Abstract: This paper briefly analyzes the types of interference that PLC control systems may encounter in practical applications. Targeted anti-interference measures are proposed from both software and hardware perspectives, emphasizing the need for a comprehensive and systematic consideration of anti-interference mechanisms and measures when applying PLCs in industrial control. Keywords: PLC; control system; electromagnetic compatibility; anti-interference. Programmable Logic Controllers (PLCs) have advantages such as simple programming, good versatility, powerful functions, and easy expansion. The reliability of PLC control systems directly affects the safe production and economic operation of enterprises, and the system's anti-interference capability is crucial to the reliable operation of the entire system. PLCs utilize highly integrated microelectronic devices, resulting in high reliability. However, the harsh working environment of industrial production sites, such as voltage fluctuations caused by the starting or stopping of high-power electrical equipment, leading to low-frequency interference and electromagnetic radiation, significantly reduces the reliability of PLC control systems. To ensure stable operation and improve reliability of the control system, certain anti-interference methods and measures must be implemented. 1. Types of Interference Affecting the Stability of PLC Control Systems 1.1 Spatial Radiated Interference Spatial Radiated Electromagnetic Fields (EMI) are mainly generated by power networks, electrical equipment, lightning, high-frequency induction heating equipment, large rectifier equipment, etc., and are usually called radiated interference. Their distribution is extremely complex. Their impact is mainly through two paths: one is radiation to the PLC communication network, which is introduced by the induction of communication lines; the other is direct radiation to the inside of the PLC, which is generated by the circuit induction. If the PLC is placed in its radiation field at this time, its signal, data lines and power lines can act as antennas to receive radiated interference. This kind of interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. 1.2 Conducted Interference (1) Interference from Power Supply In industrial sites, switching operation surges, starting and stopping of large power equipment, harmonics caused by AC and DC drive devices, and transient impacts of short circuits in the power grid can all form pulse interference in the power grid. The normal power supply of the PLC is supplied by the power grid, which will directly affect the normal operation of the PLC. Since the power grid covers a wide area, it will be subject to electromagnetic interference from all spaces and generate continuous high-frequency harmonic interference. Especially when disconnecting inductive loads in the power grid, the instantaneous voltage peak generated is tens of times the rated value. Its pulse power is enough to damage PLC semiconductor devices, and it contains a large number of harmonics that can enter the logic circuit through distributed capacitance, insulation resistance, etc. in the semiconductor circuit, causing malfunctions. (2) Interference from signal transmission lines In addition to transmitting effective information, there will always be external interference signals entering the various signal transmission lines connected to the PLC system. There are two main ways of this interference: ① Interference from the power grid introduced through the power supply of the transmitter or the power supply of the shared signal instrument; ② External induced interference on the signal lines, of which electrostatic discharge, pulse electric field and switching voltage are the main sources of interference. Interference introduced by the signal lines will cause abnormal operation of I/O signals and a significant reduction in measurement accuracy. In severe cases, it will cause damage to components. If the system isolation performance is poor, it will also cause mutual interference between signals, causing backflow of the common ground system bus, resulting in changes in logic data, malfunctions or even crashes. 1.3 Ground potential distribution interference The ground wire of the PLC control system includes system ground, shield ground, AC ground and protective ground. Ground potential distribution interference is mainly caused by uneven potential distribution at various grounding points, resulting in potential differences between different grounding points and thus causing ground loop currents. These currents may generate unequal potential distributions on the ground wire, affecting the normal operation of logic and analog circuits within the PLC. Because the logic voltage interference tolerance of PLCs is low, logic ground potential distribution interference 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. 1.4 Interference Generated Internally by the PLC System The main cause of this interference is the mutual electromagnetic radiation between internal components and circuits. Examples include mutual radiation between logic circuits and its impact on analog circuits; the mutual influence between analog and logic grounds; and mismatched use of components. 2 Hardware Measures to Improve Anti-interference Capability Hardware anti-interference technology should be the first choice in system design, as it can effectively suppress interference sources and block interference transmission channels. 2.1 Power Supply Voltage distortion or glitches caused by power supply fluctuations will adversely affect the PLC and I/O modules. According to statistical analysis, 70% of the interference in the PLC system comes from power supply coupling. In order to suppress interference and maintain voltage stability, the following anti-interference methods are often used: (1) Use isolation transformers to attenuate high-frequency interference signals from the power supply line. Twisted pairs should be used for input and output lines to suppress common-mode interference. The grounding method of the shielding layer is different, and the effect of interference suppression is also different. The general practice is to ground both the primary and secondary shielding layers. (2) Use low-pass filters to suppress high-order harmonics. The combination of inductors on the internal capacitors of low-pass filters is different, and the suppression effect of high-order harmonics is also different. In addition, the power input and output lines should be separated, and the shielding layer should be reliably grounded. Generally, both filters and isolation transformers are used in the power supply system, but it is important to connect the filter to the power supply first and then the isolation transformer. [align=center] Figure 1 Isolation transformer power supply system[/align] (3) Use spectrum equalization to suppress transient interference in the power supply. This method is not commonly used and is expensive. 