1. Characteristics of SCADA Systems SCADA (Supervisory Control and Data Acquisition) systems are computer-based automated systems for monitoring, control, and dispatch management. They enable remote data acquisition, equipment control, measurement, parameter adjustment, and signal alarms, and are widely used in power, water conservancy, petroleum, chemical, environmental protection, and municipal industries. SCADA systems generally adopt a distributed monitoring and control, centralized management approach. The entire system consists of three parts: a monitoring center, several distributed remote terminal units (RTUs), and communication media. The monitoring center, also known as the master station, is the core of the SCADA system, responsible for controlling and managing the entire system's operation. RTUs, also known as peripheral stations, are independently operating intelligent monitoring and control modules using microprocessors or DSPs. They complete the acquisition and processing of various remote field data, control of field actuators, and communication with the remote control center, featuring easy scalability and maintainability. Communication media have various implementation methods depending on actual needs and application objects, which will be briefly analyzed below. 2. Selection of Communication Media Data transmission media are divided into wired and wireless types. Wired transmission methods include power line carrier, RS-485 fieldbus, and PSTN public telephone network. Wireless transmission methods include VHF/UHF radio stations, ISM spread spectrum radio stations, GSM mobile phone networks, and satellite communication networks. Each method has its own characteristics. Power line carrier utilizes existing power lines without the need for dedicated communication lines, but power lines inherently have significant interference, and carrier signals can only be transmitted within a single transformer area, resulting in short communication tracking distances. RS-485 fieldbus offers advantages such as high communication efficiency and reliability, but the cost of laying dedicated lines for large-capacity systems is too high. PSTN public telephone network and GSM mobile phone network have low initial investment costs and no blind spots, but the accumulated operating costs are extremely high. ISM (industrial, scientific, and medical applications) spread spectrum radio stations (2.4 GHz) and microwave (above 4 GHz) can be used for long-distance, high-performance transmission, but they are expensive. VHF/UHF radio stations offer long transmission distances, require only maintenance costs but no operating costs, and are convenient for system use and site expansion. They are particularly suitable for large-capacity distributed SCADA systems, offering a high performance-to-price ratio. However, they require application for corresponding frequency points from the local radio management department. Therefore, wired and wireless methods each have their advantages and disadvantages. Wired communication excels in data transmission reliability, but the cost of dedicated lines for the monitoring and control system is prohibitively high when RTUs are distributed. Wireless communication, on the other hand, has significant advantages in large-capacity SCADA systems. Urban street light monitoring systems utilize VHF/UHF radio communication and have been operating continuously for several years in provincial capitals such as Nanjing, Fuzhou, and Changchun with positive customer feedback. Practice has proven that as long as the key issue of data transmission reliability is resolved, the entire wireless communication SCADA system can operate normally and reliably. 3. Design of Urban Street Light Wireless Communication SCADA System The overall structure of this system mainly consists of three parts: a monitoring center, several peripheral RTU stations, and VHF/UHF radio stations for data communication. The monitoring center uses TAIT 855/856 base stations, and the peripheral stations use Motorola GM300 vehicle radios. The network structure of the urban street light wireless communication SCADA system is shown in Figure 1. 3.1 Monitoring Center Composition and Functions The control center of the urban street light wireless communication SCADA system mainly consists of a main control unit, a backup unit, a server, a large-screen multimedia projector, radios, towers, and antennas. The main control unit and the server are networked in a Client/Server structure. The main control unit, acting as the client of the system, is composed of an Intel industrial control unit, programmed in VB language. Its main functions include implementing the human-machine interface and connecting to the radio via a wireless modem, sending control commands to the RTU, and receiving returned detection data and status parameters. To improve system reliability, another Intel industrial control unit is used as a backup to back up the main control unit. An IBM server is used as the database server for the wireless communication SCADA system. Data collected by the RTU is processed by the main control unit and stored in the data server. Other authorized management systems can directly access this server, facilitating data sharing between systems. The monitoring center mainly performs the following functions: (1) The large-screen multimedia projector dynamically displays the location of each RTU in the SCADA system on the city map, as well as the working status and real-time parameters of the station (including AC voltage, current, power, electricity meter reading, lighting rate, etc. of each line). (2) Based on the longitude and sunrise and sunset times of the city, the daily on/off time curves corresponding to the whole year are formulated to control the on/off of street lights in real time; in addition, the control personnel can modify the on/off time according to the specific situation and transmit it to the RTU for control. (3) Regularly inspect/fixed-point test the RTU, and obtain the switch status, data parameters and alarm codes returned by the RTU through the wireless modem. (4) During the inspection, if a fault or parameter exceeding the limit is found, an audible and visual alarm can be triggered, and a magnified map with the station location as the center and the corresponding working conditions, measured data and fault types can be dynamically displayed on the large screen. (5) It has energy-saving function and implements two working modes: full night light/half night light. The controller decides whether to use the half night light mode for each line and adjusts the start and end time of the half night light arbitrarily. It also has anti-theft monitoring function: when the current value of a certain branch is detected to be greater than the normal operating value, if the current values ​​of other branches in the same RTU are normal (indicating that the A/D chip is working normally), then there is a theft phenomenon in that branch. (6) It uses virtual instrument method to display the shape of instruments such as voltmeter, ammeter, and electricity meter, and uses the VB control Meter to intuitively display the dynamic changes of analog data on the instrument panel. (7) It performs time synchronization on all RTUs at regular intervals to ensure the time accuracy of the peripheral stations. (8) Data processing, storage and report printing functions. 