1. System Composition The basic equipment of a power plant's chemical water treatment system has good controllability. It is monitored and managed in real time using PLCs and industrial control computers. The basic system composition is shown in Figure 1. [align=center] Figure 1 System Structure Diagram[/align] Three Siemens SIMATIC S5-115U programmable logic controllers (PLCs) are used to control the primary equipment in the field. The PLC system consists of three sets: #1 makeup water treatment unit, #2 makeup water treatment unit, and #3 condensate treatment unit. Two Advantech 586 industrial control computers are used as operator stations. The software development platform is the FIX5.5 configuration software from Intellution, USA. FIX5.5 is a complete industrial automation software capable of data acquisition and control, alarm, and graphical data display. This version runs under Windows or Windows NT environments and uses a graphical user interface. Its internal graphics processing is based on third-generation graphics technology. The data communication system uses a SINEC L2 network, which connects the SIMATIC series PLCs and industrial control computers into a network. SNEC L2 is a token bus network, and the network transmission medium is twisted pair or optical fiber. Each node is connected to the bus network through a bus connector. In this system, data communication is achieved between the three PLCs and between the PLCs and two industrial control computers. This configuration reflects the advantages of a distributed control system, namely, distributed control functions and centralized operation management. Distributed control functions mean fast real-time response and dispersed system risks, while centralized operation management facilitates centralized management. The configuration also has redundancy characteristics. 2. PLC and its programming 2.1 SIMATIC S5-115U hardware composition and programming overview The SIMATIC S5-115U programmable controller adopts a standard modular structure. The power supply, CPU, and various I/O modules are all plugged into a motherboard, and expansion motherboards can be added according to different I/O points. The fine hierarchy of input, output modules, and memory makes this device highly adaptable to different configurations. Through the communication processor and local area network, communication between PLCs and with computers can be easily realized. The programming language for the SIMATIC S5-115U is STEP5, which offers three expression methods: Control System Flowchart (CSF), Ladder Diagram (LAD), and Statement List (STL). Among these, STL most closely resembles the machine's internal control program and offers significantly richer functionality than the other two methods; therefore, STL is used exclusively in the actual programming applications of this system. STEP5's greatest strength lies in its structured programming approach, providing numerous standard function blocks such as the multiplication function block FB242 and the communication function block FB244. This greatly simplifies programming, resulting in well-organized, easy-to-read, modify, and test programs—an advantage particularly evident when developing large and complex programs. To accomplish complex tasks, the entire program can be divided into independent program blocks. STEP5 offers five block types: Organization Block (OB), Program Block (PB), Sequence Block (SB), Function Block (FB), and Data Block (DB). The Organization Block (OB) manages the user program, forming the interface between the operating system and the control program; all other block types are called and executed within this block. Function blocks (FBs) are used to implement repeatedly called or particularly complex program functions. These function blocks can be provided by the system as standard function blocks, or they can be written by the user. For example, the standard function block FB242 can implement 16-bit binary multiplication, and FB244 can implement data transmission between the CPU and the communication processor. These function blocks can be called directly when these functions are needed. 2.2 Design of a Large SIMATIC S5-115U Program Taking the control program of the #1 water supply treatment unit of this system as an example, the organization block OB1 mainly contains the following statements, which complete the unconditional call of each function block. JU FB1 (Defines the data transmitted from PLC1 to two industrial computers) JU FB2 (Defines the data transmitted from two industrial computers to PLC1) JU FB231 (Completes the basic settings for communication between PLC1 and two industrial computers) JU FB232 (Completes the basic settings for communication between PLC1 and PLC2/PLC3) JU FB4 (Implements automatic control and seamless switching functions) JU FB3 (Controls pneumatic and electric doors) JU FB10 (Implements analog signal processing functions) JU FB11 (Alarm handling) In FB1 and FB2, the data to be communicated is written into the corresponding bits of a data block such as DB10, thus ensuring consistency with the variables in the communication processor. In FB231, two standard function blocks provided by STEP5 itself, FB244 (send data) and FB245 (receive data), are called. Then, some necessary parameters such as interface and job number are filled in according to the communication processor to realize the data communication function. Within FB232, the data to be transmitted between the three PLCs is defined according to the data bits allocated by the communication processor. Within FB4, based on production process requirements and operating procedures, data transmitted from other function blocks and I/O modules is fully utilized to achieve automatic control and seamless