JPS6110344A - Loopback control system in data transmission - Google Patents

Abstract

PURPOSE:To attain switching by electric components in case of the optical fiber transmission by switching the transmission line not by a transmission line part but by the inside of a station. CONSTITUTION:If a station ST2 is faulty completely, a signal from the station ST2 is lost at a main transmission line side and a detector DT1 of a station ST3 has no signal and its output is 0. Thus, a changeover switch S1 is turned on to the position of a repeater RP and a switch S4 is turned off. As a result, a station ST3 is connected by the path of sub-transmission line R2 S2 RP TM D1 main transmission line. The switch S4 is turned off and no signal is transmitted to the station ST2. The detector DT2 of the station ST1 has no signal and then the changeover switch S2 is turned on to the position of a transmitter TM. As a result, the station ST1 is connected in the path of main transmission line R1 S1 TM S2 RP S4 D2 sub-transmission line. Then only the station ST2 is disconnected and the loop back is established.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a loopback control method for automatically performing loopback in a loopback data transmission system with duplex transmission paths.

(Prior Art) In recent years, data transmission systems that connect computers (including microcomputers) or between computers and their terminals have come into widespread use, and many systems are forming complex networks.

There are many types of networks, but typical ones include the bus system and the loop system.

Figure 17 shows an example of the configuration of the loop method.
The transmission line is configured in a loop shape.

Unlike the bus system, relays are performed for each station, so
Since the signal is amplified for each station, it is more suitable for large-scale systems than bus systems. In the loop method, as shown in the figure, the signal goes around the loop in one direction. Therefore, relay control is easy. When relaying, signals are bidirectional on a line as shown in FIG. 18, making control of the repeater complicated. Therefore, in systems that require relaying, a loop is generally used.

The loop method includes the methods shown in FIGS. 18 and 17. In the method shown in FIG. 19 (loop method in a narrow sense), there is a loop controller 2 that performs transmission control, and each station performs transmission under the control of this loop controller.
In the method shown in FIG. 7 (ring method), there is no loop controller, and each station is on an equal footing. In this case as well, some kind of control of the loop as a whole is required. For example, there is a method in which each station is given a function to control the entire loop, and each station takes turns according to certain rules.

Generally, the loop method is easier to control than the ring method. However, if the loop controller fails, the system will go down. The ring system is considered to be a highly reliable system because the failure of one station does not cause the system to go down. However, in reality, there are factors that do not necessarily improve reliability, such as multiple stations attempting to control the entire loop at the same time, or conversely, times when none of the stations control the loop. In order to solve these problems, abnormality recovery means are required.

In the loop system (loop and ring in the narrow sense), backup in case of failure is important. In the loop system, even if one station or transmission path goes down, the entire system will go down. That is, in the loop system, in order to perform normal operation, the signal must be able to go around the loop. If a situation occurs where the signal cannot go around the loop, it will lead to system failure.

Therefore, in systems that require high reliability, the loop method requires a backup system.

Many methods have been proposed for backup in the loop method.

One of the typical methods is the loopback method. As shown in FIG. 20, this is a highly reliable backup system that includes duplication of transmission lines. (Note that the figure illustrates the case where the number of stations is 4). In the figure, each station ST, ~ST4 incorporates transmission equipment WiTM, -TM, and repeaters RP, ~RP. Each transmission device transmits rH-1 data via a main transmission line 1, while each repeater RP, to RP4 is connected via a sub-transmission line 2 in a cascade configuration.

Now, consider the case where a failure occurs.For example, station S
Suppose that T fails. In this case, the transmission path is switched as shown in FIG. By this switching, the failed station ST is disconnected, but the other stations are switched from TM2 to TM.
Since a loop is formed by the routes 3→TM, →RP, →RP3→RP2→TM2, the transmission devices 17M7 to TM, excluding the failed station, can perform transmission with each other.

