Fault examining apparatus for loop-shaped data transmission system using optical fiber

A fault examining apparatus is provided in a loop network system, in which a master station is connected sequentially with a plurality of follower stations in a loop transmission path to transfer digital optical signals denoting serial data information from the master station to the follower stations, from the follower stations to the follower stations or from the follower stations to the master station. The apparatus includes a device for detecting and storing a present value, a maximum value and a minimum value of a peak level value of received optical signals, and a display device for displaying the values.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a fault examining apparatus for 
a loop network system in which a master station is sequentially connected 
with a plurality of follower stations into a loop network using a single 
transmission path to transmit digital signals with serial information from 
the master station to the follower stations, from the follower stations to 
the follower stations or from the follower stations to the master station. 
In recent years, as microprocessor technology has advanced, the functions 
and utility of automatic machines for industry use has advanced. It is not 
uncommon that the number of sensors and actuators used in such machines 
exceeds 100. Once automatic machines are lined within factories, mutual 
information exchanges and the information exchanges between computers for 
controlling production lines and the automatic machines are required to be 
effected without fail. In order to reduce the wirings within such 
facilities a described hereinabove or among such facilities, inputting, 
outputting control units (remote I/O) are installed within the facilities 
as the follower stations so as to connect with the follower stations the 
sensors, the actuators, and the signal lines of the communication 
information, while a connection is effected using only one optical fiber 
cable to a master station leading to the microprocessors for effecting the 
overall control operation so as to serially transfer the information. 
FIG. 9 shows a system diagram having the remote I 10 units, wherein an 
input unit 3 and an output unit 4 are referred to hereinafter as follower 
stations, with sensors 1 and actuators 2 such as electromagnetic valves, 
and an optical fiber 6 being provided. A controller 5 for controlling all 
of the follower stations is called a master station. 
FIG. 10 shows a schematic block diagram of the follower stations in the 
conventional embodiment. The control information from the master station 
is transferred to the follower stations as serial information composed of 
pulses as the intermittent signals of light. The transmission format of 
the serial information is composed of a start bit to allow a follower 
station to detect the head bit of the serial information, an address 
portion as the control information, a data portion, and a parity check bit 
for detecting the transmission error of the signals as shown in FIG. 11. 
In FIG. 10, reference numerals 7, 8, 9 show an optical fiber, an 
(optical-to-electrical) transducer, and an (electrical-to-optical) 
transducer, respectively. The input signals are connected to the sensors 1 
of FIG. 9, while the output signals are connected to the actuators 2. All 
of the serial received signals RCV changed into electrical signals are 
once stored in a shift resister, wherein a serial.parallel conversion and 
a parallel.serial conversion may be effected. A circuit 15 checks whether 
or not the value of the address portion converted into a serial.parallel 
format conforms to the value of the address of the follower station 
established by the switch 16. When it is detected that the unit is 
accessed by an address conformity detecting circuit 14 after it is once 
stored in the shift register 14, the memory of the output data 11 of the 
shift register is effected by an output latch 17. The data portion of the 
serial information once stored within the shift register 14 is replaced 
into the input data 13 in accordance with the input signal so as to effect 
the parallel.serial conversion again for the transmission thereof as the 
transmission signal SND. When the address is not in conformity, it is 
transmitted through the parallel.serial conversion as it is. A timing 
pulse generating circuit 18 effects the controlling operation in the 
above-described procedure. 
Only when through the above-described construction, the serial information 
transmitted from the master station of FIG. 9 is transmitted in order from 
the follower station to the follower station to come into conformity with 
the established address of the follower station, the input data of the 
follower station is sent into the optical fiber as the serial information 
and is returned into the master station. The master station decodes the 
address portion and the data portion of the returned serial information. 
A transmission error detecting circuit 22 is adapted to detect the 
transmission error of the serial information. This circuit 22 detects 
whether the total sum of the bit numbers with the address portion and the 
data portion in the serial information being 1 is odd or even in number, 
and generates a transmission error signal if a predetermined parity error 
continues to be generated. In a gate circuit 23, the output of the latch 
pulse into the output latch 17 is stopped when the transmission error is 
generated and the output signal is retained in the previous condition. 
When the transmission error has been detected, the follower station 
transmits a received signal having an error as it is with respect to the 
next follower station. Accordingly, even in the master station, the signal 
returned by way of many follower stations is checked with the same circuit 
as the transmission error detecting circuit 22 being built in so as to 
detect that the transmission error has been generated within this loop. 
