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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[@ardentdfender4116]

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.

[@benhurley280]

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

[@realpars]

That's amazing to hear, Ben! Thanks for sharing.

[@boudjellalmustapha9271]

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

[@realpars]

You are very welcome! I am very happy to hear that. Keep it up!

[@rhythmandacoustics]

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

[@sytsewar]

The Indian guy who does not like the accent

[@melquiadesdelgadoramirez680]

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

[@realpars]

Thank you, Melquiades! That's great to hear!

[@kurtdecena8681]

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

[@realpars]

Glad to hear that, Kurt! Thank you very much for sharing

[@anthonybrown7964]

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 :)

[@realpars]

Hi Anthony, Thanks for your comment! Great feedback, I will make sure to forward this to our creator team. Thanks a lot for sharing this with us!

[@joew4202]

RealPars not hard. First learn all your symbols then work on learning logic

[@caden01691]

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

[@realpars]

Thank you!

[@Blades-fm6ck]

Another great video from realpars, many thanks 👍

[@realpars]

Thank you very much, great to hear that!

[@james77011]

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

[@realpars]

You are very welcome, James!

[@venkataavinashvunnam2478]

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.

[@realpars]

Thank you very much for your feedback! I will go ahead and share this with the team.

[@za1981zee]

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

[@realpars]

Great to hear! You are very welcome.

[@renegadeflower575]

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?

[@realpars]

Thank you Renegade Flower! I will ask our creator team to elaborate on it. :) Happy learning!

[@rifatuzzamanroni5283]

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

[@realpars]

Hi there, thanks for the topic suggestion! I will pass it on to our creator team.

[@thewrongbrothers7205]

Great work guys, thank you.

[@realpars]

Thank you!

[@MatrixW.]

Great vidio explain, thank you!

[@realpars]

Glad you enjoyed it!

[@asadiqbal87]

your videos very helpfull and provide great knowledge

[@realpars]

Amazing to hear that, Asad! Thanks for sharing that with us.

[@fyazdian1214]

Brilliant intro thanks

[@realpars]

Thank you very much!

[@DG_Fabrication]

thank you this explained it perfect

[@realpars]

Glad to hear that, and you're more than welcome!

[@andrewmikhael8085]

Thanks , please can you make a video about encoders ?

[@realpars]

Hi Andrew, thank you! I will pass your request on to our creator team. Happy learning!

[@shashidharadas2075]

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

[@realpars]

Hi Shashidhara, Happy to hear that you are benefiting from our course videos, we upload new videos every week! :) Happy learning.

[@rajivs385]

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

[@realpars]

What a great compliment! Thanks a lot! :))

[@richmond4884]

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

[@realpars]

Great to hear that, Richmond! Thanks for sharing.

[@hoodalialhajri]

Simple and informative thanks

[@realpars]

You are welcome! :)

[@joshuajavier5158]

Thanks for the video Do you have course which focuses on FBD and STRUCTURED TEXT programming in SIEMENS.. Thanks

[@JorgeMartinez-lp3wc]

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

[@realpars]

Thank you, Jorge! Thanks for the suggestion, I will pass your request on to our creator team. Happy learning :)

[@eaglelee952]

excellent teaching. BTW, more and more machine use Schneidar PLC now, do you have course for Schenider PLC? thanks

[@realpars]

Hi Eagle, Thanks for your comment! We currently do not have any video courses on Schneider PLC. I will happily pass this on as a topic suggestion to our course developers. Thanks for sharing and happy learning!

[@ishmaelkamara5395]

Thank you so much,it very interesting.

[@realpars]

You are very welcome!

[@mahmoudal-saad2825]

Hello, Thanks a lot for these very important videos… Do you have any pdf"s summarizes the videos? Also do you offer training period.

[@realpars]

Hi Mahmoud, Thanks for your comment! We have a transcript of each video in the 'Description Box'. Regarding your question about our training period, we don't provide any offline training, but we have a course library full of PLC Programming courses. As a subscriber, you obtain full access to our Technical Team who will answer any questions you might have along your learning journey! You can always try our free course in PLC Hardware through the following link bit.ly/2XnnUrF If you have any further questions, feel free to reach back out! Happy learning!

[@muhammadrifqisetiawan2925]

@@realpars you say that the course is free? but when I visit the link, they say that this course isn't free at all.

[@derina.maleek9408]

Thank you

[@realpars]

You are welcome!

[@aieautomation7512]

Nice information

[@mauricegasongo1053]

thank you, can please make a video of all gates and their symbols in function block d and ladder logic?

[@realpars]

Hi Maurice, Thanks for the topic suggestion, I will definitely go ahead and forward this to our creator team. Happy learning!

[@industrialautomationknowle373]

#industrialautomationknowledgebank

[@soolammobile8280]

Nice Teach

[@realpars]

Thank you!

[@puun5991]

Thanks, the video was simple to understand. Do you have any video about Sequential function chart, Structured text and Instruction list? All of those are PLC programming language

[@realpars]

Hi there, Thanks for your comment! We currently mostly focus on Ladder language. I will happily go ahead and forward that to our course developers as a topic suggestion! Thanks for sharing and happy learning

[@Venkatiuv]

How can I write a program if X number of motors using function block. Please explain. Thank you realpars.

[@realpars]

Hi Venkat, Thanks for your comment and suggestion. I will make sure to forward this to our course developers!

