious to the person skilled in the art without detaching from the inventive idea io, the scope of which is to be determined by the appended claims.

v

3 sheets of drawings

Claims (7)

672 040 1. An arrangement for signaling the state of at least one device arranged in the room of a subscriber connected to a telephone network via the telephone network, the arrangement comprising: Devices (39) provided on the premises and connected to telephones (11, 12, 13) of the network (10) for transmitting a low-frequency signal with a frequency which is in the lower listening range or below, from each of the premises to the center, if the Devices (41, 42, 43) in the premises are in a predetermined state, regardless of whether the handset is on-hook or off-hook, means responsive to at least one of the devices (41, 42, 43) that is in another predetermined state on the premises to interrupt transmission of the low frequency signal from a given location if at least one device in the given location is in switches the other predetermined state; means (35) provided in the center for detecting the interruption of the low frequency signal transmitted from the given room while the handset is on-hook or off-hook; means (30), provided in the control center and responsive to the detection device (35), for transmitting an interrogation signal in the audible telephone frequency range to the given room only if the detection device (35) detects an interruption of the low-frequency signal; and means (38), provided in the premises and responsive to the interrogation signal, for retransmitting response signals to the central, which are in the audible telephone frequency range and have features for displaying the state of the devices in the premises. 2. Arrangement according to claim 1, characterized by a device (40) for receiving status signals from at least one of these devices (41, 42, 43) and an interrogator (18) connected to the telephone network in the central, which has a receiving device (29) for the response signals. 2nd PATENT CLAIMS 3. Arrangement according to claim 1 or 2, wherein the interrogation signals are FSK-modulated signals in the upper part of the audible range. 4. Arrangement according to claim 3, wherein the response signals are also FSK-modulated signals in the upper part of the audible range. 5. Arrangement according to claim 4, wherein the FSK carrier frequencies are approximately 2,700 and 2,900 Hz. 6. Arrangement according to claim 1 or 2, wherein the low-frequency signal has a frequency substantially below the audible frequency range. 7. The arrangement of claim 6, wherein the low frequency signal has a frequency of about 25 Hz.