Furthermore, even in the case of a disconnection of the transmission line rather than a failure of the station, it is possible to deal with the case, for example, as shown in FIG. 22.

In this case, TM2→TM3→TM, →TM, →RP
, →RP4→RP3→RP, and →TM2 form a loop, and all the transmission devices can perform mutual transmission.

As described above, the loopback method is an excellent backup method because it can cope with not only station failures but also transmission line disconnections.

(Problem to be solved by the invention) However, the occurrence of failures and disconnections is not simple;
Therefore, it is extremely difficult to detect all factors and to perform appropriate switching according to the factors.

Therefore, in the conventional system described above, a specific fault condition is detected and switching is performed accordingly. That is, automatic switching is possible only for specific factors, but automatic switching is not possible for other factors and relies on manual switching. Further, the above-mentioned conventional method utilizes transmission line switching for loopback control. There is no problem in the case of electrical transmission using electric wires for the transmission line, but in optical transmission using optical fiber, switching the transmission line requires an expensive optical switch that causes signal attenuation. Therefore,
Especially in the case of optical transmission, the loopback method is less practical.

Therefore, the present invention was made in order to solve the problem described in -F of the conventional system, and by switching the transmission line inside the station instead of the transmission line part, the electric power in the case of optical fiber transmission is improved. It is possible to switch between sections, increasing economic efficiency, and it is easy to detect failures or disconnections, while being able to respond appropriately to any situation.In particular, in the case of optical fiber transmission, it is also possible to detect and respond to optical element failures. The purpose of this study is to provide a loopback control method for data transmission.

(Means for Solving the Problems) The present invention provides that each station has a transmission device and a repeater, and a loop-shaped main transmission line that connects each transmission device in a cascade, and a loop-shaped main transmission path that connects each repeater in a cascade. However, in a data transmission system having a loop-shaped sub-transmission path in which a signal travels in the opposite direction to the loop-shaped main transmission path, each station has the output of a receiver (R1) that receives signals from the main transmission path and a repeater. A changeover switch (S1) that switches between the output and the input of the transmission device.
, a switching switch 2 (S2) that switches between the output of the receiver (R2) that receives a signal from the sub-transmission line and the transmission device and supplies the signal to the input of the repeater; and the output of the repeater and the sub-transmission line. A switch (S4) turns off the connection to the input of the driver (D2) that sends a signal to the receiver (R
1) A detector (DT) that detects the presence or absence of a signal at the output of
1) and a detector (DT2) that detects the presence or absence of a signal at the output of the receiver (R2), and when the detector (DT1) detects the presence of a signal, the changeover switch (31) switches the output of the receiver (R1). When the detector (DT2) detects the presence of a signal, the changeover switch (S2) is controlled so that the side is on and the switch (S4) is on.
This is a loopback control force type in data transmission characterized in that the R2) side is controlled to be turned on.

Hereinafter, the present invention will be explained based on a first embodiment and a second embodiment with reference to the drawings.

(First example) FIG. 1 shows an embodiment of the station configuration according to the present invention.

In station ST, a receiver R1 receives the signal from the main transmission path. The receiver may be electrical, but in the case of fiber optic transmission it is an optical/electrical converter. The output of the receiver R1 is connected to one input of the selector switch S1 and the signal detector (D
T1). The changeover switch may be constituted by a logic element or gate, and the other input of the changeover switch Sl is connected to the output of the repeater RP. The output of the transmission device TM is output to one input of the changeover switch S2. The transmission device TM outputs its own signal when transmitting, and relays the input and outputs when not transmitting. The output of the transmission device is the driver D
, and is output to the main transmission path. The driver D1 may be an electric type, but in the case of optical fiber transmission, it is an electric/optical converter.

