SE462667B - SYSTEM FOR ASTAD COMMUNICATION OF COMMUNICATION BETWEEN A CENTRAL STATE AND EQUIPMENT OTHER THAN A CONVENTIONAL PHONE, WHICH SUCH EQUIPMENT IS LOCATED ON THE SITES BEFORE SUBSCRIBERS CONCERNING A PHONE - Google Patents

Description

15 20 40 462 667 2 ter sets up.

Other such previous system proposals include the use of extra signals, which are above the range of audible frequencies. However, this requires electrical filtering in the system itself and possibly also in the conventional telephone network, which is difficult to achieve and also expensive to such an extent that the economic acceptability of the system deteriorates.

Additional system proposals, in an attempt to overcome disadvantages of the kind described above, have simply sacrificed certain desirable system functions, such as reliable monitoring of the equipment at the subscriber site or the like.

Accordingly, a principal object of the present invention is to provide a system which cooperates with a conventional telephone network to provide additional functions, however, in an improved manner compared to previously proposed systems intended for the same general purpose.

Another object of the present invention is to provide such a system which does not have one or more of the shortcomings of the previously proposed systems. Yet another object is to provide such a system which does not use additional signals which are audible to the subscriber during normal use of the telephone.

Another object is to provide a system which does not rely on signals above the range of audible frequencies. Yet another object is to provide a system which does not require special electrical filtering in the conventional telephone network. Another object is to provide a system which provides highly reliable monitoring of the equipment at the subscriber site.

These and other objects which will become apparent are achieved in accordance with the present invention by a system which exhibits the features set forth in the appended claims.

For a system of the type referred to here, at a point where a group of subscriber loops, which are to be served by the system, are easily accessible, a central control unit (hereinafter referred to as "scanner") is connected over all these subscriber loops. At each subscriber location, an individual electronic unit (hereinafter referred to as "subscriber terminal unit 462 667 unit" or "STU") is connected via the subscriber loop in question. The different terminals 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.

The scanner can interrogate the STU units at appropriate times with the help of audible signals within the audible range. The STU units respond with the help of similar signals to indicate the status of the equipment at each subscriber location.

This normal pattern of inquiry and response thereto is limited to periods during which the subscriber's telephone is not busy, i.e. when the subscriber does not use the telephone for normal purposes, and ceases when the subscriber uses the telephone (i.e. provides a busy state).

A special signal at a frequency which is well below the audible range is also generated by the STU and is sent to the scanner via the telephone loop, which of course also over: for the other conventional telephone signals: voice signals, dial tone, ring tone, busy tone, etc. This special signal (hereinafter referred to as "low tone") is preferably present continuously under both unoccupied and busy conditions, but only if all the equipment at the particular subscriber location is in its predetermined normal, or safe, conditions.

If any or more of these devices deviate from this condition, i.e. takes a condition that involves danger or alarm, the generation of the low tone in the associated STU is interrupted. Such interruption of the low tone is sensed by the scanner and triggers immediate interrogation of the particular STU unit, which is the source of this phenomenon, even if the associated telephone is busy, so that this STU unit would not normally be interrogated (or provide an answer). ) At this time. The answer to such a request, which is thereby generated by the said STU, provides information about the reason for the observed phenomenon, i.e. what constitutes the cause of the danger or alarm.

As previously clarified, the interrogation and answering of this takes place within the audible frequency range and is therefore superimposed on any other use of the telephone during a busy condition, e.g. on subscriber calls. However, this only happens in response to an indication of danger or alarm. This is therefore not considered to be an objection, nor is it prohibited by telephone regulations. In fact, it has the desirable aspect that the person using the phone is simultaneously made aware of the situation.

