Distributed real-time systems that consist of multiple tasks with time constraints implemented on multiple processors have been used in various embedded systems, such as humanoid robots. These processors are interconnected with a real-time network, such as Responsive Link, in which data rate of each channel can be changed individually and dynamically. In this paper, we propose a low-power communication technique for distributed real-time systems. The power consumption is reduced by slowing down the data rate of communication link while satisfying the time constraints by exploiting the slack time, which is the difference between the arrival time of data and the deadline. We first measured power consumption of Responsive Link with various data rates and then implemented a real-time network simulator using the measured power values. We also implement a run-time communication mode change mechanism on Responsive Link and discuss the feasibility. Simulation results show that the proposed low-power packet transfer technique reduces the power consumption by up to 53.46% with a negligible degradation on the schedulability.