Method of testing switches and switching circuit

A switching circuit comprising:

• a plurality of incoming lines in predetermined number;
• a first switch of matrix structure having input ends in said predetermined number connected to said incoming lines and output ends in said predetermined number, said first switch performing a first predetermined exchange on data inputted to said input ends thereof to produce and output processed data to said output ends thereof;
• a first changeover switch having common, first and second ends, said common end being connected to said first switch;
• a signal processor having input and output ends, said input end thereof being connected to said first end of said first changeover switch;
• a second changeover switch having common, first and second ends, said first end thereof being connected to said output end of said signal processor, said second end thereof being connected to said second end of said first changeover switch;
• a second switch of matrix structure having input ends in said predetermined number connected to said common end of said second changeover switch and output ends in said predetermined number, said second switch performing a second predetermined exchange on outputs outputted to said output end of said signal processor in a normal operation and performing a third predetermined exchange on said processed data in a test operation, to produce and output data to said output ends thereof;
• outgoing lines in said predetermined number connected to said output ends of said second switch;
• and
• a switching control circuit for controlling said first and second changeover switches such that said common ends of said first and second changeover switches are connected to said first ends thereof in the normal operation and such that said common ends of said first and second changeover switches are connected to said second ends thereof in the test operation.