An article analyzing the connection between PLC and output components

1. Connection between PLC and input components

Common input components 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.

Connect to the main electrical components 1

The following figure shows the wiring diagram of the main electrical input devices such as buttons, travel switches, and conversion 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.

Connecting with rotary encoder 2

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 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 has only A phase.

As shown in the figure above, this 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.

Connect with sensor 3

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 terminal, as shown in the figure below. When the leakage current is less than 1mA, its impact can be ignored.

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.

Connect with multi-position DIP switch 4

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

As shown in the figure below, 4-position DIP switches are assembled together, and the COM terminals of each DIP switch are connected together and connected 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 is generally not used unless it is absolutely necessary.

2. Connection between PLC and output elements

PLC switch outputs are:

Relay output: can output both AC and DC, with a wide voltage range, large current, and low operating frequency, generally around 1Hz. Transistor output: can only output DC, generally below 30V, small current, high operating frequency, up to 200KHz or higher. Thyristor output: can only output AC, generally 60-450V, large current, high operating frequency, and expensive.

PLC analog outputs include:

Voltage output is generally -10V to +10V. Current output is generally 0-20mA or 4-20mA.

When the PLC is connected to the output device, the output points of different groups (different common terminals) can have different voltage types and levels of the corresponding output devices (loads), but the output points of the same group (same common terminal) should have the same voltage type and level. It is necessary to decide whether to connect in groups according to the type and level of the output device voltage. As shown in the figure below, the connection method between the PLC and the output device is explained using FX2N as an example. The connection method in the figure is that 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.

Connecting to inductive load element 1

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 the figure below.

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.

Interfacing with seven segment LED display 2

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.

As shown in the circuit above, 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 the lowest bit and D is the highest bit. LE is the latch enable input terminal. At the rising edge of the LE signal, the BCD number input from 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 PLC output terminal connected to CD4513 to prevent the input terminal of CD4513 from being suspended when the output relay contacts are disconnected. When the state of the PLC output relay changes, its contacts may shake, so the data output signal should be sent first, and after the signal stabilizes, the data should be latched into CD4513 using the rising edge of the LE signal.