PURPOSE:To detect the illegal action of a sampling synchronization function by permitting a terminal station becoming a master station in terms of synchronization to supervise sampling synchronization with a terminal station that stops synchronizing subordination in terms of sampling synchronization through the use of the sampling synchronization function. CONSTITUTION:The terminal station 2 is subordinate to the terminal station 1, and the terminal station 3 to the terminal station 2 in terms of synchronization. The terminal station 1 being a master station stops the synchronizing subordination to the terminal station 3 being a subordinate station. The terminal stations 1 and 2 execute sampling synchronization and the terminal stations 2 and 3 are synchronized with each other. The sampling timing of the terminal stations 2 and 3 synchronizes with that of the terminal station 1. In such case the terminal station 1 uses a function to obtain the difference between sampling timings among functions synchronizing with terminal station 3, and supervises that there is no difference of the sampling timing between the stations 1 and 3. Thus no additional hardware to a conventional device is needed, and the illegal action of the sampling synchronization function can be detected.
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