The signal from the sub-transmission path is input to the other input of the changeover switch S2 and the signal detector DT2 via the receiver R2. The output of the changeover switch S2 is input to the repeater RP. The output of the repeater RP is input to the other input of the changeover switch SI and the switch S4. The output of switch S4 is
The signal is output to the sub-transmission line via driver D. Receiver R
2 is the same as the receiver R2, the changeover switch S2 is the same as the changeover switch S1, and the driver D2 is the same as the driver D. The switch S4 may be composed of a gate of a logic element or the like.

The detectors DTI and DT2 output 1 when detecting the presence of a signal at the input of the detector, and output O when detecting a signal heat. Different signals are used in the transmission path, and thus at the receiver output, depending on the transmission system, and therefore, the determination of the presence or absence of a signal also depends on the nature of the signal.For example,
When the NRZI code is used as a signal, as shown in Figure 2, the presence of a signal means that there is a change from 0 to 1 or from 1 to 0 within a certain time Tc, and the signal heat is 1 or 0.
This means that either one of the above continues for a certain period of time Tc or more. Therefore, the detector may use, for example, the circuit shown in FIG. Tc in Figure 2
).

In any case, the presence of a signal does not mean whether data is actually being sent, but rather the state in which the signal is present regardless of the presence or absence of data.Signal heat also refers to the presence of a signal due to a failure or disconnection. In other words, a signal in which it is impossible to distinguish between a state in which there is no signal due to a failure or disconnection and a state in which there is a signal but is simply not transmitting data is used as a signal in the present invention. However, it is generally possible to use signals with the above-mentioned properties in data transmission, and the invention does not lose its versatility.

In the present invention, when the output of the detector DT is 1, the changeover switch S1 is turned on to the receiver output side, and the changeover switch S4 is turned on. Further, when the output of the detector DT2 is 1, the changeover switch S2 is operated so that the receiver R2 is turned on.

The station of the present invention shown in FIG. 1 is connected in a loopback manner as shown in FIG. 20. During normal operation, signals always flow through the main transmission path and transmission is performed.

Although the sub-transmission path is not involved in transmission, in order for the present invention to operate normally, it is necessary to keep the @ sign flowing at all times.

Consider the case where a failure or disconnection occurs in this state. This failure or disconnection can take various forms. A specific case will be explained below.

First, the normal state is shown in Fig. 4. In the same figure, the signal is emitted from a station above station ST, flows through the main transmission path in the order of S T H, S T 2 + S T 3, and further on. It is being sent to lower-level stations. In addition, on the sub-transmission path, signals from stations lower than ST3 are sent to ST2.
, ST, and is sent to the upper station.

Figure 5 shows the case where station ST2 has completely failed. Therefore, on the main transmission line side, the signal from station ST2 becomes null, the detector DT of station ST becomes signal hot, and the output becomes O. .

Therefore, the changeover switch SI is placed on the repeater RP side, and the switch S4 is turned off. As a result, station ST3 is connected to the sub-transmission path →
R2→S2→RP+TM→D, →Connected via the main transmission path. The switch S4 is off, so no signal is sent to the station ST2, but the 1st station ST2 is originally out of order, so there is no good luck at the 0th station ST. Similarly, the detector DT2 becomes a signal type, and therefore , changeover switch S, are on the transmission device TM side. This result station ST.

is the main transmission line→R,→S, 4TM 4S2→RP→S
Only the station ST2 is disconnected from 0 or more stations connected by the path 4→D2→sub transmission line, and a loopback is established.

FIG. 6 shows a case where the main transmission line between stations ST2 and ST3 is disconnected.
The output of the detector DT becomes signals, the detector DT detects the signals, and the output of the detector DTI becomes 0. As a result, station S1
The RP side is turned on, and the switch S4 is turned off. Therefore, sub transmission line → R2 → S2 → RP = S l = T
M ” D l → 0 station S connected via the main transmission line path
With switch S4 of T3 turned off, station S
The output of the driver D2 of T is signals. Therefore, in the station ST2, the detector DT2 detects the signals, and the changeover switch S2 is turned on on the TM side. This allows station ST
2 is the main transmission line → R0 → S, → TM-S2 → RP → S4
→D2→The disconnection portion is disconnected from 0 or more to which the sub-transmission path is connected, and a loopback is established with all stations in the operating state.

