PURPOSE: To suppress an adverse influence on a plant with the abnormality of an analog output signal to a minimum by speedily detecting the operational abnormality of a DAC by comparing circuit signals before and after the DAC as the component of a circuit in an AO card to be used for a digital controller. CONSTITUTION: This circuit is provided with a first switcher 20 for switching digital output signals outputted from hold circuits from 11-1 to 11-n, second switcher 21 for switching the output signal of a DAC 12, ADC 22 for performing analog/digital conversion for the output signal of the second switcher 21, and comparator 23 for comparing the digital signal outputted from the first switcher 20 with the digital signal outputted from the ADC 22 and corresponding to the signal of the compared result of the comparator 23, the fault detection of the DAC 12 is performed.
