0 A test probe includes an electrode (162) for contacting a circuit node of the logic circuit being tested. The signal from the electrode is passed to first and second comparators (166, 168) which compare the signal with acceptable high and low threshold voltages for the type of logic circuitry being tested. In this way, a visual indication can be given that the logic level of the monitored circuit node is high, low, invalid, or is a sequence of pulses of all three logic levels. The test probe also provides for injection of logical high pulses, logical low pulses or an alternating pulse of high and low pulses. Probe level detection and pulse injection can be asynchronous or selectively synchronized so that logic level detection or pulse injection occurs with each unit under test write or read operation.