What is the Difference between Ladder Logic and Function Block Diagrams?

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Today you will learn the difference between function block diagrams and ladder logic. Specifically what they are and when do we use them when programming PLCs.

Most PLC manufacturers around the world still support ladder logic programming. However, some have been moving towards function block programming. Let’s take a look at some of the similarities and differences.

Ladder logic is pretty well “the universal programming language” of PLCs.

As I mentioned earlier it has been around for decades and is pretty well understood by most technicians and engineers alike.

It is named Ladder Logic because it quite literally looks like a ladder.

The vertical lines are referred to as the power rails. The horizontal lines that our programming bits are located on are rungs, just like on a ladder. It is read from top to bottom and left to right.

Function block diagrams can essentially convert several lines of ladder logic into boxes.

In the middle of the function block will be a symbol or text, labeling the function block. This will describe the actual function of the block.

There can be one or several inputs and an output for each function block. Those function blocks can be inputs to other function blocks as well.

Just like ladder logic, function block diagrams should be read left to right, top to bottom. In a sense, this format can make it easier for a technician troubleshooting a machine see what is happening to the machine.

Let’s look at an “AND” function block first. If you are unfamiliar with “AND” Logic, it is actually pretty simple.

Basically, you need at least two inputs energized to turn on or energize its output. I like to think of it as you need this input, and, that input to turn on the output.

In ladder logic, it would look like a series circuit like this. In a function block diagram, the entire rung is replaced by this box or block.

Many industrial machine use “AND” logic. A large metal Press Brake uses “AND” logic for its controls. The operator must push two buttons at the same time. This energizes the output of the “AND” logic. This allows the cylinders to be activated and bend the metal.

Another common function used is the “OR” logic function block. This is like the opposite of “AND” logic.

With an “OR” function block, you would energize this input, or, that input to energize the output. In ladder logic, it would look like a parallel circuit like this.

Machines may use “OR” logic when there are more than one set of controls to operate the machine. Conveyor belts can utilize this programming. They will commonly have the “Start” and “Stop” buttons located near the conveyor. They can also have a start and stop button at another location in order to start the conveyor remotely.

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#RealPars

[ardentdfender]

As a junior programmer having done almost a dozen major power plant plc upgrades and program rewrites with my Senoir Process and Automation Engineer, many upgrading from PLC5 to ControlLogix I call Function Block programing Magic Boxes. That’s me and what I call them as to me they work like magic and make a large program work seemingly like magic with the programing inside the block. While I personally have never programmed one, I’ve seen my coworker do so, but more so I understand the power and huge benefit of Block Programing. In many of our company upgrades we’ve done on power plant projects, Function Block programing has meant the difference to our programmer in being able to vastly minimize the amount of programming required, like programing for many pumps etc. He programs and perfects just one pump Function Block and then he duplicate that Block for pump instruction where ever he needs instruction for a pump logic. That cuts down on making mistakes in the instruction logic. If a mistake, he fixes just one time in the Function Block instruction and like magic every other pump with same problem is fixed. That saves an immense amount of programming time. That’s why I call them Magic boxes or Magic Blocks. But that’s just me.

[boudjellalmustapha]

Thank you RealPars for these amazing vidoes about PLC programming, thanks to your videos I learned a lot of things about PLC programming.

[benhurley]

You guys just make learning so much simpler! Thank you 😊

[melquiadesdelgadoramirez]

Really educative manner to explain differences between the two most used programming languages. Great job RealPars!

[kurtdecena]

Watching one all programming laguange in PLC in your videos, it is easy to undedrstand. Thank you RealPars!

[zazee]

Thank you so much, really happy to see your videos always

[rhythmandacoustics]

This video is super underrated, and also who the hell dislikes this!

[Blades-fmck]

Another great video from realpars, many thanks 👍

[anthonybrown]

This is priceless.... i would love to see a more detailed video on how to read schematic diagrams... like what the symbols mean and how they all first together to create a 230v power wiring, or maybe a relay schematic for a flame scanner on a burner for instance... it can be very confusing for people :) I think your way of explaining would be really appreciated and informative :)

[caden]

10/10 explanation of the and/or logic as well as function block vs ladder logic.

[james]

thanks for this video... now i see how AND and OR compare with PLCs and digital electronics..

[rajivs]

Your videos are very helpful ..Thank you so much ♥️ God bless 😇

[asadiqbal]

your videos very helpfull and provide great knowledge

[ishmaelkamara]

Thank you so much, it very interesting.

[JorgeMartinez-lpwc]

excellent video, when you explain the difference between the other programming languages.
for example a structured text instruction list.

[renegadeflower]

Another great video from Realpars.
But I now hear there is a move now towards Structural Control Language. Are you able to expand on that?

[venkataavinashvunnam]

Hi Guys Thank You for helping us with your Learning Videos. My Request is that if you guys could arrange all the Videos in a Proper Sequence in all the Playlists for better Clarity while going through each playlist.

[richmond]

Very impormative video thank you very match godbless to you...

[shashidharadas]

please upload all video.
It will be helpful for us.
One who working in industries, get benifited.

[rifatuzzamanroni]

Please make a video about communication PLC, HMI & VFD.