2.2 Good grounding is one of the important conditions for ensuring the reliable operation of PLC and can avoid the damage caused by accidental voltage surges. The grounding wire is connected to the machine's grounding terminal. The basic unit must be grounded. If an extension unit is selected, its grounding point is connected to the grounding point of the basic unit. To suppress interference attached to the power supply and input/output terminals, the PLC should be connected to a dedicated grounding wire. The grounding wire should be separate from the grounding point of the power equipment (such as the motor). If this requirement cannot be met, it can be grounded together with other equipment. It is strictly forbidden to connect it to other equipment in series. The specific grounding method is shown in Figure 2. The grounding resistance should be less than 5Ω, the grounding wire should be thick, the area should be greater than 2 square millimeters, and the grounding point should preferably be close to the PLC device, with a distance of less than 50 meters. The grounding wire should avoid high-voltage circuits. If it cannot be avoided, it should intersect perpendicularly to shorten the length of parallel lines. [align=center] Figure 2 PLC System Grounding Method[/align] 2.3 Input/Output Section 2.3.1 Anti-interference of Input Signals Differential mode interference between input signal lines can be reduced by using input module filtering, while common mode interference between input lines and the ground can be suppressed by grounding the controller. When there is an inductive load at the input, to prevent the influence of induced electromotive force caused by sudden changes in the circuit signal, hardware reliability, fault tolerance, and tolerance design techniques can be adopted. For AC input signals, a capacitor C and a resistor R can be connected in parallel across the load. For DC input signals, a freewheeling diode D can be connected in parallel. Generally, when the load capacity is below 10VA, C should be 0.1μF and R should be 120Ω. When the load capacity is above 10VA, C should be 0.47μF and R should be 47Ω. The specific circuit is shown in Figure 3. [align=center] Figure 3 Anti-interference design of input signal[/align] 2.3.2 Anti-interference of output circuit For PLC systems, the output is a switching quantity, which can be in three forms: relay output, transistor output, and thyristor output. The specific selection depends on the load requirements. If the load exceeds the output capacity of the PLC, an external relay or contactor should be connected for normal operation. If an inductive load is connected to the PLC output terminal, sudden changes in electrical quantity when the output signal changes from OFF to ON or vice versa may cause interference. Therefore, appropriate protection measures should be taken to protect the PLC output contacts. For DC loads, a freewheeling diode D is usually connected in parallel across the coil. The diode should be as close to the load as possible, and it can be a 1A diode. For AC loads, an RC snubber circuit should be connected in parallel across the coil. Depending on the load capacity, the capacitor can be 0.1-0.47 μF, and the resistor can be 47-120 Ω, with the RC circuit as close to the load as possible. See Figure 4. [align=center] Figure 4 Protection of PLC Output Contacts[/align] 2.4 Anti-interference Design of External Wiring There are mutual inductances and distributed capacitances between external wirings, which can cause crosstalk during signal transmission. To prevent or reduce interference from external wiring, AC input and output signals and DC input and output signals should use separate cables. Shielded cables should be used for the input and output signal lines of integrated circuits or transistor devices. The shielded cables should be left floating on the input and output sides, but grounded on the controller side. For short distances under 30 meters, DC and AC input/output signal lines should ideally not use the same cable. If they must be run through the same conduit, shielded cables must be used for input signals, as shown in Figure 5. For distances of 30-300 meters, DC and AC output/input signal lines should use separate cables, and input signal lines must be shielded. For long distances over 300 meters, intermediate relays can be used to switch signals, or remote I/O channels can be used. The controller's grounding wire should be separated from the power supply line, and input/output signal lines should be separated from high-voltage, high-current power lines. [align=center]Figure 5 Shielded Cable Treatment[/align] 3 Software Anti-interference Design Although hardware anti-interference can filter out most interference signals, the causes of interference signals are complex and highly random, making it difficult to guarantee that the system is completely free from interference. Therefore, software anti-interference technology is often used as a supplement to hardware anti-interference measures. Software anti-interference methods are simple to design, flexible to modify, and consume few resources, and have been widely used in PLC measurement and control systems. For PLC measurement and control devices, their data input, output, and storage systems are low-voltage systems. If interference exists in the working environment, the data may be corrupted, leading to data errors, control failures, changes in program state and the working state of certain devices, and in severe cases, system program corruption. Therefore, data anti-interference is also very important. 3.1 Instruction Repetition Instruction repetition involves repeatedly executing instructions with the same function as needed. It is generally suitable for anti-interference of switch or digital inputs and outputs. When acquiring certain switch or digital quantities, the acquisition can be repeated multiple times until two or more consecutive acquisition results are completely identical, which is considered valid. If the signal is always fluctuating after multiple acquisitions, acquisition can be stopped and an alarm signal issued. Under the premise of meeting real-time requirements, inserting a delay between each acquired signal will improve data reliability. If the system's real-time requirements are not very high, the instruction repetition cycle should be as long as possible. 3.2 Digital Filtering During the acquisition of certain signals, random interference may increase the random error of the measured signal. To address this, digital filtering technology can be used. This method features high reliability and stability and is widely used in industrial computer measurement and control systems. Other commonly used digital filtering methods include: program-based filtering, median filtering, arithmetic mean filtering, and recursive mean filtering. 4 Conclusion With the expanding application scope of PLCs and the increasingly harsh working environments, the interference they need to overcome will increase. Therefore, studying the anti-interference problem of PLC systems has become increasingly important. Only by comprehensively analyzing the working environment, determining the nature of the interference, and taking corresponding anti-interference measures can the long-term stable operation of the system be guaranteed.