What is the definition of “PLC”?

When we refer to PLC as a Programmable Logic Controller, we are referring to an industrial computer device, mainly used in industrial environments.

PLCs became popular in the 1970s, after the first PLC was invented in 1969 by Dick Morley. As their name may well indicate, they were designed to carry out actions such as production line automation, control of the transport system and even the monitoring of the industrial facilities themselves.

Operation of a PLC

PLCs obtain or receive information through sensors or input devices that are connected to it. These sensors may comprise proximity, temperature, pressure sensors, etc. The signals sent by these sensors are connected to input modules in the PLC, which are responsible for converting the analog or digital signals from sensors into a format that the PLC may understand and process.

The actions that the PLC takes or that are executed in response to the data received by sensors, and processor by the central processing unit (CPU), will depend on the programming that was granted by the user or programmer of the PLC itself.
Regarding the programming that was configured for the PLC, it is adaptable, since it allows engineers and programmers to write these custom programs to be able to adapt to a large number of industrial processes, and they may also be modified and adjusted when desired to meet production requirements.
They are compatible with the following communication languages, listed by the International Electrotechnical Commission in 61131:

  • Ladder Logic
  • Instruction List
  • Structured List
  • Function Block Diagram
  • Sequential Function Chart

PLCs are also able to store this received information to later be used to guide possible changes and necessary improvements in industrial processes. This information is stored on SCADA (Supervisory Control and Data Acquisition) devices.
In addition, they offer great flexibility and scalability. This is because manufacturers provide a wide range of hardware options with different processing capacities, memory and I/O capabilities to be able to adapt to the specific needs of each case, as well as the possibility of growth or reduction of the PLC system by modifying the number of PLCs themselves, as well as the number of I/O modules, as necessary.

Key features of PLCs

As mentioned, PLCs have input modules as well as output modules. These are connected to the CPU. Module assembly is physical, and they are located in the rack according to the physical system arrangement and the needs of the application. Once these modules are assembled, they are connected to the CPU through the motherboard automatically through the internal communication buses.

The types of existing Input/Output modules are analog or digital, as previously mentioned when naming the signals it can receive and process from sensors.
Some examples may be:

  • Digital modules or signals:
    • Presence detectors: To detect the absence or presence of an item in a specific area.
    • Limit switches: To detect the absence or presence of a part, item or component in a specific position.
    • Inductive sensors: To detect the absence or presence of a metallic or non-metallic item in its vicinity.
    • Career endings: To detect the end of a linear or rotary motion.
  • Analog modules or signals:
    • Temperature sensors: For detecting the temperature of an environment or item.
    • Pressure Transducers: To measure the pressure of a fluid in a system. They are used in applications such as pressure control in hydraulic or pneumatic systems.
    • Flow Transducers: To measure the flow rate or amount of a liquid or gas in a system.
    • Level Transducers: To measure the level of a liquid or material in a tank or container.

To carry out communication between PLCs and other devices, communication protocols can be used, such as:

  • Modbus
  • SQL Client
  • MQTT Client
  • EtherCAT
  • EtherNet/IP
  • CANopen (Controller Area Network)

And as for communication ports, Modbus for example uses port 512 for communication in industrial Ethernet networks. This allows between PLCs, SCADA systems and other automation devices. For Ethernet/IP, port 44818 is used to communicate field devices and control systems.
In addition, if we talk about physical ports, RS-232 and RS-485 serial ports are commonly used for direct communication with peripheral devices or for PLC programming and diagnostics. You may also find CANbus, mini USB for programming, Ethernet TCP/IP or USB host.
In order for the user to interact with the PLC, a human-machine interface must be available. This plays a fundamental role in the operator’s interaction with the PLC and makes it easier to process data, control equipment, adjust settings and even provide feedback to users on the status of the system.
It displays information in real time and in a way that is understandable to users. It also allows you to control the industrial process by activating or deactivating machines, editing parameters or operating modes. All this through the use of buttons, switches or other interactive elements on the screen.

This HMI can be integrated with SCADA systems to provide greater visibility and control over the industrial process.
Finally, it should be borne in mind that, depending on the model and PLC manufacturer, port and protocol choices may vary.

Advanced PLC Functions

PLCs are evolving to be able to adapt to the demands of the Industrial Internet of Things (iIoT) and Industry 4.0. This is done by adding improvements to the connectivity, interoperability and efficiency of industrial environments.
If we name some examples, we may find communication through web browsers, using SQL and even using MQTT.

Web browsers

In the context of Industry 4.0, web-based user interfaces are being developed, which allow these industrial processes to be monitored and controlled from any location using a standard web browser.
This, for example, would allow users to access SCADA systems regardless of their geographical location and make collaboration between work teams easier due to remote access.


It is used for storing and managing data related to the industrial process, product quality, predictive maintenance, etc.
PLCs in this case can store data in
SQL databases for further analysis, reporting and decision making.

Through MQTT

As MQTT is a lightweight messaging protocol conceived for communication between devices in IoT networks, it is suitable for environments with limited bandwidth or unreliable connections. MQTT allows message publication and subscription, which makes asynchronous communication and data transmission in real time easier and efficient between the different components of the industrial system.

