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To improve the interference resistance of PLC systems, CHANGHUI Instruments recommends taking this aspect into consideration from the initial design stage. Below are some points to keep in mind during the design process; I hope everyone will take them into account and improve accordingly. When carrying out interference resistance design for specific projects, we can choose products with high interference resistance, and adopt measures such as suppressing the sources of interference, blocking or attenuating the pathways through which electromagnetic interference propagates, and using software solutions, in order to enhance the interference resistance of devices and systems. Changhui digital meters: yunrun.com.cn/product/ 1. High-performance power supplies are used to suppress interference from the electrical grid. For the power supply that powers PLC controllers, it should be supplied via non-power circuits, directly from the main bus in the low-voltage distribution room using dedicated wires. An isolation transformer should be used, and its capacity should be 1.2–1.5 times higher than the actual requirement; a filter can also be placed in front of the isolation transformer. For the power supply of transmitters and shared signal instruments, it is advisable to use the signal isolator YR9034A with power distribution capabilities, which features low distributed capacitance, multiple levels of isolation and shielding, as well as leakage inductance suppression technology. The controller and the I/O system are powered by their own isolation transformers, separate from the main circuit power supply. Try not to use the 24V DC power supply of the PLC controller to power various sensors connected to it, in order to reduce the impact of short circuits within those sensors or in their power supply circuits on the PLC controller. Furthermore, to ensure uninterrupted power supply to the grid, an online uninterruptible power supply (UPS) can be used. The UPS features overvoltage and undervoltage protection, software monitoring, and isolation from the grid, thereby enhancing the safety and reliability of the power supply. For some critical equipment, the AC power supply circuit can adopt a dual-power supply system. 2. Select the appropriate cables and carry out proper wiring to eliminate spatial radiation interference from programmable controllers and human-machine interfaces. Different types of signals are transmitted via separate cables; a separation technique is employed, with signal cables being laid in layers according to the type of signal they transmit. Cables for similar types of signals are wired using twisted-pair configuration. It is strictly prohibited to use different wires of the same cable to transmit both power supply and signals simultaneously. Avoid laying the signal lines alongside the power cables; instead, increase the angle between them to reduce electromagnetic interference. To reduce the radiated electromagnetic interference from power cables, especially those supplying frequency conversion devices, and to prevent such interference from reaching its target, shielded power cables should be used. 3. Anti-interference measures for the input and output channels of the PLC controller: Filtering of the input modules can reduce the differential mode interference between the lines of the input signals. To reduce common-mode interference between the input signal and ground, the PLC controller must be properly grounded. When there is an inductive load at the input, for AC input signals, a capacitor and a resistor can be connected in parallel across the load, while for DC input signals, a freewheeling diode can be connected in parallel. To suppress the induced electromotive force generated by parasitic capacitance between input signal lines, parasitic capacitance with other lines, or coupling, an RC surge absorber can be used. The output is an AC inductive load; an RC spark-suppression circuit can be connected in parallel across the load ； For a DC load, a freewheeling diode can be connected in parallel, and it should also be placed as close to the load as possible. For applications with digital output, surge absorbers or thyristor output modules can be used. Additionally, by using intermediate relays or optocouplers in series with the output points, it is possible to prevent the PLC controller’s output points from being connected directly to the electrical control circuit, thus achieving complete electrical isolation. 4. Software measures for noise resistance in PLC controllers: Due to the complexity of electromagnetic interference, it is not sufficient to rely solely on hardware-based noise suppression methods; software techniques for noise resistance in PLC controllers must also be employed to further enhance the reliability of the system. Measures such as digital filtering, power-frequency shaping of sampling, and timing correction of the reference potential are employed to effectively eliminate periodic interference and prevent potential drift. Use information redundancy techniques to design corresponding software flag bits ； Use indirect jumps, implement software protection, etc. For example, for digital input signals, multiple readings are taken using timer delays, and validity is confirmed only when the results are consistent, thereby improving the reliability of the software. 5. Select the grounding point properly and improve the grounding system. A proper grounding system is an essential condition for ensuring the reliable operation of PLC controllers; it helps to prevent damage caused by accidental voltage surges and also reduces interference. A proper grounding system is one of the important measures to help PLC controllers resist electromagnetic interference. PLC controllers are high-speed, low-level control devices, and should be grounded directly. To suppress interference applied to the power supply and the input and output terminals, a dedicated ground wire should be connected to the PLC controller, and this grounding point should be separate from that of the power equipment. If these requirements cannot be met, common grounding with other equipment must be achieved; series grounding with other equipment is prohibited. The ground point should be as close as possible to the PLC controller. The centrally arranged PLC controllers are suitable for a parallel single-point grounding system, with the grounding points at the center of each device’s enclosure being connected to the ground electrode via separate grounding wires. For PLC controllers arranged in a dispersed manner, a series single-point grounding method should be adopted. The grounding resistance of the grounding electrode should be less than 2Ω. It is advisable to bury the grounding electrode at a distance of 10–15 meters from the building, and the grounding point of the PLC controller must be at least 10 meters away from the grounding points of high-voltage equipment. If an extension unit is to be used, its ground point should be connected to the ground point of the basic unit. When the signal source is grounded, the shielding layer should be grounded on the signal side ； When the signal source is not grounded, it should be grounded on the PLC controller side. When there are connectors in the middle of the signal wires, the shielding layers should be firmly connected and insulated, and all shielding layers should be properly connected to one another. Choose an appropriate grounding point for single-point grounding, and avoid multi-point grounding. 6. Choose the grounding point properly and improve the grounding system. When selecting equipment, it is first necessary to understand the interference resistance specifications provided by domestic PLC manufacturers, such as common-mode rejection ratio, differential-mode rejection ratio, voltage tolerance, as well as the maximum electric field strength and magnetic field strength levels under which the equipment can operate. It is advisable to choose products with high interference resistance, such as those that utilize floating-ground technology or programmable controllers with excellent isolation capabilities, along with SWP-HMI human-machine interfaces. The interference resistance issues of programmable controllers and human-machine interfaces in field applications are complex and delicate. Anti-interference design is a highly complex systematic project that involves specific input and output devices as well as the particular conditions in industrial environments. It requires us to take all relevant factors into consideration; we must, based on the actual conditions on site, think comprehensively about ways to reduce interference sources and cut off paths for interference, and make full use of various anti-interference measures in the design of programmable controllers and human-machine interfaces. Only in this way can the interference resistance of programmable controllers and the SWP-HMI human-machine interface during field applications be truly improved, ensuring the safe and stable operation of the system.