JPS6390231A - Abnormality checking method for loop optical transmission line - Google Patents

Abstract

PURPOSE:To prevent an accident that loop communication is impossible, by detecting whether the level of the quantity of received light in an optical transmission line is reduced or not or the position of this reduction by a master station before the level of the quantity of received light in the optical transmission line is reduced to a level lower than the level where communi cation is impossible. CONSTITUTION:A checking reception station receives each test data transmitted in every intensity level and records numbers m1, m2,... of failures indicating whether respective signals are normally received or not. The masterstation transmits a transmission text F3 including at least a check end command C2, which instructs a terminal stations to terminate the abnor mality check, to each ter minal station, and checking reception stations except the master station transmit data of numbers m1, m2,... of failures to the master station as a transmission text F4 on the basis of this transmission. The master station discriminates the abnormality in the optical transmission line based on the data of numbers m1, m2,... of failures transmitted from reception stations or data of numbers m1, m2,... of failures recorded by the master station itself as the reception station and displays the abnormality. Thus, countermeasures like replacement or repair of the abnormal position before an important trouble like impossibil ity of communication is prevented.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention provides a loop data transmission system in which a main station that controls communication and a plurality of terminal stations are sequentially coupled via an optical signal transmission path (such as an optical fiber, hereinafter referred to as an optical transmission path). This invention relates to a method for checking abnormalities in a transmission line before an accident occurs. Note that in the following figures, the same reference numerals indicate the same or corresponding parts. [Prior art and its problems]

Figure 10 is a block diagram showing the basic configuration of a loop data transmission system using this type of optical transmission line, and Figure 11 is a block diagram showing the main configuration of the external communication section provided at each station in Figure 10. 9 are examples of main circuits in the external communication section. In Fig. 10, 1 is the main station, 2 (21 to 2n) are terminal stations, 3 (30, 31 to 3n) is an optical transmission line consisting of an optical fiber, etc., which sequentially connects each station 1.2, and 4 (4R ，
4S) is a digital link provided at each station 1 and 2. This digital link 4R94S belongs to the external communication unit TI shown in FIG. 11, and is distinguished by reading 4R as a reception digital link and 4S as a transmission digital link. Optical signals used when each station communicates are transmitted through the optical transmission line 3, sequentially connecting each station in a loop as shown by the arrows in the figure. The configuration of the external communication section TI in each station 1.2 is as shown in FIG. , is converted into an electric signal (receive signal 8R) and given to the transmission control section 7. Also, the electrical signal output from the transmission control section 7 (
Transmission number 8S) is E10 in the transmission digital link 4S
It is converted into an optical signal via the converter 6 (61 or 62) and sent to the next optical transmission line 3. Here, when the own station becomes a receiving station, the transmission control unit 7 takes in the received signal 8R internally, and at the same time changes the same signal again to the transmitted signal 8S (that is, bypasses the own station to receive signal 8R). send). In addition, when the own station becomes a transmitting station, the above-mentioned bypass operation is not performed when the transmitting number 8S transmitted by the own station goes around the other stations in a loop and returns to the own station as the receiving number 8R. This prevents erroneous communication. In this way, the transmission signals from each station 1.2 are transmitted to each station by going around this loop transmission path. FIG. 9 is an example of the circuit configuration inside the conventional E10 converter 6,
(2) is a DC power supply of a predetermined voltage, LEDl is a light emitting diode, R1 is a load resistance of this diode LED 1, and N1 is a NOT element that drives the light emitting diode LED1 ON10FF using the transmission signal 8S (8S1) as an input signal. By the way, a method for detecting abnormalities in such a loop transmission line is disclosed in Japanese Patent Application No. 59-2442 filed by the present applicant.
There is No. 49, ``Method for indicating loop breakage in data transmission systems''. This method is effective in detecting the location of an abnormality when a loop transmission interruption abnormality occurs in a loop transmission line, making it impossible to transmit the communication signals that circulate within the loop. However, the causes of such a loop breakage abnormality include: ■ Decrease in the amount of light emitted by the light emitting diode that outputs signal light to the optical transmission line over time, and ■ Repeated connection and disconnection of the optical fiber and digital link that make up the optical transmission line. In these cases, the amount of light received by the receiving station gradually decreases over time. or gradually deteriorate until communication becomes impossible. Therefore, in order to improve the operating efficiency of a loop type optical transmission system as shown in Fig. 10, it is insufficient to detect an abnormality after loop communication becomes impossible as described above. There is a problem in that it is desirable to detect a gradual decrease in the light intensity level (also referred to as the light intensity level) to prevent loop communication failure accidents.

