Apparatus for testing an integrated circuit device (DUT) having an input port and an output port comprises multiple state devices (10-16) each having multiple states that occur in a predetermined sequence and an output port at which it provides an event signal. A first of the state device is an emitting device (10) that emits an event marker signal at a predetermined time in advance of entering a predefined state, a second of the state devices is a receiving device (11) that responds to receipt of an event marker signal, at least one of the state devices (11) has its output port connected to the input port of the DUT, and at least one of the state devices is a measurement device (13) connected to the output port of the DUT. An interconnection matrix (30) is connected to each state device and allows each state device to communicate an event marker signal to each other.