JPS6284637A - Signal collision detection system - Google Patents

Abstract

PURPOSE:To detect a signal collision at low cost with high precision by detecting whether there is a down high frequency signal received from the computer of another station through a coaxial cable for the 1st time when data from each computer is transmitted, and transmitting the data with an up high frequency signal unless there is the high frequency signal of another station on the coaxial cable. CONSTITUTION:A pulse signal is normally inputted to a processing part as effective data when the pulse signal is communication data of another computer or this computer and the waveform 103 of a carrier detecting circuit 56 is '1', but if a collision occurs, a pulse Y7 is outputted to a line 422 to make the data ineffective and the transmission is interrupted immediately when the data is transmitted. Then, when there is no down high frequency signal, namely, when the output of the carrier detecting circuit 56 is '0', the unmodulated up high frequency signal is transmitted again to confirm that the collision does not occur, and the data is sent out to make a communication with another computer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a signal collision detection method in a local area network using high frequency signals.

[Prior art and its drawbacks] Recently, a large number of computers distributed in a relatively small area are connected via coaxial cables, distributors, and branches to form a branch-like coaxial cable network and data exchange. Local area networks that use high-frequency signals are attracting attention. A computer connected to this local area network has a line that modulates and demodulates data with a high frequency signal, and may transmit data at any time.

However, in this case, the demodulator detects the presence or absence of a downstream high-frequency signal on the coaxial cable and sends the data to another computer, but there is a delay time depending on the length of the coaxial cable that makes up the local area network. The drawback was that the data transmitted from the computer could be transmitted almost simultaneously, resulting in collision of high-frequency signals on the coaxial cable, thereby destroying the data transmitted from the computer.

Conventionally, in order to eliminate such drawbacks, a computer adds a specific preamble to the beginning of the data it sends to another computer to check whether its own data has been corrupted on the coaxial cable. If it is not destroyed, there is no way to send data to another computer or to
A method of checking whether the pulse amplitude after modulation exceeds a specified value or has increased due to a collision, and a method of checking whether the pulse width of received data has changed significantly have been proposed.

However, these methods require complicated detection circuits and signal processing methods, making the system expensive, and the demodulated waveform used to detect collisions is superimposed with a pulse signal such as a preamble, making it difficult to separate it. There is a drawback that the collision signal waveform component cannot be detected, and therefore highly accurate detection cannot be performed.

[Object of the Invention] The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a signal collision detection method that can detect signal collisions with high accuracy and at low cost in a local area network. be.

[Summary of the Invention] The signal collision detection method according to the present invention connects a plurality of distributed computers in the form of branches through distributors and branches, and transmits high-frequency signals between each computer at any time. When transmitting data from each computer in a local area network that uses a local area network to exchange data, first detect the presence or absence of a downstream high-frequency signal from another station's computer that is received via a coaxial cable, and then When there is no high-frequency signal from another station, data is transmitted using an upstream high-frequency signal. This upstream high frequency signal to be transmitted is sent as an unmodulated upstream high frequency signal carrier wave for a time sufficiently longer than the delay time of the coaxial cable, and is converted from the head end into a downstream high frequency signal and received by the demodulator of the computer at the local station. However, in order to detect the increase in noise generated by the collision of high-frequency signals transmitted from other stations during this non-modulation time, a high-gain amplifier circuit is connected to the detection circuit of the demodulation section, and after converting to a pulse signal, At the leading edge of the carrier detection circuit, a timer is activated that is longer than the delay time of the coaxial cable and shorter than the non-modulation time, thereby ensuring collision detection via the pulse signal and the AND gate circuit. It is something.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the system configuration of a local area network according to the present invention. In the figure, a plurality of computers 10a, 10b, . . . , 10n are connected to a branch cable 1.
1a, llb,..., 11n to the branch 12a,
12b, . . . , 12n. The coaxial cables 13a, 13b, -, and 13n constitute a main line, and a bidirectional trunk amplifier 14 is connected to the coaxial cables 13a, 13b, -, and 13n to correct for attenuation of upstream and downstream high frequency signals caused by the cables. Roughly speaking, the coaxial cable 13a is on the head end side, and the computer 10a is connected to the coaxial cable 13a on the head end side.
, 10b, . . . , 10n is connected to a frequency converter 15 for converting an upstream high frequency signal to a downstream high frequency signal.

