Currently, PLCs are widely used centralized control devices in industrial production and automation control. PLCs have replaced cumbersome relay cabinets, AC contactor cabinets, etc., and are gradually becoming more and more common in production and control. Correct wiring of PLCs is a prerequisite for PLCs to function properly, and mastering the wiring of PLC input and output ports is essential for every electrical worker.
Generally, the PLC power input terminal is connected to AC220V to provide operating power to the PLC. The PLC output power port is usually DC24V, which is the PLC's built-in power output. Correct wiring of the input and output terminals is crucial during PLC operation; correct wiring is a prerequisite for PLC function.
Next, we will focus on analyzing the common wiring types for the input and output terminals of a PLC:
I. Common wiring types and objects for input ports:
PLC input ports are generally inputs:
1. Switching signals:
Buttons, limit switches, changeover switches, proximity switches, DIP switches, etc.
A simple example would make it easier to explain:
PLC and push-button switch wiring diagram
Wiring diagram for buttons or proximity switches: PLC switch wiring, one end connected to the PLC input terminal (X0, X1, X2, etc.), the other end connected together to the PLC common port (COM terminal).
2. Analog signals:
These are generally various types of sensors, such as pressure transmitters, level transmitters, remote pressure gauges, thermocouples, and resistance temperature detectors (RTDs), etc.
Different analog signal acquisition devices have different wiring systems (two-wire or three-wire), and therefore the wiring methods will vary slightly. See the diagram below:
Figure 8-5 Interface circuit between PLC and sensor assembly
(a) Two-wire sensor; (b) Three-wire sensor
II. Output port wiring.
PLC output port wiring can generally be divided into the following three situations:
1. Relay output.
2. Transistor output.
3. Thyristor output.
Different PLC output methods require different power supply types for the output load. As shown in the diagram: this is the basic wiring diagram for the PLC input and output terminals, which is part of basic PLC knowledge.
III. Three essential points for PLC wiring:
1. PLC power supply circuit.
In addition to AC power, the power supply for a PLC control system also includes a DC power supply for the PLC. Generally, the AC power supply for the PLC can be directly supplied by the mains power, while the DC power supplies for input devices (switches, sensors, etc.) and output devices (relays) are best supplied by independent DC power supplies. Most PLCs have a built-in 24V DC power supply, which can only be used when the current required by the input or output devices is not very large.
2. Current ratings for PLC input and output ports.
The power supply for the input ports of a PLC is typically DC 24V. The current rating for each input port is between 5mA and 7mA. This current is the maximum current generated when the input port is short-circuited. When the input port has a certain load, the current flowing through it will decrease accordingly. The minimum current required for PLC input signal transmission is generally 2mA. To ensure the minimum effective signal input current, the total impedance of the devices connected to the input port should generally be less than 2K ohms.
Figure 8-5 Interface circuit between PLC and sensor assembly
(a) Two-wire sensor; (b) Three-wire sensor
In other words, when the sensor power at the input port is high, a separate external power supply is required.
3. The maximum current that a PLC output port can generally pass varies depending on the PLC model, but it is mostly between 1A and 2A. When the load current exceeds the maximum rated current of the PLC port, an intermediate relay is generally required to connect an external contactor or other equipment.