3.2 Wireless Communication 3.2.1 Communication Medium The SCADA system monitoring center and RTUs use VHF/UHF radios for data transmission, operating at frequencies of 203–450 Hz (the appropriate frequency must be obtained from the local radio management department). The modulation method for data communication is FSK, and the transmission rate is 300–1200 bps. Since the RTU sites are widely distributed and located far from the monitoring center, a large-area communication system is adopted. The monitoring center uses TAIT 855/856 base stations with a transmit power of 45W, operating in full-duplex mode. The coverage area depends on the height of the high-gain omnidirectional antenna tower, with a radius reaching tens of kilometers. The RTUs use Motorola GM300 vehicle-mounted radios with directional antennas, a transmit power of 25W, and operate in half-duplex mode. 3.2.2 Communication Method The monitoring center sends commands to the RTUs via broadcast or point-to-point methods. When using broadcast mode, the RTU only receives and executes commands, such as time calibration and modification of light on/off times. When using point-to-point mode, such as telemetry and remote control commands, the RTU first confirms the station number. If it is correct, it executes the corresponding operation according to the command format, such as controlling the circuit to be on/off, returning current, voltage, switch status, and fault code data frames to the monitoring center. 3.3 Peripheral Station RTU Structure Module Design The peripheral station RTU structure diagram of the wireless communication SCADA system is shown in Figure 2. The RTU mainly consists of a single-chip microcomputer measurement and control system, including data acquisition and processing and A/D conversion circuits, keyboard display circuits, clock circuits, street light full-night/half-night light control circuits, and wireless communication circuits. Each peripheral station can operate independently. Even if it cannot contact the monitoring center due to communication failures, it can still independently complete the daily monitoring of the street light system. The single-chip microcomputer measurement and control system uses 80C31 as CPU, 27C256 as EPROM, and 62256 as RAM. In the address decoding circuit, 74LS138 is used. P2.7 is connected to the chip select signal of 62256 (selected at zero) and pin G1 of 74LS138 (selected at 1). P2.4, P2.5, and P2.6 are connected to pins A, B, and C of 74LS138, respectively. The output chip select signal controls the multiplexer, ICL7109, DS12887, 74HC245, and LCD display. (1) Data acquisition and A/D conversion circuit RTU real-time measurement parameters include line voltage, current, and electricity meter reading. The module simulates three-phase AC voltage UA to UC as input signals. The total line current IA～IC, and the branch current (this system can detect up to 8 branches) I1A～I1C, …, I8A～I8C, are sent to the dual-slope A/D converter ICL7109 after passing through the V/I transmitter, multiplexer (4051×4 pieces), and signal processing circuit (including AC signal rectification, filtering, etc.) to convert it into a twelve-bit binary number. The lower eight bits D1～D8 are connected to P0.0～P0.7, and the higher four bits D9～D12 are connected to P0.0～P0.3. The CPU reads the higher four bits and lower eight bits of data from the data bus by controlling the high/low byte enable terminals HBEN and LBEN respectively. (2) The keyboard display circuit uses an LCD display. The main functions of the keyboard are: setting the station number, time, current change value of each branch, current/voltage zero adjustment correction, full night light/half night light mode, patrol inspection/fixed point detection of each branch parameter, etc. (3) The clock circuit uses the DS12C887 clock chip to provide a precise clock signal, including year, month, day, hour, and minute. The time can be modified manually or synchronized by the monitoring center. (4) The street light on/off circuit can automatically control the on/off of the street light line at the station by using the daily on/off time stored in the EPROM. It can also be controlled manually or remotely by the monitoring center. (5) The wireless communication circuit uses the TCM3105 modulation/demodulation chip to demodulate the received analog carrier signal into a digital signal and transmit it to the CPU. The digital signal to be transmitted is modulated into an analog carrier signal and transmitted to the monitoring center via the GM300 vehicle radio and directional antenna. 4 Precautions for the implementation of the street light wireless communication SCADA system The urban street light wireless communication SCADA system using VFH/UFH radio communication has been operating normally for several years. Experience shows that as long as the reliability of wireless data transmission is solved, this method is significantly better than wired transmission in urban high-capacity monitoring systems. It has a high performance-price ratio and can be easily expanded by RTU. The author believes that the following aspects should be noted during the design, debugging and installation process: (1) Special attention should be paid to the field strength test of the wireless signal. Before the installation of the peripheral station, the signal is sent by the monitoring center base station. The field strength test should be carried out at each RTU installation location. There should be no obvious co-channel interference and inter-channel intermodulation interference. The signal level should be above 20dB. If a certain RTU cannot meet the requirements, its address must be changed; when most RTUs cannot meet the requirements, the operating frequency of the radio station must be changed. (2) The communication baud rate should not be too high, otherwise it is very easy to cause code loss. (3) During the debugging of the A/D conversion circuit, it was found that pins 17 (TEST) and 27 (SEND) of the ICL7109 chip should not be left floating. They should be connected to +5V together with pin 26 (RUN/HOLD). Otherwise, the A/D conversion data will often be unstable. (4) The V/I transmitter used to measure the three-phase voltage should not be directly installed on the motherboard, otherwise it is very easy to be struck by lightning and burn out the motherboard. (5) In mountainous areas, if the wireless signal is affected, an interrupt station can be set up.