Abstract: This paper introduces a technical transformation scheme for the PLC control system of a steel plant molding production line. This system uses PROFIBUS-DP process fieldbus technology to realize data exchange and distributed control functions between the master and slave stations. Centralized control is achieved through a host computer, completing functions such as a visual human-machine interface, communication, and networking. This scheme is applicable to other industrial control systems. Keywords: PROFIBUS-DP; molding production line; PLC 1 Introduction A steel plant molding production line has been using the MEDEL 560 PLC system from TEXAS INSTRUMENTS, USA, with an analog panel display (system block diagram shown in Figure 1). Since this system is a product from the 1980s, it is severely outdated and no spare parts have been replaced. Therefore, the production line urgently needs transformation. [align=center] Figure 1, System Block Diagram[/align] Based on the site conditions, a control system is proposed to control the production equipment. The new system requires data acquisition and storage, process display and control, interlocking, alarm reports, safety maintenance, and other functions. It should also be stable and reliable, with a monitoring screen that conforms to operating habits. 2. Fieldbus Selection In the fieldbus field, several influential fieldbuses have emerged worldwide over the past decade. PROFIBUS, now a German and European standard, is an open, manufacturer-independent communication system and a relatively mature bus. This production line system has complex processes, numerous and geographically dispersed I/O points, including 184 digital I/O signals and 64 analog signals. These signals are distributed throughout the workshop, controlling and monitoring the normal operation of each machine. Based on these reasons, we adopted PROFIBUS-DP process fieldbus technology in this production line technical upgrade project, realizing functions such as communication, distributed control, centralized monitoring by the host computer, and visualization of the production process. 3. PLC Control System Design PLC Selection Based on principles of universality, standardization, and reliability, and considering a high performance-price ratio, the new system uses a German SIEMENS S7-400 PLC as the data acquisition and control system, and WINCC5.0 as the configuration software for the host computer. SIEMENS is a well-known international company. Its PLC systems are mature, stable, reasonably priced, and readily available with spare parts, making them a high-performance-price ratio PLC system. The SIEMENS S7-400 PLC system adopts an industrial PROFIBUS local area network, which is safe, stable, reliable, and highly expandable. This design considers the PROFIBUS connection between the PLC and expansion units, and the ETHERNET connection between the PLC and the central station. This design makes the PLC system relatively independent, while the connection between the operator station and other networks is more flexible. 3.1 System Design The control system network structure diagram is shown in Figure 2. [align=center] Figure 2 System Network Structure Diagram[/align] The system network structure is divided into two layers: the lower control network and the upper management network. The lower control network uses a two-core shielded cable, which is suitable for the PROFIBUS international standard and meets the requirements for field signal acquisition, processing, and controller communication, serving as a PROFIBUS-DP field communication network. The upper-level management network consists of a TCP/IP protocol-based management Ethernet, connected via network cards on the OS and ES stations, primarily for file management between engineer and operator stations. 3.2 Software and Configuration Design: Both the operator station OS and engineer station ES use Microsoft Chinese version Windows 2000 and Internet Explorer 6.0, ensuring that all information and interfaces except for engineer configuration are localized in Chinese. The operator station also loads monitoring software such as SIMATIC WINCC RT 64K Tags, NET PROFIBUS-S7, and PDM. Hardware configuration is also a graphical configuration method, which is very convenient. For a given process station, which actually has several ET200 remote I/Os, several IM153 modules are dragged and dropped onto the PROFIBUS-DP network line behind the process station in the configuration screen to form several ET200 remote I/O contacts (this system uses 7 slave stations based on actual needs). All modules in the hardware configuration can be found in the component library provided by S7, with the corresponding model and part number; simply drag and drop them to the location corresponding to the actual installation. After the hardware configuration is completed, it is downloaded to the corresponding process control station. This ensures that the actual installed hardware modules and the hardware configuration are consistent, thus determining the address of each point on the I/O module. The system is intended to have a network interface for connection to the company's local area network (LAN). Relevant information screens can be viewed via Internet Explorer on the LAN to share information and provide necessary information to management. 4. Selection and Design of Host Computer and Configuration Software The host computer selected is a DELL computer, a DELL 21" flat-screen CRT with a printer. The configuration software selected is SIMATIC WinCC 5.0 configuration software. The main software flowchart of the main station is shown in Figure 3. [align=center]Figure 3 Main Station Software Flowchart[/align] The main features of the system are: ⑴ Graphical User Interface (GUI) WinCC 5.0 allows users to quickly develop customized screens for their applications using easy-to-understand and configurable tools. ⑵ Distributed Historical Data System The distributed historical trend data system allows users to dynamically assign different historical file data sources to each pen of the trend chart. This feature allows operators to view local WinCC historical data and SQL Server historical data in the same trend chart. ⑶ Dynamic Reference Address Users can change the reference to the data source so that multiple data sources can be located using the same tag name. The operating system is based on PC and is standard; different capacity sizes are available; all SCADA functions (open system kernel): graphics system, alarm information system, variable archive, user archive, report system standard interface, programming interface; driver software for various PLC systems. The software is fully open, easy to learn and use, and conducive to development, application, maintenance and management. 5 Conclusion Practice has proven that the industrial network control management system built with PROFIBUS-DP and Ethernet has realized distributed control, which can greatly reduce the workload and cost of field connection, and improve the accuracy and flexibility of signal transmission. The use of SIEMENS S7-400 PLC and WinCC5.0 has realized the updating of control methods and the improvement of control efficiency, making human-machine interaction visualization and production management and control integration, which brings convenience to the system installation, debugging and equipment maintenance. References: [1] Yang Xianhui. Fieldbus Technology and Its Application. Beijing: Tsinghua University Press, 1999 [2] Zheng Sheng. Modern Programmable Controller Principles and Applications. Beijing: Science Press, 1999