Abstract: This paper presents an instrument designed to accurately, efficiently, and conveniently measure multi-core cables using a 51 microcontroller and CPLD. This instrument can quickly perform offline testing on multi-core cables, accurately measure their continuity, withstand voltage, and insulation resistance, and display the test results in real time. Prototype operation results demonstrate that the instrument is simple to operate, reliable in operation, and has high testing accuracy. Keywords: multi-core cable; short circuit; insulation resistance; test; microcontroller Abstract: This paper introduces a multi-core cable test equipment based on an 8051 single-chip microcomputer and CPLD. This system has the function of testing multi-core cables offline, which can quickly and correctly test short circuits and voltage withstand insulation resistance, and display the test results in real time. The experimental results of the prototype show that the system has the performance of simple implementation, reliable operation, and high accuracy in testing. Key words: multi-core cable; short circuit; insulation resistance; test; MCU 1 Introduction A wide variety of communication cables and control cables are widely used in various instruments and control equipment. Whether the cable conducts well and whether the insulation resistance between the wires meets the requirements directly affects the normal operation of electrical equipment. Withstand voltage insulation resistance is an important indicator for measuring the performance of electrical insulation materials. Traditional methods for measuring insulation resistance using megohmmeters have several drawbacks: large measurement errors, inability to guarantee the accuracy of the high-voltage power supply used for withstand voltage testing, inability to automatically save and print measurement results, and cumbersome wiring changes, susceptibility to errors, and high manual labor requirements in measuring the insulation resistance between cores of multi-core cables. This article introduces a multi-core cable tester that can automatically measure the continuity and withstand voltage insulation resistance of cables with up to 48 cores. It ensures the accuracy of test results and can output them from an LCD screen and printer, avoiding errors from manual measurement and significantly improving work efficiency. 2 System Introduction 2.1 System Functions The system has relatively complete functions, the specific functions are as follows: (1) Function to detect cable continuity and insulation resistance; (2) Self-diagnosis function upon startup; (3) Real-time display function of test results; (4) Automatic switching function of cable cores. After the detection starts, it automatically switches to detect the continuity of each core of the cable and the insulation resistance between each core and whether the insulation resistance value meets the requirements; (5) Function to set the number of cores of the cable to be tested; (6) Insulation resistance over-limit setting function, which can be used to judge whether the insulation resistance value of different levels is qualified. 2.2 System Composition The working principle of the multi-core cable tester is shown in Figure 1. The system is mainly composed of the following three parts: Input circuit: including keyboard circuit, insulation test circuit, A/D conversion circuit and continuity test circuit; Output circuit: composed of CPLD system circuit, LCD liquid crystal display module, printer, relay group and 500V DC high voltage circuit; Control circuit: composed of 8051 single-chip microcomputer system circuit. This system uses a 500V DC power supply generated by a high-voltage circuit as the test voltage source for insulation resistance. The insulation resistance measurement adopts the nationally standardized constant voltage method. During testing, the core value of the cable to be tested is first set via the keyboard. The microcontroller controls the opening and closing of the relay group through the CPLD system. Then, the A/D conversion or continuity test circuit converts the signal into a digital signal. The microcontroller collects, processes, and analyzes the signal, and the results are displayed, output, and saved via an LCD screen, printer, and data storage device. 3 System Circuit Design 3.1 Cable Insulation Resistance Detection Design The insulation resistance detection principle is shown in Figure 2. The test circuit consists of three parts: sampling, operational amplifier, and AD7705. The core device for insulation resistance testing is the AD7705, a 16-bit Σ-Δ A/D converter from Analog Devices, which can be used to measure low-frequency analog signals. The AD7705 has a programmable gain amplifier, which can be programmed by software to directly measure various small signals output by the sensor. The AD7705 features high resolution, wide dynamic range, and self-calibration, making it very suitable for high-precision detection and measurement. The AD7705 features two fully differential input channels and its main characteristics are as follows: 16-bit lossless code; non-linearity of 0.0003%; self-calibration and system calibration capabilities; a three-wire SPI serial interface; and low power consumption. The specific test procedure for insulation resistance is as follows: In Figure 2, Rx represents the insulation resistance of the cable under test. During testing, a 500V DC voltage is first supplied, and then relays S1 and S2 are closed. The 500V DC voltage is divided by R2 and R3 and sent to the operational amplifier. After conditioning by the operational amplifier, it is sent to the AD ref terminal of the AD7705 as the test reference voltage to eliminate the influence of 500V DC power supply fluctuations on the test results. The voltage after being divided by Rx and R1 is used as the insulation test sampling voltage, sent to the operational amplifier for conditioning, and then sent to the AD in terminal of the AD7705. The voltage at the AD in terminal is the actual sampled voltage, ranging from 0 to 2.5V. The smaller the insulation resistance, the higher the corresponding sampling voltage. 