PURPOSE: To facilitate the test of a port by providing a switch to select a port, a test signal input means and a means displaying it that a test signal reaches normally an end of a serial line.

CONSTITUTION: A 2nd port to be tested is selected by selecting switches 6a-6f inserted in each serial connection line and a port connecting a serial line with each signal line leading to a 1st port through serial connection line. Then a test signal input means 7 inputs a test signal of a fixed level to a line from a start part of a serial line between the 1st and 2nd ports. When the test signal reaches the end of the serial line normally, display means 8a, 8b display it. Thus, the port is Simply tested.