Abstract: This paper briefly introduces the software and hardware design of an unattended heat exchange station system. This remote monitoring system adopts wireless communication and uses a PLC as the slave device on site, which has the advantages of safety, economy, and high automation. Keywords: Wireless communication; PLC; Remote monitoring The heat exchange station system is designed for user heating. It converts the high-temperature hot water supplied from the boiler room into low-temperature hot water through a heat exchanger, and then supplies it to various buildings for winter heating. At present, most heat exchange stations are monitored manually, which is wasteful of manpower, poses potential safety hazards, and makes it difficult to coordinate between stations, easily causing thermal imbalance. This system is a renovation project of eight traditional heat exchange stations of Qinhuangdao Port Authority in Hebei Province. A computer is used as the master device to remotely monitor the working status of the eight heat exchange stations through wireless communication. On site, an OMRON PLC is used for data acquisition, processing, operating condition monitoring, fault protection, and accident alarm. Under normal circumstances, the monitoring personnel on the master device can start and stop the water pumps of the station in real time based on the uploaded data, realizing unattended operation of the heat exchange station. 1 System Function Introduction The computer monitoring system of the heat exchange station can be divided into two parts according to its functions: the host computer (monitoring center) and the slave computer (local PLC). 1.1 Main functions of the host computer (1) To monitor the real-time parameters of each heat exchange station and pump room, including the working status, overload status, pump current of each pump in each station, secondary side return water pressure, outlet water pressure, return water temperature, outlet water temperature, water level, etc. (2) To receive and record the alarm signals transmitted by the slave computer, including power failure, fire alarm, and theft alarm signals. (3) To remotely start and stop pumps. Each pump in each station can be started and stopped separately. (4) To display real-time data and historical data and print tables using graphs and tables. 1.2 Main functions of the slave computer (1) To collect and process on-site data, send execution action signals, and exchange information with the host computer. (2) To monitor the secondary side return water temperature, outlet water temperature, return water pressure, and outlet water pressure. (3) Monitoring of the current of each pump on the secondary side and automatic pump shutdown when the current exceeds the high or low limit. (4) Monitoring of the water level on the secondary side and alarm for exceeding the high or low limit. 2 System Composition [align=center] Figure 1 System Structure Diagram [/align] This system consists of a computer as the host computer, a wireless data transmission module, an antenna, various sensors, and a PLC as the slave computer, forming a host computer link system. The host computer provides a good human-machine interface to monitor and manage the entire network; the slave computer directly participates in the field control, collects real-time parameters through various sensors, and issues control commands. The host computer and the slave computer communicate wirelessly through a wireless data transmission device. Its hardware structure is shown in Figure 1. The PLC model selected by the system is C200HE_CPU42, which has a built-in host computer link unit and is equipped with a digital signal input unit ID212, a digital signal output unit OC224, an analog signal input unit AD003, and a temperature sensor unit TS102. 2.1 Special I/O Units The special I/O units AD003 and TS102 require unit number settings to determine the IR and DM areas occupied by each analog input. In this system, there are 9 analog inputs, so two AD-003 modules are needed. Their corresponding unit numbers are set to 0 and 1 respectively, i.e., assigned to IR100～IR109 and DM1000～DM1099, IR110～IR119 and DM1100～DM1199. There are 2 temperature inputs, so one TS102 module is selected, with unit number set to 2, and assigned to IR100～IR109 and DM1000～DM1099. 2.2 Wireless Data Transmission Module This system uses the SA68D wireless data transmission module as the data transmission medium between the computer and the programmable logic controller (PLC). Communication between the computer (or PLC) and the module is accomplished through an RS-232 asynchronous serial port. This module is only used for data transmission in this system, and its wiring to the computer and PLC is a simple three-wire connection. In this system, modules send information via broadcast, and the sender and receiver are determined by the node number in the frame format linked to the PLC and host computer. Information with mismatched node numbers will be rejected by the PLC. 3. Host Computer Link Communication Host computer link communication is used to transfer data between the PLC and the host computer, allowing the host computer to monitor the PLC's operating status and data