I. Introduction Programmable Logic Controllers (PLCs) are specifically designed for industrial control. During the design and manufacturing process, manufacturers employ multi-layered anti-interference measures to enable the system to operate alongside high-powered equipment in harsh industrial environments. They exhibit high stability and reliability, with a mean time between failures (MTBF) of tens of thousands of hours. With the development of computer technology, PLCs have become increasingly powerful and convenient to use, leading to their widespread adoption in industrial control systems. However, high overall reliability is only a prerequisite for reliable system operation. Appropriate measures must also be taken during the design and installation of the PLC system to ensure its reliable operation. This paper mainly discusses interference measures during the design and installation of PLC systems. II. Basic Components and Structure of a PLC System The programmable logic controller (PLC) hardware system consists of a PLC host, functional I/O units, and external devices, as shown in Figure 1. The PLC host comprises a CPU, memory, basic I/O modules, I/O expansion interfaces, peripheral interfaces, and a power supply, all connected by an internal system bus. 3.1 Hardware Measures for Anti-interference in PLC System Design (1) Shielding: The main components such as the power transformer, central processing unit, and programmer are shielded with materials that have good conductivity and magnetic permeability to prevent the influence of external interference signals. (2) Filtering: Various forms of filtering are used for the input lines of the power supply system to eliminate and suppress high-frequency interference signals and weaken the mutual influence between modules. (3) Power Adjustment and Protection: Power fluctuations cause voltage distortion or glitches, which will have an adverse effect on the PLC and I/O modules. The +5V power supply required by the core components of the microprocessor is filtered in multiple stages and adjusted with an integrated voltage regulator to adapt to the fluctuations of the AC power grid and the effects of overvoltage and undervoltage. The power lines are run parallel as much as possible and the power lines have low impedance to ground to reduce power noise interference. The grounding method of the shielding layer has different interference suppression effects. Generally, the secondary coil cannot be grounded. The input and output lines should use twisted pair cables and the shielding layer should be reliably grounded to suppress common-mode interference. (4) Isolation: Opto-isolation measures are adopted between the microprocessor and the I/O circuit to effectively separate them to prevent external interference signals and noise signals generated in the ground loop from entering the PLC through the common ground wire, thereby affecting its normal operation. (5) Modular structure: This structure helps to repair the fault in a short period of time. Once a fault is found in a module, it can be quickly replaced to restore the system to normal operation. It also helps to accelerate the search for the cause of the system fault. 3.2 Software measures In order to improve the signal-to-noise ratio of the input signal, software digital filtering is often used to improve the authenticity of the useful signal. For systems with large-amplitude random interference, the program amplitude limiting method is adopted, that is, five consecutive samples are taken. If the amplitude of a certain sample is much larger than that of the other samples, then it is discarded. For parameters such as flow rate, pressure, liquid level, and displacement, which often fluctuate frequently within a certain range, the arithmetic average method is adopted. That is, the average value of n samples is used to replace the current value. It is generally believed that: flow rate n=12 and pressure n=4 are most suitable. (1) Fault diagnosis: The system software periodically detects the external environment, such as power failure, undervoltage, low lithium battery voltage, and strong interference signals, so as to respond and handle them in a timely manner. (2) Signal protection and recovery: When an occasional fault occurs, the information inside the PLC is not destroyed. Once the fault disappears, it can return to normal and continue the original work. (3) Set the warning clock WDT: If the program loop scan execution time exceeds the time specified by the WDT, it indicates that the program has entered an infinite loop and an alarm will be triggered immediately. (4) Strengthen the inspection and verification of the program: Once there is an error in the program, an alarm will be triggered immediately and the program will be stopped. (5) Battery backup for the program and dynamic data: When there is a power failure, the backup battery will be used to power the system and keep the relevant information and status data from being lost. IV. Anti-interference measures during PLC system installation The composition of each part of the PLC and the system connection and assembly method must be strictly carried out in accordance with the installation requirements in the instruction manual. This is very important and is the basic condition for ensuring the reliable operation of the system. 