Abstract: A method and apparatus allows fault recovery in a digital computer based control system whereby system upsets induced by external transient noise conditions can be accommodated. A CPU is coupled to its main memory and its I/O interfaces by a common address/data bus, these three elements being susceptible to having data thereon or therein corrupted by transient noise. Also coupled to the bus, but in a hardened environment, are first and second supplemental memories which, under memory control, operate on alternating even and odd computational frames defined by the CPU's real-time clock to store the same words as are then being entered into the CPU's main memory. As computational frames are entered into one or the other of these two memories by eaves-dropping on the common bus, the other supplemental memory is transferring its contents to a backup memory which is also housed in the noise-immune environment.