Abstract: This paper introduces a modern digital communication speed control scheme adopted in the retrofitting of a paper machine speed control system. The scheme involves configuring a master-slave station using a SIEMENS S7-300 and an EMERSON TD2000 AC frequency converter, based on the PROFIBUS-DP communication protocol, to achieve overall speed control of the paper machine. This system employs an open-loop speed chain control and a SIMATIC HMI-TPI70A human-machine interface, achieving precise, rapid, and simple speed control. It facilitates operators' observation of speed control results and judgment of system operation, improving production safety, and the retrofitting cost is relatively low. Keywords: Paper machine; PLC; Frequency converter; HMI; Speed ​​chain; PROFIBUS-DP With the continuous maturation of AC frequency converter technology and information technology, since the 1990s, AC variable frequency speed control systems have gradually replaced older DC speed control, slip speed control, electromagnetic speed control, and pole-changing speed control systems. This retrofitting is mostly done through DCS systems, especially prevalent in small and medium-sized paper mills. However, DCS systems have inherent limitations, such as system isolation and low openness. The goal of fieldbus technology development is to overcome these limitations, establishing an open control system independent of any vendor and a unified factory-level information network. Any equipment from different manufacturers adhering to the same standards can connect to it, enabling information exchange and interoperability. PROFIBUS is a highly influential fieldbus internationally, supported by products from many well-known intelligent device manufacturers. This article will discuss the application and implementation methods of PROFIBUS-DP in the electrical drive system of a paper machine. 1. Basic System Composition The stability of paper machine operation primarily ensures the synchronization of speeds in each section during paper production, guaranteeing constant tension during the winding process. Although actual production conditions may vary, a basic speed chain between sections is still required. The basis for establishing this speed chain is the equal linear speed of each section. This paper machine system, according to the process flow, includes ten sections: wire section, first press, second press, first drying, second drying, soft calendering I, soft calendering II, third drying, calendering, and winding. [b][align=center]For details, please click: Paper Machine Variable Frequency Speed ​​Control System Based on PROFIBUS-DP Bus[/align][/b]