Programmable Logic Controllers (PLCs) are devices used for automated control in industrial production. However, many external factors can interfere with their operation, causing program errors or calculation mistakes, resulting in incorrect inputs and outputs, which can lead to equipment malfunction and misoperation. To improve the reliability of PLC control systems, PLC manufacturers need to enhance the anti-interference capabilities of their equipment; on the other hand, engineering design, installation, construction, and maintenance must pay close attention to this issue, and multi-party cooperation is necessary to effectively solve the problem and enhance the system's anti-interference performance. With the increasingly widespread application of PLCs, their anti-interference issues have become increasingly important. This article proposes some anti-interference measures. 1 Main Sources and Pathways of Interference in PLC Systems 1.1 Radiated Interference from Space Radiated electromagnetic fields (EMI) in space are mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and are commonly referred to as radiated interference, the distribution of which is extremely complex. If a PLC system is placed within a radio frequency field, it will receive radiated interference. This interference primarily occurs through two paths: first, direct radiation into the PLC's internal circuitry, causing interference; second, radiation into the PLC's communication network, introducing interference through the communication lines. Radiated interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially its frequency. Protection is generally achieved by using shielded cables, local shielding of the PLC, and high-voltage discharge components. 1.2 Interference from external system leads is mainly introduced through power and signal lines, commonly referred to as conducted interference. This type of interference is quite serious in industrial settings in my country. 1.2.1 Interference from Power Supply PLC systems are normally powered by the power grid. Due to the wide coverage of the power grid, it is susceptible to electromagnetic interference from all areas, inducing voltage and circuits on the lines. Especially changes within the power grid, such as surges from switch operations, start-ups and shutdowns of large power equipment, harmonics from AC/DC drives, and transient impacts from power grid short circuits, are all transmitted to the primary side of the power supply through transmission lines. PLC power supplies typically use isolated power supplies, but their structure and manufacturing processes make their isolation less than ideal. In reality, absolute isolation is impossible due to the presence of distributed parameters, especially distributed capacitance. 1.2.2 Interference Introduced from Signal Lines Various signal transmission lines connected to the PLC control system, besides transmitting valid information, are always subject to external interference signals. This interference mainly occurs through two pathways: Power grid interference introduced through the power supply of transmitters or shared signal instruments, which is often overlooked; and interference induced by spatial electromagnetic radiation, i.e., external induced interference on the signal lines, which is very serious. Interference introduced by signals can cause abnormal I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, damage to components. For systems with poor isolation performance, it can also lead to mutual interference between signals, causing backflow in the common ground system bus, resulting in changes in logic data, malfunctions, and system crashes. The number of I/O module damages caused by signal-introduced interference in PLC control systems is quite high, and many system failures are caused by this. 1.2.3 Interference from a Chaotic Grounding System Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can suppress the effects of electromagnetic interference and prevent the equipment from emitting interference; while incorrect grounding can introduce serious interference signals, making the PLC system unable to function properly. The grounding of a PLC control system includes system ground, shield ground, AC ground, and protective ground. A chaotic grounding system interferes with the PLC system primarily due to uneven potential distribution at various grounding points. Potential differences between different grounding points create ground loop currents, affecting normal system operation. For example, a cable shield must be grounded at one point. If both ends (A and B) of the cable shield are grounded, a potential difference exists, causing current to flow through the shield. In abnormal conditions such as lightning strikes, the ground current will be even greater. Furthermore, the shield, grounding wire, and earth may form a closed loop. Under the influence of a changing magnetic field, induced currents may appear within the shield, interfering with the signal loop through the coupling between the shield and the core wire. If the system ground and other grounding methods are chaotic, the resulting ground loop currents may create unequal potential distributions on the ground wires, affecting the normal operation of the PLC's logic and analog circuits. PLCs have low logic voltage interference tolerance; interference from logic ground potential distribution easily affects PLC logic operations and data storage, causing data corruption, program crashes, or system freezes. Analog ground potential distribution will lead to decreased measurement accuracy, causing serious distortion and malfunctions in signal measurement and control. 1.3 Interference from within the PLC system is mainly generated by mutual electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits and its impact on analog circuits, the mutual influence between analog ground and logic ground, and the mismatch between components. These are all aspects of the electromagnetic compatibility design performed by the PLC manufacturer, which is quite complex and cannot be changed by the application department. Therefore, it is not necessary to worry too much about these aspects, but it is important to choose systems with a proven track record or proven performance. 