[Purpose of the invention]

The present invention solves the above-mentioned problems, and enables the main station to detect whether or not the amount of light received in the optical transmission line has decreased and the location thereof before the level decreases below a level that makes communication impossible. The purpose of this paper is to provide a method for inspecting loop-type optical transmission lines for abnormalities.

[Key points of the invention]

The gist of the present invention is to sequentially connect a main station (such as 1) that controls communication with a plurality of terminal stations (such as 2 (21 to 2n)) through an optical transmission line (such as an optical fiber) (3 (30 to 3n)). In a loop data transmission system that is connected via a network (e.g.), the main station sends a command (inspection start command 01, etc.) instructing to start an abnormality inspection of the optical transmission line, and sends a command (inspection start command 01, etc.) Data that specifies two adjacent stations, and specifies one of the two stations as a transmitting station and the other station as a receiving station (sending side address Ai, receiving side address Aj, etc.). An inspection start signal (such as transmission text F1) containing at least one of the signal lights is transmitted to each terminal station, and based on this transmission, the transmission station selects one of the signal lights of a plurality of predetermined intensity levels (Ll to LN, etc.). While sequentially selecting signal lights with low intensity levels from high intensity levels (Ll, etc.), for each selection, predetermined inspection data (test data T1 to TN, etc.) corresponding to the intensity level is (transmitted). (as sentences F21 to F2N, etc.) a predetermined number of times, using the signal light of the intensity level, to the receiving station, and the receiving station receives each of the inspection data transmitted for each of the intensity levels. At the same time, inspection result data indicating the success or failure of each signal (number of failures ml, m2,
-, etc.), and then the main station sends an inspection completion signal (transmission message F' 3, etc.) to each terminal, which includes at least a command (inspection completion command 02, etc.) instructing the abnormality inspection to end. Based on this transmission, the receiving stations other than the main station transmit the inspection result data (as transmission message F4, etc.) to the main station, and the main station receives the inspection result data sent from the receiving station. inspection result data,
Alternatively, the own station serves as the receiving station and determines an abnormality in the optical transmission line based on the recorded inspection result data, and displays an abnormality, or furthermore, the transmitting station The current flow (IF) of a light emitting diode (LEDI, etc.) that sends the signal light to the optical transmission line is (
(via switching load resistors R1 and R2, or a combination of D/A converter DACI and operational amplifier OPI, etc.)
The present invention is characterized in that the intensity level of the signal light can be varied by changing the intensity level of the signal light.

[Embodiments of the invention]

