A DSP-based optical fiber monitoring network system solution

The neutral beam injection accelerating electrode power supply is a set of high-voltage and high-power pulse power supply developed for the National Large Science Project Superconducting Tokamak Fusion Experimental Device (EAST). This power supply adopts Pulse Step Modulation technology and is composed of 80 identical 1100v/100A output power modules in series. Its rated output is: 80kv/80A, and the maximum pulse width is 1000s. Since the power supply is placed in a high-voltage and strong electromagnetic interference environment, in order to ensure the safety and stability of the power supply system, the status of each power module must be monitored in real time. Through the comparison of various schemes, the RS232 port of the computer and the RS232 port of the DSP (Digital Signal Processor) are used to form a fiber optic ring network through a fiber optic converter. The whole system has the advantages of simple structure, high cost performance, and easy expansion.

1 Fiber Optic Network Structure

1.1 Fiber Optic Network Hardware

The fiber optic monitoring network adopts a single master and multiple slaves structure of RS485 bus, with PC as the master station and DSP as the slave station, connected by multimode optical fiber, forming a fiber optic ring network with dual-ring self-healing function. The advantage of the fiber optic ring network with dual-ring self-healing function is that when the fiber optic converter of a node on the network fails, other nodes can still communicate normally. The fiber optic converter adopts MWF5100P produced by Wuhan Maiwei Company, which converts RS232 signals into unified optical signal transmission. The DSP adopts TMS320LF2407A of TI Company.

The master station serial port COM1 communicates with each slave station. The master station sends a signal, and the slave station determines whether to send data to the main control computer. When a fault occurs, the master station can send a signal to reset all power modules. The network topology is shown in (Figure 1).

1.2 Fiber Optic Network Software

The master station uses the communication interface of the WINDOWS system written by the MSCOMM control, which is simple and easy to use. Different parameters can be set as needed. The slave station uses an integrated development environment CC (CODECOMPOSE) developed by TI for TMS320C2XX. CC uses a graphical interface, provides editing instructions and parameter modification tools, can perform instruction-level simulation and visual real-time data analysis on the TMS320C2000 series DSP, which can greatly improve the work efficiency of developers and shorten the application system development cycle.

There are two methods for DSP multiprocessor communication: idle line mode and address bit mode. Since the address bit mode is not fast enough to avoid a 10-bit idle in the transmission stream at a high transmission speed, and the idle line mode is compatible with RS232 communication, the fiber optic network adopts the idle line communication mode.

The master station communicates with the slave station through the COM1 port and receives the information uploaded by the slave station using the software interrupt method. The data received or sent by the master station is in NRZ (non-return-to-zero) format. The NRZ format includes the following components: a start bit, 1 to 8 data bits, a parity bit (optional), and 1 or 2 stop bits.

When the slave receives a frame of data, it generates an interrupt and determines whether it is consistent with its own virtual address. The settings of the master and slave are as follows: the slave virtual address ranges from 01 to 80. When the data received by the slave is consistent with its own address, it sends information to the master, otherwise it is in a waiting state (Figure 2). The slaves do not communicate with each other, and only one slave sends information to the master at the same time. DSP has an RS232 communication port, and the program communicates through receiving interrupts, sending interrupts, receiving instructions, and sending instructions.

The slave station can set communication parameters such as baud rate, parity bit and data bit length through the program.

2 Simulation debugging

A fiber ring network was built with a PC, two SEED-DSK2407 evaluation boards from Hezhongda, three fiber converters and multimode jumpers, and the communication time was tested. The test instrument was a Tektronix TDS3032B oscilloscope. The first rising edge of the 3rd pin of the PC COM1 communication port was used as the trigger signal, and the trigger level was 5V. The communication time test process is as follows:

The first rising edge of data sent by pin 3 of COM1 communication port is taken as the starting time, and the first rising edge of data uploaded by the slave station is received by pin 2, which is the communication time between the master station and the slave station. After multiple tests, there is no communication error, and the communication time is 560us (Figure 3), with fluctuation within 20us, and the data transmission speed is set to 9600bps.

Figure 3 Communication time waveform (1 is sending, 4 is receiving) The reset signal time is measured as follows: the first rising edge of the 3rd pin of the PC COM1 port is used as the starting time (trigger level 5V), and the first falling edge generated by the IOPF0 pin of the slave station is used as the reset time of the slave station. After multiple measurements, the average reset time is about 3.24ms (Figure 4), and the fluctuation is within 80us.

Figure 4 shows the reset signal time (1 is the signal sent by the master station, 4 is the reset signal). After being measured at three different transmission rates of 9600bps, 38400bps, and 115200bps, if the program is debugged successfully, the communication between the master station and the slave station can be established once, and there will be no bit errors in the communication. The fluctuation in time is determined by the data transmission path and measurement errors.

The conversion delay test of the fiber optic converter is triggered by the No. 3 pin of the COM1 communication port, and the RS232 ports of the two DSPs receive the signal. The test result is: the delay after a fiber optic converter is 500ns. In the actual fiber optic network in the future, 80 fiber optic converters are connected, and the fiber optic ring network with dual ring self-healing mode is adopted. The longest communication delay will pass through 40 fiber optic converters, and the transmission delay is 20us, which can be ignored compared with the 560us of one communication time. Because asynchronous serial communication is adopted, the 20us delay has little effect on network communication and can be ignored from the monitoring perspective.

3 Conclusion

Through the analysis of multiple test results, the average communication time between the master station of the monitoring system and one of the slave stations is 42.3ms (the slave station sends 31 characters). No communication failure occurred during the test, and the fiber optic network meets the design indicators. Compared with the PLC-based fiber optic monitoring network we are currently using, it has higher reliability and saves about 100,000 yuan in costs. The fiber optic network we developed is also suitable for other monitoring systems in high voltage isolation and strong electromagnetic interference environments.