In the absence of a phenomenon of the kind described above, the scanner would normally interrogate the individual STUs in a predetermined, cyclic sequence. However, other deviations from this sequence are preferably also inserted. For example, when a certain STU unit for the first time assets as part of the system it is preferably interrogated immediately, even if this would be outside its normal sequence, this in order to determine the status of its associated equipment without any delay. Furthermore, whenever a subscriber ends a call, the associated STU unit is preferably interrogated immediately, even if a low tone had not indicated a state of danger or alarm.

W In accordance with the present invention, polling takes place in another way. In this case, the low-tone signal is the initial source of information regarding the conditions of the equipment at an STU unit.

Instead of starting by transmitting interrogation signals and receiving responses thereto, the scanner monitors the presence or absence of the low tone signal both during unoccupied and busy state of the telephone. Only after the low tone signal is lost, which indicates a change in the condition of at least one equipment, does the scanner send a request to the corresponding STU unit to determine the reason why the low tone signal has been lost.

Brief Description of the Drawing For further details, reference is made to the following discussion in the accompanying drawing, in which: Fig. 1 is an overall system block diagram of a system of the type contemplated herein; Fig. 2 is a block diagram of a scanner forming part of the system of Fig. 1; and Fig. 3. (or STU) forming part of the system of Fig. 1.

In the various figures, the same reference numerals are a block diagram of similar elements of a subscriber terminal unit. Referring to Fig. 1, there is shown a telephone network 10, which in itself is completely conventional in all respects. It is in fact a feature of the present system that it can be used practically in connection with a conventional telephone network, without its operation being disturbed in the slightest way.

This network 10, shown in very simplified form in Fig. 1, comprises a plurality of subscriber telephone sets, which are marked by the reference numerals 11, 12 and 13, respectively. Each of these sets is connected to its own local telephone line or loop. The latter are marked by the reference numerals 14, 15 and 16, respectively. These local lines in turn all lead to a switching system 17, which is normally located at a central telephone station. All these elements included in the system can have in principle any conventional form. The subscriber devices 11, 12, 13 can be of the finger-dial type or of the push-button type. The central station switch lf can have a rotary selector or coordinate selector or even be completely electronic. The local lines 14, 15, 16 may also be of any conventional design, including coatings, equalizers, etc., as appropriate and provided that they are capable of transmitting the low tone signals. As can be seen from Fig. 1, the scanner further comprises a scanner 18. This scanner is preferably physically located in a place where it can be connected in a suitable and convenient manner to the various individual subscriber lines. This can be at the central station, where the exchange equipment 17 is also located. Fig. 1 shows the connections 19, 20 and 21 between the scanner 18 and the respective lines 14, 15 and 16. The system also applies that there is a separate subscriber terminal unit at each individual subscriber point, designated STU. These units are marked by the reference numerals Z2, 23 and 24 respectively.

It should be noted that Fig. 1 only shows equipment for three subscribers. It should, of course, be understood that this is only to simplify the presentation. The system is intended to be used together with a larger number of subscribers, in which case the devices, connections and STU units would all be present in a correspondingly increased number. This is symbolically indicated in Fig. 1 by bridges by means of dashed lines between all the preceding elements in the overall system.

Referring now to Fig. 2, which shows the "inside measurement" of the scanner 18, the first, second and third multiplexers 25, 26 and 27, respectively, are included. These all operate with time multiplexing. They thus select in a conventional manner the signals on the different connections 19, 20 and 21 at different times and transmit the signals thus selected to their respective outputs. In addition, the multiplexer 25 is a two-way device which is also capable of selectively distributing at different times signals, which are applied to the other "end" of this multiplexer 25, to the different connections 19, 20 and 21. Thus, both ends of the multiplexer 25 can serve either as input or output, as appropriate. The multiplexer 25 is connected to a transmission / reception switch 28 (also called S / M switch). The part of the switch 28, which operates in reception mode, in turn supplies signals from the multiplexer 25 to a demodulator 29 for FSK signals (frequency shift modulated signals). The part of the switch 28, which operates in transmission mode, receives its input signal (to be transmitted) from a modulator 30 for FSK signals.