It is to be noted that the main causes for generating transmission errors 
are a strong noise presence with respect to the electric circuits of the 
follower stations, a wiring condition of the optical fiber cables, changes 
in the bending condition of the optical cables with the cables being wired 
in a movable portion, and the received signal level of the optical signal 
of the follower station being beyond the proper range caused by an 
impropriety in the splicing portion of the light connector. 
However, in above-described construction, if the master station can find 
the transmission error through a check of the returned signal, it cannot 
be easily checked how the level of the received signal under the 
conditions where the transmission error is likely to be generated is 
provided in an optional follower station. It is necessary to manually 
measure the level of the received signal in the follower station with a 
light-amount measuring instrument in order to have another look at the 
wiring of the optical fiber cable and the splicing portion of the optical 
connector when the level of the received signal at the follower station is 
excessively large or small. As a means for outputting the level of the 
received signal with respect to an optional follower station from the 
master station is not provided, the above-described operation is required 
to be effected one by one with respect to the follower stations in order 
to see which follower station within the loop is inferior in the wiring 
condition thereof, thus requiring increased time and labor consumption. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a fault 
examining apparatus for a loop-shaped data transmission system using an 
optical fiber, which is capable of, on each of the follower stations, 
detecting the received-light-amount levels of the received optical signals 
so as to store the maximum value and the minimum value of the 
received-light-amount levels of the optical signals received one after 
another, so that the master station may read the values in the serial 
information so as to examine the fault. 
Another object of the present invention is to provide a fault examining 
apparatus for a loop-shaped data transmission system using an optical 
fiber, which is capable of effecting the above-described judgments even at 
the follower station the provision, at the follower stations, of a 
received-light-amount level display instrument for effecting renewals at 
each reception and a display instrument of the maximum light level values 
and the minimum values. 
In accomplishing these and other objects, according to one preferred 
embodiment of the present invention, there is provided a loop-shaped data 
transmission system which is capable of transmitting and receiving the 
serial information and decoding it, with one master station being 
connected with a plurality of follower stations in a loop optical 
transmission path. In a first embodiment, the follower station is composed 
of a means for detecting and storing as the received-light-amount level 
the peak value of the received signal at the moment of the serial 
information composed of pulse rows of intermittent signals of light, and a 
display instrument for displaying the values. 
A second embodiment is composed of a means for rewriting it into the value 
of this time so as to store it if the received-light-amount level received 
this time is larger through the comparison of the detecting means output 
at the received-light-amount level of the serial information at that 
moment with a maximum value given so far in the received-light-amount 
level received before that, a means for rewriting into the 
received-light-amount level of this time if the received-light-amount 
level received this time is smaller through the comparison of the output 
thereof likewise with a minimum value given so far, and a display 
instrument for displaying the above-described three peak values. 
In a third embodiment and a fourth embodiment, the content of the serial 
information is composed of an address portion for specifying the 
conventional particular follower station, a data portion and a mode 
portion added into it so as to load from the master station the level of 
the received-light-amount of the follower station corresponding to the 
first and second inventions. In the follower station, there are a means 
for detecting the value of the new received portion of the 
received-light-amount level of the received signal, a minimum value, a 
maximum value given up to that time point so as to turn them into digital 
values for storing them, and a means for coding the contents of the mode 
portion of the serial information to distinguish the cases among (1) a 
case of reading the received-light-amount level of the new received 
signal, (2) a case of reading the maximum value provided up to this time, 
(3) a case of reading the minimum value provided up to this time, and (4) 
a case of resetting the memories of the maximum value and the minimum 
value, for replacing the data portion of the serial information into the 
received-light-amount level in accordance with each of the above-described 
cases (1) through (3) so as to transmit it. The master station is composed 
of a means for transmitting the serial information having the contents of 
the mode portion, a means for decoding the returned serial information so 
as to load, as the digital values, the level of the received-light-amount 
of the received signal of each follower station, a means for deciding 
whether the value is higher or lower as compared with the proper level, 
and a display means for displaying the results together with the address 
of the follower station. 
In the first embodiment of the present invention, the level of the 
received-light-amount is renewed and displayed every time each follower 
station receives the serial information. A follower station which cannot 
effect the normal data transmission may be found easily through judgment 
as to whether or not the display value stays within the proper range. 
Thus, main causes such as inferior wirings of the optical fibers, inferior 
connector portions, light-emitting power shortage of the previous follower 
station are easier to find. The failures of the whole transmission system 
may be prevented from being effected through the above-described coping 
operation before the received-light-amount level becomes completely 
abnormal. 
In the second embodiment of the present invention, as the display content 
of the level of the received-light-amount of the follower station, there 
are shown the maximum value and the minimum value before that time, in 
addition to the present value which is a value for each reception. When 
the long hours' surrounding noises changes or the bending condition 
changes with the optical fiber being mounted on a movable portion, the 
varying range of the level of the received signal can be seen, so that it 
is easier to make sure of a measure for improving the reliability of the 
transmission of the system, and the effects thereof. 
According to the third and fourth embodiments of the present invention, the 
master station can load at an optional timing the level of the 
received-light-amount of the follower station by the change in the 
contents of the mode portion and the address portion of the serial 
information, and display the value thereof together with the address of 
the follower station. The levels of the received-light-amount of the 
follower stations may be seen collectively in the established place of the 
master station although the user does not go near the follower station on 
purpose, so that the transmission system may be easily maintained.

DETAILED DESCRIPTION OF THE INVENTION 
Before the description of the present invention proceeds, it is to be noted 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
Referring now to the drawings, there is shown in FIGS. 1 a block diagram of 
a follower station of a loop-shaped data transmission apparatus in an 
embodiment of the present invention, which includes an optical fiber 7, an 
(optical-to-electrical) transducer 8, an (optical-to-electrical) 
transducer 9 into which the SND signal of the digital signal as the 
transmission signal is inputted, an address conformity detecting circuit 
15, an address setting switch 16, an output latching circuit 17, an input 
interface circuit 20, an output interface circuit 21, a circuit for 
detecting transmission errors 22 which has the same function as that of 
the conventional embodiment in that it outputs an ERR signal in the error 
generation and which detects transmission error using a parity bit, a mode 
deciding circuit 24 which analyzes the information of the mode portion as 
shown in FIG. 2 so as to output RIN, XIN, NIN, RS signals in addition to 
IN, OUT signals, AND gate circuits 25, 26, 27, 28, an inverter 29, and an 
OR gate circuit 30. The input interface circuit 20 has an output buffer 
circuit built in, wherein the input signal appears on the output side only 
when the IN signal is on. A shift register circuit 31 is provided which is 
I.sub.7 in bit as I.sub.7 in the serial information, wherein the 
information I.sub.7 through I.sub.0 of the input interface circuit 20 and 
the information I.sub.R7 through I.sub.R0, I.sub.X7 through I.sub.X0, 
I.sub.N7 through I.sub.N0 of the output buffer circuits 40, 41, 42 are 
loaded in parallel by a LD.sub.2 signal and a RCV signal may be inputted 
in series at a timing of the SFT signal as a shift pulse, and at the same 
time a serial information is outputted sequentially from a 
most-significant-bit (MSB) by a SFT signal. A timing control circuit 32 is 
provided for controlling the latch timings into the shift register 31, the 
output latch circuit 17, a parity generating circuit of the input data 33 
for generating all the parity bits of parallel information inputs I.sub.7 
through I.sub.0 to the shift register, addresses A.sub.3 through A.sub.0 
and mode information M.sub.2 through M.sub.0, the output signal I.sub.P as 
parity bit which is given a least-significant-bit (LSB) information of the 
parallel input information of the shift register 31, with a RCV signal 
being a received signal with an optical signal being shaped into a digital 
signal, and an AOUT signal is an analog output signal proportional to the 
strength of the light, with an optical signal being. 
The parity bit of the transmission format of FIG. 2 and the transmission 
error detection will be described hereinafter. When the master station 
transmits the serial information, a setting operation (in the embodiment, 
"1" is provided in the odd number) into the parity bit through the 
removing of all the parities of the mode portion, the address portion, the 
data portion except for the start bit of the contents thereof. The 
transmission error detecting circuit 22 of the follower station inputs a 
received signal RCV and outputs a transmission error signal ERR when a 
serial information which does not conform to the rule is received. When 
the ERR signal does not become "1", the timing pulses LD.sub.0 through 
LD.sub.2 of the outputs of the AND gate circuits 25, 26, 27 are not 
generated. 
Returning to FIG. 1 again, an amplifier 34 amplifies the analog output 
signal AOUT of the transducer 8. A peak value hold circuit 35 receives the 
output of the amplifier so as to detect and retain the peak voltage 
corresponding to a maximum light-amount value, the resetting of the peak 
value retention being effected in accordance with a T.sub.0 signal. An 
analog.digital conversion circuit 36 converts the value of the analog 
output V.sub.p of the peak value holding circuit 35 into a digital value, 
and the output thereof is inputted into the display instrument 37 and 
represented in numerals. The conversion start timing of the analog to 
digital conversion circuit 36 is effected in accordance with a T.sub.1 
signal. The maximum value decision memory circuit 38 compares a maximum 
value of the peak value of the digitized AOUT signal with a new 
received-light-amount level value in accordance with a timing of the 
T.sub.3 signal so as to rewrite the value, when the new value is larger, 
as the maximum value and to store it, with the memory circuit having a 
function of resetting the memory contents with a LD.sub.1. A minimum value 
decision storing circuit 39, which is similar in construction, keeps the 
minimum value stored. Output buffer circuits 40, 41, 42, wherein either 
data of the new value or the maximum value, the minimum value of the peak 
values is selected when one of the output RIN, XIN, NIN signals of the 
mode decision circuit 24 is turned on, so that the output thereof becomes 
the parallel input information of the shift register 31. It is to be noted 
that +V.sub.cc shows that the input logic is "1". Also, further two 
numeral display apparatuses not shown are respectively connected with the 
output of a maximum value decision storing circuit 38 and the output of 
the minimum value decision circuit 39. 
FIG. 3 is a chart showing the normal and abnormal condition abnormality, 
abnormality of the level of the received-light-amount of the transducer 8, 
wherein the received-light-amount is abnormal at E.sub.1 or lower and at 
E.sub.4 or more in voltage. The received light level is set within the 
range of the normal level 2 through the adjustment of the pattern on the 
light-emitting side of the previous station. The received light amount 
becomes a level 1 or a level 3 condition because of an inferior wiring 
condition, optical fiber modification or the like. If the digitized 
received signal RCV comes out correctly, the received light amount is 
slightly insufficient or excessive. In this condition, by another look at 
the wiring condition of the optical fiber of the follower station, the 
troubles of the facility may be prevented from being caused. As the master 
station specifies an address so that the received-light-amount level of 
the follower station may be checked for such reasons as described 
hereinabove, the master station transmits the serial information of "100" 
through "111" in the mode portion so as to examine the 
received-light-amount level of the follower station. FIG. 4 is a timing 
chart of one example thereof. A user may check the level of the 
received-light-amount of the follower station at each time by the numeral 
display instrument 37 of FIG. 1. 
FIG. 4 is a timing chart of the follower station, showing a case where two 
serial information have been transmitter from the master station. The 
former half thereof shows a case where the mode portion has received the 
information of "100", while the latter half shows a case where it has 
received the information of the content of the "111". In the former half, 
the follower station outputs as a transmission signal SND the data of the 
new received-light-amount of the follower station, and thus expresses the 
state of loading into the shift register 31. The latter half shows the 
state of resetting the maximum value and the minimum value of the level of 
the received-light-amount. 
The timing control circuit 32 starts its operation by the rising of the 
start bit of the received signal RCV so as to sequentially output the 
shift pulse SFT synchronized with the received serial information by 17 
pulses of the whole bit length portion. At the same time, the signal 
G.sub.1 for controlling the output of the transmitting signal SND is made 
"1", the data of the previous time portion is outputted sequentially into 
the transducer 9 by the shift register 31. The contents of the outputs 
M.sub.2 through M.sub.0 of the shift register at a time point when 17 
shift pulses SFT have come out are "100". Thus, the mode decision circuit 
24 turns the RIN signal only into "1". At the head of the RCV signal, the 
T.sub.0 signal is outputted, so that the output V.sub.p of the peak value 
hold circuit 35 is once turned to zero. When the address of the follower 
station is turned to "0101", it conforms to the address portion of the 
serial information, so that the output AEQ of the address conformity 
detecting circuit 15 becomes "1". As the RIN signal is "1" at a time point 
when 17 SET signals have been outputted, the output selection of the 
output buffer circuit 40 is maintained, so that the data I.sub.R7 through 
I.sub.R0 of the level of the new received-light-amount which is the 
content thereof becomes the parallel data input of the shift register 31. 
At this time, the parity bit I.sub.P is produced by the parity generating 
circuit 33. 
As the inputting conditions of the AND gate circuit 26 are adjusted when 
the T.sub.2 signal is outputted, a LD.sub.2 signal which becomes a load 
timing of the parallel data of the shift register 31 is outputted and the 
content of the data portion of the shift register 31 is replaced by the 
content of the above-described I.sub.R7 through I.sub.R0. In the 
embodiment, it is "00000111", with the parity portion becoming "0". When 
the information of the 17 bits produced thus receives the next received 
signal, it is outputted as the SND signal of the latter half portion of 
the drawing. 
The timing chart of the latter half portion of FIG. 4 will be described. 
As the content of the mode portion is "111" in this case, the output of the 
mode deciding circuit 24 becomes "1" only in the RS signal at a time point 
where seventeen SFT signals have been outputted. As the inputting 
condition of the AND gate circuit 27 is adjusted when the T.sub.1 signal 
is outputted from the timing control circuit 32, the LD.sub.1 signal is 
outputted, so that the memory contents of the maximum value decision 
circuit 38 and the minimum value decision circuit 39 are reset. When the 
load timing signal LD.sub.1 into the shift register 31 is not outputted, 
it is outputted as it is and becomes a transmission signal SND when a 
receiving signal RCV receives the next serial information. 
The above-described description is presented in which no transmission 
errors are provided. When the transmission errors exist, the ERR signal 
becomes "1", so that the inputting conditions of the AND gate circuits 25, 
26, 27 are not met, thus resulting in no operation to be effected. In this 
case, the master station finds the transmission error of the returned 
serial information and transmits the same contents again. 
When the content of the maximum value storing circuit is inputted, the 
serial information with the content of the mode portion being "101" 
therein is transmitted by the master station, so that the content is 
carried out with only the XIN signal becoming "1" as shown in the timing 
chart of the former half portion of FIG. 4. Also, the content of the 
minimum value storing circuit is carried out . when the NIN signal becomes 
"1" with the contents of the mode portion being "110". 
As described in the explanation of FIG. 3, the object may be achieved if 
the level of the received light amount in the follower station may be 
found at the four stages of E.sub.1 through E.sub.4. A voltage comparator 
and an light emitting diode, instead of an analog.digital transducer 36 
and a numeral display instrument 37, may be used in the circuit as in 
shown in FIG. 5. 
In FIG. 5, the analog signal AOUT proportional to the received light signal 
from the transducer 8 is amplified by an amplifier 34. The peak value hold 
circuit 35 retains the voltage V.sub.P of the peak thereof. The outputs of 
the voltage comparators 43 through 46 are inputted into the shift register 
31 of FIG. 1 through the output buffer circuit 40. Also, the condition 
thereof may be seen by the light emitting diodes 47 through 50. The input 
voltages V.sub.1 through V.sub.4 of the voltage comparators 43 through 46 
are divided in voltage by the resistors R.sub.1 through R.sub.5 and are 
set into the voltage corresponding to E.sub.1 through E.sub.4 of FIG. 3. 
The T.sub.0 signal is a signal which once discharges the peak voltage 
retained by a reset signal of the peak value hold circuit 35 and turns it 
into zero. 
FIG. 6 shows a block diagram of the master station of the present invention 
FIG. 6 shows a timing chart in the case of a transmission. FIG. 7 is a 
timing chart in the case of a reception. 
FIG. 6 shows a transmission error detecting circuit 60 of a received signal 
RCV which is completely the same as that of FIG. 1, a microprocessor 61, 
an input timing controlling circuit 62 for receiving the received signal 
RCV so as to output an interruption iRTl into the microprocessor 61 and a 
shift pulse SFT, an address decoding circuit 63, an input buffer circuit 
64 as a interface circuit with respect to the microprocessor, a shift 
register 65 for reception use, a shift register 66 for transmission use 
which is capable of loading the parallel input data, a shift pulse 
generating circuit 67 for generating a transmission signal SND, AND gates 
69 through 72, and buffer gate circuit 73. 
