Abstract: A portable tester for testing tape drive units, which provides means for generating a selective repetitive composite data pattern, which includes a direct pattern and a reverse pattern which is an inverted image of the direct pattern, and writing the composite pattern on tape. Means are provided for reading the composite pattern off the tape and providing a read signal which is the result of amplifying and differentiating the signal from the read head. Means are provided for applying this read signal to an oscilloscope in a conventional manner, and to trigger the sweep of the oscilloscope at a selected time in advance of a critical portion of the read signal, for each of the direct and reverse patterns. Thus, an analog pattern signal will be displayed on the oscilloscope screen for the critical parts of both the direct and reverse portions of the pattern, for tape movement both direct and reverse.