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.

Abstract: A novel mapping and protection circuit arrangement comprises a plurality of checking mechanisms that collectively cooperate to verify the accuracy of I/O addresses generated by input/output (I/O) controllers of a fault-tolerant computer. These verified I/O addresses are translated into system addresses to enable direct memory access (DMA) transactions between the controllers and the computer's host memory. Specifically, certain of the checking mechanisms cooperate to ensure that the DMA accesses are directed to correct pages in host memory, while other checking mechanisms are provided to ensure that memory access operations are performed at correct locations within the page. Additional checking mechanisms are provided to further verify the accuracy of generated I/O addresses.

Abstract: A multinodal system is one-way interconnected, two-way interconnected or, more generally, (n)-way interconnected, where (n) is an integer. In a one-way interconnected system, only one connection element couples any two nodes. Or, put another way, only one communication path exists between every node and every other node. A two-way interconnected system, on the other hand, has two connection elements coupling each pair of nodes. Likewise, an (n)-way interconnected system provides (n) independent connection paths between each pair. Such systems are characteristic in that the relationship between the number of independent buses (b), the number of nodes (v), the number of ports (r), and the degree of interconnectedness (n) can be expressed by the equation ##EQU1## Two-way and (n)-way interconnect arrays may be adapted for use in fault-tolerant communications.