switching of the system. Considering the similarities among multiple controlled objects, several representative function blocks FB20, FB30, and FB40 are developed. For example, FB20 is called multiple times within FB4 to ensure consistency between the program step time within the PLC and the display time on the industrial control computer screen. Furthermore, FB20 calls the multiplication function block FB244. FB3 controls field equipment such as pneumatic doors, electric doors, and pumps based on the automatic program step instructions issued by FB4. FB10 handles all analog signal processing, calling the square root function block FB5. FB11 performs alarm processing on the analog signals based on the data converted from FB10. It is crucial to pay attention to the correspondence between the analog signals and the digital signals within the PLC to ensure the accuracy of analog signal display and alarms. 3. Design of Industrial PC Monitoring and Management Software [table][tr][td][/td][/tr][/table] The industrial PC monitoring and management software is completed on the FIX5.5 software platform. FIX5.5 is a high-precision modular software system, including more than a dozen software modules. Here, we mainly introduce several software modules used in the development of this application software. (1) System Configuration Module (SCU): It mainly completes the configuration of network, I/O driver, database name, system startup parameters and initial startup tasks. Intellution and third-party manufacturers have written more than 300 I/O drivers for PLCs and I/O cards, such as drivers for products of SIEMENS, OMRON, MODICON, ABB and other companies, and provide I/O driver development toolkits for users to develop their own I/O drivers. (2) Data Scan, Alarm and Control Module (SAC): It is used to scan field data, condition signals, convert data formats and data types, determine alarm conditions, and realize remote control output. The SAC sends the processed field data to the real-time database or sends the remote control output data to the I/O driver to realize the remote control output function. (3) Real-time Database Management Module (DATABASE BUILDER): It provides the function of interactively creating a real-time database and online display/modification of the real-time database. It is the main data source for system operation. Users need to do a lot of work here. The most important thing is to fill in the variable label name so that the field data and the variable label in the database are matched one by one so that the data can be called in other modules. When filling in the variable label name, both standardization and skill are required. First, the syntax requirements of the FIX software should be followed. Second, the label name should be defined according to a certain classification standard to improve efficiency in future data query and application. (4) Drawing Module (DRAW): FIX has an intuitive, object-based graphical user interface (GUI), which simplifies the graphics development process. To create a screen, you can use the toolbox provided by DRAW to generate certain objects such as valves, pumps, and record tables, and define their dynamic characteristics. That is, the objects change their state, size, color, rotation, and movement based on the field data. For example, the opening or closing of valves and the turning or stopping of pumps can be done in a dialog box. The main purpose is to link the variable labels in the real-time database with the corresponding objects, so that the object state changes with the field data. In order to reduce the graphics development time, Intellution also provides a graphics library of commonly used equipment objects, which contains a variety of international standard graphics that can be pasted into the user screen at any time. At the same time, users can save their own commonly used graphics into the graphics library for future use. (5) Display module (VIEW): Its main function is to dynamically display the screen created by DRAW. It can switch between multiple screens, change the screen form, input data, and realize monitoring, which is the actual operation screen provided to the user. 4. System functions The system has upper computer monitoring function and simulation panel monitoring function. The two are hot standby and run in parallel. The host computer screen offers three operating modes: host computer manual, host computer automatic, and host computer monitoring. Switching between modes is seamless. When the system requires monitoring by the analog panel, the host computer screen selects the host computer monitoring mode. In this mode, the system status is determined by the analog panel's M/A switch. When M/A is set to manual, one-to-one manual operation can be performed on the analog panel using the buttons. When M/A is set to automatic, the system can achieve automatic control of water treatment via an automatic program. When the system requires host computer monitoring, the host computer screen allows selection of host computer manual or host computer automatic functions. Selecting host computer manual allows one-to-one manual operation of local devices on the host computer screen. Selecting host computer automatic allows automatic start/stop control on the host computer screen; the automatic control program is the same as the analog panel's automatic control function. 5. Conclusion In large and medium-sized production systems, the era of using programmable logic controllers (PLCs) as standalone machines is over. Their organic integration with industrial control computers and the application of computer networks have greatly improved the level of automation control and management in production sites, which is an inevitable trend.