JPH0553337B2 - - Google Patents

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 function of modulating and demodulating 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. , almost simultaneous transmission occurs, causing collision of high-frequency signals on the coaxial cable,
For this reason, there was a drawback that the data transmitted from the computer was destroyed.

Conventionally, in order to eliminate such drawbacks, computers add a specific preamble to the beginning of data sent to other computers.
Check whether your own station's data has been destroyed on the coaxial cable, and if not, find out how to send data to other computers, and if the pulse amplitude after demodulation is higher than the specified value or if there is a collision. A method of checking whether the pulse width of the received data has changed significantly has been proposed.

However, these methods require complicated detection circuits and signal processing methods, making the systems expensive.Also, pulse signals such as preambles are superimposed on the demodulated waveform used to detect collisions, and are separated and detected when collisions occur. The disadvantage is that the 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. It is in.

[Summary of the Invention] The signal collision detection method according to the present invention connects a plurality of distributed computers in a branched manner via distributors and branches, and transmits high-frequency signals between each computer at an arbitrary time. When transmitting data from each computer in a local area network where data is exchanged using If there is no high frequency signal from another station on the uplink, 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 demodulation section of the computer at the local station. In order to detect the increase in noise generated by the collision of high-frequency signals transmitted from other stations during this non-modulated time, a high-gain amplifier circuit is connected to the detection circuit of the demodulation section, and after converting to a pulse signal, the carrier The first rising edge of the detection circuit activates a timer 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.

[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 branch cables 11a, 11b, ...
..., 11n, branchers 12a, 12b, ...
..., 12n. Further, the coaxial cables 13a, 13b, . . . , 13n constitute a trunk line, and a bidirectional trunk amplifier 14 is connected to the coaxial cables 13a, 13b, . Further, the coaxial cable 13a is located on the head end side, and a frequency converter 15 is connected to its terminal end for converting an upstream high frequency signal from the computers 10a, 10b, . . . , 10n into a downstream high frequency signal.

FIG. 2 shows the internal structure of the frequency converter 15. In this frequency converter 15, computers 10a, 10b, ..., 10n
When data for mutual communication is modulated into an upstream high frequency signal at the appropriate time, this upstream high frequency signal is sent to the duplexer 21 via the coaxial cable 13a. 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
The signal is sent to a mixer 25 via a line 24, mixed with a local oscillation signal sent from a local oscillation circuit 26 via a line 27, and a sum or difference frequency signal is output. This sum or difference frequency signal is sent to a branching filter 29 via a line 28.
9, one of them 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 each computer 10a, 10b, .
sent back to the side.

FIG. 3 shows the internal configuration of the computer 10a. In the figure, data storage,
When transmitting data addressed to another computer from the processing unit 40 that performs processing such as arithmetic operations, the demodulation unit 41
A carrier detection signal, which indicates the transmission status from another computer, is monitored via the line ₄₂₁ . here,
If no other computer is transmitting, line 4
3 ₁ to control the modulating section 44, and output the upstream high frequency signal to the branch cable 11a via the line 45 and the splitter 46. At this time, the processing unit 40
3 Since the sending data has not been output to ₂ yet,
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 branch 12a,
The frequency converter 1 via the coaxial cable 13a
5 and undergoes frequency conversion to become a downstream high-frequency signal. This signal is transmitted to the coaxial cable 13a,
The signal returns via the branching device 12a and the branching cable 11a, and is further sent to the demodulating section 41 via the branching device 46 and line 47. If the signal demodulated by this demodulator 41 is no signal, there is no transmission from other computers, and if the collision detection signal is not output from the line ₄₂₂ , that is, if no collision has occurred, the processor 40 is the computer 10a
determines that it is ready for transmission, and outputs data destined for another computer via line ₄₃₂ .

The modulation unit 44 connects the processing unit 40 to the line 43 ₂
to other computers 10b,...
...,10n data addressed to FSK modulation function,
It has a function of turning on or off a high frequency signal, which is a transmission wave, with a signal to the line ₄₃₁ . Duplexer 4
6 sends the upstream high frequency signal from the modulation section 44 to the frequency converter 15, and also
The downstream high frequency signal from 5 is demultiplexed and sent to the demodulator 41.

Next, the internal configuration of the demodulator 41 is shown in FIG.
Further, FIG. 5 shows signal waveforms at various parts inside the demodulator 41.

