Common input devices of PLC and how to correctly connect input and output circuits

Common input devices of PLC include buttons, travel switches, proximity switches, conversion switches, encoders, various sensors, etc. Output devices include relays, contactors, solenoid valves, etc. Correctly connecting input and output circuits is the prerequisite for ensuring the safe and reliable operation of PLC.

1. Connection between PLC and main electrical equipment

Figure 1 is a wiring diagram for input devices such as buttons, travel switches, and transfer switches. The PLC in the figure is a DC junction input, that is, all input points share a common terminal COM, and the COM terminal has a DC24V power supply. If it is a group input, you can also refer to the method in the figure below for group connection.

▲Figure 1 Connection between PLC and main electrical input devices

2. Connection between PLC and rotary encoder

The 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 is counted by the high-speed counter of the PLC to obtain the measurement result. Different types of rotary encoders have different output pulse phases. Some rotary encoders output three-phase pulses of A, B, and Z, some only have two phases of A and B, and the simplest one has only A phase.

▲Figure 2 Connection between rotary encoder and PLC

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

3. Connection between PLC and sensor

There are many types of sensors, and their output methods are also different. When using two-wire sensors such as proximity switches and photoelectric switches, due to the large leakage current of the sensor, an erroneous input signal may occur, causing the PLC to malfunction. In this case, a bypass resistor R can be connected in parallel to the PLC input end, as shown in Figure 3. When the leakage current is less than 1mA, its impact can be ignored.

▲Figure 3 Connection between PLC and two-wire sensor

Where: I is the leakage current of the sensor (mA), UOFF is the upper limit of the low level of the PLC input voltage (V), RC is the input impedance of the PLC (KΩ), and the value of RC varies depending on the input point.

4. Connection between PLC and multi-position DIP switch

If some data in the PLC control system needs to be modified frequently, a multi-bit DIP switch can be used to connect to the PLC and set the data outside the PLC. As shown in Figure 4, it is a schematic diagram of a single-bit DIP switch. A single-bit DIP switch can input a decimal number 0 to 9, or a hexadecimal number 0 to F.

Assemble the 4-position DIP switches together, connect the COM terminals of each DIP switch together, and connect them to the COM terminal on the PLC input side. The 4 data lines of each DIP switch are connected to the 4 input points of the PLC in a certain order. As can be seen from the figure, the use of DIP switches will occupy many PLC input points, so this method should not be used unless it is absolutely necessary.

5. Connection between PLC and output device switch

When the PLC is connected to the output device, the voltage type and level of the corresponding output device (load) can be different for the output points of different groups (different common terminals), but the voltage type and level of the output points of the same group (same common terminal) should be the same. It is necessary to decide whether to connect in groups according to the type and level of the output device voltage. As shown in Figure 6, the connection method between the PLC and the output device is explained by taking FX2N as an example. The connection method in the figure is the case where the output devices have the same power supply, so the common terminals of each group are connected together, otherwise they must be connected in groups. The figure only shows the connection between the Y0-Y7 output points and the output device, and the connection methods of other output points are similar.

6. Connection between PLC and inductive load

The output end of the PLC is often connected to an inductive output device (inductive load). In order to suppress the voltage generated when the inductive circuit is disconnected and cause damage to the internal output components of the PLC, when the PLC is connected to an inductive output device, if it is a DC inductive load, a freewheeling diode should be connected in parallel at both ends; if it is an AC inductive load, a RC absorption circuit should be connected in parallel at both ends. As shown in Figure 7.

In the figure, the freewheeling diode can be selected with a rated current of 1A and a rated voltage greater than 3 times 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 between PLC and seven-segment LED display

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

In the circuit, the chip CD4513 with latching, decoding and driving functions is used to drive the common cathode LED seven-segment display. The data input terminals A to D of the two CD4513 share the four output terminals of the PLC, where A is a bit and D is a bit. LE is the latch enable input terminal. At the rising edge of the LE signal, the BCD number input at the data input terminal is latched in the register on the chip, and the number is decoded and displayed. If the input is not a decimal number, the display goes out. When LE is high, the displayed number 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-down resistor should be connected to each output terminal of the PLC connected to the CD4513 to avoid the input terminal of the CD4513 being suspended when the contact of the output relay is disconnected. When the state of the PLC output relay changes, its contact may shake, so the data output signal should be sent first, and after the signal is stable, the data should be latched into the CD4513 using the rising edge of the LE signal.