Sequence control system for numerically controlled machine tool

A numerical control system for a numerically controlled machine tool whose sequence operation results from command data applied to the machine tool from an NC unit and is controlled through a programmable sequence controller interposed between the NC unit and the machine tool comprising:

• a first non-volatile storage area means, provided in the programmable sequence controller, for storing command data applied to the machine tool;
• a second non-volatile storage area means, provided in the programmable sequence controller, for storing a duplicate of the command data stored in said first non-volatile storage area means when the machine tool issues a signal indicating that it has completed an operation in accordance with the command data;
• and
• a comparator means, provided in the programmable sequence controller, for comparing the contents of said first and second non-volatile storage area means, when electric power is connected to the machine tool.