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

As for the multiplexer 26 and the multiplexer 27, these supply their respective outputs to different filters 32, 33. The filter 32 in turn supplies outputs to a detector 34, while the filter 33 supplies its outputs to a detector 35.

Both of these detectors eventually supply their outputs to the aforementioned system controller 31.

Due to the mutual reading between the operations of the various elements of the scanner 10, which are shown in Fig. 2, and the elements of each STU unit 22, 23 and 24 (see Fig. 1), it seems necessary to describe the later in more detail, before the respective operating characteristics are reported in more detail.

For this purpose, reference is now made to Fig. 3, which shows the "inner measurement" of the STU unit 22. It will be appreciated that all of the STU units may be substantially identical in their construction, so that the The description given of the STU unit 22 applies equally to the STU units 23 and 24, as well as to each additional STU unit symbolized by the dashed line between the STU unit Z3 and the STU unit 24 in Fig. 1.

The STU unit 22 (Fig. 3) is connected to the line 14, which also connects the subscriber apparatus 14 to the mains exchange 17 and to the scanner 18.

Signals received from line 14 are fed in the STU unit 22 to a filter 36. The output signal from this filter is applied to a demodulator 37 for FSK signals.

To line 14, signals are fed from a modulator 38 for FSK signals.

Signals from a signal generator 39 are also fed to line 14.

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

The signal sources 41, 42 and 43 in Fig. 3 represent electrical outputs from corresponding equipment (not shown), which are located in the same place as the STU unit 22 and the apparatus 11 and which are intended to be monitored using the system in question.

For example, the signal source 41 may be a relay which closes and thereby establishes an electrical signal path whenever a fire alarm device at the subscriber site senses an alarm condition. The signal source 42 may also be a relay which operates in a similar manner when a burglar alarm is activated, and the signal source 43 may function in a similar manner in the event of an interruption in a temperature control system.

As far as the operating characteristics of the elements described above are concerned, it is desirable to first emphasize that there is what could be called a "closed loop" relationship between the scanner 18 and each STU unit 22, 23 and 24.

In other words, the scanner generates signals which are sent to the STU unit, which responds by sending signals back to the scanner, and so on back and forth or as in the form of a closed loop.

Therefore, for the purpose of description, let initially enter the closed loop at the output of the system controller 31, which is connected to the FSK modulator 30 (Fig. 2). This system control unit is designed to generate FSK modulation signals on a predetermined pattern in a predetermined pattern. This pattern is such that the STU units 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 an FSK modulated equivalent to the signals from the control unit 31. The two carrier frequencies used are preferably in the vicinity of 2700 and 2900 Hz, i.e. near the upper end of the band of frequencies transmitted by a typical local telephone line.

These FSK modulated signals from the modulator 30 then pass through the S / M switch 28 to the multiplexer ZS.

Control signals from the system control unit 31 are also supplied there in such time coordination with the signals from the FSK modulator 30 that the latter are channeled to one of the connections 19, 20 and 21 depending on which of the STU units 22, 23 and 24 is the intended destination for these signals from the FSK modulator 30 at the time in question.

Assuming that the intended destination for a certain signal train is the STU unit 22, the multiplexer 25 feeds this train to the connection 19, whereupon the same reaches the STU unit 22 via the telephone line 14. It should be noted that the same signal train also reaches the apparatus 11 via the line 14. More information on this will be given later.

Referring now to Fig. 3, the signal train described above, after reaching the STU 22, is demodulated in the FSK demodulator 37, after passing through the filter 36, which is preferably a bandpass filter designed to selectively transmit the band of frequencies occupied by the two FSK carrier frequencies (eg the band from 2700 to 2900 Hz). Thus, at the output of the demodulator 37, the modulation signals originally given by the control unit 31 (Fig. 3) are recovered. These reconstructed signals are fed to the signal processing circuit 40 in the STU 22.

This processing circuit or processor 40 is designed to generate a train of modulation signals in response to the above signals, which in turn are fed to the FSK modulator 38, where they provide a corresponding train of FSK modulated signals, preferably at the same carrier frequencies as those received by the scanner 18. This train of FSK modulated signals returns to the scanner via line 14 and the connection 19. It can be noted here that this response signal train of course also reaches the apparatus 11.

In the scanner, the multiplexer 25 is activated by the control unit 31 in a time-coordinated manner with the expected arrival of this response signal train from the STU unit 22 in order to transmit the signals on the connection 19 to the FSK demodulator 29 via the receiving part of the S / M switch 28. The demodulator 29 recovers the signals which had been used to modulate the FSK signals in the STU unit 22 and feeds them to the system control unit 31. Returning now to the STU unit 22 (Fig. 3), the modulation signals generated by the signal processing circuit 40 provide information on the condition of the signal sources 41, 42 and 43. If, for example, there is no relay closure at the STU unit 22 which would indicate an alarm condition of the various equipment under monitoring, a certain, predetermined pattern of modulation signals is generated by the processing circuit. 40. If one or more relay terminals are present, deviating patterns are generated to a corresponding degree, and the (demodulated) signals which finally reach the control unit 31 from the STU unit 22 are also different. The system controller 31 responds in a particular manner to the particular received signal pattern. If, for example, a signal pattern is received which indicates that there is an alarm condition at the signal source 41 in the STU unit 22 (Fig. 3), the system control unit 31 gives a corresponding alarm signal to any desired indicator. It can e.g. be an indicator of the fire station responsible for the subscriber location where the STU 22 is located. Such uses of signal patterns received from each STU unit are in themselves conventional and can take any of a large number of forms without affecting the aspects of the overall system which constitute the present invention. The means for achieving these uses are therefore not further described or illustrated in the present description. 10 15 20 25 30 462 667 10 Attention should now again be drawn to the fact (previously noted) that the FSK signals which pass back and forth between the scanner 18 and an arbitrary STU unit also reach the telephone set with which the STU the device is associated. This is not a problem as long as the telephone is not busy, as these FSK signals remain unobserved by the subscriber under such unoccupied condition. This is not the case when the occupied state prevails. The FSK signals are within the audible frequency range. They are therefore perceived by the subscriber as he tries to use the telephone for normal call purposes. This is clearly disturbing, especially as these FSK signals are intentionally repeated at short intervals, in order to give indications of the condition of the monitoring standing equipment, which are relatively fresh. Furthermore, even if the audible effects of these FSK signals were assumed to be acceptable to the subscriber, they would in many cases be in conflict with regulations issued by telephone companies and / or authorities, which prohibit the introduction of such foreign audible signals in the network. during a busy condition.

This relationship can be handled as follows.

The generator 39, which is arranged at each STU unit (Fig. 3), is designed to provide a signal at a frequency which is substantially below the audible frequency range.

This generator 39 can e.g. be able to produce a signal with a frequency of approximately 25 Hz. It is because the generator operates at such a low frequency that the generator 39 is referred to as the "low tone generator".

Generator 39 does not operate continuously. It is instead controlled by the signal processing circuit 40 to be in one or the other of two states. As long as the signal sources 41, 42 and 43 are all in their state without alarm, the signal processing circuit 40 causes the low tone generator 39 to generate its low frequency output signal, which then reaches the scanner 18 via the telephone line 14 and the connection 19. In contrast, one or more of the signal sources 41, 42 and 43 are in the standstill signal processing circuit 40 causing the low tone generator 39 to stop generating its low frequency output signal. This low frequency signal therefore also ceases to reach the scanner 18. As previously stated, each STU unit is equipped in a substantially similar manner to the STU unit 22, which is shown in particular in Fig. 3, and operates in substantially the same manner.

In the scanner 18, the multiplexers 26 and 27 are normally activated by the system control unit 31 to sequentially sample the signals appearing on the terminals 19, 20 and 21, respectively.

From the multiplexer 26, via the filter 32 and the detector 34, a signal representing, in regular recurring sequence, the state (busy or not busy) of the various devices 11, 12 and 13 is derived. This signal is fed to the system control unit 31 .

From the multiplexer 27, a signal representing, also in a regularly recurring sequence, the presence or absence of the low-frequency signals from the various low-tone generators in the STU units 22, 23 and 24 is derived via the filter 33. This signal is fed to the system control unit 31.

It should now be observed what happens during each of the four possible situations that may prevail at each of the STU units, e.g. The STU unit 22 in Fig. 3.

A possible situation is that the telephone set 11, which is associated with the STU unit 22, is not busy and that no alarm condition prevails at the STU unit 22. The unoccupied condition is sensed in the scanner 18, and the output signal from the FSK modulator 30 is fed periodically to the STU unit 22 in question, which responds with a "non-alarm" output signal from its FSK modulator: 38. This is detected by the FSK demodulator 29, and the system control unit 31 does not take any alarm action but simply continues to periodically sample the connection to this STU unit both in terms of continued "not busy" signal and continued "non-alarm" FSK signals.

A second possible situation is that the telephone set 11 is still not busy, but that an alarm condition prevails at the STU unit 22. The unoccupied condition is still sensed in the scanner 18, and the output signal from the FSK modulator 30 is still periodically fed to the STU unit 22 in question. However, the STU now responds with an alarm signal from its FSK modulator 38. This signal is detected by the FSK demodulator 29, and the system controller 31 takes appropriate alarm action, as previously described.

A third situation is that the telephone set 11 is busy and that there is no alarm condition at the STU unit 22. This busy condition is now sensed in the scanner 18, which then does not supply output signals from the FSK modulator 30 to the STU unit 22 for as long as this third situation prevails, provided that the sensor 18 also senses a low tone signal received from the same STU unit. As previously mentioned, a low tone is emitted from the generator 39, when no alarm condition prevails.

The fourth situation means that the telephone set is again occupied, but that there is at least one alarm condition at the STU unit 22. The occupied condition at the STU unit ZZ is still sensed by the scanner 18, but the same applies to the absence of the low tone signal from the STU unit in question. Under these circumstances, however, the scanner 18 supplies output signals from the FSK modulator 30 to the STU unit 22, which signals are of the same type that would have been fed if the apparatus 11 had not been busy (ie if the handset had been turned on instead of being lifted) . This will, of course, cause a response from the STU 22 in the form of FSK modulated signals from its modulator 38, which signals will in turn be detected in the scanner 18 and used to provide the desired alarm response.

It should now be understood that as long as there is no alarm condition at the STU unit, there will be no signal within the audible area when the associated device is used by the subscriber. This satisfies the subscriber's wishes and is also in accordance with rules and regulations. On the other hand, in the event of an alarm condition, there will be such audible signals, even when the device is used, namely in the form of the FSK-modulated signals that go back and forth between the location of the scanner and the subscriber location. When an alarm condition prevails, however, the presence of such audible signals is not only permitted by rules and regulations, but it is also really desirable from the subscriber's point of view, as it gives a quick warning to the subscriber that an alarm condition has occurred.

It should also be understood that thanks to the multiplexers 25, 26 and 27 in the scanner 18 (Fig. 2) the same treatment given to the STU unit 22 is also given to the STU units 23 and 24 in regular recurring sequence. In other words, the scanner 18 is first connected so that it cooperates with the STU unit 22 in the manner described above, after which the scanner is connected to the STU unit 23 for corresponding purposes and then to the STU unit 24 and so on on a regular basis. way.

In addition, if there are more than three STUs, which will normally be the case in each practical application, all of them will be treated in a regular sequence in the manner described above with respect to the single STU 22.

A number of additional observations should be in place in connection with the system operating above.

A particularly "happy" combination of cooperating features appears to be achieved by primarily using the high frequency end portion of the audible range to report alarms in the unoccupied state, while using a signal below the audible range for initial alarm report in connection with busy condition.

Use of the high-frequency end part enables a relatively high data rate, which means that relatively detailed information about the equipment under surveillance can be obtained, as well as that this can be repeated often and that relatively many STU units can be included in the system. The use of signals below the audible range makes it possible to make calls without being disturbed during the (probably overwhelming) majority of occasions when there is no alarm.

By switching from frequencies below the audible range to high frequencies, one can nevertheless obtain the desired detailed information when required, e.g. when an alarm condition occurs, this also during periods of busy equipment.

It should also be noted that the low tone provides reliability for the system's alarm function in connection with unoccupied conditions. Under such a condition, the transmission of low tone from each STU unit to the scanner continues as long as no alarm is present at the STU unit. Each interruption of the low tone during an unoccupied state is also sensed by the scanner and gives what could be called a "rough" supporter alarm indication before the more detailed indication, which is normally given by the FSK-modulated response from the STU unit.

The alarm indication during periods of busy equipment is also absolutely safe, as it is the disappearance of low-tone signals that indicates an alarm condition. Faults in the equipment in the STU unit will thus give a pseudo-alarm indication which contributes to immediate correction.

In a preferred embodiment of the described system, a further case, in addition to that described above, when a certain STU unit receives the FSK-modulated signal train from the scanner 18, even if this STU unit is not in line in the regular recurring sequence.

This occurs when the STU unit's associated telephone set returns from the occupied state to the unoccupied state.

As previously stated, the condition of the device (handset lifted or on) is detected for each STU unit in the scanner 18. When a change from busy state to unoccupied state is detected, the STU unit is preferably placed in the subscriber location next in turn to receive the FSK-modulated the signal from the scanner, so it will also be the next device to provide its FSK-modulated response. The reason for this is that only no more detailed information about the status of the equipment at this location has been available during the previous period with the occupied condition, and it is considered desirable to provide an update without delay, as soon as the occupied the condition no longer prevails.

A highly desirable feature of the system is that the individual components may have any of a number of known embodiments.

Thus, all elements included in the scanner 18 (Fig. 2) can be of known design, which also applies to the elements in the STU unit 22 (Fig. 3). If we first consider the scanner 18, it will be appreciated that the multiplexers 25, 26 and 27 may be of known construction. The filters 32 and 33 may be of known design, suitable for selecting the particular frequencies which it is desired to pass through to the busy detector 34 and the low tone detector 35, respectively. These detectors may themselves be of known performance.

The detector 34 is thus a conventional part of a telephone system, and the low tone detector 35 is a detector for detecting the presence or absence of a 25 Hz low tone signal. The S / M switch 28 may be of known design, and the system controller 31 may include arbitrary circuits capable of providing the uncomplicated sensing and response control functions described.

The system control unit 31 may thus comprise a conventional source for timing signals, from which the sequential control signals for the multiplexers 25, 26 and 27 are derived. Signals derived from the timing signals can also drive sources, such as shift registers, for the trains of modulation signals for the FSK modulator 30. AND gate circuits can be used to detect the simultaneous occurrence of detected busy signals and low tone signals. Such detection is used by conventional circuits to inhibit signals to the FSK modulator 30. Conversely, failure to detect these simultaneous signals results in transmission of such signals to the FSK modulator.

As for the STU unit 22, the FSK demodulator 37 and the FSK modulator 38 can also be of any known design, as can the filter 36, which precedes the demodulator 37. The low tone generator can also be of any known design capable of generating the desired 25 The Hz signal when activated. The signal sources may be simple relay circuits affected by respective alarms, and the signal processing circuit or processor may be in the form of any known digital logic or of microprocessor circuits programmed in a conventional manner to produce the appropriate timed signal responses as described above. .

The telephone system itself is completely unaffected.

