FIG. 1 is a front perspective view of a data transmission device showing our new design; FIG. 2 is a front elevation view thereof; FIG. 3 is a rear elevation view thereof; FIG. 4 is a top plan view thereof; FIG. 5 is a side view thereof; FIG. 6 is a bottom plan view thereof; FIG. 7 is the other side view thereof; and, FIG. 8 is a sectional view through FIG. 1 as indicated.