In addition, it is not a disconnection, but a failure of the driver D1 of station ST2.
When receiver R of station ST3 fails, it operates in the same way, only the failed part is isolated, and station ST2 and station ST3 are operable. In order to replace the failed driver D of the station ST2 or the receiver R1 of the station ST3, it is necessary to temporarily stop the station ST2 or ST3, but in that case, the same condition as shown in Fig. 5 will occur, and the automatic Loopback is established.

Figure 7 shows a case where the main transmission line and the sub-transmission line are disconnected at the same time.Generally, the main transmission line and the sub-transmission line are often routed along the same route, so disconnections are expected to occur at the same time. be done. In this case as well, as shown in FIG. 7, the sixth and third circuits operate in the same manner as C, forming a loopback in which only the broken transmission line is disconnected.

FIG. 8 shows a case where the transmission device TM of the station ST2 is out of order. As a result, first of all, the detector DT of the station ST detects signals. This causes the changeover switch S1 to switch to RP.
side is turned on and switch S4 is turned off. Therefore, in station ST, a loopback path from the sub-transmission line to the main transmission line is completed.

On the other hand, since the switch S4 of the station ST3 is off, the detector DT of the station ST2 detects the signals, and the changeover switch S2 becomes TM side neon. However, since the transmission device TM of the station ST2 is out of order, the output of the driver D2 of the station ST2 becomes signals. Similarly, the detector DT of the station ST
2 detects the signals, and the station ST creates a loopback path from the main transmission line to the sub transmission line.From 0 or more, the station ST, whose transmission equipment has failed, eventually performs a loopback in a state where the station ST is disconnected. holds true.

A situation similar to that shown in FIG. 6 or 8 also occurs on the sub-transmission line. In this case as well, the detector DT2 that has detected the signals operates. However, in this case, only the switch S2 is changed over, and the main transmission path side continues to transmit data.

Therefore, transmission is not affected.

Inoperability of a station also occurs not due to a failure but due to power cutoff to the station.According to the method of the present invention, even if a station becomes inoperable due to power cutoff to one station, the same problem as shown in FIG. Loopback is established. Power outages can also be caused by partial power outages. In this case, as shown in Figure 9,
It is possible that some subsequent stations may lose power (
In the case of the figure, three stations ST2 to ST4 are out of power).

In this case, according to the method of the present invention, the power outage station is automatically disconnected and loopback is established.

Note that, as shown in FIG. 10, if distant stations fail or experience a power outage at the same time, no loop is formed as a whole. Therefore, communication with stations belonging to other loops becomes impossible. However, this is a drawback of the loopback method itself, and cannot be solved even if a control method other than the method of the present invention is adopted.

In addition, when the loopback method operates and enters the loopback state, a station disconnected from the loop by the loopback may have a loop controller in the loop method (in a narrow sense) or a controller for abnormality recovery in the ring method. If it contains a device, it will become inoperable if left as is. Therefore, it is necessary to have a backup controller and switch to it. However, although this occurs incidentally to the present invention, it is a separate matter from the present invention. Therefore, the specific method will not be mentioned.

In the present invention, it is necessary that a signal is routed to the sub-transmission line side even in normal conditions. Various methods can be considered for this purpose, but the simplest method is to do nothing; at this time, at startup, no signal is circulating in the sub-transmission path.

However, this causes the detection station DT2 of one of the stations to operate and introduces the signal on the main transmission path to the sub-transmission path. Thereafter, the signal automatically circulates through the sub-transmission path.

