CLOCK SUPPLY SWITCHING CIRCUIT

PURPOSE:To prevent communication from interrupting or malfunctioning due to manual switching by switching an external switching signal into an internal switching signal depending on a device alarm signal outputted from an external control circuit so as to execute the switching of an input clock at a fault of the external control circuit. CONSTITUTION:When an external control circuit 8 is normal, an external switching signal 54 from the external control circuit 8 is selected by a selector 3 and outputted to a selector 4 as a switching signal 56 to switch the output clock 102 into an input clock 101. When the external control. circuit 8 is faulty, a device alarm signal 55 is outputted from the external control circuit 8, then the selector 3 selects the internal switching signal 53 and outputted as a switching signal 56 and it is fed to the selector 4 to switch the input clock 100 and the input clock 101 and the output clock 102 is outputted. Thus, interruption or malfunction of communication due to manual switching is prevented.