Abstract: A numerical control system for a machine tool. The relay controls for the machine tool are simulated in the read only memory of a computer which is used as a controller for the flow of digital information between the control display panel, the tape reader, a memory, the machine tool and other parts of the system. Since the simulation is based on relay diagrams, the user can customize the control system to his requirements without having been previously trained in computer programming or computer systems. Also, since the control system is adapted to the machine tool by software changes, the same hardware is usable without modification by a variety of machine tools, such as turret lathes and milling machines, and under a variety of operation conditions. Data debouncing and diagnostic aids are included in the system.