The present invention will be described in detail below based on FIGS. 1 to 8. FIG. 1 is an explanatory diagram of the main communication sequence between the main station 1 and the terminal station 2 (in this case, an inspection transmitting station and an inspection receiving station) as an embodiment of the present invention. 3, 4, and 5 are flowcharts for explaining the main operations of the inspection transmitting station, inspection receiving station, and main station, respectively. FIGS. 7 and 8 are explanatory diagrams of light intensity level loss characteristics of optical signals in optical transmission lines between adjacent stations, and are circuits showing the main part configurations of E10 converters in each station as different embodiments of the present invention. It is a diagram. In the light intensity level loss characteristics shown in Figure 6, the vertical axis is the light intensity level of the signal light, and the horizontal axis is the adjacent stations 1 and 2) or 2 and 2.
In the optical transmission line 3 from the E10 converter 6 (or 61.62 described later) of one (transmitting side) station to the O/E converter 5 of the other (receiving side) station between The positions of each point are shown in the order of the signal light path. Note that this optical transmission line 3 is an optical fiber not shown. It consists of optical connectors, etc. Here, the normal characteristics are AO-BO-Co-DO-EO (that is, the signal light output from the transmitting station at the light intensity level of AO is attenuated sequentially to the light intensity level of BO to DO in the optical transmission path, and the receiving station For example, if a part of the optical fiber (CO part of the above characteristics) is scratched, the characteristics will be AO-BO-CO-FO-Go-HO, and the damage will be AO-BO-CO-FO-Go-HO. The light intensity level decreases in some parts like Co-FO. Lwin indicates the lower limit value of the light reception level, and when the input light amount level at the time of reception by the receiving station becomes less than this level Lmin, a reception failure occurs. In the present invention, the output light amount level is set in stages when one of the adjacent stations transmits test data to the other station to check for deterioration of the signal light transmission capability (therefore, the loss characteristics) of the optical transmission line 3. This is done by reducing the amount by a fixed amount. A in the same figure
The characteristic of l-B1-C1-DI-El is an example of the loss characteristic when the transmitting station reduces the output light level to AO-A1 when the optical transmission line 3 is normal at the time of this inspection. Even if the output light quantity of the adjacent receiving station decreases, the light reception level of the adjacent receiving station (the light quantity level of point E1) is greater than the lower limit value Lmin, and reception failure will not occur. However, as mentioned above, if the optical fiber is damaged, the loss characteristic during this inspection will be A.
l-Bl-CI-Fl-Gl-Hl, and the received light level of the adjacent receiving station (point H1) falls below the lower limit L min.
Communication in this case will fail. In this way, by reducing the amount of transmitted light from an adjacent transmitting station by a predetermined amount and determining whether there is a corresponding communication failure, deterioration in the transmission characteristics of the optical transmission line can be detected before an accident occurs. Returning now to the circuit of the conventional E10 converter 6 in FIG. ) is given by the formula. IF = (VCVF −Vex) /R1−=−・
(1) However, ■: DC power supply voltage supplied to this circuit,
■: Forward voltage drop of the light emitting diode LED1; ■ct: Collector-emitter voltage of the transistor forming the NOT element N1. Since the amount of light output from the light emitting diode LED1 is determined by this current IF, the resistor R1 is adjusted so that the input light amount level at the point EO of the adjacent receiving station when the optical transfer path is normal in FIG. The value and therefore the current IF is set. FIG. 7 is a circuit diagram of the E10 converter 61 in the external communication section TI as an embodiment of the present invention. In this figure, compared to FIG. 9, NOT element N2. Resistor R2 and transmission signal 8S (8
S2) is added, and the output light level of the light emitting diode LED1 can be changed in two stages. That is, the value of the resistor R1 is changed by the transmission signal 8S1 to the NOT element N1.
The output light amount of the light emitting diode LED1 is determined to be the light amount level at the point AO of the adjacent transmitting station in FIG. Similarly, the value of the resistor R2 is changed by the transmission signal 8S2 to the NOT element N.
When only the transistor constituting LED 2 is conductive, the output light level of the light emitting diode LED1 is at point A1 in FIG.
The amount of light is determined to be . FIG. 8 is a circuit diagram of an E10 converter 62 in the external communication section TI as another embodiment of the present invention. Transmission signal 8S (
8S3) consists of an n-bit digital signal, and this signal is converted to a voltage of 1 through the D/A converter DACI. 1 analog signal. This voltage ■. 1 is operational amplifier O
Input to PI and its output voltage■. The value of 2 is the voltage ■. ,
is equal to Here, the current IF flowing through the light emitting diode LEDI is given by equation (2). 1 F = (VO2Vy) / R1-=----
