Today, NC means computer numerical control. Computer numerical control CNC is defined as an NC system whose MCU is based on a dedicated microcomputer rather than on a hard-wired controller.

Features of CNC
*Storage of more than one part program
*Various forms of program input
*Program editing at the machine tool
*Fixed cycles and programming subroutines
*Interpolation
*Positioning features for setup
*Cutter length and size compensation
*Acceleration and deceleration calculations
*Communication interface
*Diagnostics

The MCU is the hardware that distinguishes CNC from conventional NC.

The Machine Control Unit for CNC
MCU consists of the following components and subsystems:
(1) Central processing unit
(2) Memory
(3) I/O interface
(4) Controls for machine tool axes and spindle speed
(5) Sequence controls for other machine tool functions
These subsystems are interconnected by means of a system bus.

Central Processing Unit
Manages the other components in the MCU based on software contained in memory. The CPU can be divided into three sections:
(1) Control section
(2) Arithmetic-logic unit
(3) Immediate access memory

Memory
Consists of main memory and secondary memory.
Main memory (Primary storage) consists of ROM (read-only memory) and RAM (Random access memory) devices.
Operating system software and machine interface programs are generally stored in ROM. Numerical control part programs are stored in RAM devices. Current programs in RAM can be erased and replaced by new programs as jobs are changed.
High-capacity secondary memory (also called auxiliary storage or secondary storage) devices are used to store large programs and data files, which are transferred to main memory as needed.

Input/Output Interface
Provides communication between the various components of the CNC system, other computer systems, and the machine operator.

CNC Software
The computer in CNC operates by means of software. There are three types of software programs used in CNC systems: (1) operating system software, (2) machine interface software, and (3) application software.
The principal function of the operating system software is to interpret the NC part programs and generate the corresponding control signals to drive the machine tool axes
The machine interface software is used to operate the communication link between the CPU and the machine tool to accomplish the CNC auxiliary functions.
The application software consists of the NC part programs that are written for machining (or other) applications in the user’s plant.