4.1 Power supply and grounding wiring should be arranged reasonably. High-voltage and low-voltage power lines must be strictly separated, and low-voltage power lines should be reinforced as much as possible. Grounding plays a significant role in eliminating interference. AC ground is essential for the PLC control system, forming one of the two power supply circuits through the transformer's center point. The current and various harmonic currents on this circuit are serious interference sources. Therefore, AC ground, DC ground, analog ground, and digital ground must be separated. The common ground point of digital and analog grounds is best placed in a floating configuration. The potential difference between grounding points should be minimized, and grounding wires should be thickened as much as possible; a ring ground wire can be used if conditions permit. The system ground terminal (LG) is the neutral terminal for interference suppression and usually does not need to be grounded. However, when electromagnetic interference is severe, this terminal needs to be connected to the earth-grounded terminal (GR). To prevent current surges, a dedicated 14# grounding wire with a cross-sectional area greater than 2mm² should be used to connect the GR terminal to the earth. The grounding resistance should be less than 100Ω, and the grounding length should be less than 20m. 4.2 Wiring of Output Terminals (1) When several external devices are connected to a single power supply, shorting pieces should be used to short-circuit the common terminals corresponding to their output terminals. Different voltages can be used for the output terminals, and their corresponding common terminals should be connected to different voltage sources respectively. (2) AC output lines and DC output lines cannot use the same cable. Output lines should be kept away from high-voltage lines and nuclear power lines, and should not run in parallel. External devices should not be connected to the output terminals marked with "·". (3) There should be fuses in the output circuit to protect the output components of the PLC. The maximum current flowing into the output terminal should not exceed the allowable value of the PLC, otherwise an external contactor or relay must be connected. Similarly, if the load current is lower than the specified minimum value, a resistor-capacitor absorption circuit should be connected in parallel, as shown in Figure 2. The resistor is 50Ω and the capacitor is 0.1μF. (4) When an inductive load is de-energized, it will generate a large self-induced electromotive force. When the circuit is turned on, an electric arc will be generated at the contact point. In severe cases, the contact point will burn out. Therefore, a freewheeling diode should be connected in parallel with the inductor coil, as shown in Figure 3. 4.3 Cable Laying When power cables exceed 10A/400V or 20A/220V, if they are required to be laid parallel to input/output cables, then there must be a minimum distance of 300 mm between them. If they are placed in the same conduit, they must be at least 100 mm apart and must be shielded with grounded metal. Special attention should be paid to the cable between the PLC's basic unit and expansion unit, which transmits low-voltage, high-frequency signals and is highly susceptible to interference; therefore, it should not be laid in the same conduit as other cables. Additionally, the cable used should be a shielded cable with a cross-sectional area less than 1.5 mm². It is best to use cable conduits for cable laying. When using cable trays, the length should be sufficient to encompass all input/output connections and should be separated from other cables. Twisting the input lines reduces common-mode interference by changing the direction of electromagnetic induction in the conductors, thus canceling out the induction. See Figure 4. When signal acquisition is an analog circuit, the wires can be bundled together. Data lines and pulse lines must not be close to or bundled together. Otherwise, if all data lines are "1", interference will occur on the pulse line, and vice versa. When using shielded cables as input lines, only one end needs to be grounded. If both ends are grounded, current will flow through the shielding layer due to the potential difference, causing interference. To discharge high-frequency interference, a potential equalization line should be connected in parallel to the shielding layer of the digital signal line, with a resistance less than one-tenth of the shielding resistance. Both ends of the shielding layer should be grounded. Alternatively, one end can be grounded to suppress low-frequency interference. In conclusion, PLC application systems operate in harsh environments with various types of interference, despite the high reliability of the PLC itself. Therefore, a comprehensive environmental analysis must be conducted during system design and installation to determine the nature of the interference and implement appropriate anti-interference measures to ensure long-term stable system operation.