However, to ensure even more reliability, it is sufficient to install a signal generator at one location in the loop on the sub-transmission line side and output the signal from this signal generator. Fig. 11 is a block diagram of an example of the configuration. It is. Under normal conditions, the signal generator SG side of the changeover switch S5 is on. Therefore, the signal generator SG is outputted to the sub-transmission line via the changeover switch SSr driver D2, goes around the loop of the sub-transmission line, and returns to the receiver R2. However, since it is necessary to have a relay function when the loopback state is entered, the changeover switch S5 is turned on on the repeater side during the loopback, and the signal is relayed. It is sufficient that the @ signal generator SG and the changeover switch S5 are located at one location in the loop in the sub-transmission line, but there is no problem in having a plurality of them for backup purposes in case of a failure.

In order to switch the sea lion, it is necessary to determine whether it is a normal state or a backup state. Therefore, the transmitted signal and the output signal from the signal generator SG must be distinguishable;
That is, the output signal from the signal generator SG needs to be a dummy signal. In general, such a dummy signal varies depending on the transmission system, but consecutive 0's or consecutive 1's are often used, and the transmission signal is supposed to alternate between 0's and 1's during data transmission. Even in the transmission signal, when data is not being transmitted, O and I may be consecutive. Therefore, it is often impossible to distinguish between a transmission signal that is not transmitting data and a dummy signal.However, since it is a transmission signal, it cannot continue without transmitting data forever, so a dummy signal and a transmission signal that are not transmitting data are different. If you wait, it will definitely be possible to determine.

Now, in the loopback system, during normal operation, a dummy signal is circulating in the sub-transmission line. On the other hand, during loopback, the transmission signal rotates. This transmission signal goes around to the station with the signal generator SG, so here:
The determination is made by a determiner ID that determines whether the signal is a transmission signal or a dummy signal, and when the transmission signal is determined, the selector switch S is switched to the repeater side.ID in FIG. 11 indicates this determiner.

Although this determiner differs depending on the nature of the signal, it uses an NRZI code as a transmission code and uses continuous O as a dummy signal.
Fig. 12 shows an example of the configuration when using a dummy signal.When a dummy signal is used, the retriggerable mono multivibrator SS is always triggered, and when a transmission signal comes and l continues for a predetermined number of bits or more, The output of the SS changes to identify the transmitted signal. Generally, in order to prevent switching errors, it is necessary to continuously determine whether the signal is a dummy signal or a transmission signal. Even if a transmission signal is temporarily detected,
A method is required, such as determining that the signal is a dummy signal when O continues for a certain period of time or more.

After detecting the transmission signal and switching the changeover switch S5 to the repeater side to complete the loopback state, when the loop returns to the normal state again, this can be automatically detected and the return to normal state can be performed. Various methods can be considered for this detection as well. Generally, in the loop system, the relay is often performed by digital regenerative relay, and this digital internal relay is usually performed using a PLL (phase-locked loop). FIG. 13 shows an example of a configuration of a digital regenerative relay. A PLL extracts a clock component contained in an input signal and outputs a stable clock. In this PLL, it is generally possible to detect when the PLL operates normally and outputs a clock synchronized with the input clock component, but when the operation becomes abnormal, that is, out-of-synchronization. Note that 13 in the figure is a D-type flip-flop.

In a loop data transmission system synchronized by a PLL, as shown in Figure 14, a system is used in which a signal is output based on the reference clock at one point during the loop, and all others are synchronized by the PLL. In other words, the reference station generates a reference clock using a reference clock generator CG, and sends out a signal using that clock. All other stations (including the receiving side at the reference station) operate by extracting clocks using the PLL.

In a system with such a configuration, there is no reference station, and it is impossible for all stations to operate by PLL, and in this case, an out-of-synchronization condition occurs.

In the case of a transmission system using PLL as described above, the first
The signal generator SG in FIG. 1 also serves as a reference station for this synchronization. During normal operation, this station is in the loop, and therefore synchronization is achieved using the PLL. During loopback, this signal generator SG switches to relay. Therefore, it is no longer a synchronous reference station. However, in this case, one of the stations on the main transmission path must be the reference station for synchronization, and therefore synchronization will be achieved normally.