------------- (21 However, ■. 1 = VO2 < VC1 VF Forward voltage drop of dual light emitting diode LED1, R1 = resistance value of load resistor R1. In this circuit, changing the voltage VOW is the transmission signal Since this is easily possible with 8S3, the light intensity level output by the adjacent transmitting station can be changed in more steps than in the method shown in Fig. 7. Fig. 1 shows main station 1 and terminal station 2 of the present invention. In this example, main station 1 specifies two adjacent terminal stations 2 (2i) and 2 (2i+1), and communicates between these terminal stations 2 i and 2 t+. Optical transmission line 3 (31)
This shows the case where an inspection is performed. Here, the former terminal station 2
1 is the adjacent transmitting station, which is also called a transmitting terminal station or a check transmitting station. The latter terminal station 21+1 is the adjacent receiving station and is also referred to as a receiving terminal station or a check receiving station. The figure shows the flow of communication data between each station 1, 2i, and 2i+1 in the horizontal direction, and the sequence (11 to 2i+1) in the vertical direction.
The time course is shown in the order of (4). That is, sequence (1): The main station 1 first issues an inspection start command CI as a command to start an inspection, the address (also referred to as the transmitting side address) A1 of the default terminal station (inspection transmitting station) 2i, and the receiving side. Transmission text F containing the address (also called the receiving side address) Aj of the terminal station (inspection receiving station) 2i+1
Send 1. As a result, all terminal stations 2 send the transmitted message F1
The terminal station 21 detects that the address Ai is its own address, and the terminal station 21+1 detects that the address Aj is its own address. Sequence (2-1) to (2-N): Next, the terminal station 21
are the respective transmission messages F21 . . . containing known test data Tl, T2 . F
22. -., F2N are sequentially transmitted once or a predetermined number of times as necessary. Then, in this order, the terminal station 21 changes (decreases) the sight level of the signal light to be transmitted, for example, as shown in FIG. In this case, the value of the step N for switching the light amount level may be selected from a plurality of arbitrary steps depending on the accuracy of the inspection. Note that the light level L1 here is the normal light level (
For example, the light amount level at point AO in FIG. In addition, in order to change the light intensity level in multiple stages N in this way, the configuration of the E/○ converter in the external communication section TI at the inspection transmitting station is as follows.
If the number of stages N is only two, a configuration like the E10 converter 61 shown in FIG. 7 may be used, and to switch to a larger number of stages, a configuration like the E10 converter 62 shown in FIG. 8 may be used. Now, on the other hand, the receiving terminal station 21+1 receives the received data F21,
Test data T1 to TN in F22...F2N
The success or failure of the communication is determined each time, and the light intensity level Ll, Ll - the number of successes or failures of another m
1. m2 -... is calculated and stored. Sequence (3): The main station 1 sends an inspection mode end command C2 as a command for instructing the end of inspection.) A transmission message F3 including the address AI of the transmitting terminal station 21 and the address Aj of the receiving terminal station 2i+1. Sequence (4): In this way, the receiving terminal station 2i+
When l receives the inspection mode end command C2 and an address Aj that matches its own address, l receives its own address A.
j1 light level L, 1. Sends a message F4 including the number of failures ml and m2-mN for each L2--LN. Therefore, the main station 1 receives this transmission message F4 and transmits the corresponding optical transmission line 3.
It is possible to know the presence or absence of deterioration of the optical transmission characteristics of a portion and its degree. In Fig. 1, one of the adjacent terminal stations 2 is a transmitting station and the other is a receiving station, and the optical transmission line 3 between these two terminal stations 2 is inspected. It is also necessary to carry out the optical transmission line 3 between the terminal station 2 and the terminal station 2 adjacent thereto, and in this case, the terminal station 2 t in FIG.
Either one of 2 i+1 is replaced with the main station 1, and the other terminal station becomes the terminal station 2) adjacent to the main station 1, that is, 21 or 2n in FIG. Here, if the terminal station 21 in Fig. 1 is replaced with the main station 1 and the terminal station 21+1 becomes 21, the main station 1 becomes the inspection transmitting station and the terminal station 21+1 becomes the inspection receiving station, and optical transmission is performed. Road 30 will be inspected. Also, the terminal station 21+1 in FIG. 1 is replaced with the main station 1, and the terminal station 21 is also replaced with 2n.
In this case, the terminal station 2n becomes an inspection transmitting station and the main station 1 acts as an inspection receiving station to inspect the optical transmission line 3n. In the former case, the address Ai in the transmitted messages Fl and F3 in FIG. , and address Aj is each address of the main station 1. FIGS. 3, 4, and 5 are flowcharts illustrating the operations of the inspection transmitting station 9, the inspection receiving station, and the main station in the optical transfer path inspection mode, respectively. In FIG. 3, the station before being designated as an inspection transmitting station determines in step 101 whether or not it has received the transmission F1 containing the inspection start command CI from the main station 1, and if no
