Programmable logic controllers or PLC can be compared to your personal computer you have at home when it comes to some features. They both have operating software, memory, inputs and outputs, a CPU or Central Processing Unit and a power supply. But, there is one major difference, PLC can perform continuous and discrete functions that your computer can’t and can withstand tough environments.

If PLC programming is something new to you, always start with basics and start practicing work as soon as you can because you will learn the best through experience. It isn’t as difficult as it sounds but it is a very responsible job if you work at high demand manufacturing facilities. Also, learn from different examples because everyone has their own way which may be difficult to understand at the beginning.

Professionals will always mention that it has a crucial part in the field of automation which is very true. Each process has its own operational requirements and this type of controller can adapt to it with proper coding. Of course, each industry has changes in production after a certain period so there will be reprogramming needed also. But, that issue can be overcome with programmable logic controller-based control systems.

Basic Functions of Controllers

Dick Morley invented them in 1964 and it was a part of the revolution when it comes to the manufacturing and industrial sector. It has many functions like processing, comparing, calculating, counting and timing various analog signals. One of the advantages it has compared to “hard-wired” control systems is that you will only pay the programmer’s time to reprogram it after a certain change needs to be made instead of getting a new one.

Before they were invested, you would need to get rid of the wires and start from the beginning which takes longer and is more expensive. For example, when you have a light connected to a switch, you will have two options on and off. You can set a task that the light should glow after 30 seconds when you turn the switch on. With a hard-wired setup, you would need to rewire the circuit to add a timing relay.

Having to change the circuit is time-consuming and hard work so this is the time where PLC comes into the picture. The difference you would need to make is only in the code where you can set a timer for 30 seconds after the switch is turned on. It basically means that you can easily manipulate and incorporate various inputs and outputs. This was one of the simplest examples there are more complex processes it can manage that you can customize.

How To Operate and What Are The Parts?

The Scan cycle or also known as a cyclic scanning method is the easiest way to understand how a programmable logic controller works. The first step it takes is starting the monitoring and cycling of time. The next step is done by a CPU that reads the information from the input module and examines the status of all inputs. Now, it can start executing the application or user program made in relay-ladder logic or any available language. The last steps include performing communication tasks and internal diagnosis and writing the data into the output module so the outputs can be updated and these processes continue as long as it is in the run mode.

You can compare a computer’s architecture to the structure of a PLC. They are constantly monitoring the input values from multiple input sensing mechanisms. There are five elements you should learn in your basic training: communication interface module, input and output module, CPU or Central Processing Unit, Power Supply Module and Rack or chassis. They are the hardware parts you will see in training.

Rack and Power Supply

Rack or chassis are considered as the backbone and the most important module of all PLC systems. They come in all shapes and sizes and more complex systems will have a larger chassis. A fixed I/O pin configuration is in small-sized types and can accept different kinds of I/O modules with slide and fit in concept.

Like in PCs, the power supply is used to provide power to the system but you probably didn’t know that it converts the usable AC power to DC which is needed by the I/O module and CPU. You will rarely find ones that use isolated PS, most of them work on a 24V DC supply. It isn’t the most important part but it is necessary.

The Central Processing Unit

The CPU module has RAM, ROM memory and a central processor. ROM memory involves application programs, drivers and operating systems and RAM is meant to store data and programs. You can consider the CPU as the brain that has a hexagonal or octal microprocessor. Being a microprocessor, it replaces counters, relays and timers.

There are 2 types of processors, word processor and a single bit type that can be in the PLC. Word type is used for recording data, controlling, numerical data and text. One bit type is made to perform logic functions. With the use of sensors, it reads the input data and processes it and in the end transfers the command to controlling devices.

First Steps in Ladder Logic

There are countless cases that need a separate design that you will code and implement. To be able to do this, we need to know the specifics of programming. There are two types of instructions that the program consists of, textual or graphical. Textual language consists of structured text and instruction list and graphical form consists of Sequential Function Chart or SFC, Function Block Diagram or FBD and Ladder Diagrams or LD. Find more information here:

One of the reasons why most professionals will tell you to focus on ladder logic is that it’s the simplest form of programming. You can find many examples online where they explain how a lamp would work based on this program. You would have switches, rungs and rails where rungs form the electrical connectivity among negative rail and positive rail.

Functional Block Diagrams

FBD is also used to program multiple functions as a simple and graphical method. When properly executed, FB is an instruction unit that produces 1 or more output values. You will see it in the program as a block that has inputs entering from the left and output lines exiting at the right. One of the advantages of using this type of graphical form is that any number of outputs and inputs can be used in the functional block.

Responsibilities and Tasks

Besides learning the basics, you would want to know what is expected from you when you get hired. Some of the tasks include visualization of processes, maintenance and execution of fault diagnostics, development and optimization and design of control and regulation programs. Most of the companies would expect that you have knowledge of relay ladder logic, programming, that you would have a high sense of responsibility and knowledge of control programs. One important thing to mention is that there isn’t a degree program that trains you to become a PLC specialist so you can only rely on the training you get afterward or on yourself if you learn by yourself.