However, when the loopback is restored and the state returns to normal, the synchronized reference station no longer exists. Therefore, an out-of-synchronization phenomenon occurs. Conversely, by detecting this loss of synchronization, it is possible to know that the loopback state has returned to the normal state.

FIG. 15 shows an embodiment in which the following functions are added.

In the figure, SY is an out-of-sync detector, C0NT is a control circuit, and the other components are the same as those described above. Basically, the control circuit C0NT switches the switch S5 to the repeater RP side when the determiner ID determines that there is a transmission signal, and the signal generator SG when the out-of-sync detector SY detects out-of-sync.
All you have to do is generate an output that turns on the side, but,
In reality, transient states occur, such as a temporary out-of-sync state when transitioning from normal state to loopback state.
Auxiliary circuits may be required to confirm the continuation of a fixed period of time, etc.

Here, one configuration example of the control circuit is shown in FIG.

The output of the determiner ID that determines the presence or absence of a transmission signal is l when the transmission signal is present. However, when synchronization occurs due to switching, the input of this determiner itself cannot be trusted.Therefore, it outputs l only when it is in synchronization and there is a transmission signal (a). Set flip-flops. The output of this set of flip-flops causes the changeover switch Ss to switch between repeaters RP and RP.
so that the side is turned on. Further, a timer TM is provided so that the flip-flop is reset not by a temporary loss of synchronization but by a continuation of a loss of synchronization for a certain period of time.

(The following is a blank space) (Second Example) Next, a second example of the present invention will be described. FIG. 23 shows the station configuration of the second embodiment. The differences from the first embodiment are as follows.
On the main transmission path side, the output of the transmission device TM and the driver D
1 is provided with a switch S3 between the two. Switch S3 is equivalent to switch S4.

From the above, the main transmission line side and the sub transmission line side operate in exactly the same way. As a result, the states shown in #I24 to FIG. 27 correspond to those in FIGS. 5 to 8.

Although there is a difference in whether the switch S3 is activated or not, the result is exactly the same function.

However, when the phenomenon shown in FIG. 6 or 8 occurs on the sub-transmission path side, loopback does not occur in the first embodiment and transmission continues. On the other hand, in the second embodiment, loopback occurs.

This difference is because failures (including disconnections) in the sub-transmission line
The WII2 embodiment seems to be at a disadvantage in that it causes loopbacks even though the main transmission line is fault-free.

However, in the first embodiment, even though a failure has occurred on the sub-transmission line side, there is a possibility that the failure may occur without being aware of it.

In this case, it means that a failure has occurred on the main transmission line side and no transmission is performed even if looped back.

From the above, the second embodiment has the meaning of warning of a failure on the sub-transmission line side.

In the second embodiment as well, generation of dummy signals and switching thereof can be performed in the same manner as in the first embodiment.

