Abstract: Signals at selected points on a test device place indicia on a record indicating, for each selected point, a selected logical active level and an initial polarity level. At preselected time intervals, levels, level changes and the number of transitions per change appear on the record as human-readable symbols. Each selected points signals are translated into digital representations which are stored for comparison with subsequent digital representations from the point. Counters record the number of changes thus detected and provide to the record appropriate symbols depicting the pre-change value, the number of changes and the post change value.