An upper piggybacked unit of a tandem unit is provided with a hinge unit and at least one motion lock to prevent the upper unit from falling away from the lower unit when it is tandemly coupled to the lower unit and the lower unit is pivoted to a non-horizontal position. A main processor communicates a configuration instruction to an interface controller. A separator structure can be attached to the frame and extends between the frame and the wind turbine tower to position the frame at a predetermined distance away from the wind turbine tower. In each subfield group S, a light emitting state of OFF is continued until a writing is performed, after which ON is kept in each sustain period.