(Effects of the Invention) As explained above, according to the present invention, it is possible to provide a loopback control method that appropriately responds to all conditions. , can be easily and economically applied.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the first embodiment of the present invention, Fig. 2 is a waveform diagram for explaining the presence or absence of a signal, Fig. 3 is an example of the configuration of the detector shown in Fig. 1, and Figs. FIG. 8 is a block diagram for explaining the operation of the first embodiment, FIGS. 9 and 10 are block diagrams for explaining the operation of the first embodiment when the power is cut off, and FIG. 11 is a block diagram for explaining the operation of the first embodiment when the power is cut off. A block diagram when a signal generator is provided in a repeater, and FIG. 12 is a configuration example for detecting the presence or absence of a signal when using an NRZI code.
Figure 3 shows an example of the configuration of the main part of a digital regenerative relay using PLL, Figure 14 shows an example of the configuration of a loop data transmission system that performs synchronization using PLL, Figure 15 shows an example of the configuration for detecting out-of-synchronization, Figure 18 is an example of the configuration of the control circuit in Figure 15, Figure 17 is an example of a loop type configuration, Figure 18 is an example of a configuration where each station is connected in a line, and Figure 18 is an example of a loop type configuration in a narrow sense. , s20 through FIG. 22 are diagrams for explaining the conventional loopback method, FIG. 23 is a block diagram of the second embodiment of the present invention, and FIGS. 24 through 27 are diagrams showing the operation of the second embodiment. FIG. 2 is a block diagram for explaining. 1-m-main transmission line. 2-m-subtransmission line, lo---buffer. II-m-exclusive OR, ! 2---Retriggerable mono multivibrator (S
S). ST, ~ST, o---station, TM-m-transmission device, RP-m-middle *S, DT,, DT, -m-signal detector, R,, R2-m-receiver, DI + 02--- Driver, S l *
s2. S S--All changeover switch, Ss, 5a---Swee, Chi, SG-m-Signal generator. ID-m-determiner, PLL-m-phase locked loop, SY-m-control circuit, SY-m-out-of-synchronization detector, TM-m-timer. Fig. 15 R, P Fig. 161!1 -9 [- Fig. 18 Fig. 191!1 2011 Fig. 21

Claims (5)

[Claims] (1) Each station has a transmission device and a repeater, and a loop-shaped main transmission path that connects each transmission device in a cascade, and a loop-shaped main transmission path that connects each repeater in a cascade, and is opposite to the loop-shaped main transmission path. In a data transmission system that has a loop-shaped sub-transmission path in which a signal circulates in both directions, each station switches between the receiver (R_1) output that receives the signal from the main transmission path and the output of the repeater and supplies it to the input of the transmission device. a changeover switch (S_1) to
a changeover switch (S_2) that switches between the output of the receiver (R_2) that receives a signal from the sub-transmission path and the transmission device and supplies the signal to the input of the repeater; and a switch (S_2) that sends the signal to the output of the repeater and the sub-transmission path. A switch (S_4) that turns on/off the connection with the input of the driver (D_2) to
a detector (D) that detects the presence or absence of a signal at the output of
T_1) and a detector (DT_2) for detecting the presence or absence of a signal at the output of the receiver (R_2).
When T_1) detects the presence of a signal, the selector switch (S
_1) is controlled so that the receiver (R_1) side is turned on and the switch (S_4) is turned on, and the detector (D
When T_2) detects the presence of a signal, the selector switch (S
_2) is a loopback control method in data transmission characterized in that the receiver (R_2) side is controlled to be turned on. (2) At least one of the repeaters in each station has a dummy signal generator, and in normal conditions outputs the dummy signal output from the device to the sub-transmission line via the driver (D_2),
2. The loopback control system for data transmission according to claim 1, wherein during loopback, the repeater output is sent to the sub-transmission path via a driver. (3) A determiner is provided to identify whether the output of the receiver (R_2) from the sub-transmission line is a dummy signal or a transmission signal, and when the determiner determines that it is a dummy signal, it is normal, and when it determines that it is a transmission signal, it is normal. The loopback control method in data transmission according to claim 2, characterized in that the loopback control method is performed during loopback. (4) When the repeater is synchronized by PLL,
It has a synchronization detector that detects whether the PLL is in a synchronized state or an out-of-synchronization state, and when looping back, it is determined that the state has returned to normal when the PLL shifts from a synchronized state to an out-of-synchronized state. A loopback control method in data transmission according to claim 2. (5) Each station has a switch (S_3) that turns on/off the connection between the output of the transmission device and the input of the driver (D_1) that sends the signal to the main transmission path, and the detector (DT_2) detects the presence of a signal. 2. The loopback control method for data transmission according to claim 1, wherein the switch (S_3) is controlled to be turned on when detected.