The ornamental design for a serialtransmitter, as shown and described.
FIG. 1 shows a front view of a serial transmitter showing my new design;
FIG. 2 shows a rear view thereof;
FIG. 3 shows a top view thereof;
FIG. 4 shows a bottom view thereof;
FIG. 5 shows a left side view thereof;
FIG. 6 shows a right side view thereof;
FIG. 7 shows a top, front and right side perspective view thereof; and,
FIG. 8 shows a bottom, rear and left side perspective view thereof.
The portions of the design shown in broken lines in FIGS. 2 and 6 are for the purpose of illustrating the environment only and form no part of the claimed design.