What is a PLC?

– PLC is an acronym for Programmable Logic Controller or Programmable Controller. Unlike conventional controllers that only have a certain control algorithm, PLCs have the ability to change the user-written custom control algorithms through a programming language. Therefore, it allows flexible implementation of all control problems.

– Currently, there are many PLC manufacturers such as Allen-Bradley (USA), Siemens (Germany), Omron (Japan), Mitsubishi (Japan), Delta (Taiwan)..etc.

– Common programming languages are LAD (Ladder Logic – Trapezoid), FBD (Function Block Diagram – Function Block), STL (Statement List – List of instructions) and Ladder logic is the most popular PLC programming language. most favored.

PLC relies on input signals and internal control algorithms written by the programmer, it will output output signals to control other devices.

For example, in the image below (Fig.1):, the Omron PLC switches the relay to let the inverter control the motor to run/stop, outputs a 4-20mA signal to the inverter to adjust the motor’s fast or slow running speed.

Fig.1: PLC in a control cabinet

Nowadays, modern control systems are indispensable for PLCs, it is widely applied in all fields of electrical automation, serving many industries and machines such as: water supply, wastewater treatment. , energy monitoring, electrical system monitoring, packaging machines, food processing machines, conveyor lines…etc.


All PLCs today include the following main components:

– Program memory RAM, ROM

– A central processor CPU, has the role of processing algorithms

– Signal I/O modules


First, signals from peripheral devices (such as sensors, contacts, etc.) are fed to the CPU through the input module. After receiving the input signal, the CPU will process and send the control signals through the output module to external controlled devices according to a pre-programmed program.

A cycle consisting of reading the input signal, executing the program, communicating internally, checking for errors, and sending updates to the output signal is called a scan cycle.

Usually, the execution of a scan cycle occurs in a very short time (from 1ms-100ms). The execution time of this scan cycle depends on the instruction processing speed of the PLC, the short length of the program, and the communication speed between the PLC and the peripheral devices.

Advantages and Disadvantages of PLC


  • Easily change the program at will
  • Perform complex algorithms with high accuracy.
  • Compact electrical circuit, easy to maintain and repair.

– Modular PLC structure, allowing easy replacement, expansion of input/output, expansion of other functions

  • Good anti-interference ability, completely reliable in industrial environment.

– Communicate with other smart devices such as: Computer, connect to communication network with other devices.


  • High hardware cost, some firms have to buy additional software for programming.
  • Requires users to have a high level of expertise.

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