FIG. 2 shows the internal structure of the frequency converter 15. In this frequency converter 15, when the computers toa, iob, . and is sent to the duplexer 21. The splitter 21 is a filter that separates an upstream high frequency signal and a downstream high frequency signal, and the upstream high frequency signal is sent to the amplifier circuit 23 via the line 22. The high frequency signal amplified by the amplifier circuit 23 is sent to the mixer 25 via the line 24, where it is mixed with the local oscillation signal sent from the local oscillator circuit 26 via the line 27, and the frequency of the sum or difference is determined. A signal is output. This sum or difference frequency signal is sent to a branching filter 29 via a line 28, where either one is selected and becomes a downstream high frequency signal. Then, this signal is sent to an amplifier circuit 31 via a line 30, and after being amplified, it is sent to a line 32 and a branching filter 2.
1 and then sent back to each computer 10a, 10b, . . . , 10n via the coaxial cable +38.

FIG. 3 shows the internal configuration of the computer 10a. In the figure, when transmitting data addressed to another computer from a processing unit 40 that performs processing such as data storage and calculation, a demodulation unit 41 sends a carrier detection signal from the other computer via a line 421 to know the transmission status. Monitor. Here, if the other computer is not in the transmitting state, the modulator 44 is controlled via the line 431, and the upstream high frequency signal is sent to the branch cable 11 via the line 45 and the branching filter 4G.
Output to a.

At this time, since the processing unit 40 has not yet outputted transmission data to the line 432, the upstream high frequency signal is "0" data, that is, it is an unmodulated upstream high frequency signal.

Next, this upstream high frequency signal is sent to the frequency converter 15 via the branch 12a and the coaxial cable 13a, where it is frequency converted and becomes a downstream high frequency signal.

This signal is transmitted to the coaxial cable 13a and the branch 12a.
, returns via the branch cable 11a, and is further connected to the branching filter 46.
, and are sent to the demodulator 41 via a line 47. When the signal demodulated by this demodulation section 41 is no signal, there is no transmission from other computers, and furthermore, when the collision detection signal is not output from the line 422, that is, when no collision has occurred, the processing section 40 determines that computer 10a is ready for transmission, and outputs data destined for another computer via line 432.

The modulation unit 44 has the function of performing FSKf modulation on the data sent from the processing unit 40 via the line 432 to the other computers 10b, . . . It has the function to control on or off. The demultiplexer 46 sends the upstream high frequency signal from the modulator 44 to the frequency converter 15, and also demultiplexes the downstream high frequency signal from the frequency converter 15 to the demodulator 4.
1.

Next, the internal configuration of the demodulator 41 is shown in FIG.
The figure shows signal waveforms at various parts inside the demodulating section 41.

In Fig. 5, waveforms 101a and 101b represent the case where any two of the computers 10a, 10b, -, 10n transmit uplink high frequency signals with a difference in time To in order to communicate with other computers. , the frequency converter 15
It is converted into a downstream high frequency signal and 11! When input to the tuning section 41, there is a collision during transmission, resulting in a waveform like 102. Here, the waveforms 101a and 101b are 1"1. H
2 shows a waveform of an unmodulated part, and DI and D2 show waveforms modulated by computer communication data. Waveform 102
When a collision occurs, the change in the imaging width of the downstream high-frequency signal occurs because the frequencies of the waveform 101a and the waveform 101b are slightly different, and therefore their phases are also different, so the change in amplitude of the sum and difference occurs. This is because it occurs.

This waveform 102 is sent to the high frequency amplification circuit 51 of the demodulation section 41 via the line 47 and amplified, and then mixed with the local oscillation signal from the local oscillation circuit 53 in the a gold separation 52 and converted to an intermediate frequency. . Unnecessary wave components outside the intermediate frequency band are removed from this intermediate frequency signal by an intermediate frequency filter 54, and the signal is guided to the next intermediate frequency amplification circuit 55.

The intermediate frequency amplification circuit 55 is provided with a carrier detection circuit 5G, which outputs a pulse as shown in waveform 103 when there is an intermediate frequency signal that has a correlation with the downlink high frequency signal input to the demodulator 41 via the line 47. do.

