Data exchange capable of minimizing loss of a data block

A data exchange for use in an asynchronous transfer mode communication system for exchanging an input data block into an output data block, said data exchange comprising first buffer means for memorizing said input data block as a first buffered data block to produce said first buffered data block as a first output data block, second buffer means for memorizing said input data block as a second buffered data block to produce said second buffered data block as a second output data block, receiving means for receiving a received data block to selectively supply said received data as said input data block to said first and said second buffer means, and transmitting means for carrying out transmission of one of said first and said second data blocks as said output data block;

• wherein said data exchange further comprises:
• control signal producing means for producing first and second control signals, said receiving means being responsive to said first control signal to supply said input data block to said first buffer means, said receiving means being responsive to said second control signal to supply said input data block to said first and said second buffer means;
• test data supplying means for supplying a test data block as said input data block to said first and said second buffer means in response to said second control signal;
• detecting means for detecting whether or not said first output data block is coincident with said test data block to produce a first detecting signal, said detecting means detecting whether or not said second output data block is coincident with said test data block to produce a second detecting signal;
• control means responsive to said first and said second detecting signals for controlling said receiving means and said transmitting means to connect said receiving means and said transmitting means to said second buffer means.