If so, proceed to step 104; if YES, proceed to step 102. In step 102, the sender address At sent in the transmission message F1 is compared with the own address, and if they do not match (No), the process proceeds to step 104, but in this case they match (YES), so step Proceeding to 103-1, operation in inspection mode is started. In step 103-1, test data T1 is sent to the inspection receiving station.
The transmission text F21 including the above is transmitted multiple times with the transmission light amount level L1. Similarly, step 103-2. ~． 103-N
The test data T2. ~． TN to transmit light level L2. ~． Transmit multiple times as LN,
Proceed to step 104. In step 104, it is determined whether or not the transmission message F3 containing the inspection end command C2 has been received from the main station 1. If No, the process directly proceeds to a process not shown, but if YES, the process proceeds to step 105. The station in question finishes operating in inspection mode,
Return to normal communication mode. However, if the station matches the main station 1, in steps 101 and 104, it is determined whether the own station (main station 1) has transmitted the transmission messages F1 and F3, respectively. Further, in step 102, it is determined whether the transmitter address At in the transmission F1 transmitted by the own station matches the address of the own station. In FIG. 4, the station before being designated as an inspection receiving station determines in step 201 whether or not it has received the transmission F1 containing the inspection start command C1 from the main station 1. If NO, step The process proceeds to step 204, and if YES, the process proceeds to step 202. In step 202, the receiver address Aj sent in the transmission message F1 is compared with the address of the local station. If they do not match (No), the process proceeds to step 204, but in this case they match (YES). , the process proceeds to step 203-1 to start operation in the inspection mode. In step 203-1, the transmission message F21 sent from the inspection transmission station is received, and in the same manner, in step 203-2
．． ~． In step 203-H, each of the transmission messages F22 to F2N is received. Next step 203-M
, the transmitted messages F21 to F2N received in this way
Compare each of the test data T1 to TN in the test data with its own known test data, and if they match, it is determined that the communication is successful,
If there is a mismatch, it is determined that the communication has failed, and each test data (
Therefore, after counting and storing the number of reception failures for each of the light intensity levels 1 to LN), the process proceeds to the next step 204. In step 204, similar to step 104 in FIG.
Sentence F that includes the inspection end command C2 from the main station
It is determined whether or not 3 has been received, and if NO, the process directly proceeds to a process not shown, but if YE or S, the process proceeds to 205-1. In step 205-1, the station transmits to the main station 1 the number of communication failures (or successes) for each light intensity level Ll-LNO created in step 203-M using a transmission message F4, and then in step 205-2 Proceed to , complete inspection mode operation, and return to normal communication mode. However, if the station loses to the main station 1, in steps 201 and 204, it is determined whether the own station (main station 1) has transmitted the transmission messages F1 and F3, respectively. In step 202, the transmission text F1 sent by the local station is
It is determined whether or not the receiving side address Aj in the middle is defeated by the address of the own station. Further, the step 205-
1 is omitted. In FIG. 5, the main station 1 transmits a transmission message F1 including an inspection start command CI in step 301. This step 301 corresponds to steps 101 and 201 described above. Next, after a certain period of time, the main station transmits a transmission message F3 including an inspection end command C2 in step 304. This step 304 corresponds to step 104.204 above. Next, in step 305, the transmission message F4 transmitted from the inspection receiving station (that is, the station having the receiving side address Aj) is received. Next, in step 306, the main station 1 checks the number of reception failures m1 to mN for each of the light quantity levels 1 to LN included in this transmission message F4. When the light loss in the optical transmission line 3 between adjacent stations is normal, the light reception level at the inspection receiving station is above the lower limit value Lmin, and when the light loss is abnormal, the light reception level is less than the lower limit Lmin, and communication fails. Therefore, for example, if communication is successful at the output light level L1 (normal level) of the inspection transmitting station, and communication fails at the same level L2 or lower, it is determined that there is an abnormality. In step 307, based on the determination result in step 306, if the above-mentioned abnormality exists, the process proceeds to step 309, and an abnormality display is performed. In this abnormality display, for example, the respective addresses At and Aj of the inspection transmitting station and the inspection receiving station may be displayed. If it is determined in step 307 that there is no abnormality, step 3
Proceed to 08 and display normally. However, in FIG. 5, if the main station 1 matches the inspection receiving station, step 305 stores the stored data of the number of reception failures for each light intensity level within the own station (main station 1), which corresponds to the data in the transmission message F4. Replace it with a read operation.

