Abstract: A method of testing connecting and/or switching devices, particularly of a space-division multiplex switching matrix (K), in which one or more of a plurality of input lines (D.sub.i, C.sub.j) is connectable with one or more of a plurality of output lines (Y.sub.k), and which is carried out to check the formation of a check word is described. A first check word PB1 is derived from the signals on the input lines (D.sub.i, C.sub.j), and also, according to the same rules of formation, a second check word PB2 is derived from the signals on the output lines (Y.sub.k), both derived continuously. The two check words are compared to each other. In case of a disagreement, an error indication is provided.