area contents using host computer link commands. The TXD(-) instruction can also be used to transfer data from the PLC's data area to the host computer. Host computer link communication is a master-slave bus communication method, with the host computer acting as the master station in the industrial LAN, and all other PLCs connected to the network as slave stations. In this system, the host computer monitors the operation of each heat exchange station, and each station operates independently without data exchange between them; therefore, it is a master-slave communication method in terms of data transmission. After the master station establishes a connection with a slave station, communication is conducted using a response method. The master station sends commands in the form of command frames, and the slave station responds with response frames. Command frames and response frames contain the data to be communicated. Through the exchange of command and response frames, the data to be exchanged can be successfully delivered to the other party. The formats of command and response frames are described below. Command frame format: Response frame format: The FCS checksum is used to check for data errors during transmission. The end code indicates the status of command completion. If the PLC correctly executes the command issued by the host computer, the end code is 00; if an error occurs, the end code is non-zero. 4 System Software Design This system mainly uses a host computer polling method for communication. This method avoids data conflicts on the communication bus and ensures that the real-time data of each PLC in the monitoring host is refreshed periodically. For alarm information actively sent by the PLC, the host computer uses an interrupt method for communication. The host computer first sends a command to the PLC to receive data. After receiving the command from the host computer, the PLC judges whether it is correct. If correct, it transmits the data (including the first and last checksum bytes) to the host computer; otherwise, the PLC refuses to transmit data to the host computer. After receiving data from the PLC, the host computer must also determine its correctness. If correct, it accepts the data; otherwise, it rejects it. 4.1 Host Computer Program The host computer uses VB to write the communication program and establish the database. The command frame format issued by the communication program must fully conform to the communication protocol of the OMRON host computer link. The host computer program must disassemble and identify the response frames sent back by the PLC according to their format in order to correctly separate the exchanged data and useful status information. The MSCOMM communication control in VB can be used to easily initialize the serial port and receive and send information through the serial port. Figure 2 is the flowchart of the host computer polling communication program. 4.2 PLC Programming The response frame is automatically generated in the host computer link unit of the PLC, and the user does not need to write the communication program in the PLC. Therefore, PLC programming mainly solves the on-site start and stop implementation; analog data processing; and actively uploading alarm signals using the TXD command when there is a fire alarm, theft alarm, or power failure. The analog quantity output by the A/D003 module is not the actual value and needs to be processed according to the specific situation. The process is shown in Figure 3(a). Figure 3(b) is the alarm procedure flowchart. It should be noted that the power failure signal is a transition from 1 to 0, while the theft alarm signal is a pulse signal. Once they occur, they need to be held to obtain a long alarm signal. [align=center] Figure 2 Polling Communication Flowchart (a) Analog Signal Processing Flowchart (b) Alarm Flowchart[/align] Figure 3 PLC Flowchart Conclusion In the design of this remote monitoring system for the heat exchange station, two main problems were solved: one is the local real-time monitoring of the PLC, and the other is the wireless communication between the host computer and the PLCs at each station. The system has a simple structure, is economical and reliable, can save a lot of manpower, and can improve the safety operation level of the heat exchange station. After more than a year of actual operation, this system has achieved the expected design requirements, reduced the operating cost of the heating network, and achieved significant economic benefits. In addition, this multi-point wireless monitoring network can also be widely used in many fields where it is inconvenient to have on-site personnel, and has good application prospects. References [1] Guo Zongren. Programmable Controller and its Communication Network Technology. Beijing: People's Posts and Telecommunications Press, 1999: 291-361 [2] Qiu Gongwei. Programmable Controller Network Communication and Application. Beijing: Tsinghua University Press, 2000: 1-172 [3] Fan Yizhi, Chen Liyuan. Visual Basic and RS-232 Serial Communication Control. Beijing: Tsinghua University Press, 2002: 54-90 [4] Wen Xianfa. Visual Basic 6 Database Programming Master. Beijing: Science Press, 2001: 226-264