PLC Programmable Logic Controller

The question “What is a PLC?” It is very common for those who are starting to study industrial electrical, technological advances provide that today an electrical system is controlled and supervised in an increasingly automatic, simple way, occupying less space and with increasingly lower costs.

PLC stands for Programmable Logic Controller or PLC in English, Programmable Logic Controller, basically a PLC is a device that resembles a computer (hardware) where it is possible to insert a program (software) to control and monitor loads (output devices) according to with parameters sent to the PLC (input devices). The program developed for a PLC is fully customizable, consisting of a series of instructions or specific functions such as logic, sequencing, timing, counting and arithmetic, which makes the PLC very dynamic equipment that can be used in any automatic activation process and or monitoring of machines and processes.

Programmable logical controller.

According to ABNT (Brazilian Association of Technical Standards ), a PLC is digital electronic equipment with hardware and software compatible with industrial applications. Despite this definition today, there are PLCs with characteristics and uses in home automation and security systems.

Components of a PLC.


From the English CPU (Center Processing Data) or central data processing is the element responsible for reading the values ​​of the inputs, executing the program functions and transferring the orders according to the functions to the outputs. Basically composed of the processor and the memories.


The processor has the basic function that is the responsibility for reading and executing the program developed by the user, for this, it is composed of an operating system (cannot be modified by the user). The processor also has the function of analyzing faults of the program that is being executed and internal faults of the PLC itself (self-diagnosis) and managing data communication. A CPU can have more than one processor depending on the model and manufacturer.


The memories of a PLC (volatile and non-volatile) are responsible for storing all the information necessary for a PLC to function and perform its functions, the operating system, for example, is stored in non-volatile memories, that is, they cannot be accessed and changed by the user, as well as factory and other information that cannot or should not be changed. The program that will be executed and made by the user is stored in volatile memories, that is, they can be modified and deleted.

Power supply

The power supply is responsible for regulating voltages to the appropriate levels for each PLC. Generally, the PLCs are supplied with 24VDC and connected to 220VAC networks, the source makes the necessary transformations and supplies the modules with their specific voltages, such as the processors that generally work with 5VDC,

Input modules

This part of the PLC is responsible for making the connection between the actuators and input sensors with the PLC. Here are some types of sensors and actuators that can be connected to the PLC inputs:

  • Buttonholes;
  • Limit switches;
  • Level switches;
  • Buoys;
  • Various sensors;
  • Thermostats;
  • Thermocouples;
  • Pressure switches;
  • Flow switches;
  • Encoders.

Inputs can be either digital or analog according to the characteristics of the PLC’s

Output module

Responsible for communicating the PLC with the various loads of a process or equipment, it can be composed of analog or digital outputs such as the inputs. They work as the result of a program that aims to act on different loads, these can be:

  • Electropneumatic actuators;
  • Electro-hydraulic actuators;
  • Contactors (interfacing with various loads such as motors, lighting, resistors and etc.);
  • Coils and timers internal to the PLC.

Programming devices

They are the devices in which the program is inserted in the PLC, a computer can be communicated with PLC to transfer the program to the process or equipment, but due to certain difficulties of having a computer in the field, most manufacturers provide devices for transferring Software.