Abstract: A test unit for identifying output signals at test points of a logic circuit on an oscilloscope and being operable in a real time mode or a memory mode. In the real time mode, the oscilloscope is triggered by any selected one of several signals and, at this time, output signals sensed by test probes are displayed on the oscilloscope. In the memory mode, the output signals are written into a memory. In response to the selected one signal, the output signals written into the memory before or after the occurrence of the selected one signal can be selected for display on the oscilloscope.