| 摘要 |
In a receiving circuit which serves as a system in use, a gate circuit is enabled in response to a specifying signal. In a receiving circuit which serves as a backup system, a gate circuit is enabled in response to a specifying signal. When no signal is applied to an input terminal, alarm detection signals are outputted. However, a system-in-use/backup-system specifying circuit is first supplied with the alarm detection signal of the backup system. Thus, the system-in-use/backup-system specifying circuit determines or detects that the receiving circuit serving as the system in use is normal.
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