A direct current and a modulation signal are simultaneously applied to contact pads on a device under test, such as a laser diode. In one embodiment an optical reference apparatus includes a body having an outer surface. In this way, a stair which is easier to walk on and which automatically shows a more regular nosing line at the outer string is obtained FIG. The receiving part is defined at an end of the first sidewall. The display device responds with a status signal to the host according to the first and the second signal attribute specifications. Transition-history information generating means generates, as transition-history information, information concerning a transition history regarding the node to be used.