[Abstract] This article, based on a practical project related to PLC design, illustrates the importance of ingenious design ideas and appropriate hardware selection in system optimization by proposing, analyzing, and comparing several design schemes. [Keywords] PLC; optimization; selection 1. Overview With the development of the times and the continuous improvement of science and technology, industrial engineering is gradually moving towards large-scale and integrated operations. At the same time, more and more industrial products with various functions are surrounding us designers. How to better utilize these numerous products to achieve engineering optimization is a goal that designers have always strived for. This article, based on practical experience, discusses how PLCs can be cleverly combined with other products in engineering projects to make the entire system more economical, efficient, and reliable. 2. Project Background In the centrifugal blower electrical automatic control system of the CNOOC Refining & Chemical Company Huizhou Project, the system configuration is as follows: six sets of locally installed low-voltage electrical control cabinets are used to complete the start-up and shutdown functions of their respective blower subsystems; one remote PLC control cabinet, using the Siemens S7-300 series, collects analog quantities such as temperature, flow rate, and oil pressure of the entire blower system and outputs them to the host computer for display and alarm. The fan system has numerous temperature acquisition points. In this project, each fan has 3 motor stator temperatures, 2 motor support bearing temperatures, 2 fan support bearing temperatures, and 1 thrust bearing temperature, totaling 48 temperature points. If Siemens 8-point analog input modules are selected, a total of 6 modules would be needed (as shown in Figure 1). Adding analog inputs such as shaft vibration, oil pipe pressure, and outlet air pressure, the number of analog input modules required for the PLC becomes too large, resulting in high cost and installation difficulty. So, how can we minimize the number of expansion modules while ensuring or even improving system performance? 3. Proposing a New Approach Some CPUs in Siemens' S7-300 series PLCs are equipped with PROFIBUS DP master/slave interfaces, so we can use the PROFIBUS bus transmission protocol to acquire field temperature signals in batches. PROFIBUS-DP is used for high-speed data transmission at the field level. The master station periodically reads the input information from the slave station and periodically sends output information to the slave station. The bus cycle time must be shorter than the master station (PLC) program cycle time. In addition to periodic user data transmission, PROFIBUS-DP also provides non-periodic communication required by intelligent devices for configuration, diagnosis, and alarm handling. We know that temperature signals can be centrally collected through a monitoring system. Therefore, based on the actual situation, I selected three SS7-16 16-channel temperature monitoring instruments manufactured by Shaanxi Industrial Automation Engineering Co., Ltd. However, this model of temperature monitoring instrument uses the Modbus RTU protocol (RS485 port) for communication. The Modbus protocol is a common language used in electronic controllers. Through this protocol, controllers can communicate with each other and with other devices via a network (such as Ethernet). As a general industrial standard, it has been widely used in industrial networks composed of control equipment from different manufacturers to achieve centralized monitoring. Figure 1 shows a schematic diagram of the temperature acquisition structure using an analog input module. Figure 2 shows a schematic diagram of the temperature signal acquisition structure using a master-slave transmission method. The two diagrams respectively illustrate the acquisition of temperature signals using an analog expansion module and a temperature monitoring instrument. A direct comparison of the two diagrams reveals that the master-slave structure has the following advantages: 1. Saves on hardware quantity and investment. 2. Saves on installation costs. 3. Saves on maintenance expenses. 4. Users have a high degree of initiative in system integration. 5. Improved system accuracy and reliability. Since this approach has significant advantages, we can discard the analog input module and replace it with a temperature monitoring instrument. Below, we will analyze this in more detail. 4. Two Solutions For situations where two devices use two different protocols, the lower-level machine needs to add a device to convert the Modbus RTU protocol to the Pofibus protocol. Although changing the selection approach brings new problems, the wide variety of products on the industrial control market can easily solve this. Regarding the Modbus RTU protocol to Pofibus protocol conversion problem, two solutions are readily apparent. Solution 1: Utilizing the Siemens CP341 communication module. The CP341 module is a serial communication module in the Siemens S7-3001400 series PLC. This module has one optional serial communication port (RS485 port selected in this project). The CP341 module can communicate with multiple serial communication devices simultaneously, such as connecting multiple frequency converters and monitoring instruments at the same time. Its processor can be parameterized particularly conveniently and simply. If RS422/485 Modbus RTU communication is used, the transmitted data packet needs to include information such as station number, data area, and read/write instructions. This allows the slave devices connected to the CP341 module to identify which station the data packet is addressed to and which data area it is used to read or write. The advantage of using the CP341 in Option 1's communication hardware selection table is that this communication module is from the same manufacturer as the CPU, ensuring good compatibility and reliability. The disadvantage is that this module requires additional software and a dongle, which can be costly for small projects. Option 2: Using a protocol converter for protocol conversion. A protocol converter, also known as a bridge or industrial gateway, is used at the link layer to store and forward data packets between LANs or between a LAN and a WAN, as well as to convert between different physical interfaces. If the Ethernet network types are different (such as Ethernet and Token Ring), they cannot be interconnected by relying solely on the data link layer protocol because they do not recognize each other's frame format transmission methods. Therefore, this problem is solved at the network layer: two gateways are set up for each network type, and the networks are connected through the gateways, thus the gateways act as protocol converters. In recent years, the quality and performance of protocol converter products have become increasingly mature, with a wide variety of brands and powerful functions. We will take the Swedish "Anybus" as an example to illustrate how to use it in the project. According to the requirements of this project, we can choose the Profibus-DP Serial Gateway series products. Through the accompanying software, different types of serial ports (RS232/422/485) can be selected. Anybus Communicator provides a Modbus RTU master mode. In this mode, the gateway polls one or more Modbus slave devices and maps the data obtained from the slave devices to the gateway's internal storage area. Users can choose to use predefined standard Modbus commands or use the gateway's advanced data processing functions to define communication messages. In this way, only the selected data is exchanged between the gateway and the fieldbus master. This function is well-suited for using a temperature monitoring instrument as a slave station, and then sequentially reading the data collected by it through a gateway. Comparing the communication hardware selection table for Scheme Two, the former is more reliable and stable because the communication module and CPU are from the same manufacturer. However, the cost is higher due to the need to purchase software and a dongle; the latter is more economical, and the protocol converter is small, easy to use, and more suitable for certain special applications. Furthermore, when project requirements are broad, we can use slave devices with the same protocol as the PLC, thus eliminating the protocol conversion process and optimizing the system. 5. Conclusion The purpose of selecting products and equipment is to ensure that the design, installation, commissioning, normal production operation, and maintenance of the control system all demonstrate superiority. A wide variety of hardware and modules are like tangram pieces; different combinations will produce different results. Therefore, when we receive a project and begin considering hardware selection, we can consider several approaches and document each approach on drawings for clearer comparison and refinement. The progress of science and technology is evident to all, and the functions of products are changing rapidly. There are no longer any unchanging methods in industrial design. As a technical designer in industrial control, we should use our brains, broaden our thinking, and solve problems quickly, efficiently, and economically.