Abstract: In the environment of high voltage and strong electromagnetic interference, the use of fiber optic network is the most ideal communication method. The accelerating electrode power supply system of the superconducting tokamak fusion experimental device operates in the environment of high voltage and strong electromagnetic interference. In order to ensure the stability and safety of the power supply system, the status of the power supply module must be monitored in real time. After software and hardware design and hardware circuit fabrication, the data acquisition waveform of the system at a transmission speed of 115200bps was given through debugging, which proved the feasibility of the method. Keywords: DSP, fiber optic data acquisition network 1. Introduction The neutral beam injection accelerating electrode power supply is a high voltage and high power pulse power supply developed for the national major scientific project superconducting tokamak fusion experimental device (EAST). This power supply adopts the Pulse Step Modulation[1] technology and is composed of 80 identical 1100v/100A output power supply modules connected in series (Figure 1). Its rated output is 80kv/80A and the maximum pulse width is 1000s. Because the power supply is placed in a high-voltage, strong electromagnetic interference environment, the status of each power module must be monitored in real time to ensure the safety and stability of the power supply system. Through comparison of various schemes, a fiber optic network was constructed using the RS232 ports of a computer and a DSP (Digital Signal Processor) via fiber optic converters. The entire system has advantages such as simple structure, high cost-effectiveness, easy expansion, and strong anti-interference capability. Figure 1 shows the block diagram and voltage output waveform of the accelerating electrode power supply. 2. Fiber Optic Network Structure 2.1 Fiber Optic Network Hardware This fiber optic data acquisition network adopts a single-master, multi-slave structure using an RS232 bus. The master station uses a PC, and the slave stations use DSPs, connected by multimode fiber optics to form a bus-type fiber optic network. The fiber optic converter is a self-made 1-to-2 converter that converts the computer's RS232 port data into optical signals. The DSP uses TI's LF2407A. This fiber optic network consists of: a master computer, a single-channel fiber optic converter, a 1-to-8 fiber optic converter, and a DSP. The network topology is shown in Figure 2. Figure 2 Network structure topology diagram The DSP hardware system structure includes: A/D conversion part, SCI interface, I/O port, etc. as shown in Figure 3. The functions that the DSP needs to implement are: monitoring the status of 8 switching quantities of the power module; monitoring the voltage of the input terminal of the power module to determine whether the IGBT can be closed; controlling the opening and closing of IGBT and CKJ; and communicating with the master computer. Figure 3 DSP hardware system structure 2.2 Fiber network software The master station uses the data acquisition interface of the Windows system written by LabVIEW [2] (Figure 4), which is simple and easy to use. Different parameters can be set as needed. The slave station uses a set of integrated development environment CC (CODE COMPOSE) developed by TI for TMS320C2XX [3]. CC uses a graphical interface, provides editing instructions and parameter modification tools, and can perform instruction-level simulation and visualized real-time data analysis of TMS320C2000 series DSPs, which can greatly improve the work efficiency of developers and shorten the system development cycle. There are two methods for DSP multiprocessor communication: idle line mode and address bit mode [4]. Because the address bit mode is insufficient to avoid a 10-bit idle time in the transmission stream at high transmission speeds, and the idle line mode is compatible with RS232 communication, this fiber optic network adopts the idle line communication mode. Figure 4 shows the data acquisition interface. The master station communicates with the slave station via COM1 port, receiving information uploaded by the slave station using a software interrupt method. The data received and transmitted by the master station is in NRZ (Non-Return-to-Zero) format. The NRZ format includes the following components: a start bit, 1-8 data bits, a parity bit (optional), and 1 or 2 stop bits. When a slave station receives a frame of data, it generates an interrupt and checks if it matches its own virtual address. The master and slave station settings are as follows: the slave station's virtual address ranges from 01 to 80. When the data received by the slave station matches its own address, it sends information to the master station; otherwise, it remains in a waiting state (Figure 5). Slave stations do not communicate with each other; only one slave station sends information to the master station at a time. Figure 5. Program Flowchart 3. Simulation Debugging A fiber optic network was constructed using a PC, two self-designed DSP development boards, and a 1-to-2 fiber optic converter (Figure 6). The data acquisition network was then simulated and debugged. A Tektronix TDS 3032B oscilloscope was used for testing. The first rising edge of pin 3 of the PC's COM1 communication port was used as the trigger signal, with a trigger level of 5V. The communication time test process was as follows: the first rising edge of data transmission from pin 3 of the COM1 communication port was used as the start time, and the time until pin 2 received the first rising edge of data transmitted from the slave station was recorded. This time was the duration of one communication between the master and slave stations. After multiple tests, no communication errors occurred, and the communication time was 560µs (Figure 7), with fluctuations within 20µs. The data transmission speed was set to 115200bps. Figure 6. Hardware circuit of the fiber optic data acquisition network (1 is the fiber optic cable, 2 is the LF2407A chip, 3 is a 1-to-2 fiber optic converter). The data acquisition test process is as follows: The master PC sends an address 76 to the slave DSP. Each DSP judges and waits. Finally, the DSP with virtual address 76 sends the operating status of the power module (including IGBT on/off, fuse on/off, etc.) and port voltage to the master computer. After receiving the data, the master station analyzes it. If the operating status of the power module received by the I/O is abnormal, the DSP sends a warning signal to the master computer, reminding the operator to turn off the module power, and stores the acquired data in the designated folder. If the operating status of the power module received by the I/O is normal, no data is stored. Through multiple tests and analyses, the average communication time between the master station and one of the slave stations in this fiber optic data acquisition network is 42.3ms (the slave station sends 31 characters). No communication failures occurred during the test, and the fiber optic network meets the design specifications. Figure 7. Communication Time Waveform Diagram (1 Send, 4 Receive) 4. Comparison of Anti-interference Effects RS232 is the most commonly used serial communication interface between microcomputers, but its anti-interference capability is very poor. This is because RS232C uses single-ended signal transmission, and its connecting cable connects the grounds of the two machines together. Therefore, when the ground potentials between the two ground wires are inconsistent, common-mode interference voltage is generated. This causes serious interference and may even burn out the interface devices. As the frequency increases, the attenuation of the twisted pair increases rapidly, and twisted pairs also have so-called near-end crosstalk, that is, electromagnetic coupling interference between the transmitting and receiving pairs. If optical fiber communication is used, the connection between the two grounds can be isolated, thereby greatly improving its anti-interference capability. Furthermore, optical fiber does not radiate energy, is non-conductive, has no inductance, and there is no crosstalk or mutual interference between optical signals in the optical cable, nor are there safety issues caused by inductive coupling at the line "joint". Using a PC and an LF2407 development board, interference signals were received at pin 2 of COM1 port when communicating via twisted-pair cable (Figure 7). The interference signal was so strong that it overwhelmed the effective transmission signal, causing the twisted-pair cable to fail in this environment. Using a fiber optic network in this environment demonstrated significant anti-interference effects, with the interference signal having no impact on communication. Therefore, fiber optics is an ideal communication medium in this environment. Figure 8 shows the interference waveform . 5. Conclusion The innovation of this paper is the combination of SCI bus and fiber optic communication medium. With the continuous improvement of DSP functions, DSPs are increasingly being applied in process control and monitoring fields. Furthermore, this network can be combined with industrial Ethernet, fieldbus, and other technologies to form distributed control systems and fieldbus control systems, realizing decentralized control of industrial processes. The fiber optic converter used in this paper is low-cost and easy to expand the network. The fiber optic monitoring network described in this paper can be applied to other high-voltage isolation and strong electromagnetic interference environments. 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