The rising edge of this pulse triggers the timer circuit 57 at the next stage, and a pulse of time T4 shown in waveform 104 is output to the gate circuit 58. The time T4 of the timer circuit 57 is set to be longer than the time from the computer 10n connected to the end of the network to the time when the frequency is converted by the frequency converter 15 on the head end side and returned.
This is made shorter than the unmodulated time of the uplink high frequency signals transmitted from 0a, 10b, -, 10n, so that the data sent to other stations does not fall into the collision detection time. On the other hand, the intermediate frequency amplification circuit 55 is provided with a detection circuit 59 that performs FSK detection and demodulation. The output waveform of the detection circuit 59 is, as shown in the waveform 105, during the time T1. Noise is generated during T2 and the time T2- during which the collision occurs.

Here, this noise signal is amplified by a high gain amplifier circuit 60,
After clamping, a waveform 106 is obtained, and the above-mentioned timer circuit 5
With the output pulse of No. 7 and the gate circuit 58, a noise pulse is obtained only at the collision occurrence time T6 of the non-modulated time as shown in the waveform 107. Further, the monostable multivibrator 61 generates a time T7 and outputs it to the line 422, and the processing section 40 detects the occurrence of a collision. The Schmitt circuit 62 shapes the waveform 105, which is the demodulated waveform of the detection circuit 59, and outputs it to the line 423 as a pulse signal.

That is, normally this pulse signal is communication data from another computer or the own computer, and is taken into the processing unit 40 as valid data when the waveform 103 of the carrier detection circuit 56 is "1", but as described above, a collision occurs. If this occurs, a T7 pulse is output to the line 422, invalidating it, and immediately interrupting the transmission if data is being transmitted. After that, when there is no downstream high-frequency signal, that is, when the output of the carrier detection circuit 56 is "0", an unmodulated upstream high-frequency signal is transmitted again, and after confirming that no collision occurs, the data is transmitted, and the other data is transmitted. communicate with computers.

[Effects of the Invention] As described above, according to the signal collision detection method according to the present invention,
When sending data from multiple computers to a coaxial cable at any time using an upstream high-frequency signal, in order to output an unmodulated upstream high-frequency signal in advance, the frequency-converted downstream high-frequency signal is demodulated and then converted into a data pulse. The configuration detects the presence or absence of collisions by separating them in time. During this non-modulation period, the noise caused by collisions with other downlink high-frequency signals is amplified with high gain and detected, enabling highly sensitive collision detection. becomes. Furthermore, as with conventional collision detection methods, is it possible to store the transmitted data and compare it bit by bit with the data returned from the headend over the coaxial cable? ! Since it does not require complicated circuits or processing, it becomes an inexpensive local area network with a signal collision detection method.

[Brief explanation of drawings]

FIG. 1 is a system configuration diagram of a local area network according to an embodiment of the present invention, FIG. 2 is a block diagram showing the internal configuration of a frequency converter in the system, and FIG. 3 is a block diagram showing the internal configuration of a computer. Similarly, FIG. 4 is a block diagram showing the internal configuration of the demodulating section, and FIG. 5 is a waveform diagram of each part of the demodulating section. 10a, 10b, -, 10n·ml Nbuta, 11a. 11b, -, l1n-...branch cable, 12a
, 12b, . 12n... Turnout, 13a, 13b,..., 13n
... Coaxial cable, 15 ... Frequency converter, 41.
... demodulation section, 44 ... modulation section, 56 ... carrier detection circuit, 57 ... timer circuit, 58 ... gate circuit.

Claims (1)

[Claims] A plurality of distributed computers are branch-connected to a branch-like bidirectional coaxial cable network, and each of the computers is provided with means for modulating and demodulating a data signal using a high-frequency signal, and the head end side is provided with means for modulating and demodulating a data signal with a high-frequency signal. In a local area network provided with means for converting the frequency of a high frequency signal into a downstream high frequency signal,
When the computer communicates with other computers,
First, it sends out an unmodulated upstream high frequency signal for a predetermined period of time, then modulates and sends communication data, monitors the unmodulated time with a demodulation circuit, and detects collisions caused by transmissions from other computers. Therefore, the demodulation circuit is provided with a carrier detection circuit that detects the presence or absence of a downstream high-frequency signal, and a timer circuit that is triggered by the pulse edge of the detection start of this carrier detection circuit, and the collision noise after demodulation during the non-modulation time is amplified. This signal collision detection method is characterized by separating and detecting the collision using a gate circuit.