【Effect of the invention】

A main station 1 that controls communication and a plurality of terminal stations 2 (21 to 2n
) are sequentially coupled via optical transmission lines 3 (30 to 3n), the main station issues an inspection start command C1 instructing to start an abnormality inspection of the optical transmission line; An address that specifies two adjacent stations among each station, including the own station, and specifies one of these two stations as the inspection transmitting station and the other station as the inspection receiving station, that is, the sending side address. Ai, the receiving side address Aj, and transmits a transmission message F1 including at least Aj to each terminal station, and based on this transmission, the inspection transmitting station selects one of the signal lights of a plurality of predetermined light intensity levels L1 to LN. , high intensity level L
While sequentially selecting signal lights with low intensity levels starting from signal light 1, for each selection, predetermined test data T1 to TN corresponding to the intensity level are transmitted a predetermined number of times as transmission texts F21 to F2N. transmitting it to the receiving station using signal light, and the inspection receiving station receives the test data transmitted for each intensity level, as well as the number of failures ml indicating the success or failure of each signal; m2-. Next, the main station sends a transmission message F3 including a check completion command C2 instructing the abnormality check to end to each terminal station, and based on this transmission, the main station The inspection receiving stations other than those in which the number of failures ml, m2-
and the main station receives the number of failures m1. transmitted from the receiving station. m
2. An abnormality is displayed by determining an abnormality in the optical transmission path based on the data of .-- or the number of failures ml, m2-. As a result, the following effects can be obtained. ■When using optical fibers as optical transmission lines, it becomes possible to detect abnormalities such as an increase in light loss in the optical fiber between each station or an increase in dirt on the connector, making maintenance and inspection of the transmission system easier. Moreover, it is possible to take measures such as replacing or repairing abnormal parts before the light intensity decreases and serious problems such as communication being impossible occur. ■There is little cost increase due to the increase in circuits for changing the optical signal level, and communication in inspection mode can be created by reusing most of the conventional software.
The transmission system can be constructed at low cost. ■During post-construction inspections when installing a transmission system or changing the layout of signal cables, it becomes easier to check for optical fiber cables that have been damaged due to improper handling. ■Compared to inspections using measurement equipment such as optical power meters,
Inspections can be completed in a short time.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the main communication sequence between the main station, the inspection transmitting station, and the inspection receiving station as an embodiment of the present invention, and FIG. Similarly, Figures 3, 4, and 5 are for the inspection transmitting station 1.
A flowchart to explain the operation of the main parts of the inspection receiving station and the main station. Figure 6 is also an explanatory diagram of the light intensity level loss characteristics of the signal light in the optical transmission line between adjacent stations. Figures 7 and 8 are from this book. E10 in each station as different embodiments of the invention
A circuit diagram showing the main part configuration of the converter, FIG. 9 is similar to the above-mentioned FIG. 7,
FIG. 8 is a circuit diagram showing the main configuration of a conventional E10 converter, FIG. 10 is a block diagram showing the basic configuration of the present invention and a conventional loop data transmission system, and FIG. FIG. 2 is a block diagram showing the main part configuration of an external communication unit TI in each station. 1: Main station, 2 (21~2n): Terminal station, 3 (30~
3n): Optical transmission line, T■: External communication section, 4 (4R. 4S): Digital link'), 5: O/E converter, 6
1゜62: E10 converter, 7: Transmission control section, LEDI:
Light emitting diode, R1, R2: Load resistance, Nl. N2: NOT element, DACl: D/A converter,
OPl: Operational amplifier, Fl, F2i~・F2N, F3°F4: Transmission text, C1: Inspection start command, C1 inspection end command, Ai: Sending side address, Aj: Receiving side address, T1~TN: Test data, Ll~ LN: light intensity level, Lmin: lower limit of light reception level, m1 to mN = number of failures. Gradually; photo Q Kakekebono 2-64 Figure 5 N1 62 Niji mo Henkyōki ■c-

Claims (1)

[Claims] 1) In a loop data transmission system in which a main station that controls communication and a plurality of terminal stations are sequentially coupled via an optical transmission line, the main station starts checking for abnormalities in the optical transmission line. A command to instruct that the
An inspection start signal containing at least data specifying a station and specifying one of the two stations as a transmitting station and the other station as a receiving station is transmitted to each of the terminal stations. , Based on this transmission, the transmitting station sequentially transmits signal lights of a plurality of predetermined intensity levels, starting from the highest intensity level.
While selecting a signal light of a low intensity level, for each selection, predetermined inspection data corresponding to the intensity level is transmitted to the receiving station a predetermined number of times using the signal light of the intensity level, and The receiving station receives each of the inspection data transmitted for each intensity level and records inspection result data indicating the success or failure of each signal, and then the main station finishes the abnormality inspection. An inspection completion signal containing at least a command to indicate that the The apparatus is characterized in that an abnormality in the optical transmission path is determined based on the inspection result data transmitted from the receiving station or the inspection result data recorded by the own station serving as the receiving station. How to check for abnormalities in loop optical transmission lines. 2) In the method according to claim 1, the transmitting station varies the intensity level of the signal light by changing the energization current of a light emitting diode that sends the signal light to the optical transmission path. 1. An abnormality inspection method for a loop optical transmission line, characterized in that: