Abstract:
Disclosed is a cross-connect apparatus having n-number of working cross-connects for cross-connecting, on a per-bit basis, n-bit input signals that arrive from respective ones of a plurality of input paths; n-number of first logic circuits for calculating the exclusive-ORs of the n bits of each input signal and applying their outputs to a standby cross-connect; n-number of second logic circuits for calculating the exclusive-ORs of signals output from the n-number of working cross-connects and from the single standby cross-connect; and third logic circuits having a function for selecting output signals of the n-number of working cross-connects and outputs of the second logic circuits. The cross-connects apparatus detects the occurrence of an abnormality in a working cross-connect by monitoring the outputs of the second logic circuits, identifies the faulty cross-connect by successively turning off one input among the n-number of inputs to the first and second logic circuits, and selectively outputs the output signals of the second logic circuits instead of the output signals of the faulty cross-connect by using the third logic circuits.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a cross-connect apparatus having a plurality of cross-connects. More particularly, the invention relates to a cross-connect apparatus which, while reducing the burden of control performed by a controller, is capable of detecting cross-connect abnormality, detecting the cross-connect in which the abnormality has occurred, switching over to a standby cross-connect and switching back to a working cross-connect at the time of recovery.  
           [0002]    It is believed that the interfacing of optical signals to optical transmission devices will undergo a shift in emphasis from the handling of serial signals to the handling of parallel signals in the future. The present invention furnishes a cross-connect apparatus, the input to which is an optical parallel signal, with a highly reliable redundant structure and the ability to detect errors within the apparatus at minimum cost.  
           [0003]    Structure of Cross-Connect Apparatus According to Prior Art  
           [0004]    [0004]FIG. 13 is a diagram illustrating the structure of an optical cross-connect apparatus according to the prior art. If m(=4) represents the number of input paths and n(=4) represents the number of bits of each channel arriving in time-shared fashion from each input path, then n-number of m x m working optical cross-connects  101   1  to  101   n  are provided and one standby optical cross-connect  102  is provided for the n-number of working cross-connects. As Illustrated in FIG. 14, an m×m cross-connect has m-number of Input terminals and m-number of output terminals (where m=4 holds in the Figure), m-number of input lines and m-number of output lines are arrayed to intersect in the form of a matrix, and a switch is provided at each intersection. In order to implement a cross connection in such a manner that a signal that has arrived at an ith input terminal will be output from a jth output terminal, the switch at the intersection (i, j) is turned on (see the black circles). The Figure illustrates a case where a first input signal is cross-connected to a fourth output, a second input signal to a first output, a third input signal to a third output and a fourth input signal to a second output. It should be noted that although signals on a plurality of channels arrive in time-shared fashion from a single input path, each channel that arrives from an input path #i shall be referred to as an ith channel for the sake of explanation.  
           [0005]    Optical signals of first to fourth channels that arrive from optical input paths #1 to #4 are input to optoelectronic transducers (O/E)  103   1  to  103   4 , respectively, upon being converted to n(=4)-bit parallel signals by serial/parallel converters, which are not shown. The optoelectronic transducers  103   1  to  103   4  convert the optical signals that enter from each of the input paths to electrical signals. A distributor  105   i  (i=1 to 4) inputs the first to fourth bit signals of a channel i that arrives from input path #i to ith input terminals of respective ones of the first to fourth cross-connects  101   1  to  101   4  and to a selector  106 .  
           [0006]    By way of example, the distributor  1051  inputs the first to fourth bit signals of the first channel that arrives from input path #1 to the first input terminals of respective ones of the first to fourth cross-connects  101   1  to  101   4  and to the selector  106 . Similarly, the distributor  105   2  inputs the first to fourth bit signals of the second channel that arrives from input path #2 to the second input terminals of respective ones of the first to fourth cross-connects  101   1  to  101   4  and to the selector  106 . If an abnormality occurs in the ith optical cross-connect  101   i , the selector  106  selects the ith bit signals of the first to fourth channels that enter the ith optical cross-connect  101   i  and inputs these to the standby optical cross-connect  102 .  
           [0007]    Electro-optic transducers (E/O)  107   1  to  107   4 ,  108  are provided on the input side of the working and standby optical cross-connects  101   1  to  101   4 ,  102 , respectively, and optoelectronic transducers (O/E)  109   1  to  109   4 ,  110  are provided on the output side of these cross-connects. Signals sent from the first output terminals of the first to fourth cross-connects  101   1  to  101   4  are converted to electrical signals by the optoelectronic transducers  109   1  to  109   4 , and the electrical signals are input to an electro-optic transducer  104   1  via a selector  111   1 . Similarly, signals sent from the second output terminals of the first to fourth cross-connects  101   1  to  101   4  are converted to electrical signals by the optoelectronic transducers  109   1  to  109   4 , and the electrical signals are input to an electro-optic transducer  104   2  via a selector  111   2 . Signals sent from the third output terminals of the first to fourth cross-connects  101   1  to  101   4  are converted to electrical signals by the optoelectronic transducers  109   1  to  109   4 , and the electrical signals are input to an electro-optic transducer  104   3  via a selector  111   3 . Signals sent from the fourth output terminals of the first to fourth cross-connects  101   1  to  101   4  are converted to electrical signals by the optoelectronic transducers  109   1  to  109   4 , and the electrical signals are input to an electro-optic transducer  104   4  via a selector  111   4 . The electro-optic transducers  104   1  to  104   4  convert the cross-connected electrical parallel signals input thereto to optical parallel signals and input these parallel signals to parallel/serial converters (not shown). The parallel/serial converters convert the optical parallel signals to optical serial signals and send these signals to prescribed optical output paths #1 to #4.  
           [0008]    The first to fourth optical signals of the standby optical cross-connect  102  are converted to electro-optic signals by the optical electro-optic transducer  110 , and these signals are input to a distributor  112 . If an abnormality occurs in an ith optical cross-connect  101   i , the distributor  112  inputs the first output signal of the standby optical cross-connect  102  to an ith terminal on the standby side of the selector  111   1 , inputs the second output signal of the standby optical cross-connect  102  to the ith terminal on the standby side of the selector  111   2 , inputs the third output signal of the standby optical cross-connect  102  to the ith terminal on the standby side of the selector  111   3 , and inputs the fourth output signal of the standby optical cross-connect  102  to the ith terminal on the standby side of the selector  111   4 . When operation is normal, the selectors  111   1  to  111   4  select parallel signals of four bits input to the four input terminals of the working channels and input these signals to the electro-optic transducers  104   1  to  104   4 , which constitute the next stage. If an abnormality occurs in the ith cross-connect  101   i , however, the selectors  111   1  to  111   4  select a four-bit signal that is the result of replacing the signals input to the ith input terminals on the working side by signals input to the ith input terminals on the standby side.  
           [0009]    A signal cut-off detecting circuit  113  detects cut-off of the signal output from each output terminal of the working optical cross-connects  101   1  to  101   4 , thereby detecting an abnormality in a working cross-connect, and proceeds to notify a controller  115  of the result of detection. A signal cut-off detecting circuit  114  detects cut-off of signals output from the distributor  112 , thereby detecting an abnormality in the standby optical cross-connect  102 , and proceeds to notify the controller  115  of the result of detection.  
           [0010]    Operation  
           [0011]    Assume that signals on the first channel that enters from the input path #1 are output upon being cross-connected to the output path #2. As indicated by the dashed lines in FIG. 15, the working optical crossconnects  101   1  to  101   4  each cross-connect the first input to the second output. Further, the standby optical cross-connect  102  also cross-connects the first input to the second output. Operation is similar with regard to the other input paths as well. Generally, if a signal on an ith channel that arrives from input path #i is output upon being cross-connected to an output path #j, the working optical cross-connects  101   1  to  101   4  and the standby optical cross-connect  102  each cross-connect the ith input to the jth output.  
           [0012]    If the signal cut-off detecting circuit  113  detects cut-off of the signal from, e.g., the working optical cross-connect  101   1  in a state in which a signal on the first channel that is input from the input path #1 is cross-connected to the output path #2, then the signal cut-off detecting circuit  113  notifies the controller  115  of result of detection. The controller  115  controls the selector  106 , which, as indicated by the dashed lines, selects the first bit signals of the first to fourth channels that are input to the working optical cross-connect  101   1  and inputs these signals to the four input terminals of the standby optical cross-connect  102 . As a result, the standby optical crossconnect  102  attains an input state identical with that of the working optical cross-connect  101   1 .  
           [0013]    Further, the controller  115  controls the distributor  112  so that the four output signals of the standby optical cross-connect  102  are input to the first input terminals on the standby side of respective ones of the selectors  111   1  to  111   4  that are the same as those that receive the four output signals of the working optical cross-connect  101   1 . Furthermore, the controller  115  instructs the selectors  111   1  to  111   4  to select the signal from the standby optical cross-connect  102  instead of the signal from the working optical cross-connect  101   1  as the first bit. As a result, the selectors  111   1  to  111   4  select the signal from the standby optical cross-connect  102  as the first bit and select the signals from the working optical cross-connects  101   2  to  101   4  as the second to fourth bits.  
           [0014]    By virtue of the foregoing, rescue is possible and cross-connect control can continue even if the working optical cross-connect  101   1  develops an abnormality. Similarly, rescue is possible if an abnormality should occur in the other working optical cross-connects  101   2  to  101   4 . If the working optical cross-connect that malfunctioned returns to normal after the changeover is made to the standby optical cross-connect  102 , then the original normal state is restored and cross-connect control continues.  
           [0015]    Problems  
           [0016]    With the conventional optical cross-connect system, it is necessary to control the two selectors, namely the selector  106  on the input side and the selectors  111   1  to  111   4  on the output side, simultaneously even though changeover of. the signal to the standby optical cross-connect  102  is performed when a malfunction occurs. As a consequence, there is an increase in the number of optical parallel signals and an increase in the number of optical cross-connects, resulting in an apparatus of large size. A problem which arises is that there is an increase in load with regard to selector control.  
           [0017]    Further, what can be detected by the signal cut-off detecting circuit  113  is only the fact that a signal has been cut off. A problem which results is that signals cannot be checked for error.  
         SUMMARY OF THE INVENTION  
         [0018]    Accordingly, an object of the present invention is to so arrange it that an abnormality in a cross-connect apparatus can be detected by a simple arrangement of logic circuits.  
           [0019]    Another object of the present invention is to so arrange it that the occurrence of abnormality in any working cross-connect can be detected by a simple arrangement of logic circuits.  
           [0020]    Another object of the present invention is to so arrange it that changeover of working and standby optical cross-connects can be performed by a simple arrangement of logic circuits, thereby alleviating the control load involved in a working and standby changeover operation performed by a controller.  
           [0021]    Another object of the present invention is to so arrange it that an abnormality can be detected not only at cut-off of a signal from a cross-connect but also in a case where a signal changes from “1” to “0” or from “0” to “1” in a cross-connect. A further object is to so arrange it that cross-connect control can be continued by changing over to a standby cross-connect when such an abnormality occurs.  
           [0022]    The present invention discloses a cross-connect apparatus having first to nth cross-connects for changing over output paths of n-bit input signals that arrive from respective ones of m-number of input paths, and a connection architecture for inputting first to nth bit data of an input signal that arrives from an ith input path (i=1 to m) to respective ones of ith input terminals of the first to nth cross-connects and sending bit data that is output from jth output terminals (j=1 to m) of the first to nth cross-connects to a Jth output path. The cross-connect apparatus comprises (1) m-number of first logic circuits for calculating the exclusive-OR of the first to nth bit data of input signals that arrives from an ith input path (i=1 to m); (2) a standby cross-connect having an ith terminal to which is input an output signal of a first logic circuit that corresponds to the ith input path (i=1 to m); (3) m-number of second logic circuits for calculating the exclusive-OR of signals output from the jth terminals (j=1 to m) of the first to nth working cross-connects and of the standby cross-connect; and (4) abnormality detecting means for detecting that an abnormality has occurred in any of the working cross-connects. When the working and standby cross-connects have all been set to the same cross-connect state, the abnormality detecting means monitors output signals from the second exclusive-OR circuits and, in response to generation of an output signal that differs from that when operation is normal, decides that an abnormality has occurred in any -of the n-number of working cross-connects.  
           [0023]    In accordance with the present invention, an abnormality in the cross-connect apparatus can be detected by a simple arrangement of logic circuits. Moreover, an abnormality can be detected in a case where a signal changes from “1” to “0” or from “0” to “1” in a working cross-connect.  
           [0024]    The cross-connect apparatus of the present invention further comprises abnormal cross-connect specifying means for specifying, in a manner described below, whether an abnormality has occurred in any cross-connect when an abnormality has been detected. Specifically, first, without inputting signals that enter an ith (i=1) cross-connect to each of the first logic circuits and without inputting signals that are output from the ith cross-connect to each of the second logic circuits, the abnormal cross-connect specifying means monitors whether the output signals of the second logic circuits at this time are the same as when operation is normal. Thereafter, the abnormal cross-connect specifying means increments i successively, changes over the signals that are not input to each of the first and second logic circuits and decides that an abnormality has occurred in the ith cross-connect when the output signals of the second logic circuits become the same as when operation is normal. In accordance with the present invention, whether an abnormality has occurred in any of the working cross-connects can be detected by a simple arrangement of logic circuits.  
           [0025]    Further, the cross-connect apparatus according to the present invention further comprises a controller for exercising control in such a manner that a signal output from an ith cross-connect will not be input to the second logic circuits when an abnormality has occurred in the ith cross-connect, and third logic circuits for replacing m-number of signals output from the ith cross-connect with output signals of m-number of the second logic circuits and outputting the latter signals. The output signals of the m-number of second logic circuits are the same as m-number of ith bit signals that enter from the input paths #1 to #4. In accordance with the present invention, changeover of working and standby cross-connects can be performed by a simple arrangement of logic circuits and the load involved in the working and standby changeover control can be alleviated.  
           [0026]    Further, the cross-connect apparatus of the present invention comprises fourth logic circuits for checking whether output signals of the m-number of second logic circuits agree with m-number of signals output from an ith cross-connect in which an abnormality has been detected, and means for determining that the ith cross-connect has returned to normal in response to agreement of the signals that continues for a predetermined period of time. In accordance with the present invention, recovery can be detected by a simple arrangement of logic circuits and it is possible to switch back to a working cross-connect from the standby cross-connect unerringly.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a diagram illustrating the overall structure of a cross-connect apparatus according to the present invention;  
         [0028]    [0028]FIG. 2 is a diagram useful in describing abnormality detection, identification of an abnormal cross-connect and working/standby changeover according to the present invention;  
         [0029]    [0029]FIG. 3 is a diagram useful in describing outputs of a logic circuit in a case where an input path #i is cross-connected to an output path #j;  
         [0030]    [0030]FIG. 4 is a diagram showing the main structure of a cross-connect apparatus in which only a portion that cross-connects an input path #i to an output path #j has been extracted for illustration;  
         [0031]    [0031]FIG. 5 is a diagram showing the structure of a first logic circuit;  
         [0032]    [0032]FIG. 6 is a diagram showing the structure of a second logic circuit;  
         [0033]    [0033]FIG. 7 is a diagram showing the structures of third and fourth logic circuits;  
         [0034]    [0034]FIG. 8 is a diagram useful in describing placement of signal cut-off detecting circuits;  
         [0035]    [0035]FIG. 9 is a flowchart of processing for detecting abnormality and controlling output changeover;  
         [0036]    [0036]FIG. 10 is a flowchart of processing in a case where abnormality has been detected by the signal cut-off detecting circuit;  
         [0037]    [0037]FIG. 11 is a flowchart of recovery processing;  
         [0038]    [0038]FIG. 12 is another flowchart of recovery processing;  
         [0039]    [0039]FIG. 13 is a diagram illustrating the structure of an optical cross-connect apparatus according to the prior art;  
         [0040]    [0040]FIG. 14 is a diagram useful in describing cross-connect; and  
         [0041]    [0041]FIG. 15 is a diagram useful in describing operation of the optical cross-connect apparatus according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    (A) Overall Structure of Cross-Connect Apparatus According to the Present Invention  
         [0043]    [0043]FIG. 1 is a diagram illustrating the overall structure of a cross-connect apparatus according to the present invention. If n(=4) represents the number of bits of each channel arriving in time-shared fashion from each input path, then n-number of m x m working optical cross-connects  101   1  to  101   n  are provided and one standby optical cross-connect  202  is provided for the n-number of working cross-connects. It should be noted that although signals on a plurality of channels arrive in time-shared fashion from a single input path, each channel that arrives from an input path #i shall be referred to as an ith channel for the sake of explanation.  
         [0044]    Optical signals of first to fourth channels that arrive from input paths #1 to #4 are input to optoelectronic transducers (O/E)  203   1  to  203   4 , respectively, upon being converted to n(=4)-bit parallel signals by serial/parallel converters, which are not shown. The optoelectronic transducer  203   1  inputs first to fourth bit signals of the first channel arriving from the first path to first input terminals of respective ones of the first to fourth optical cross-connects  201   1  to  201   4  and to a logic circuit  205   1 . Similarly, an optoelectronic transducer  203   i  connected to an input path #i (i=1 to 4) inputs first to fourth bit signals of the ith channel arriving from the abovementioned path to ith input terminals of respective ones of the first to fourth optical cross-connects  201   1  to  201   4  and inputs the first to fourth bit signals to a first logic circuit  205   1  (i=1 to 4), which calculates the exclusive-OR. The logic circuit  205   1  calculates the exclusive-OR of the first to fourth bit signals of the ith channel and inputs the result of this operation to the standby cross-connect  202  via an electro-optic transducer.  
         [0045]    Electro-optic transducers (E/O)  207   1  to  207   4 ,  208  are provided on the input side of the working and standby optical cross-connects  201   1  to  201   4 ,  202 , respectively, and optoelectronic transducers (O/E)  209   1  to  209   4 ,  210  are provided on the output side of these cross-connects.  
         [0046]    Signals sent from the first output terminals of the first to fourth cross-connects  201   1  to  201   4  are converted to electrical signals by the optoelectronic transducers  209   1  to  209   4 , and the electrical signals are input to logic circuits  211   1  , and  212   1  corresponding to output path #1. The logic circuit  211   1  calculates the exclusive-OR of these input signals, and the logic circuit  212   2  performs signal selection in accordance with the absence or presence of an abnormality. Similarly, signals sent from ith output terminals (i=1 to 4) of the first to fourth cross-connects  201   1  to  201   4  are converted to electrical signals by the optoelectronic transducers  209   1  to  209   4 , and the electrical signals are input to logic circuits  211   1  and  212   i  corresponding to output path #i. The logic circuit  211   i  calculates the exclusive-OR of these input signals, and the logic circuit  212   i  performs signal selection in accordance with the absence or presence of an abnormality. The parallel signals of four bits selected by each of the logic circuits  212   1  to  212   4  are converted to optical parallel signals by electro-optic transducers  212   1  to  213   4 , respectively, and the optical parallel signals are input to parallel/serial converters, not shown. The parallel/serial converters convert the entered optical parallel signals to optical serial signals and send these signals to prescribed optical output paths #1 to #4. The four signals output from first to fourth output terminals of the standby optical cross-connect  202  are converted to electrical signals by the optoelectronic transducer  210 , and the electrical signals are input to the logic circuits  211   1  , to  211   4  .  
         [0047]    As a result of the foregoing, five signals output from the first output terminals of the optical cross-connects  201   1  to  201   4  and of the standby optical cross-connect  202  are input to the logic circuit  211   1  , that corresponds to the output path #1. Further, five signals output from the second output terminals of the optical cross-connects  201   1  to  201   4  and of the standby optical cross-connect  202  are input to the logic circuit  211   2  that corresponds to the output path #2. Further, five signals output from the third output terminals of the optical cross-connects  201   1  to  201   4  and of the standby optical cross-connect  202  are input to the logic circuit  211   3  that corresponds to the output path #3. Further, five signals output from the fourth output terminals of the optical cross-connects  201   1  to  201 , and of the standby optical cross-connect  202  are input to the logic circuit  211   4  that corresponds to the output path #4.  
         [0048]    A controller  215  (1) monitors the output signals from the logic circuits  211   1  , to  211   4  and detects any abnormality in the optical cross-connects; (2) changes over the input signals of the logic circuits  205   4  to  205   4  and of logic circuits  211   1  to  211   4  and monitors the output signals of the logic circuits  211   1  to  211   4  at this time to thereby specify whether an abnormality has occurred in any of the optical cross-connects; and (3) inputs changeover signals to the logic circuits  212   1  to  212   4  based upon whether an abnormality has occurred in any of the optical cross-connects, thereby outputting the correct signals.  
         [0049]    (B) Abnormality Detection Control  
         [0050]    When cross-connecting. the working optical crossconnects  201   1  to  201   4  and standby optical cross-connect  202  are controlled so as to attain the same crossconnect state. Accordingly, as shown for example in FIG. 2, assume that signals on the first channel that arrives from input path #1 are cross-connected to output path #2. As indicated by the dashed lines, the working optical cross-connects  201   1  to  201   4  and standby optical cross-connect  202  each cross-connect the first Input to the second output. Further, the output signal of the logic circuit  205   1  and the cross-connected first to fourth bit signals of the first channel are input to the logic circuit  211   2  corresponding to output path #2. Since the logic circuit  205   1  calculates the exclusive OR of the first to fourth bit signals of the first channel prevailing prior to the cross connection, the logic circuit  211   1  outputs the exclusive-OR between the exclusive-OR signal of the correct signals that prevailed prior to the cross connection and the four signals prevailing after the cross connection.  
         [0051]    When Operation is Normal  
         [0052]    If the number of 1 &#39;s of the first to fourth bits of the first channel (i=1) prior to cross connection is odd, as indicated at {circumflex over ( 1 )} in FIG. 3( a ), then the output of the logic circuit  205   1  is “1”. If the working optical cross-connects  201   1  to  201   4  are normal, then the number of “1”s of the signals output from the second output terminals (j=2) of respective ones of these cross-connects will be odd. Accordingly, the number of “1”s input to the logic circuit  211   2  corresponding to output path #2 will be even when totaled, and the output of the logic circuit will be “0”. Further, if the number of “1”s of the first to fourth bits of the first channel prior to cross connection is even, as indicated at {circumflex over ( 2 )} in FIG. 3( a ), then the output of the logic circuit  205   1  is “0”. If the working optical cross-connects  201   1  to  201   4  are normal, then the number of “1”s of the signals output from the second output terminals of respective ones of these cross-connects will be even. Accordingly, the number of “1”s input to the logic circuit  211   2  corresponding to output path #2 will be even when totaled, and the output of the logic circuit will be  
         [0053]    When Operation is Abnormal  
         [0054]    If the number of “1”s of the first to fourth bits of the first channel prior to cross connection is odd, as indicated at {circumflex over ( 3 )} in FIG. 3( b ), then the output of the logic circuit  205   1  is “1”. If any one of the working optical cross-connects  201   1  to  201   4  becomes abnormal, then the number of “1”s of the signals output from the second output terminals of respective ones of these cross-connects will be even. Accordingly, the number of “1”s input to the logic circuit  211   2  corresponding to output path #2 will be even when totaled, and the output of the logic circuit will be “1”. Further, if the number of “1”s of the first to fourth bits of the first channel prior to cross connection is even, as indicated at {circumflex over ( 4 )} in FIG. 3( b ), then the output of the logic circuit  205   1  is “1”. If any one of the working optical cross-connects  201   1  to  201   4  becomes abnormal, then the number of “1”s of the signals output from the second output terminals of respective ones of these cross-connects will be odd. Accordingly, the number of “1”s input to the logic circuit  211   2  corresponding to output path #2 will be odd when totaled, and the output of the logic circuit will be “1”.  
         [0055]    Thus, if all of the cross-connects  201   1  to  201   4  are normal, the output of the logic circuit  211   1  will be “0”; if any one of the cross-connects becomes abnormal, then the output of the logic circuit  211   1  will be “1”.  
         [0056]    The foregoing is described with regard to a case where the signals that enter from the first input path #1 are cross-connected and output from the second output path #2. However, operation is similar also for a case where the signals that enter from any input path #i are cross-connected and output from any output path #j.  
         [0057]    Further, when an abnormality has developed in one working cross-connect, all of its output signals do not necessary change from “1”to “0” or vice versa. When an abnormality develops in one cross-connect in the above example, therefore, not all of the output signals from the logic circuits  211   1  , to  211   4  become “1” but the output of at least one logic circuit does, thereby enabling the abnormality to be detected.  
         [0058]    Accordingly, the controller  215  performs monitoring to determine whether one output signal from any of the logic circuits  211   1  to  211   4  has become “1”. If the output of at least logic circuit becomes “1”, then the controller  215  determines that an abnormality has occurred in a cross-connect.  
         [0059]    Description of abnormality detection using simplified drawing  
         [0060]    [0060]FIG. 4 is a diagram showing the main structure of a cross-connect apparatus In which only a portion that cross-connects input path #1 to output path #2 has been extracted for illustration. Components identical with those shown in FIGS. 1 and 2 are designated by like reference characters. A serial/parallel converter (S/P)  222   1  converts an optical serial signal that arrives from input path #1 to optical parallel signals and inputs these signals to the optoelectronic transducer (O/E)  203   i . A parallel/serial converter (P/S)  231   2  converts the 4-bit parallel signal that is output from the electro-optic transducer  213   2  to a serial optical signal and sends this signal to output path #2.  
         [0061]    The working optical cross-connects  201   1  to  201   4  cross-connect first to fourth bit signals S 11 , S 12 , S 13 , S 14 , which have been input to the first input terminals of respective ones of these cross-connects from the optoelectronic transducer  203   1  to their second output terminals, as indicated by the dashed lines.  
         [0062]    The first logic circuit  205   1  calculates the exclusive-OR of the first to fourth bit signals S 11 , S 12 , S 13 , S 14 , of the first channel that arrives from input path #1 and inputs the operational result A to the first input terminal of the standby optical crossconnect  202 .  
         [0063]    An output signal A′ of the first logic circuit  205   1  prevailing after the cross connection and first to fourth bit signals S 11 ′, S 12 ′, S 13 ′, S 14 ′ of the first channel prevailing after the cross connection are input to the second logic circuit  211   2 . Since the first logic circuit  205   1  calculates the exclusive-OR of the first to fourth bit signals S 11 ′, S 12 ′, S 13 ′, S 14 ′ of the first channel prevailing prior to the cross connection, the second logic circuit  211   2  outputs the exclusive-OR between the exclusive-OR signal of the correct signals that prevailed prior to the cross connection and the four signals S 11 ′, S 12 ′, S 13 ′, S 14 ′ prevailing after the cross connection.  
         [0064]    As described above based upon {circumflex over ( 3 )} and {circumflex over ( 4 )} in FIG. 3( b ), when any one of the working optical crossconnects  201   1  to  201   4  develops an abnormality and the logic of the output signal thereof becomes inverted (“1”→“0” or “0”→“1”), the output B of the second logic circuit  211   2  becomes “1”. Accordingly, the control circuit  215  performs monitoring to determine whether the output of the second logic circuit  211   2  has become “1” and decides that an abnormality has occurred if this output becomes “1”.  
         [0065]    (C) Control for Specifying Cross-Connect in which Abnormality has Occurred  
         [0066]    If at least one of the output signals of the logic circuits  211   1  , to  211   4  (FIG. 2) becomes “1”, thus indicating that an abnormality has occurred in any one of the optical cross-connects  201   1  to  201   4 , the controller  215  specifies the cross-connect in which the abnormality has occurred.  
         [0067]    To accomplish this, the controller  215  inhibits entry, to the logic circuits  205   1  to  205   4 , of the signals applied to the first optical cross-connect  201   1  and simultaneously inhibits entry of the signals output from the first optical cross-connect  201   1  to the logic circuits  211   1  , to  211   4 . The controller  215  then performs monitoring to determine whether the output signals of the logic circuits  211   1  to  211   4  are the same as during normal operation (=“0”). If the first optical cross-connect  201   1  has developed an abnormality, the input and output signals of the cross-connect  201   1  will have been excluded from the exclusive-OR operation and, as a result, the output signals of the logic circuits  211   1  to  211   4  will all be “0”. This makes it possible to specify that the first optical cross-connect  201   1  is the faulty location in which the abnormality occurred.  
         [0068]    If the first optical cross-connect  201   1  is normal, however, the output signal of any of the logic circuits  211   1  to  211   4  will be “1”, owing to the existence of an abnormal cross-connect, even though the input and output signals of the cross-connect  201   1  have been excluded from the exclusive-OR operation. As a result, it can be determined that the first optical crossconnect  201   1  has not developed an abnormality and is normal.  
         [0069]    If the controller  215  determines that the first optical cross-connect  201   1  is normal, then It inhibits entry, to the logic circuits  205   1  to  205   4 , of the signals applied to the second optical cross-connect  201   2  and simultaneously inhibits entry of the signals output from the second optical cross-connect  201   2  to the logic circuits  211   1  to  211   4 . The controller  215  then performs monitoring to determine whether the output signals of the logic circuits  211   1  to  211   4  are the same as during normal operation (=“0”), thereby determining whether the second optical cross-connect  201   2  is normal or abnormal. If the second optical cross-connect  201   2  is found to be normal, then the controller  215  subsequently determines the normality or abnormality of the third and fourth optical crossconnects  2013 ,  2014  in similar fashion until the faulty location can be identified.  
         [0070]    Description of Abnormality Detection using Simplified Drawing  
         [0071]    The above will be described with reference to the simplified diagram of FIG. 4. Assume that the output of the second logic circuit  211   2  is “1” owing to occurrence of an abnormality. The controller  215  first performs control in such a manner that the first bit signals S 11 , S 11 ′ will not enter the first logic circuit  205   1  and second logic circuit  2112 , respectively. If the first optical cross-connect  201   1  develops an abnormality, the input and output signals S 11 , S 11 ′ of the cross-connect  201   1  are excluded from the exclusive-OR operation and therefore the output signal of the logic circuit  211   2  becomes “0”. As a result, the controller  215  is capable of determining that an abnormality has occurred in the first optical cross-connect  201   1  and can specify the faulty location.  
         [0072]    If the first optical cross-connect  201   1  is normal, however, the output signal of the logic circuit  211   2  will be “1”, owing to the fact that another abnormal cross-connect exists, even though the input and output signals S 11 , S 11 ′ of the cross-connect  201   1  have been excluded from the exclusive-OR operation. As a result, it can be determined that the first optical cross-connect  201   1  has not developed an abnormality and is normal.  
         [0073]    If the controller  215  determines that the first optical cross-connect  201   1  is normal, it then performs control in such a manner that the second bit signals S 12 , S 12 ′ will not enter the first logic circuit  205   1  and second logic circuit  211   2 , respectively. If the second optical cross-connect  201   2  develops an abnormality, the input and output signals S 12 , S 12 ′ of the cross-connect  201   2  are excluded from the exclusive-OR operation and therefore the output signal of the logic circuit  211   2  becomes “0”. As a result, the controller  215  is capable of determining that an abnormality has occurred in the second optical cross-connect  201   2  and can specify the faulty location.  
         [0074]    If the second optical cross-connect  201   2  is normal, however, the output signal of the logic circuit  211   2  will be “1”, owing to the fact that another abnormal cross-connect exists, even though the input and output signals S 12 , S 12 ′ of the cross-connect  201   2  have been excluded from the exclusive-OR operation. As a result, it can be determined that the second optical cross-connect  201   2  has not developed an abnormality and is normal. The controller  215  thenceforth executes similar control until it is able to specify the faulty location.  
         [0075]    (D) Control for Changing over Output Signal at Time of Abnormality  
         [0076]    Described next will be changeover control in a case where the signals of the first channel arriving from input path #1 are cross-connected to output path #2 (see the dashed lines in FIG. 2) and the signals of the fourth channel arriving from input path #4 are cross-connected to output path #3 (see the dot-and-dash lines in FIG. 2). If the fact that an abnormality has occurred in a kth cross-connect  201   k  (e.g., k=3) is detected in the cross-connect state described above, the controller  215  inhibits the signals that are output from this kth cross-connect from entering the logic circuits  211   1  , to  211   4 .  
         [0077]    As a result, output signal A′ of the logic circuit  205   1  and all of the first to fourth bit signals of the first channel prevailing after cross connection, with the exception of the signal of the kth bit (k=3), are input to the logic circuit  211   2  corresponding to output path #2 (see FIG. 4). The logic circuit  205   1  calculates the exclusive-OR of the first to fourth bit signals of the first channel that prevailed prior to the cross connection. Therefore, it we let S 11  to S 14  represent the first to fourth bits signals of the first channel that prevailed before cross connection and let S 11 ′ to S 14 ′ represent the first to fourth bits signals of the first channel prevailing after cross connection, and if k=3 holds, then the output of the logic circuit  211   2  corresponding to output path #2 will be the exclusive-OR of the following signals:  
         [0078]    S 11 , S 12 , S 13 , S 14 , S 11 ′, S 12 ′, S 14 ′ Since the signals S 11 ′, S 12 ′, S 14 ′ are the outputs of the normal cross-connects, they agree with the signals S 11 , S 12 , S 14 . As a result, whether the number of “1” signals among the above-mentioned seven signals is even or odd depends upon the third bit signal S 13 . In other words, the output of the logic circuit  211   2  is the third bit signal S 13 .  
         [0079]    The logic circuit  212   2  includes four selectors SEL 21 , SEL 22 , SEL 23 , SEL 24  having first input terminals connected to the second output terminals of the working optical cross-connects  201   1  to  201   4  and second input terminals connected to the output terminals of the logic circuit  211   2 . The controller  215  instructs each of the selectors of the logic circuit  212   2  to select the signals output from the second output terminals of the cross-connects  201   1 ,  201   2 ,  201   4  as the first, second and fourth bit signals and to select the signal S 13  output from the logic circuit  211   2  as the third bit signal. As a result, the logic circuit  212   2  selects and outputs S 11 ′, S 12 ′, S 13 , S 14 ′ as the first to fourth bit signals.  
         [0080]    Further, the output signal of the logic circuit  205   4  and all of the first to fourth bit signals of the fourth channel prevailing after cross connection, with the exception of the signal of the kth bit (k=3), are input to the logic circuit  211   3  corresponding to output path #3 (see FIG. 2). The logic circuit  205   4  calculates the exclusive-OR of the first to fourth bit signals of the fourth channel that prevailed prior to the cross connection. Therefore, it we let S 41  to S 44  represent the first to fourth bits signals of the fourth channel that prevailed before cross connection and let S 41 ′ to S 44 ′ represent the first to fourth bits signals of the fourth channel prevailing after cross connection, and if k=3 holds, then the output of the logic circuit  211   3  corresponding to output path #3 will be the exclusive-OR of the following signals: S 41 , S 42 , S 43 , S 44 , S 41 ′, S 42 ′, S 44 ′.  
         [0081]    Since the signals S 41 ′, S 42 ′, S 44 ′ are the outputs of the normal cross-connects, they agree with the signals S 41 , S 42 , S 44 . As a result, whether the number of “1” signals among the above-mentioned seven signals is even or odd depends upon the third bit signal S 13 . In other words, the output of the logic circuit  211   3  is the third bit signal S 43 . The controller  215  instructs each of the selectors of the logic circuit  212   3  to select the signals output from the third output terminals of the cross-connects  201   1 ,  201   2 ,  201   4  as the first, second and fourth bit signals and to select the signal S 43  output from the logic circuit  211   3  as the third bit signal. As a result, the logic circuit  212   3  selects and outputs S 41 ′, S 42 ′, S 43 , S 44 ′ as the first to fourth bit signals.  
         [0082]    Similarly, the logic circuits  212   1  and  212   4  select, and send to output paths #1 and #4, the output signals of the logic circuits  211   1 ,  211   4  instead of the signals output from the third cross-connect  201   3  (k=3) as the third bit signal.  
         [0083]    In the description given above, it is assumed that an abnormally occurs in the third cross-connect. However, operation is similar even if an abnormality occurs in any kth cross-connect, and the logic circuits  212   1  to  212   4  select and output the output signals of the logic circuits  211   1  to  211   4  instead of the signals output from the kth cross-connect  201   k .  
         [0084]    Further, in the description given above, the case described is one in which the signals of the first channel arriving from input path #1 are cross-connected to output path #2 and the signals of the fourth channel arriving from input path #4 are cross-connected to output path #3. However, it should be obvious that the invention will hold true for any state of cross connection.  
         [0085]    (E) Recovery Control  
         [0086]    Logic circuits (not shown in FIGS. 1 and 2) are provided for checking whether output signals of the logic circuits  211   1  to  211   4  agree with respective ones of four signals output from the first to fourth output terminals of an ith cross-connect  201   i  in which an abnormality has been detected, and the controller  215  monitors the outputs of these logic circuits and determines that the ith cross-connect  201   i  has returned to normal in response to signal agreement that continues for a predetermined period of time. If the controller  215  determines that the normal state has been restored, then it switches the state of each logic circuit back to the state that prevails under normal operation, and the logic circuits  212   1  to  212   4  select, and output to the output paths #1 to #4, signals that have been cross-connected by the working optical crossconnects  201   1  to  201   4 .  
         [0087]    (F) Structure of Logic Circuits  
         [0088]    (a) First Logic Circuits  
         [0089]    The logic circuits  205   1  to  205   4  are identical in structure. Among these logic circuits, the logic circuit  205   1  comprises four AND gates AG 11 , to AG 14  for controlling passage of the first to fourth bit signals S 11  to S 14  of the first channel, and three exclusive-OR gates EOR 11  to EOR 13  for calculating the exclusive-OR of the first to fourth bit signals S 11  to S 14 , as illustrated in FIG. 5. It is so arranged that gate signals G 11  to G 14  usually are at the high level, as a result of which the logic circuit outputs the exclusive-OR signal A of the first to fourth bit signals S 11  to S 14 . When a faulty cross-connect is specified, however, the controller  215  sends the gate signals G 11  to G 14  to the low level in the following order: G 11 →G 12 →G 13  →G 14 , thereby inhibiting passage of one signal from among the first to fourth bit signals S 11  to S 14  so that the logic circuit outputs an exclusive-OR signal of the other three signals.  
         [0090]    (b) Second logic circuit  
         [0091]    The second logic circuits  211   1  to  211   4  are identical in structure. Among these logic circuits, logic circuit  211   2  comprises four AND gates AG 21  to AG 24  for controlling passage of the signals (signals S 11 ′ to S 14 ′ in the example of FIG. 4) output from the second output terminals of the first to fourth cross-connects  201   1  to  201   4 ; an AND gate AG25 for controlling passage of the signal (signal A′ in the example of FIG. 4) output from the second output terminal of the standby optical cross-connect  202 ; and four exclusive-OR gates EOR 21  to EOR 24  for calculating the exclusive-OR of the outputs from each of the above-mentioned AND gates, as illustrated in FIG. 6. It is so arranged that gate signals G 11  to G 14  usually are at the high level, as a result of which the logic circuit outputs the exclusive-OR signal A of the first to fourth bit signals S 11  to S 14 . It is so arranged that gate control signals G 21  to G 25  usually are at the high level, as a result of which the second logic circuit  211   2  outputs a signal B obtained by calculating the exclusive-OR between the first to fourth bit signals S 11 ′ to S 14 ′ prevailing after cross connection and the output signal A′ of the first logic circuit in the case of FIG. 4.  
         [0092]    When a faulty cross-connect is specified, however, the controller  215  sends the gate signals G 11  to G 24  to the low level in the following order: G 21 →G 22 →G 23  →G 24 , and the second logic circuit  211   2  outputs the signal B obtained by calculating the exclusive-OR between three signals remaining after any one signal is excluded from the first to fourth bits signals S 11 ′ to S 14 ′ prevailing after cross connection, and the output signal A′ of the first logic circuit.  
         [0093]    Further, if it is found that the faulty cross-connect is the kth cross-connect, then the controller  215  sends the gate signal G 2k to the low level, thereby inhibiting passage of the signal S 1k  that is output from the second output terminal of the kth crossconnect, so that the logic circuit outputs the signal B by calculating the exclusive-OR between the other three signals and the output signal A′ of the first logic circuit. Since k=3 holds in the example of FIG. 4, the gate signal G 23  assumes the low level and passage of the signal S 13 ′ output from the second output terminal of the third cross-connect  201   3  is inhibited. As a result, the second logic circuit  211   2  outputs the third bit signal S 13  of the first channel as the output signal B.  
         [0094]    (c) Third and Fourth Logic Circuits  
         [0095]    The third logic circuits  212   1  to  212   4  are identically constructed. The third logic circuit  2122  corresponding to the output path #2 has the four selectors SEL 21  to SEL 24 , as described above with reference to FIG. 4. In the case of FIG. 4, the first input terminals of the selectors SEL 21  to SEL 24  are connected to the second output terminals of the working cross-connects  201   1  to  201   4 , and the second input terminals are connected to the output of the logic circuit  211   2 . If an abnormality has occurred in the third cross-connect  201   3 , the controller  215  instructs the selectors SEL 21 , SEL 22 , SEL 24  of the logic circuit  212   2  to select the signals that are output from the second output terminals of the optical cross-connects  201   i ,  201   2 ,  201   4 , and instructs the selector SEL 23  to select the signal S 13  that is output from the logic circuit  211   2 . As a result, the logic circuit  212   2  selects and outputs S 11 ′, S 12 ′, S 13 , S 14 ′ as the first to fourth bit signals.  
         [0096]    Four of the fourth logic circuits are provided and form pairs with the third logic circuits. In FIG. 7, one logic circuit  301   2  forming a pair with the third logic circuit  212   2  is shown. The four fourth logic circuits each have exclusive-OR gates EOR 41  to EOR 44  and check to determine whether the four output signals of the second logic circuits  211   1  to  211   4  agree with the four output signals of the faulty kth cross-connect  201   k , and the controller  215  determines that the kth cross-connect  201   k  has returned to normal in response to agreement that continues for a predetermined period of time. More specifically, the fourth logic circuit  301   2  corresponding to output path #2 in FIG. 7 performs monitoring to determine, based upon the output of the gate EOR 43 , whether the second output signal S 13 ′ of the faulty third cross-connect  201   3  and the output signal S 13  of the second logic circuit  211   2  agree. The other fourth logic circuits also monitor for agreement in the same manner.  
         [0097]    (d) Implementation having Signal Cut-Off Detecting Circuits  
         [0098]    In the foregoing, abnormalities in cross-connects are detected based upon “1” outputs from the second logic circuits  211   1  to  211   4 . However, detection of abnormality can also be performed in response to the output signals of the optical cross-connects  201   1  to  201   4  being severed continuously in excess of a predetermined period of time.  
         [0099]    [0099]FIG. 8 is a diagram useful in describing placement of signal cut-off detecting circuits. Components in FIG. 8 identical with those shown in FIG. 6 are designated by like reference characters. The second output signals of the optical cross-connects  201   1  to  201   4  are input to the second logic circuit  211   2  corresponding to output path #2. Signal cut-off detecting circuits  401   1  to  401   4  detect the fact that the second output signals of these cross-connects  201   1  to  201   4  have been severed continuously in excess of a predetermined period of time and so notify the controller  215 .  
         [0100]    (G) Flow of Various Processes  
         [0101]    (a) Abnormality Detection and Control of Output Changeover  
         [0102]    [0102]FIG. 9 is a Flowchart of Processing for Detecting Abnormality and Controlling Output Changeover.  
         [0103]    The controller  215  monitors whether any of the outputs of the second logic circuits  211   1  to  211   4  has become logical “1” (step  501 ). If an output becomes “1”, the controller decides that an abnormality has occurred in any one of the working optical crossconnects  201   1  to  201   4 , establishes the relation n=1 and starts processing to specify the faulty location (step  502 ).  
         [0104]    The controller  215  turns off the nth input signal to each of the first logic circuits  205   1  to  205   4  and the nth input signal to each of the second logic circuits  211   1  to  211   4  (step  503 ). In actuality, the controller sends the gate signals G 1n  and G 2n  in FIGS. 5 and 6 to the low level, thereby arranging it so that the nth input to each of the first and second logic circuits will not take part in the exclusive-OR operation.  
