Abstract: A combined cycle electric power plant includes two gas turbines, a steam turbine, and a digital control system with an analog or manual backup. Each of the gas turbines has an exhaust heat recovery steam generator connected to a common header from which the steam is supplied by one or both of the steam generators for operating the steam turbine. Both the digital and the analog systems provide a digital input to an interface for controlling the steam turbine valves. The analog system is controlled to operate a respective valve by an input to its interface which determines valve position in accordance with its duration. The digital system is controlled to operate a respective valve by an input to the interface in accordance with the repetitive duration of the signal. The analog system input and digital system input is applied to an interface for each valve.