In FIG. 5, waveforms 101a and 101b are
In the case where any two of the computers 10a, 10b _, .
When the frequency converter 15 converts the signal into a downstream high-frequency signal and inputs the signal to the demodulator 41, a collision occurs during transmission, resulting in a waveform 102. Here, waveform 1
H ₁ and H ₂ of 01a and 101b indicate waveforms of non-modulated portions, and D ₁ and D ₂ indicate waveforms modulated by computer communication data. When a collision occurs in waveform 102, the amplitude of the downstream high-frequency signal changes because the frequencies of waveform 101a and waveform 101b are slightly different, and therefore their phases are also different. This is because change occurs.

This waveform 102 is transmitted to the demodulator 4 via a line 47.
After being sent to the high frequency amplification circuit 51 of No. 1 and amplified,
A mixer 52 mixes the signal with a local oscillation signal from a local oscillation circuit 53 and converts it into an intermediate frequency. This intermediate frequency signal has unnecessary wave components outside the intermediate frequency band removed by an intermediate frequency filter 54, and is guided to the next intermediate frequency amplification circuit 55.

The intermediate frequency amplification circuit 55 includes a carrier detection circuit 5.
6 is provided, and when there is an intermediate frequency signal that has a correlation with the downlink high frequency signal inputted to the demodulator 41 via the line 47, a pulse like the waveform 103 is output. The rising edge of this pulse triggers the next stage timer circuit 57, and the time shown in the waveform 104 is
A pulse of T ₄ 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 frequency conversion by the frequency converter 15 on the head end side and back, and furthermore, 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 returns. This is made shorter than the unmodulated time of the uplink high-frequency signal transmitted from the station, 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. As shown in waveform 105, the output waveform of the detection circuit 59 is generated at times T ₁ and T ₃ when there is no downstream high-frequency signal in the demodulator 41, and at times T ₂ ' when a collision occurs.
Noise is generated during this period.

Here, this noise signal is amplified and clamped by the high gain amplifier circuit 60 to obtain a waveform 106, which is combined with the output pulse of the timer circuit 57 and the gate circuit 58.
As shown in waveform 107, a noise pulse is obtained only at collision occurrence time T ₆ of non-modulated time. Furthermore, create a time T ₇ with a monostable multivibrator 61,
The signal is output to the track ₄₂₂ , and the processing unit 40 learns of 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.

In other words, normally this pulse signal is communication data from another computer or your own computer.
When the waveform 103 of the carrier detection circuit 56 is "1", it is taken into the processing unit 40 as important data, but if a collision occurs as described above, the line 42
If a _T7 pulse is output to ₂ , invalidating it and transmitting data, the transmission is immediately interrupted. 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 sent to the other device. communicate with the computer.

[Effects of the Invention] As described above, according to the signal collision detection method of the present invention, when data is sent from multiple computers to a coaxial cable at an arbitrary time using an upstream high-frequency signal, an unmodulated upstream high-frequency signal is sent in advance. In order to output, the frequency-converted downlink high-frequency signal is demodulated, and then separated in time from the data pulse to detect the presence or absence of a collision. Since the time noise is amplified and detected with high gain, highly sensitive collision detection is possible. Furthermore, unlike conventional collision detection methods, there is no need for complex circuitry or processing to store the transmitted data and compare it bit by bit with the data returned from the head end over the coaxial cable.
It becomes a local area network with an inexpensive 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 same 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 structure of the demodulating section, and FIG. 5 is a waveform diagram of each part of the demodulating section. 10a, 10b,..., 10n... Computer, 11a, 11b,..., 11n... Branch cable, 12a, 12b,..., 12n... Branch, 13a, 13b,..., 13n... Coaxial cable , 15... Frequency converter, 41... Demodulator,
44...Modulation section, 56...Carrier detection circuit, 5
7...Timer circuit, 58...Gate circuit.

Claims (1)

[Claims] 1 A plurality of distributed computers are branch-connected to a branch-like bidirectional coaxial cable network, each of the computers is provided with means for modulating and demodulating a data signal with 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 frequency converting a high frequency signal into a downstream high frequency signal, when the computer communicates with another computer, it first sends out a fixed time non-modulated upstream high frequency signal. , after that, the communication data is modulated and sent, and a demodulation circuit monitors the non-modulation time,
In order to detect collisions caused by transmissions from other computers, 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. The setting time is the round-trip time required from the end computer connected to the transmission path to the time it is sent back and received via the head end, and is set to be shorter than the computer's non-modulation time. After amplifying the collision noise after demodulating the non-modulated time, the output of the timer circuit and the gate circuit perform a logical sum to separate and detect the collision noise, and further detect the collision occurrence on the output side of the gate circuit using a computer. A signal collision detection method characterized by having a configuration in which a monostable multivibrator is provided to output the pulse width necessary for detection.