Abstract : This paper introduces the design and implementation of a semi-closed-loop master-slave CNC system in the CNC retrofitting of ordinary machine tools. To meet the requirements of strong control functions and minimal modification of the CNC system, a standard computer printer interface is adopted for communication, combining the powerful computing capabilities of the host computer (PC) with the anti-interference and real-time control capabilities of the slave computer (microcontroller). The specific strategy for communication via the standard computer printer interface and the main hardware modules and software structure of the system are presented in the paper. This CNC system is characterized by easy retrofitting, convenient operation and maintenance, low cost, and high precision, making it an economical CNC system. Keywords : CNC system; CNC machine tool; parallel communication; printer interface 0 Introduction Currently, the number of CNC machine tools has become one of the important indicators for measuring the level of a country's machinery manufacturing industry. China has a large number of ordinary machine tools, but the production scale of enterprises is small. Adopting an economical CNC system to retrofit existing equipment is reasonable and feasible both economically and technically, and is a very meaningful endeavor. In the CNC retrofitting of ordinary machine tools, most adopt 8-bit processors and microcontrollers, which have advantages such as high integration, reliability, flexibility and convenience, and a good performance-price ratio. When higher requirements are placed on manufacturing equipment, a master-slave CNC system composed of upper and lower level computers is often used. The upper computer can be an industrial computer or a personal computer. Industrial computers have high reliability and are suitable for complex and changing industrial environments. However, in recent years, personal computers have become increasingly powerful and cheaper, and are increasingly being chosen for experimental research in research institutes and universities. This paper selects the PC486t100 as the upper computer and cleverly utilizes the microcomputer's printer port to achieve communication, thus retrofitting the ordinary machine tool CQ9107. [b][align=center]For details, please click: Application of Microcomputer Printer Port Communication in CNC Retrofitting of Machine Tools[/align][/b]