FIG. 7 is a timing chart, wherein a master station outputs the transmission 
signals SND of "100" into a mode portion of the serial information, and of 
"0101" into an address portion thereof. This is a timing chart depicting 
an operation which tries to check the new value of the 
received-light-amount level of the received signal of the follower 
station, corresponding to the former half portion of the timing chart of 
the above-described FIG. 4. A starting operation is effected by a program 
of the fault examination of the follower station of the microprocessor 61 
of the master station. The mode portion "100" and the address portion 
"0101" of the follower station, "00000000" as the data portion, "1" as a 
parity bit for them are computed, "1000101000000001" as the data 
corresponding to them are set into the data buses DB.sub.15 through 
DB.sub.0. Then, a storing signal WR.sub.1 is outputted and latched into a 
shift register 66. Then, after some hours, the starting signal WR.sub.2 is 
outputted into the shift pulse generating circuit 67. As the eighteen 
shift pulses SP are outputted into the shift register 66 for transmission 
use by the shift pulse generating circuit 47, the above-described contents 
are outputted from the MSB sequentially as shown in FIG. 7 and become a 
transmission signal SND. 
A case where the above-described signal has returned by way of the 
transmission path of the optical fiber loop will be described with 
reference to FIG. 6 and FIG. 8. Upon receiving the received signal RCV, 
the shift pulse SFT signal which is synchronized with the received signal 
RCV by the input timing controlling circuit 62 is outputted by seventeen, 
so that the received signal RCV becomes a parallel data by a shift 
register 65 for reception use. As an interruption iRTl is outputted into 
the microprocessor 61 at the same timing as that of the seventeenth SFT 
signal, it is received so as to start the reception program of the 
microprocessor 61. The contents of the program first outputs the RD.sub.2 
signals to load the contents of the received signal RCV through an input 
buffer circuit 64. In order to check the absence of the transmission 
error, the RD.sub.2 signal is outputted and the ERR signal is inputted. If 
"0" is provided, the mode portion and the address portion are decoded to 
effect the corresponding control and the fault examination. In the example 
of FIG. 8, the content of the mode portion is "100" so that the 
received-light-amount of the follower station of "0101" in address is 
found out to be "00000111" (=7). If the ERR signal is "1", the serial 
information of FIG. 7 is transmitted again. When the content of the mode 
portion is turned into "101" or "110", the minimum value and the maximum 
value of the received-light-amount level of an optional follower station 
may be provided in a similar procedure. 
In the manner as described hereinabove, the master station obtain the 
received-light-amount level of the follower station and judges where the 
value is in the range of the above-described FIG. 3. The master station 
displays a warning together with an address denoting a follower station 
having something wrong with the received-light-amount level. 
After the completion of the check, the memories of the maximum value 
decision storing circuit of the follower station and the minimum value 
memory deciding circuit thereof are reset, so that the serial information 
of the contents of "111" as the model portion is transmitted. 
As is clear from the foregoing description, according to the first 
embodiment of the present invention, as the respective follower stations 
renew the peak of the optical signal received every time to retain it, and 
digitalized the peak thereof so as to display it, it may be immediately 
found out whether or not a wiring of the optical fiber of which follower 
station is inferior, deteriorated or the like or whether or not the level 
thereof is such just prior to the time the wiring becomes inferior, so 
that the fault detection and maintenance are extremely simplified. 
According to the second embodiment, the maximum value and the minimum value 
among the peak values of the serial information to be received one after 
another are stored, are digitized, and are displayed. It may be judged by 
the difference among them whether or not the influences are applied 
directly upon the optoelectronic transducer by the light amount variation, 
noises or the like caused through the bending of the fiber established on 
the movable portion for many hours. Accordingly, the reliability of the 
system may be positively judged. 
According to the third and fourth embodiments, in order to know the 
received-light-amount level in accordance with the follower station of the 
first embodiment, and the follower station of the second embodiment, the 
information coded so that the contents of the mode portion of the serial 
information between the master station and the follower station may be 
discriminated from the normal inputting and outputting operations may be 
transmitted and received so as to allow the received-light-amount level of 
an optional follower station to be loaded in the master station. Thus, the 
address numbers of the defective follower stations may be collectively 
displayed through the judging operation of the value. 
As the levels of the received-light-amount of the all the follower stations 
may be collectively observed at the master station, the fault discovery 
and maintenance of the follower stations are simplified, thus contributing 
greatly towards improvements in the reliability of the automatic machines. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as 
included therein.