No additional filtering is required in the telephone system. In particular, there is no leakage of low tone signals from one side to the other of the mains gear 17, due to the fact that the mains switch contains adequate filtering for other reasons, namely to isolate different DC voltage levels on 10 15 20 25 30 462 667 16 opposite sides of the gear from each other.

Since the described system is not used to signal an alarm condition as such but instead other conditions of equipment at the site of a telephone subscriber, such as meter readings, meteorological measurements, etc., the low tone can be used to indicate that all measuring instruments are working. continuously, even while the telephone is busy. When the device is not busy, the signals from an arbitrary STU unit can provide more detailed data as desired regarding the equipment.

The system thus illustrated in Figs. 1, 2 and 3 operates in accordance with the invention in a special manner. Instead of starting the monitoring procedure by transmitting interrogation signals and receiving a response thereon, the scanner 18 is arranged to detect the presence or absence of the low-tone signals and to interrogate it only in the event of loss of the low-tone signal from a certain STU unit. - the signal is lost. In this mode of operation, the low-tone signals give initial indications of the condition of the equipment. As long as the low tone signal is received by the scan signal from a particular STU unit, the scanner does not interrogate this STU unit, since the presence of the low tone signal is an indication that the STU unit does not need to be interrogated (ie all the equipment connected to this STU unit). device is in "no alarm" mode). In the event of a loss of the low-tone signal, the scanner notices a change and sends interrogation signals to the special STU unit in order to determine the reason for the loss of the low-tone signal, which reason is stated by the response signals sent from the STU unit.

It will be appreciated that various modifications appear to be possible to those skilled in the art within the scope of the invention, as defined by the appended claims. lb *

Claims (7)

10 15 20 25 30 35 40 ß '462 667 17 PATENTKRAV A system for providing communication between a central location and equipment other than a conventional telephone, such equipment being located at the locations of subscribers belonging to a telephone network, the system comprising means at said locations connected to the telephones in said network. to send to the central location a signal having a frequency below the audible range from each of said locations, when the respective location equipment is in a predetermined state; means at said locations arranged to, in response to at least one equipment being in a different, predetermined state, interrupt the transmission of said signal at a frequency below the audible range from resp. subscriber location, when at least one equipment at the location in question passes to said second, predetermined condition; means at the central location for sensing the interruption of the transmission of said signal at a frequency below the audible range from any subscriber location; means at the central location responsive to said sensing means for transmitting interrogation signals in the area of audible telephone frequencies to said subscriber location, said sensing means sensing an interruption of said signal with a frequency below the audible area transmitted from said subscriber location; and means at said locations responsive to such an interrogation signal for returning to the central location response signals in the range of audible telephone frequencies and having characteristic indicating status of the respective location equipment, characterized in that said means for transmitting a signal with a frequency below the the audible area is arranged to transmit the signal regardless of whether the associated telephone is busy or not; that said means at the central location for sensing the interruption of the transmission of said signal at a frequency below the audible range is arranged to detect such interruption regardless of whether the associated telephones are busy or not; 18 462 667 10 15 20 25 30 35 40 and that said means at the central location for transmitting interrogation signals are arranged to transmit interrogation signals only when sensing interruption of the transmission of said signal at a frequency below the audible range from any subscriber location. ß 2. A system according to claim 1, characterized in that the interrogation signal is FSK-modulated signals in the upper part of the audible range. A system according to claim 2, that the response signals are also FSK-modulated signals in the upper part of the part of the audible range. The system of claim 3, that the FSK carrier frequencies are in the vicinity of 2700 and 2900 Hz. k ä n n e t e c k n a t av 5. A system according to any one of the preceding claims, characterized in that the signal with a frequency below the audible range has a frequency which is substantially below the audible frequencies. 6. A system according to claim 5, characterized in that the signal with a frequency below the audible range has a frequency of approximately 25 Hz. System according to one of the preceding claims, in that a plurality of equipment are arranged on the site of a telephone and are connected to the means of the associated telephone. (9: n