Working principle and wiring method of PLC input and output terminals

PLC input/output (I/O) refers to the circuits that a PLC system uses to connect input and output devices. Input refers to the signals received by the PLC, such as changes in state or signals detected by a sensor or switch, while output refers to the signals that the PLC outputs to the devices it controls, such as control signals for motors or relays. Typically, a PLC's I/O modules have multiple channels and can connect multiple input or output devices at the same time. This allows the PLC to control multiple devices and receive multiple input signals at the same time. I/O modules are usually programmable, meaning they can be configured in the PLC program to provide the desired inputs and outputs.

The input and output design of PLC (Programmable Logic Controller) adopts circuit isolation and signal conversion technologies, which can realize precise control of various field devices. The principle is as follows:

1. Input port principle:

When a sensor or switch is triggered, it will output an electrical signal. The input port of the PLC will isolate the signal and convert it into a digital signal, and transmit the signal to the CPU through the internal circuit.

2. Output port principle:

The output port of the PLC converts the digital signal into an electrical signal. The electrical signal drives a relay or another output device in the output port to control the field device.

In short, the PLC input port inputs the electrical signal of the field equipment into the digital circuit of the control system, and performs logical operations and judgments. The judgment results are sent to the output port, and then converted into electrical signals and output to the actual equipment for control and operation.

PLC input and output signals are mainly divided into three types:

1. Digital signal: Digital signal is a binary signal with only two states, "1" and "0". PLC collects and outputs digital signals through digital input and output modules.

2. Analog signal: Analog signal is a continuously changing signal that can take any value. Physical quantities such as temperature, pressure, flow rate, etc. are analog signals. PLC needs to collect and output analog signals through analog input and output modules.

3. Special signals: Special signals refer to other signal types except digital signals and analog signals, such as encoder signals, pulse signals, communication signals, etc. PLC can collect and output special signals through special modules.

The wiring method of the PLC input and output depends on the PLC model and application scenario. Generally speaking, the common PLC input methods are as follows:

1. DC input: usually use 2 or 3 wire input. 2-wire input requires connecting the negative pole of the power supply to the input terminal, and 3-wire input requires connecting the positive and negative poles of the power supply to the input terminal.

2. AC input: The input end generally has two wires, and you need to pay attention to whether the input voltage is consistent with the rated input voltage of the PLC.

3. Optocoupler input: The input end is a photocoupler, which can use external voltage as the input signal, which is safe and reliable.

There are also many types of PLC outputs:

1. DC output: Generally there are two types: solid-state relay and mechanical relay, and the output voltage also has different options.

2. AC output: Similar to AC input, the output signal needs to closely match the device parameters.

3. Relay output: A common way to connect external devices to PLC. It is necessary to make sure that the power supply and current match.

4. Analog output: Outputs proportional digital signals, the most common ones are 0-10V and 4-20mA.

In short, ensuring the safety, matching, grounding and correct circuit connection of the input and output ports is the key to PLC operation. In actual application, parameter settings and connection methods are generally determined according to equipment requirements and PLC manuals or installation instructions.