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The challenge in conserving energy in embedded real-time systems is to reduce power consumption while preserving temporal correctness. Much research has focused on power conservation for the processor, while power conservation for I/O devices has received little attention. In this paper, we analyze the problem of online energy-aware I/O scheduling for hard real-time systems based on the preemptive periodic task model with non-preemptible shared resources. We extend the concept of device slack proposed in  to support non-preemptible shared resources; and propose an online energy-aware I/O scheduling algorithm: Energy-efficient Device Scheduling with Non-preemptible Resources (EEDS NR). The EEDS NR 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.