[@bac4998]

Subscribed. Thanks

[@realpars]

You are welcome!

[@محمدمحمد-خ5ع2ص]

Useful channel

[@realpars]

Thank you!

[@amitkumar5602]

Sir kindly make a video on IBMS. and descriptions on IBA and DALI.

[@realpars]

Hi Amit, thanks a lot for your topic suggestion, I will send this through to our creator team.

[@braulioamador6514]

Thanks, very much!!!

[@realpars]

You are very welcome!

[@anlee95]

rất bổ ích . thank very much

[@realpars]

You are very welcome!

[@刘轩-h4r]

Thank you so much.GOOD

[@realpars]

You are very welcome!

[@anupnandi3481]

Excellent

[@MD-cd7em]

YES SIRR..

[@adisharr]

If you can read ladder, you can read FBD and vice versa. I don't see any advantage to FBD as it generally takes up more screen space to accomplish the same operation.

[@derina.maleek9408]

Please answer Hello. I'm want to start my journey in learning motion control I think I just need plc and driver and stepper motor I think I don't need a encoder I wondar that I am right or not

[@realpars]

Hello, The encoder is used to give you the feedback of the motor's position so that you can give it a command according to the feedback you are receiving. But take care of the resolution of the encoder you are choosing. For your application, normally there is no need for a high-resolution encoder. Thanks

[@_Iokanaan_Marusidze_]

Thanks !

[@realpars]

You are welcome!

[@jermanskingsly7773]

nice sir,thanks please can you make a video about encoders ?

[@realpars]

Hi there, thanks for the tip! I will pass your request on to our creator team. Happy learning!

[@jianbinchen1428]

I think they are basically the same, only with the difference in terms of the vision

[@ArunKumar-sg6jf]

bro please tell best book for plc and industrial automation

[@realpars]

Hi Arun, We don't have any specific books that we would truly recommend at the moment. However, maybe other students over here might have suggestions :). Happy learning.

[@cyrox1859]

step seven in seven steps by c.t.jones

[@filipkramaric6636]

Is there a way to use function blocks instead ladder logic with s7-300,1200,1500 on tia portal?

[@MaNemUmar]

Filip Kramarić if im not mistaken, step 7 allowed the user to alternate between ladder logic and other languages such as function block diagram and structured list. So yes, a PLC program MAY be written entirely in a language other than ladder logic

[@ziborgbe]

As i know 1200 & 1500 plc support FBD I am not sure about s7 300.

[@MaNemUmar]

ziborgbe it does.

[@filipkramaric6636]

@@MaNemUmar I have seen that it has some variation of FBD but since I have used LOGO 8! I was wondering if it is possible to use that kind of FBD? The FBD in step 7 is some kind of ladder logic with function blocks which is something I don't personally like. Wish I could program s7 in the LOGO comfort.

[@cyrox1859]

when creating an FB, fC or On you have the chose betwin ladder, fbd, and instruction list,

[@charliechimples]

Well explained, thanks for sharing. 🐵

[@realpars]

You're very welcome!

[@mr.lubynets]

Realy cool

[@realpars]

Thanks!

[@derina.maleek9408]

Hi I want buy Siemens 1200 ac/dc/rly CPU. Wondar that I need a memory card or not

[@zorro4622]

Hi! Basically, you don't have to buy a memory card for the S7-1200 series, because it has a built-in program memory (work memory). But if you want, you can extend the CPU with a memory card, but it's used to store project on that or do a password reset or update the firmware. And be careful, because normal SD cards won't work, only special formatted Siemens cards (and never format these cards from PC, because you'll brick the card!). But if you want to start programming with it, you'll be happy without having a memory card. :)

[@derina.maleek9408]

@@zorro4622 thank you

[@szwecmylove]

The volume on the voice fades in and out on each section and it is hard to hear sometimes

[@gerbawyasgat4869]

how can i get the software

[@realpars]

Hi Gerbraw, We currently only have the following links to download different versions of STEP7 sie.ag/2eH6GEM sie.ag/2Jt80F0 Hopefully, that is helpful. Happy learning! p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; line-height: 17.0px; font: 12.0px Helvetica}

[@abdulazeemadesanya825]

Please help!!! Does anyone know how to export a Function Block Diargram in Codesys as an XML file and open it on PLC open or simply export it to PLC open? Your help will be much appreciated. Thank you.

[@realpars]

Abdulazeem, what CODESYS editor are you using? Typically, the export tool is in the PROJECT menu item or you can do a "save as" and select the file type as Project which saves the file in XML. In the CODESYS 3.5 SP1 editor, you can also select "Export as PLCOpenXML" from the PROJECT menu. Repeat the process at the other device, but use the Import tool.

[@abdulazeemadesanya825]

​@@realpars Thank you for your quick response. I use version 2.3. I have tried both methods it wouldn't work. The PROJECT menu has EXPORT as an option but it only saves files as an *EXT file. What I am trying to achieve is to Export the file to a PLC open XML file or if I can covert the Ext file which already has all the FBD program as a PLC open XML file. Your help will be much appreciated. Thank you!

[@prasenjitdeb9139]

I would still prefer ladder logic over fbd

[@jothain]

Exact opposite for me. It's much easier to simulate and visualize functions imo in fbd :)

[@phuciendj1572]

😍😍😍😍😍