Abstract: A fault-isolating digital data processing apparatus includes plural functional units that are interconnected for point-to-point communications by a plurality of buses. The functional units monitor the buses to which they are attached and signal the other units in the event there are bus communication errors. The functional units can simultaneously enter into an error isolation phase, e.g., in response to a bus error signaled by one of the units. During this phase, each unit transmits test data (e.g., predetermined patterns of O's and 1's) onto at least one of its attached buses. The functional units continue to monitor the buses and to signal bus errors while the test data is being transmitted. In addition to signaling bus errors, the functional units can signal unit-level (or "board") faults when they detect fault in their own operation.