Method and system for diagnosing external signal input/output units

A system having automated machines, a numerical controller for controlling the automated machines, a plurality of external signal input/output units connected to the numerical controller, each of the external signal input/output units having an external signal output line and an external signal input line, and a diagnostic means for checking whether each of the external signal input/output units is operating normally or not, with no diagnostic circuit provided to the external signal input/output unit, wherein;

• the numerical controller has a diagnostic signal transmission means for transmitting a diagnostic signal to each of the external signal input/output units,
• each of the external signal input/output units has a diagnostic signal return means for receiving the diagnostic signal transmitted by the diagnostic signal transmission means and returning the diagnostic signal to the numerical controller through a loop formed by short-circuiting an input line to and an output line from the external signal input/output unit at input and output terminals of digital output (DO) and digital input (DI) signals on an external signal transmitting/receiving block,
• the diagnostic means is provided in the numerical controller and checks whether each of the external signal input/output units is operating normally or not based on the diagnostic signal returned by the diagnostic signal return means to the numerical controller, and
• an identification is made of the source of the diagnostic signal returned to the numerical controller which the numerical controller receives when carrying out diagnosis so that it can be determined from which one of the external signal input/output units connected to the numerical controller the diagnostic signal has been returned to the numerical controller.