2. Main Anti-interference Measures 2.1 Using a high-performance power supply to suppress interference introduced by the power grid. In a PLC control system, the power supply plays a crucial role. Power grid interference enters the PLC control system mainly through coupling with the PLC system's power supply (such as CPU power supply, I/O power supply, etc.), transmitter power supply, and instrument power supply that has a direct electrical connection to the PLC system. Currently, power supplies for PLC systems generally use those with good isolation performance. However, the power supplies for transmitters and instruments directly electrically connected to the PLC system have not received sufficient attention. Although some isolation measures have been taken, they are generally insufficient, mainly because the isolation transformers used have large distributed parameters and poor interference suppression capabilities, leading to common-mode and differential-mode interference introduced through power supply coupling. Therefore, for the power supply of transmitters and shared signal instruments, power distribution units with small distributed capacitance and large suppression band (such as those using multiple isolation and shielding technologies and leakage inductance technology) should be selected to reduce interference to the PLC system. In addition, to ensure uninterrupted power grid feed, online uninterruptible power supplies (UPS) can be used to improve the safety and reliability of the power supply. UPS also has strong interference isolation performance, making it an ideal power supply for PLC control systems. 2.2 Cable Selection Considerations To reduce electromagnetic interference radiated from power cables, especially for frequency converter feeder cables, the author used copper-tape armored shielded power cables in a certain project, thereby reducing electromagnetic interference generated by the power line. The project achieved satisfactory results after commissioning. Different types of signals are transmitted by different cables. Signal cables should be laid out in layers according to the type of signal transmitted. It is strictly forbidden to transmit power and signals simultaneously by different conductors of the same cable. Signal lines and power cables should not be laid close to each other to reduce electromagnetic interference. 2.3 Hardware Filtering and Software Anti-interference Measures Due to the complexity of electromagnetic interference, it is impossible to completely eliminate the effects of interference. Therefore, in the software design and configuration of the PLC control system, anti-interference measures should also be implemented in the software to further improve the reliability of the system. Some commonly used measures: digital filtering and power frequency shaping sampling can effectively eliminate periodic interference; timed correction of reference point potential and the use of dynamic zero point can effectively prevent potential drift; information redundancy technology and design of corresponding software flag bits; indirect jump and setting of software traps can improve the reliability of software structure. For analog signals with low signal-to-noise ratio, large fluctuations often occur due to instantaneous interference on site. If only instantaneous sampling is used for control calculation, large errors will occur. Therefore, digital filtering methods can be used. The analog signals on site are converted into discrete digital signals after A/D conversion, and then the formed data is stored in the PLC memory in time sequence. The signal is then processed using a digital filtering program to remove noise and obtain a pure signal. The current value can be replaced by the average of m samples, but instead of averaging every sample, it is added to the most recent m-1 historical samples after each sample. This method offers fast response and excellent real-time performance. After processing, the input signal is displayed or used for loop adjustment, effectively suppressing noise interference. Due to harsh industrial environments, numerous interference signals, and long I/O signal transmission distances, errors in the transmitted signals are common. To improve system reliability and enable the PLC to promptly detect and eliminate errors, software fault-tolerant technology can be used in program development. 2.4 Correctly Selecting Grounding Points and Improving the Grounding System The purpose of grounding is usually twofold: safety and interference suppression. A proper grounding system is one of the important measures for PLC control systems to resist electromagnetic interference. There are three grounding methods: floating ground, direct grounding, and capacitor grounding. For PLC control systems, which are high-speed, low-level control devices, direct grounding should be used. The PLC control system grounding system adopts both single-point grounding and series single-point grounding methods. Centrally arranged PLC systems are suitable for parallel single-point grounding, with each device's cabinet center grounding point connected to a separate grounding wire leading to the grounding electrode. If the spacing between devices is large, series single-point grounding should be used. Connect the center grounding points of each device's cabinet using a large-section copper busbar (or insulated cable), and then directly connect the grounding busbar to the grounding electrode. The grounding wire uses copper conductors with a cross-section greater than 22 mm², and the main busbar uses copper busbars with a cross-section greater than 60 mm². The grounding resistance of the grounding electrode should be less than 2Ω. The grounding electrode is preferably buried 10–15 m away from the building (or no more than 50 m away from the controller), and the PLC system grounding point must be at least 10 m away from the grounding point of high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multiple grounding points must be avoided; when the shielded twisted pair of multiple measurement point signals is connected to the multi-core twisted pair shielded cable, each shielding layer should be connected to each other and insulated. Select an appropriate grounding point for single-point connection. 3 Conclusion The above measures, as shown by the actual operation of several PLC control systems in the field, can basically eliminate the influence of field interference signals and ensure the reliable operation of the system. Interference in the PLC control system is a very complex problem. Therefore, in the anti-interference design, all aspects of factors should be considered comprehensively, and anti-interference should be suppressed reasonably and effectively. For some interference situations, specific analysis should be carried out, and targeted measures should be taken to ensure that the PLC control system can work normally. [References] [1] Chen Zaiping, Zhao Xiangbin. Programmable Logic Controller Technology and Application System Design [M]. Beijing: Machinery Industry Press, 2002. [2] Chang Hengyi. Programmable Logic Controller [M]. Beijing: People's Posts and Telecommunications Press, 1991. [3] Yuan Renguang. Programmable Logic Controller Application Technology and Examples [M]. Guangzhou: South China University of Technology Press, 1997. [4] Wang Weibing, et al. Programmable Logic Controller Principles and Applications [M]. Beijing: Machinery Industry Press, 1997.