| 摘要 |
PROBLEM TO BE SOLVED: To solve the problem that an I/O device has to be surely stopped when abnormality of a master controller occurs and at the time of changing the master controller 1 in a master controller simplex system in a conventional field bus system. SOLUTION: A dummy master relay unit 4 is arranged between the master controller 1 and the I/O device 2 polled through a transmission line 3 by the master controller 1. While the dummy master relay unit 4 only relays signals when the master controller 1 is normal, when the master controller 1 is stopped, it detects an abnormality reporting signal 6 from the master controller 1 and polls the I/O device 2 instead off the stopped master controller 1 and the I/O device 2 is continuously operated even when the master controller 1 is stopped. COPYRIGHT: (C)2003,JPO
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