A datagram with a destination network address is received at a router. The protection relays are provided at different terminals of the power transmission line. The control device allows a primary laser pulse emitted from the laser rod during the emission period to impinge on a workpiece and blocks from the workpiece secondary laser emission occurring during the emission period after emission of the primary pulse. The L-shaped supports have a plurality of engagement points so that the step can be adjusted upward and downward using the different attachment points. It is thus unnecessary to prepare a new description of a signal line exclusively for indication of error generation and it is possible to simulate a memory operation containing an error only by the normal descriptions of an address, data, and the like.