JPH0149222B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a two-way communication system that enables two-way communication between a plurality of terminal devices connected to one signal cable. [Background technology] Recently, with the development of home automation and office automation, multiple signals such as intercom signals, video signals such as wired television, and security information such as crime prevention or disaster prevention are branched and connected to a single signal cable. A two-way communication system is being considered in which information is exchanged between two terminal devices. Figure 1 shows an example _of a contention type bidirectional communication system.
Keying) A terminal device that sends out a modulated transmission signal V _T , 2 is a transmission signal V _T from each terminal device 1A to 1D
This is a central control device equipped with a cone converter section _2a that sends out a return signal _VB obtained by converting the carrier frequency _f1 into f2.Each terminal device 1A to 1D and the central control device 2 are connected via a coaxial cable via a , and is connected to a signal cable 3 made of optical fiber or the like. On the other hand, the return signal V _B is sent to the terminals 1A to 1D.
When a return signal V _B containing address data that matches the own address after FSK demodulation is received,
A data receiving section is provided to receive the data of the return signal _VB , and terminal devices 1A to 1D
Two-way communication is carried out between the two via the central control device 2. In the figure, 2b is a data processing circuit, 5 is a television camera, and 6 is an intercom device, and the signal cable 3 is also used as a signal transmission path of the television intercom system. By the way, in such a conventional example, each terminal device 1A to 1D determines whether or not a transmission signal V _T is being sent from another terminal device 1A to 1D. A transmission suppression means is provided for suppressing the transmission of the own transmission signal V _T when the transmission signal V _T is being transmitted. However, within the response time of this transmission suppression means, the transmission signal _V was generated, causing interference. In other words, the transmission signals V _T are generated almost simultaneously, resulting in missing data signals or erroneous transmission, which may confuse the communication system and cause a serious situation. [Object of the Invention] The present invention has been made in view of the above points, and its purpose is to prevent malfunctions when transmission signals are simultaneously sent from multiple terminal devices. The purpose is to provide a two-way communication system. [Disclosure of the Invention] Embodiment FIG. 2 is a block circuit diagram of main parts of terminal devices 2A to 2D according to an embodiment of the present invention, in which reference numeral 10 indicates a local oscillation circuit 11, a mixing circuit 12, and a limiter amplification circuit 13.
, an FM detection circuit 14, and a comparison circuit 15. This FSK demodulator 10 amplifies the received return signal _VB , removes the amplitude modulation component with a limiter, and then outputs the FM signal. By performing detection and detecting the level of the detected output V _F in a comparison circuit 15, a data signal V _D ' consisting of a baseband signal is demodulated. 16 is
This is a data processing circuit that appropriately takes in the data signal V _D ' demodulated by the FSK demodulator 10, including its own address data, controls the load, stores the data, and processes the received data. Note that this data processing circuit 16 is formed using a microcomputer, and also serves as a data processing circuit that forms a data signal V _D to be transmitted from the terminal device 1 and sends it to the FSK modulator 17. 18 is FSK demodulator 1
This is an interference discrimination section that determines the presence or absence of interference by detecting the amplitude modulation component (frequency 2Δf) included in the detected output V _F of the FM detection circuit 14 of 0. The transmission of the transmission signal _VT is stopped at the signal _VM .
Note that 19 detects the amplitude distortion that cannot be removed by the limiter amplification circuit 13 among the amplitude distortion contained in the amplitude modulation component contained in the output of the limiter amplification circuit 13, that is, the amplitude distortion contained in the reply signal _VB . This is a second interference discriminator that determines whether or not there is interference.The outputs of both interference discriminators 18 and 19 are input to the data processing circuit 16 via an OR circuit 20, and interference is detected from either of the mixture discriminators 18 and 19. When the detection signals V _M ′, V _M ′ are obtained, the transmission of the transmission signal V _T is stopped. The operation of the embodiment will be described below. now,
Transmission signal as shown in Figure 4a from the terminal 1A
V _TA is sent out, and almost at the same time, terminal device 1B sends
When a transmission signal V _TB as shown in the diagram is sent out, the current transmission signals V _TA and V _TB will interfere and beats due to interference will occur in the parts T ₁ and T ₂ (parts where the data are different) where the frequencies are different. As shown in c, it appears as an amplitude modulation component, and the frequency of the amplitude modulation component is
It is twice the shift frequency Δf in FSK modulation,
It is also included in the return signal V _B subjected to frequency conversion f ₁ →f ₂ in the converter section 2a of the central control device 2. This amplitude modulation component is removed by the limiter amplification circuit 13 and FM detected by the FM detection circuit 14. Third
The figure shows the FM detection characteristics, where the lower side of the S-shaped curve is the detection input, and the right side is the detection output _VF . By the way, the FM detection circuit 1 obtained in this way
A beat due to interference also appears in the detected output V _F of No. 4 as shown in FIG. 5, and the amplitude of the beat component changes according to the level difference between the two transmission signals V _TA and V _TB as shown in the table below.

〔Effect of the invention〕

As described above, the present invention includes a large number of terminal devices that send out transmission signals obtained by FSK modulating a carrier wave with a data signal, and a converter unit that sends out return signals obtained by converting the carrier frequency of the transmission signals from each terminal device. In a two-way communication system, each terminal device is provided with a data receiving section which connects a central control device equipped with a central control device to a single signal cable, demodulates the returned signal by FSK, and receives the data signal as appropriate. FSK
Each terminal device is equipped with an interference discrimination section that determines the presence or absence of interference by detecting the amplitude modulation component of the FM detection circuit output of the demodulator, and the transmission section transmits signals based on the interference detection signal output from the interference discrimination section. Since the presence or absence of interference is determined by detecting the amplitude modulation component of the FM detection circuit output of the FSK demodulator, the circuit configuration is simple and the presence or absence of interference can be determined reliably. This has the effect of preventing malfunctions when transmission signals are sent out simultaneously from a plurality of terminals.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a schematic configuration of a two-way communication system according to the present invention, FIG. 2 is a main block circuit diagram of an embodiment of the present invention, FIGS. 3 to 6 are operation explanatory diagrams, and FIG. FIG. 7 is a circuit diagram of a main part of another embodiment. 1A to 1B are terminal devices, 2 is a central control unit, 2a
1 is a converter section, 3 is a signal cable, and 18 is an interference discrimination section.

Claims (1)

[Claims] 1 A central control unit equipped with a large number of terminal devices that send out transmission signals obtained by FSK-modulating carrier waves with data signals, and a converter unit that sends out return signals obtained by converting the carrier frequency of the transmission signals from each terminal device. 1
Connect the branch signal cable to the main signal cable and send the return signal.
In a two-way communication system in which each terminal is equipped with a data receiving section that demodulates FSK and receives data signals as appropriate, interference can be avoided by detecting the amplitude modulation component of the FM detection circuit output of the FSK demodulator. 1. A two-way communication system characterized in that each terminal device is provided with an interference discriminating section for determining the presence or absence of interference, and transmission of a transmission signal is stopped in response to an interference detection signal output from the interference discriminating section.