A fault-tolerant computer system employs multiple identical CPUs executing the same instruction stream, each with their own independent memory. The multiple CPUs are loosely synchronized, as by counting events such as operating cycles and stalling any CPU ahead of others. Data output references via separate busses are voted at separate ports of each of the CPUs by voting circuits which detect when all CPUs have made the same reference, and only then pass on identical references to external I/O busses. The ports may include FIFO buffers to allow output references from the asynchronous CPUs to be handled as the CPUs load the FIFOs at different times. Input data to the CPUs from the I/O busses is not voted, but is buffered to allow the CPUs to accept it at their own clock rate.