         [0105]    Under these conditions, the controller  215  performs monitoring to determine whether all outputs of the second logic circuits  211   1  to  211   4  are “0” (step  504 ). If all outputs are not “0”, then the controller decides that the nth cross-connect  201   n  is not abnormal, increments n by the operation n+1→n (step  505 ) and repeats processing from step  503  onward. If it is found at step  504  that all outputs of the second logic circuits  211   1  to  211   4  are “0”, on the other hand, then the controller decides that the nth cross-connect  201   n  is the abnormal cross-connect and exits control for the faulty location.  
         [0106]    Next, the controller  215  continues holding the nth input to each of the second logic circuits  211   1  to  211   4  in the off state restores the nth input of each of the first logic circuits  205   1  to  205   4  from the off to the on state (step  506 ). As a result, the output signals of the second logic circuits  211   1  to  211   4  become correct signals that should be output by the nth cross-connect  201   n .  
         [0107]    Further, the controller  215  instructs the third logic circuits  212   1  to  212   4  to select the output signals of the second logic circuits  211   1  to  211   4  instead of the output signals of the nth cross-connect  201   n , (step  507 ). As a result, the third logic circuits  212   1  to  212   4  change over the selected signals from the output signals of the nth cross-connect  201   n  to the output signals of the second logic circuits  211   1  to  211   4 , thereby outputting the correct signals.  
         [0108]    (b) Abnormality Detection and Control of Output Changeover by Signal Cut-Off Detecting Circuit  
         [0109]    [0109]FIG. 9 relates to a case where abnormality in a cross-connect is detected by monitoring the output signals of the second logic circuits  211   1  to  211   4 . FIG. 10, however, shows a processing flowchart in a case where abnormality is detected by the signal cut-off detecting circuits  401   1  to  401   4  (see FIG. 8).  
         [0110]    The controller  215  monitors whether any output signals SF 1  to SF 4  of the signal cut-off detecting circuits  401   1  to  401   4  has become logical “1” (step  601 ). If an abnormality has occurred in the nth cross-connect  201   n  and the output signal thereof is cut off in excess of a predetermined period of time, then the signal cutoff detecting circuit  401   n  outputs a cut-off signal SF n . The controller  215  responds by recognizing that the nth cross-connect  201   n  has developed an abnormality (step  602 ).  
         [0111]    Next, the controller  215  turns off the nth input to each of the second logic circuits  211   1  to  211   4  (step  603 ). As a result, the output signals of the second logic circuits  211   1  to  211   4  become correct signals that should be output by the nth cross-connect  201   n .  
         [0112]    Further, the controller  215  instructs the third logic circuits  212   1  to  212   4  to select the output signals of the second logic circuits  211   1  to  211   4  instead of the output signals of the nth cross-connect  201   n  (step  604 ). As a result, the third logic circuits  212   1  to  212   4  change over the selected signals from the output signals of the nth cross-connect  201   n  to the output signals of the second logic circuits  211   1  to  211   4 , thereby outputting the correct signals.  
         [0113]    (c) Recovery Processing  
         [0114]    [0114]FIG. 11 is a Flowchart of Recovery Processing.  
         [0115]    Under conditions in which the output signals of a kth cross-connect  201   k  have been changed over (the condition prevailing at step  507  in FIG. 9 or at step  604  in FIG. 10) owing to occurrence of an abnormality in this cross-connect, the controller  215  monitors whether the kth cross-connect  201   k  has been restored to normal based upon exclusive-OR signals output from four fourth logic circuits  301   1  to  301   4  (only logic circuit  301   2  is shown in FIG. 7). Specifically, the controller  215  performs monitoring to determine whether the four output signals of the second logic circuits  211   1  to  211   4  and the four output signals of the kth cross-connect  201   k  agree (step  701 ). If the signals agree, then the controller checks to see whether agreement continues in excess of a set period of time (step  702 ). If agreement does not continue in excess of the set period of time, the controller repeats processing from step  701  onward. If agreement does continue in excess of the set period of time, the controller  215  decides that the kth cross-connect  201   k  has returned to normal (step  703 ).  
         [0116]    If it has decided that the normal state has been restored, the controller  215  terminates automatic changeover of the outputs due to occurrence of abnormality and restores the normal state that originally prevailed (step  704 ). That is, the controller sends all gate signals G ij  (FIGS. 5, 6) in the second logic circuits to the high level and instructs the third logic circuits (FIG. 7) to select he output signals from working cross-connects.  
         [0117]    [0117]FIG. 12 is another flowchart of recovery processing. This is a flow of processing for deciding hat recovery has been achieved in a case where agreement of signals is obtained a set number of times in succession.  
         [0118]    Under conditions in which the output signals of a kth cross-connect  201   k  have been changed over owing to occurrence of an abnormality in this cross-connect, the controller  215  performs monitoring to determine whether the four output signals of the second logic circuits  211   1  to  211   4  and the four output signals of the kth cross-connect  201   k  agree (step  801 ). If the signals agree, then the controller checks to see whether agreement continues in excess of a set period of time (step  802 ). If agreement does not continue in excess of the set period of time, then the controller clears a count value c to zero (step  803 ) and repeats processing from step  801  onward. If agreement does continue in excess of the set period of time, the controller  215  counts up the value c (step  804 ), checks to see whether the count value c has become equal to a set value m (step  805 ) and repeats processing from step  801  onward if equality has not been achieved.  
         [0119]    If the count value c has become equal to the set value m, the controller  215  decides that the kth cross-connect  201   k  has returned to normal (step  806 ). The controller  215  then terminates automatic changeover of the outputs due to occurrence of abnormality and restores the normal state that originally prevailed (step  807 ).  
         [0120]    Thus, in accordance with the present invention, abnormality in a cross-connect can be detected by a simple arrangement of logic circuits. Moreover, an abnormality can be detected not only at cut-off of a signal from a cross-connect but also in a case where a signal changes from “1” to “0” or from “0” to “1” in a cross-connect.  
         [0121]    In accordance with the present invention, occurrence of abnormality in any working cross-connect can be detected by a simple arrangement of logic circuits.  
         [0122]    In accordance with the present invention, changeover of working and standby optical crossconnects can be performed by a simple arrangement of logic circuits, thereby alleviating the control load involved in a working and standby changeover operation.  
         [0123]    In accordance with the present invention, recovery can be detected by a simple arrangement of logic circuits and it is possible to switch back to a working cross-connect from the standby cross-connect unerringly.  
         [0124]    In accordance with the present invention, it is possible to provide a highly reliable optical cross-connect apparatus at minimum cost in comparison with the cross-connect apparatus of the prior art.  
         [0125]    In accordance with the present invention, the specifying of faulty locations and the changeover/switch-back of output signals can be carried out at high speed by gating control.