Patent ID: 8259595

Claim:
A diagnostic method for checking whether a plurality of external signal input/output units are operating normally or not, with no diagnostic circuit provided to the external signal input/output unit, in a system in which the plurality of external signal input/output units, each of which has an external signal output line and an external signal input line, are connected to a numerical controller to control automated machines, comprising the steps of: transmitting a diagnostic signal from the numerical controller to each of the external signal input/output units; returning the diagnostic signal, received by the external signal input/output unit, 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; and checking whether the external signal input/output unit is operating normally or not based on the diagnostic signal returned to the numerical controller, wherein 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.