A fault-tolerant digital data processing system comprises at least a first input/output controller communicating with at least one peripheral device over a peripheral device bus. The peripheral bus includes first and second input/output buses, each having means for carrying data, address, control, and timing signals. The input/output controller includes an element for applying duplicate information signals synchronously and simultaneously to the first and second input/output buses for transfer to the peripheral device. The input/output controller further includes a bus interface element for receiving, in the absence of fault, duplicative information signals synchronously and simultaneously from the first and second input/output buses.