| 摘要 |
PURPOSE:To normally operate a CPU even at the abnormality of an optical input level by providing a transmitted data loopback circuit in the optical transmitter of a multi-CPU system in the coarse coupling multi-drop serial system. CONSTITUTION:If power supply interruption takes place in the 1st station, since the level of optical input of a photoelectric converter 22b is abnormal, the level of a selection terminal S of a multiplexer 24 constituting a loopback circuit goes to 1 and a sent data from a CPU 21 is a reception data as it is. Since the transmitted data of the CPU 21 and the loopback reception data are coincident, the CPU 21 is not system-down and the normal operation is continued. When the input level is normal after the 1st station is normal, the level of the selection terminal S of the multiplexer 24 goes to O, the loopback circuit is restored and the system is reconstituted.
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