In a signal supply apparatus such as might find use in an electro-optical device,
signals that are supplied from multiple digital-to-analog converters are subject
to impedance conversion by respective voltage followers, and provided as outputs
to control the operation of the device. In the signal supply apparatus in a diagnostic
examination mode, switching elements are operated so that output lines of the voltage
followers are short-circuited by an output examination line. A current value measured
as a result of the short circuit is compared with a specified current value to
make a good-or-bad determination for the signal supply apparatus.