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A Programmable Logic Controller (PLC) is a new type of general-purpose automation control device that integrates traditional relay control technology, computer technology, and communication technology. It boasts advantages such as strong control capabilities, high reliability, flexibility in use, and ease of expansion, which have led to its increasing widespread use. However, in actual use, due to the harsh working conditions in industrial production environments and the numerous sources of interference, such as voltage fluctuations in the power grid caused by the start-up or shutdown of high-power electrical equipment, which result in low-frequency interference, as well as power-frequency interference generated through electromagnetic coupling by welding machines, electric discharge machining tools, motor brushes, etc., all of these can affect the proper operation of the PLC. Although PLCs are control devices designed for use in the field, and many measures have been taken during their design and manufacture to make them suitable for industrial environments, to ensure the stability of the entire system, it is still necessary to provide PLCs with favorable operating conditions and to implement appropriate anti-interference measures. 2 Issues to Consider During the Installation and Maintenance of PLCs 2.1 PLC Installation PLCs are suitable for most industrial environments, but they do have certain requirements regarding the application scenario and ambient temperature. Controlling the working environment of the PLC can effectively improve its operational efficiency and lifespan. When installing the PLC, avoid the following locations: (1) Areas where the ambient temperature exceeds the range of 0 ~ 50°C ； (2) Relative humidity exceeding 85% or the formation of dew (caused by sudden temperature changes or other factors) ； (3) Direct sunlight exposure ； (4) There are corrosive and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc ； (5) There is iron shavings and dust to be cleaned up ； (6) Frequent or continuous vibration, with a vibration frequency of 10 ~ 55 Hz and an amplitude of 0.5 mm (peak-to-peak) ； (7) Impact exceeding 10g (gravity acceleration). The housing of the small programmable controller has mounting holes at all 4 corners. There are two installation methods: one is to fix it with screws, and different units have different installation dimensions ； Another option is DIN (German Republican Standard) track fixation. Installation clamps for use with DIN rails, one pair on the left and one pair on the right. On the track, first install the left and right clamps, then attach the PLC, and finally tighten the screws. To ensure the proper operation of the control system, programmable controllers are usually installed in control cabinets with protective enclosures to prevent dust, oil, and water splashes. To ensure that the temperature of the programmable controller remains within the specified range while it is in operation, the machine should have sufficient ventilation, with a distance of at least 30 mm between the basic unit and the expansion units. If the surrounding temperature exceeds 55°C, an electric fan should be installed to ensure forced ventilation. To avoid electrical interference from other peripheral devices, the programmable controller should be placed as far away as possible from high-voltage power lines and high-voltage equipment; at least 200 mm of distance should be maintained between the programmable controller and such equipment and power lines. When the programmable controller is installed vertically, it is essential to prevent wire ends, metal shavings, and other debris from falling into the controller through the ventilation openings, as this could cause short circuits in the printed circuit board, preventing it from functioning properly or even leading to permanent damage. 2.2 Power Supply Connection: The PLC’s power supply is AC electricity with a frequency of 50Hz and a voltage of 220V±10%. The FX series of programmable controllers have DC 24V output terminals. This terminal can provide a 24V DC power supply for input sensors, such as photoelectric switches or proximity switches. If the power supply fails, the interruption time is less than 10 ms, and the PLC’s operation is not affected. If the power supply is interrupted for more than 10 ms or the voltage drops below the allowable level, the PLC stops operating, and all output points are disconnected simultaneously. When power is restored, if the RUN input is activated, the operation proceeds automatically. The PLC itself has sufficient capability to resist interference from power lines. If power supply interference is particularly severe, an isolation transformer with a 1:1 turns ratio can be installed to reduce interference between the equipment and ground. 2.3 Grounding A proper grounding is an important condition to ensure the reliable operation of the PLC, as it helps to prevent damage caused by accidental voltage surges. The ground wire is connected to the machine’s grounding terminal, thus grounding the basic unit. If an extension unit is to be used, its ground point should be connected to the ground point of the basic unit. To suppress interference applied to the power supply and the input and output terminals, a dedicated ground wire should be connected to the programmable controller, and this grounding point should be separate from the grounding point of power equipment such as motors. 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. 2.4 DC 24V terminal: When passive-contact input devices are used, the PLC’s internal 24V power supply supplies a current of 7 mA per point to the inputs via these input devices. The 24V terminal on the PLC can also supply current to external sensors, such as proximity switches or photoelectric switches. When the 24V terminal is used as the power supply for the sensor, the COM terminal serves as the DC 24V ground. If extended crew members are used, the 24V terminals of the basic unit and the extended unit should be connected. Additionally, no external power supply may be connected to this terminal. If overload occurs, the voltage will drop automatically, and the input at that point will become ineffective for the programmable controller. The number of input points for each type of PLC is specified. For each unused input point, no power is consumed; therefore, in this case, the capability of the 24V power terminal to supply current outward can be increased. The vacant terminals of the FX series PLCs must not be used under any circumstances. 2.5 Input Wiring: PLCs generally accept digital signal inputs such as those from travel switches and limit switches. The input terminal is the port through which the PLC exchanges signals with external sensor loads. Input wiring generally refers to the wiring between external sensors and the input ports. The input device can be any passive contact or a NPN transistor with an open collector. When the input device is turned on, the input terminal is activated, the input circuit is closed, and at the same time the LED indicating input lights up. Opto-coupling is used for isolation between the primary circuit and the secondary circuit at the input side. The secondary circuit is equipped with an RC filter to prevent the PLC from malfunctioning due to input contact jitter or electrical noise introduced from the input lines. If a diode is connected in series with the input contact circuit, the voltage across the series diode should be less than 4V. If a reed switch with LEDs is used, the number of diodes in series must not exceed two. In addition, special attention should be paid to the following points regarding the input wiring: (1) The length of the input wiring should generally not exceed 30m. However, if the environmental interference is low and the voltage drop is not significant, the input wiring can be made appropriately longer. (2) The input and output wires cannot be the same cable; they must be separate. (3) The width of the pulse signal that can be accepted by the programmable controller should be greater than the duration of the scanning cycle. 2.6 Output Wiring (1) Programmable controllers come in three types of outputs: relay output, thyristor output, and transistor output. (2) The output terminal wiring is divided into independent output and common output. When the output relay or thyristor of the PLC operates, the two output terminals with the same number are connected. In different groups, output voltages of various types and voltage levels can be used. However, within the same group, the outputs can only use power supplies of the same type and voltage level. (3) Since the output components of the PLC are mounted on the printed circuit board and connected to the terminal block, short-circuiting the load connected to these output components will damage the printed circuit board; therefore, fuses should be used to protect the output components. (4) When using a relay output, the size of the inductive load it must handle affects the relay’s service life; therefore, a long service life for the relay is required. (5) The output load of the PLC may generate noise interference, so measures must be taken to control it. Furthermore, for dangerous loads that could cause harm to users, in addition to taking them into account in the control program, external emergency stop circuits should also be designed so that, in the event of a failure in the programmable controller, the power supply to those hazardous loads can be cut off. Do not use the same cable for the AC output line and the DC output line; the output lines should be kept as far away as possible from high-voltage lines and power lines to avoid running them in parallel. 3 Conclusion PLCs are widely used in industrial control due to their significant advantages. There are many issues associated with their practical application, and this article merely presents some considerations regarding their on-site installation and maintenance for those involved in PLC design and application as a reference.