The application research of power network monitoring systems for program-controlled exchanges is attracting increasing attention in this field. Currently, commonly used power monitoring systems are generally large-scale monitoring systems. These systems use 2Mbit/s ports to transmit information, have high requirements for transmission lines, and consume optical cable resources. Due to the comprehensive monitoring of the power supply of program-controlled exchanges, the signal acquisition volume is very large, the structure is complex, the investment cost is high, and the technical requirements for personnel are also very high. They are suitable for large-capacity computer rooms, but not for power monitoring in the numerous small-capacity computer rooms in rural areas. This paper proposes a new type of power network monitoring system for program-controlled exchanges suitable for small-capacity computer rooms, based on the research of power monitoring systems for large-scale program-controlled exchanges. System Hardware Circuit Design The monitoring system consists of field monitors, a transmission network, and a central monitor. The field monitors have functions such as signal acquisition, judgment, transmission, alarm, and reception control; the central monitor is the control center of the system; the transmission network utilizes telecommunications network lines and switching equipment. [align=center] Figure 1 Schematic diagram of the composition principle of the field monitor[/align] The field monitor is the most important and basic component of the communication power network monitoring system in this paper, and one must be placed in each monitored computer room. The field monitor consists of a microcontroller control system and various sensors and control components. All operating parameters and statuses of the monitored objects are monitored, recorded, judged, and alarmed by the field monitor. Alarms from the field monitor are transmitted via telephone lines. It can actively dial to transmit alarm information to the monitoring center and can also call the responsible person for voice alarms. Therefore, the field monitor can also work independently without relying on the central monitor. The block diagram of the field monitor's composition principle is shown in Figure 1. The field monitor is a microcontroller control system, consisting of a DC/DC power supply module, dialing and DTMF transceiver circuits, voice circuits, a general-purpose data acquisition unit, and control circuits. The transmission network between the central monitor and each field monitor utilizes the existing telecommunications network, connecting via dial-up. The system network diagram is shown in Figure 2. [align=center]Figure 2 System Network Diagram[/align] The central monitor consists of a host, monitor, speaker keyboard, mouse, printer, interface circuit, etc. The functions of the central monitor are: receiving alarm information from the field monitors, generating screen displays, sound alarms, and storing fault information; sending control information to the field monitors; setting parameters for the field monitors; viewing field power parameters; and also having management functions such as querying, statistically analyzing, printing, and reporting historical faults. 1 Microcontroller System Design [align=center]Figure 3 Microcontroller System Schematic Diagram[/align] The field monitor uses an MCS51 series microcontroller for control. The microcontroller system schematic diagram is shown in Figure 3. The CPU chip is an 8032 microcontroller, the peripheral expansion circuit uses a 74HC373 as the address latch, the program chip is a 27C256, and the external memory is a 62256. A 74HC138 decoder is used for address decoding, selecting one 74LS374 and two 8255 chips respectively. The 74LS374 is used as the control indicator output. 8255 (1) is used as analog input control, and 8255 (2) is used as digital input and output control. 2 Acquisition of AC and DC current The acquisition of AC and DC current firstly requires the current signal to be converted into a DC voltage signal of 0~5V through AC and DC transformers, and then converted into a voltage signal (0~10V) that meets the requirements of the A/D converter (AD574A) after voltage conversion. The circuit principle block diagram is shown in Figure 4. [align=center] Figure 4 Circuit principle diagram of AC and DC current acquisition[/align] 3 Digital signal acquisition circuit The acquisition of digital signals first requires various sensors such as smoke detectors, infrared detectors, water immersion detectors, etc. to convert the measured physical quantity into a digital signal, and then perform isolation and level conversion, output to 8255A, and then read by the microcontroller. The circuit principle diagram is shown in Figure 5. [align=center] Figure 5 Circuit principle diagram of digital signal acquisition[/align] System software design The system software mainly includes two parts: central monitoring software design and field monitoring microcontroller software design. The central monitoring host software is programmed in Delphi and runs on Windows 98, 2000, XP, and NT operating systems. The program employs a modular design and utilizes dynamic link libraries. Serial communication uses multi-threading (serial port interrupts), and the database uses an SQL database system. The software function of the central monitoring interface circuit mainly completes the communication process with the central monitoring microcomputer. The central monitoring interface circuit and the microcomputer communicate via an RS232 interface. If there is an alarm from the field monitor, the interface circuit sends an alarm message to the microcomputer; if the central monitor sends a command to the field monitor, the interface circuit receives the command message from the microcomputer; if neither of these situations occurs, the interface circuit sends a handshake message to the microcomputer. The flowchart of the RS232 serial port receiving and sending information program of the interface circuit is shown in Figure 6. [align=center] Figure 6 Serial Communication Program Flowchart[/align] The field monitoring software is written in MCS51 assembly language and consists of an initialization program and an interrupt service routine. In the initialization program, timer T0 is defined as a 6.25ms interrupt, meaning the interrupt service routine is executed every 6.25ms. All operations are placed within the interrupt service routine, and no actual operations are performed after the main program initialization is complete. The interrupt service routine for timer T0 mainly includes the following subroutines: indicator light timing subroutine, signal acquisition subroutine, fault judgment subroutine, fault alarm subroutine, control function subroutine, parameter setting subroutine, parameter reading subroutine, active monitoring subroutine, and automatic patrol subroutine. System Reliability Design In the actual operating environment of the power supply, there are often various interferences such as power fluctuations, impacts, and electromagnetic interference. To ensure that the microcontroller program does not "crash," an automatic reset circuit is used in the design of the microcontroller system. Its circuit principle is shown in Figure 7. In this circuit, DS1232 is the "watchdog," which ensures that the CPU (8032) is automatically reset when the program is not running, allowing the microcontroller to start running again. S1 in the figure is the manual reset button. [align=center]Figure 7 Schematic diagram of automatic reset circuit[/align] To test the various functions of the system, a simulated real-world environment was used in the laboratory to test the centralized monitoring function, parameter setting function, parameter reading function, automatic testing function, query and statistics function, and distributed notification function. The results show that all designed functions of the system can be implemented. Extreme case tests were also conducted. For example, when 10 field monitors alarmed simultaneously, the system ensured that all alarm information was transmitted to the monitoring center sequentially without information loss; when two field monitors continuously alarmed for more than 12 hours, the system continued to operate normally; when the alarm information database reached 20,000 alarm messages, the system still operated normally, with a query time of less than 30 seconds. Conclusion This system is highly reliable, convenient, practical, and low-cost. The comprehensive cost per monitoring point (computer room) is around 8,000 yuan, while a large monitoring system requires around 100,000 yuan. This solves the problem of power monitoring in small computer rooms being impossible due to excessive investment.