In short, they enable interconnection, display, storage and data management, as well as efficient transmission.

All-in-One PLC

All-in-one PLCs are the type of PLC that integrates multiple functions into a single device. They are conceived to offer the most complete and convenient solution for industrial control applications, since they have the control capabilities, input/output modules, communication and data display in the same device.
They usually include an integrated touch screen that allows users to monitor and control the PLC, getting rid of the need to use external devices such as panels or separate HMI screens.
All of this helps to simplify system installation and maintenance, as well as a possible cost reduction associated with the purchase and integration of individual components, wiring and time.

Programming a PLC

For programming a PLC, programs are normally written to a computer and then transferred to the PLC. Most PLC programming software offer the use of Ladder Logic or C.

Ladder Logic is the traditional programming language, which imitates circuit diagrams using logical “rungs” that are read from left to right. Each rung is used to represent a specific action controlled by the PLC. Being a visual programming language, it presents graphic symbols such as contacts, coils, timers, counters, etc. It is also easy to understand for electrical engineers and control technicians familiar with electrical circuits and relay logic.
In relation to this, it allows a modular design of the program to be carried out, being able to separate different sections of the program separated in the contact diagram, and also makes debugging tasks easier by being able to follow the flow of the control logic.
As for the use of C, this is a more recent implementation. It is a structured programming language and allows for more direct control over PLC functions and features compared to graphical languages. This may help perform more advanced operations or access PLC-specific hardware features, but may be less accessible and error-prone than programming in graphical languages.

PLC Types

If we talk about PLC types, in addition to the traditional PLC described above, there are several types of PLCs with different specific features for certain types of industrial applications. Some examples are:

  • PLC with integrated security features: Designed for applications that require safety functions, such as emergency downtimes, safe speed or position controls, among others. They comply with industrial safety standards and offer integrated features to ensure worker and machinery safety.
  • High Speed PLC: Optimized for applications that require high-speed control such as packaging machines, label printers, plastic injection… They offer very fast cycle times and advanced data processing capabilities to keep up with production.
  • PLC for hostile environments: EThese PLCs are designed to operate in difficult industrial environments or where they may be exposed to extreme conditions, such as very high or very low temperatures, vibrations, humidity, dust, corrosive products, etc.
    They are built with resistant and sealed materials to protect them from these damages.
  • PLC for power and utilities applications: This type is designed for building automation applications, such as heating, ventilation and air conditioning systems, lighting control, etc.
  • PLC for transport and logistics applications: Transport and logistics PLCs are designed for the control of automated transport systems (AGVs), package sorting, warehouse management, port and terminal control..
  • PLC+HMI: This type of PLC is basically the combination between a PLC of any type and a human-machine interface (HMI) in a single integrated device, which gives both the control capabilities of the PLC and the display and operation capabilities of the HMI.

Regarding the PLC HMI, we find Unitronics, which also include input/output modules, which you may call an all-in-one.
In addition, Unitronics offers several rugged PLC product lines to meet a wide range of system requirements.
By being compact, they make the installation process easier and save time-consuming tasks such as wiring organization or communications configuration.
This project of all-in-one PLCs together with revolutionary software allows Ladder control programming (Ladder logic), design with human-machine interface, and programming of all hardware and communications configuration in a single, simple environment.

Monitoring PLC with Pandora FMS

PLCs are a complex and heterogeneous world, which means that there is no single way, or even a recommended way, to approach every integration project.
Complex ones often include different PLCs with different data acquisition interfaces, so you need to have the ability to integrate different data sources into one platform. Pandora FMS is a perfect tool for this task because it can incorporate data through agents that are executed on a machine to which the PLC is connected and obtain data from log files, command or query execution in a registry, access to a serial port or other more exotic forms of communication.

Pandora FMS also supports MQTT, which is one of the most common ways to communicate with this type of devices. Whatever the way you communicate, Pandora FMS can be adapted to get that information and put it together with other pieces of the puzzle.


To sum up, PLCs are essential equipment when working in industrial environments thanks to their precise and reliable control over a wide range of processes and machinery in multiple industrial sectors. The most important highlights can be:

1. Versatility and adaptability:
Because it can be easily adapted to a wide variety of industrial applications, from simple processes to complex systems.

2. Programming languages:
They support multiple programming languages, allowing engineers to choose the most suitable language for their specific application.

3. HMI Integration:
Some PLCs are integrated with HMIs, allowing for the most direct human-machine interface from a single device.

4. Connectivity and communication:
They offer extensive connectivity and communication capability, allowing integration with other devices and systems in the same floor.

5. Security and reliability:
They are conceived to ensure safety and reliability in industrial environments. They offer built-in safety features as well as advanced diagnostic capabilities to ensure reliable and safe operation.

Overall speaking, PLCs play an important role in improving efficiency, safety and productivity in the industry, allowing process automation, resource optimization and decision-making based on the data obtained.

Can one tool have global visibility?