[Technology] How does PLC connect to other devices? Input and output once clear!

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Guide: PLC common input devices include buttons, travel switches, proximity switches, transfer switches, dialers, various sensors, etc. The output devices include relays, contactors, solenoid valves, etc. Properly connecting the input and output circuits is a prerequisite for ensuring safe and reliable operation of the PLC.

1. Connection between PLC and main electrical equipment

Figure 6-4 shows the wiring diagram of the main electrical input devices such as buttons, travel switches, and transfer switches. The PLC in the figure is a DC sink input, that is, all input points share a common COM, and the COM terminal has a DC24V power supply. For grouped input, you can also perform group connection by referring to the method in Figure 6-4.


2. Connection between PLC and rotary encoder

A rotary encoder is a photoelectric rotary measuring device that directly converts the measured angular displacement into a digital signal (high-speed pulse signal). Therefore, the output pulse signal of the rotary encoder can be directly input to the PLC, and the pulse signal of the PLC is counted by the high-speed counter of the PLC to obtain the measurement result. Different types of rotary encoders have different phase numbers of output pulses. Some rotary encoders output A, B, and Z three-phase pulses, and some have only two phases of A and B phases. The simplest is phase A.


Figure 6-7 shows the connection between the rotary encoder that outputs the two-phase pulse and the FX series PLC. The encoder has 4 leads, 2 of which are pulse output lines, 1 is a COM end line, and 1 is a power line. The power supply of the encoder can be an external power supply, or it can directly use the DC24V power supply of the PLC. The "-" end of the power supply should be connected to the COM end of the encoder, and the "+" should be connected to the power supply end of the encoder. The COM end of the encoder is connected to the COM input terminal of the PLC. The A and B two-phase pulse output lines are directly connected to the input end of the PLC. Pay attention to the response time of the PLC input when connecting. Some rotary encoders also have a shielded wire. When using it, ground the shielded wire.

3. PLC and sensor connection

There are many types of sensors, and their output methods are also different. When a two-wire sensor such as a proximity switch or a photoelectric switch is used, the leakage current of the sensor is large, and an erroneous input signal may occur, which may cause a malfunction of the PLC. At this time, the bypass resistor R may be connected in parallel at the input end of the PLC, as shown in the figure. 6-8. When the leakage current is less than lmA, the influence can be ignored.


4. Connection of PLC and multi-digit DIP switch

If some data in the PLC control system needs to be modified frequently, you can use the multi-digit DIP switch to connect to the PLC and set the data outside the PLC. Figure 6-5 shows a schematic diagram of a DIP switch. A DIP switch can input 0~9 of a decimal number or 0~F of a hexadecimal number.


Figure 6-5 Schematic diagram of a DIP switch

As shown in Figure 6-6, the 4-digit DIP switches are assembled together. Connect the COM terminals of the DIP switches together and connect them to the COM terminals on the PLC input side. The four data lines of each DIP switch are connected to the four input points of the PLC in a certain order. It can be seen from the figure that the use of the DIP switch requires a lot of PLC input points, so it is not very necessary. Generally, this method is not used.


5. Connection between PLC and output device switch

When the PLC is connected to the output device, the output points of different groups (different common terminals) may have different voltage types and levels of corresponding output devices (loads), but the output points of the same group (same common terminal), their voltage types and levels It should be the same. It is necessary to decide whether or not to connect in groups according to the type and level of the output device voltage. As shown in Figure 6-9, the FX2N is used as an example to illustrate how to connect the PLC to the output device. The connection in the figure is the case where the output devices have the same power supply, so the common ends of the groups are connected together, otherwise the packets are connected in groups. Only the Y0-Y7 output points are connected to the output device, and the connection methods of other output points are similar.


6. Connection of PLC to inductive load

The output of the PLC is often connected to an inductive output device (inductive load), which causes damage to the internal output components of the PLC in order to suppress the voltage generated when the inductive circuit is disconnected. Therefore, when the PLC is connected to the inductive output device, if it is a DC inductive load, the freewheeling diode should be connected in parallel at both ends; if it is an AC inductive load, the RC absorption circuit should be connected in parallel at both ends. As shown in Figure 6-10.


Figure 6-10 Connection of PLC to inductive output device

In the figure, the freewheeling diode can be rated at 1A, the rated voltage is greater than 3 times of the power supply voltage; the resistance value can be 50~120Ω, the capacitance value can be 0.1~0.47μF, and the rated voltage of the capacitor should be greater than the peak voltage of the power supply. Pay attention to the polarity of the freewheeling diode when wiring.

7. Connection of PLC and seven-segment LED display

PLC can directly use the switch output to connect with the seven-segment LED display, but if the PLC controls a multi-bit LED seven-segment display, the required output points are many.


As shown in Figure 6-11, the chip CD4513 with latching, decoding, and driving functions drives the common cathode LED seven-segment display. The data input terminals A to D of the two CD4513 share the four output outputs of the PLC. A is the lowest bit and D is the highest bit. LE is the latch enable input. The number of BCDs input to the data input terminal is latched in the on-chip register on the rising edge of the LE signal, and the number is decoded and displayed. If the input is not a decimal number, the display goes out. When LE is high, the number displayed is not affected by the data input signal. Obviously, the number of output points occupied by N displays is P=4+N.


If the PLC uses a relay output module, a pull-up resistor should be connected to each output of the PLC connected to the CD4513 to avoid the input of the CD4513 floating when the contact of the output relay is disconnected. When the state of the PLC output relay changes, its contact may be jittery. Therefore, the data output signal should be sent first. After the signal is stabilized, the data is latched into the CD4513 with the rising edge of the LE signal.