Abstract: Diagnostic apparatus and method are disclosed for testing a Small Computer System Interface (SCSI), by passively monitoring the interface between initiator and target devices or systems, upon sensing a user-selected phase of operation at the interface, disconnecting the initiator and target devices from one another, forcing a user-selected fault for the appropriate time, and reverting to a passive monitor. A pair of analog switches are provided for each line of the interface. The switches selectively connect the initiator system and the target system to each other or to a set of line drivers for providing high or low SCSI signals. A set of sense lines are connected to the initiator and the target for monitoring so as to sense the phase of operation of the initiator and target and to sense the response of the interface to the forced fault.