Much research has focused on power conservation for the processor, while power conservation for I/O devices has received little attention. In this paper, the authors analyze the problem of online energy-aware I/O scheduling for hard real-time systems based on the preemptive periodic task model. They propose an online energy-aware I/O device scheduling algorithm: Energy-Efficient Device Scheduling (EEDS). The EEDS algorithm utilizes device slack to perform device power state transitions to save energy, without jeopardizing temporal correctness. An evaluation of the approach shows that it yields significant energy savings with respect to no Dynamic Power Management (DPM) techniques.