3.2 Cable Continuity Detection Design The principle of cable continuity detection is shown in Figure 3. Before starting the continuity test, all core wires at one end of the cable should be short-circuited using a short-circuit ring, and the other end connected to a relay array. The relay array is used to switch the connection of the cable core wires. All core wires are grounded through resistors R1 and R2 via the normally closed contacts of the corresponding relays. When testing core wire #1, relay S1 is closed, causing the normally open contact of relay S1 to close. +15V is applied through the core wire to resistors R1 and R2, and after voltage division by R1 and R2, it is sent to the microcontroller. If the tested core wire is open-circuited, Vo is 0V; otherwise, Vo is +5V. 3.3 CPLD System Circuit Testing The maximum number of cable cores that the system can detect is 48. The 51 microcontroller has 24 I/O ports. If the microcontroller is used directly to control the relay group, the microcontroller must use expansion chips to expand the I/O ports to meet the system requirements, such as the 8255, which will increase the complexity of the system. Therefore, it was decided to use a CPLD to control the relay group. A CPLD (Complex Programmable Logic Device) is a complex user-programmable logic device. CPLDs are standard large-scale integrated circuit products that can be used in the design of various digital logic systems. In recent years, due to the adoption of advanced integration processes and mass production, the cost of CPLD devices has been continuously decreasing, while integration density, speed, and performance have been significantly improved. A single chip can implement a complex digital circuit system. Coupled with user-friendly development tools, using CPLD devices can greatly shorten product development cycles and significantly facilitate design modifications. This paper uses Altera's MAX7000s, a high-precision, high-performance, in-system programmable CPLD chip based on the second-generation MAX architecture. It is fabricated using advanced CMOS technology and contains an electrically erasable read-only memory (EPROM), providing 600-5000 usable strobe pins, an ISP, a delay of only 5ns, and a high-speed counter with a frequency up to 175.4MHz. After the relay on/off program is programmed, it is burned into the CPLD using a dedicated download line. The CPLD and the microcontroller are connected via an analog serial connection, which greatly simplifies the system circuit. After the system uses a CPLD for I/O port expansion, the microcontroller only needs to send the core wire number of the cable under test to the CPLD, and the selection of the relay group is handled by the CPLD. This greatly simplifies programming, making the main program structure compact and the control flexible. 3.4 Other Circuits In addition to the circuits mentioned above, the system also has power supply, keyboard, and system reset circuits. The power supply circuit provides power to the DC high-voltage circuit, as well as the microcontroller, LCD display module, relay group, and detection circuit. The keyboard circuit operates at +5V and has 7 keys. These 7 keys allow for setting system parameters, such as the number of cores in the cable under test, setting the insulation resistance over-limit value, and viewing test results. 4 System Program Design The software part of this system is written in assembly language. The executable code generated by assembly language is fast and compact, and its running efficiency is superior to that of C language programs. The main program flow of the system is shown in Figure 4. The system software mainly includes a system self-test program, a cable parameter setting program, a program for viewing the last test results, a cable continuity test program, a continuity test result display program, a cable insulation test program, and an insulation test result display program. The self-test program initializes the system upon power-up, performing a self-test to ensure the correctness of the instrument's operating status. The parameter setting program is used to set the cable insulation resistance over-limit value, ranging from 1 to 20 MΩ, as well as the total number of cable cores and the number of cores in each branch, ranging from 2 to 48 cores. The test result viewing program is used to review the last cable continuity and insulation resistance test results. The continuity test and insulation test programs are used to perform continuity and insulation tests on the cable, respectively, and the test results are displayed by the continuity test result display program and the insulation test result display program, respectively. 5. Test Result Analysis First, the system's accuracy was verified. High-precision resistors of 1MΩ, 2MΩ, 5MΩ, 10MΩ, and 20MΩ were selected for testing. The units of the results are all in MΩ. The measured data are shown in Table 1. As can be seen from Table 1, the relative error of the obtained test results is within 3.5%, achieving high accuracy and meeting the design requirements. In addition, the testing speed of the instrument was tested. Taking a 48-core cable as an example, the insulation resistance test between the cable plug cores using a megohmmeter took 30 minutes, while the tester used this instrument took 1 minute and 20 seconds; the continuity test took less than 30 seconds. This system can complete a continuity and insulation resistance test within 2 minutes. Therefore, the instrument's testing speed is high. 6. Conclusion The author's innovation lies in the organic combination of a 51 microcontroller and a CPLD chip in the tester, which designs a testing system with high testing speed, flexible control, adaptability to continuity and insulation testing requirements for cables with different core counts, and a high degree of intelligence and automation. This product features strong environmental adaptability due to its anti-interference measures. The instrument can be used for testing various cables in industries such as power, communications, railways, and defense. References [1] Su Jing, Meng Shang, Li Wenhai. Design of cable testing based on microcontroller [J]. Fiber Optics and Cables and their Application Technology, 2005, 2: 24-26. [2] Song Xingyuan, Li Wei, Yan Xu. Digital insulation resistance tester based on MSP430F149 [J]. China Instrument and Meter, 2003. 7: 26-28 [3] He Limin. Advanced tutorial on microcontrollers [M]. Beijing University of Aeronautics and Astronautics Press, 1999 [4] LCM12864B graphic dot matrix liquid crystal display module user manual [Z]. Beijing: Beijing Qingyun Innovation Technology Development Co., Ltd., 2005 [5] Huang Zhengjin, Xu Jian, Zhang Xiaoli, Xiong Mingzhen, et al. Introduction and application of CPLD system design technology [M]. Beijing: Electronic Industry Press, 2002. 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