How PLCs Are Programmed

Programmable Logic Controllers (PLCs) are programmed by professional programmers who are trained in at least one of the programming languages used for PLCs. There are five common programming languages for PLCs. 

Are you hoping to learn how PLCs are programmed by professionals? Would you like to be prepared to ask questions when your PLC programming contractor meets with you? Keep reading to learn more about the very basics of PLC programming.

If you’re not sure if your business is ready to venture into automation, learn more about the pros and cons of industrial automation here.

This Access Industrial Automation blog post will walk you through the basics of Programmable Logic Controller (PLC) programming with a focus on introducing the five most common PLC programming languages.

When you finish reading this article, learn more about the technologies used in automation in this blog post.

Programmable Logic Controller (PLC) Programming

Programmable Logic Controllers (PLC) are programmed by a trained professional. This professional considers what functions the PLC must do and then uses a programming language to dictate what the PLC does in response to inputs. PLC programming can be adjusted over time.

Learn more about finding a good PLC programmer with this blog post if needed.

The languages used to program PLCs are comparable to coding languages used to write programs and apps. They represent methods of communicating with computers and machines.

Programmable Logic Controller (PLC) Languages

Programmable Logic Controllers (PLCs) have programming languages that programmers use to dictate how a PLC functions. There are five most common languages: Ladder/Logic Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Sequential Function Charts (SFC), and Instruction Lists (IL).

Languages may also be referred to as methods for programming. These 5 languages or methods consist of two basic types: graphical and textual. 

Let’s take a brief look at each of the five most common PLC languages and discuss their application and any pros or cons associated with specific methods.

Ladder/Logic Diagram (LD)

Ladder/Logic Diagram (LD) is one of the most common languages used to program PLCs. It’s a graphical arrangement that appears like a ladder. Each rung of the ladder represents inputs from sensors and monitoring equipment as well as desired outputs associated with the given range of data.

LD is the most common PLC language used to program PLCs in North America. LD is favored because it’s fairly easy to learn and can be flexibly programmed and adjusted. It’s also versatile and easy to troubleshoot problems.

The main drawback of the Ladder/Logic Diagram language is that, at some point, the amount of rungs becomes unwieldy. Then, more than one ladder or using a different language becomes important.

This language or method has multiple variations for its name:

  • Ladder Logic
  • Ladder Diagram
  • Logic Diagram

Structured Text (ST)

Structured Text (ST) is a text-based PLC programming language. The syntax of ST is similar to C or C++, so it’s easier for coders to get a handle on it. The main drawback is that it’s not as versatile as languages like Ladder/Logic Diagram and has fewer functional applications.

Function Block Diagram (FBD)

Function Block Diagram (FBD) is a language used to program Programmable Logic Controllers (PLCs). It’s a graphical language composed of blocks to represent inputs and outputs. One block’s output can become another block’s input to connect them.

Function Block Diagram (FBD) is a popular choice for safety PLC functions, complex building automation (PID Loops), and in situations where a lot of the coding is repetitive. 

Sequential Function Charts (SFC)

Another graphical PLC programming language is Sequential Function Charts (SFC). These charts consist of steps and transitions. A step is an action to be taken, and the transition is the conditions that must be met before the next step can begin.

This popular PLC language is easy to troubleshoot. Sequential Function Charts are best for complex multi-state processes and linked processes.

Instruction List (IL)

Instruction List (IL) is a textual-based PLC language that’s being phased out. It’s found in older PLCs and systems. This low-level programming language is being replaced by Structural Text (ST) in applications where a textual PLC language is most appropriate.

PLC Programming Services in California

Programmable Logic Controller (PLC) programming services are available through Access Industrial Automation in central California near Modesto, Stockton, and the surrounding areas. Contact us online or call (209)-577-1491.

We have additional PLC topics for you to explore to learn more:

Looking ahead? AI Automation has a commitment to understanding manufacturing automation in the future. 

Frequently Asked Questions

What is the best technique for PLC programming?

The best technique for programming PLCs involves planning and designing a system that will function as needed and monitoring and adjusting as needed once programmed. During the planning, a programmer will select the best language that integrates with the system and will perform the needed tasks. 

What programming languages do PLCs use?

There are five common languages used by PLCs. These include Ladder Logic Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), Sequential Function Charts (SFC), and Instruction List (IL).

Are Programmable Logic Controllers (PLCs) programmed using a computer?

PLCs are programmed using languages and methods specific to PLCs. PLC programmers are trained to work with specific brands of PLCs like Siemens or Rockwell and are versed in the programming languages used by PLCs. These languages are different than the programming languages used by computers.

What are the four most common methods for programming PLCs?

The four most common methods for programming new PLCs are Ladder Logic Diagram (LD), Structured Text (ST), Function Block Diagram (FBD), and Sequential Function Charts (SFC). Another common PLC method is Instruction List (IL), but this low-level method is being phased out.

What is the most common method for programming PLCs?

Ladder Logic (LD), also known as Ladder Diagram or Logic Diagram, is the most common method for programming PLCs. It’s relatively easy to learn. LD is also flexible, versatile, and easy to troubleshoot. 

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