What is Ladder Logic?

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If you are involved in any way with industrial machinery, you almost have to have heard of ladder logic.
Since there are almost never ladders involved, it probably doesn’t make a lot of sense that we call it that, but the reasons for the name are buried in the history of automatic controls.
At one time, automatic systems were controlled with relays. Relays are electromechanical devices and fail quickly as the mechanical parts wear out and the electrical parts are destroyed as the electricity they switch burns away the contacts.
They are big and use lots of electricity, generating lots of heat in the process.
In order to understand why we call it ladder logic, you need to look at how we document relay logic controls.
The actual “ladder” was the drawing of the control logic, an illustration of how the relays were wired together.
Even today, a lot of the conventions that led to “ladders” are still in use, and I will use a recent example drawing to illustrate this.
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[@manny2092]

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Hi Tom, Thanks a lot for your kind compliment! We are always extremely happy to hear such positive feedback! If you ever have any questions, feel free to reach out to us. Happy learning!

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I really appreciate the effort put in this video. This very video made me to click the subscribe button. Trurly worth watching! Keep it up!

[@realpars]

That's great, Muhammad! Thanks for sharing and happy learning!

[@gvenagas]

If I understand correctly, between the physical switches and the make and brake symbols of a plc, the bits mediate. The bits have an initial value of '0', so it coincides with the normally open state of a contact, while the same is not true of the initial closed state of a contact in the ladder diagram, which for any contact to open, the input bit must change from 0 to 1

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Great to hear that! Thanks for your support. Let us know if you have any questions!

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[@ardentdfender4116]

I greatly enjoyed this video. Ladder Logic is the underlying basis of all the automation programing we deal with in every program whether it’s its with Rockwell Automation products or Citect Software. I didn’t start off as a programmer but cane from Operations and being a Technician. I’ve been in automations 5 years now but it’s not only thing I do as it includes Field Service and commissioning of OEM equipment also. But, I’m going to also admit, I’m very slow at reading Ladder Logic as it has to make sense as I’m reading it. My Senoir coworker just zips through it scrolling up and down at lightning speed and I never get how he can read it all so fast. The simple symbol elements I can more easily get. But it seemingly gets very complicated to read and understand when it has the Function Block elements in the ladder to read or understand what exactly it’s saying as well how the bit is programmed especially in ControlLogix which is easier to understand than PLC 500 or the dinosaur of PLC 5. Elements like MOV, LIMS, SQC, EQL and many others as there is a whole chart for all the other ladder rung elements. I honestly think a whole series of videos for ladder logic on how all the common advance Function elements each work or is “read” in a ladder similar to this video could be immensely helpful in learning to read and understand Ladder Logic. My coworker always tells me it’s easy and I know he thinks it is and probably is, but when your not quite clear what it’s even saying as well how that bit is programmed, your not sure how to even read what that programmed bit is saying or doing. Love the video though.

[@rigosandoval282]

Great Information. Thanks

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[@kistamganesh]

Yeah it's great. And I received a lot of information from this channel. And if you provide subtitles then it will be out standing.

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Hi Ganesh, Great to hear that our course videos have been so beneficial to you! We currently only provide our course videos in English, you are always able to turn on the English subtitles as that might make it a bit easier for you. Happy learning

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subtitles are already there, just press CC button in right corner of the video (next to pause/play and volume button)

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Great content👍

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Please upload a video for PLC programming using ''analog input and output''

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Kevin Huffman what is the scope of industrial automation in future.i am a fresher graduate ,I am interested in automation

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Thanks a lot, Kevin! That's an amazing compliment! Happy learning!

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thx

[@lhaellor]

Thankyou

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You're very welcome!

[@jorgeyumarasilemagneliyim2988]

in video time 7:12, there is a little mistake, last rung isn't correct in Ladder logic, M4 coil must be changed by M3 coil (electrical seal); then it matches with control circuit. Also, in first rung normally close contact M3, it shouldn't this be normally open?

[@realpars]

To be honest, the last rung of both the PLC program and the electrical ladder should be deleted. You should never hold a relay on by its own output.

[@dangyeuhiepsi9430]

thank you

[@realpars]

You're welcome!

[@solomonmarfo2968]

Please can you explain more on how to read ladder logic

[@realpars]

Thanks for your feedback, Solomon! I will happily pass this on to our course developers. Happy learning!

[@PbPomper]

I find well structured ST much easier to read; especially for more complex scenario's. For simple stuff Ladder is fine. It's easy for Ladder to become bloated which makes you lose the bigger picture.

[@tl8142]

4 days now with this ladder logic and still it literally confuses me to death. Believe me… i cant understand what and how it wants me to think 😂😂

[@surendrakverma555]

Very good

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You're very welcome!

[@reaksalong8947]

I want knows control system for maintenance in building

[@bookquotes5]

Thanks .......

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[@MaNemUmar]

Can we expect any DCS programming videos in the future?

[@realpars]

Maybe :). I should discuss this with the team.

[@MaNemUmar]

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[@mackanic3]

DeltaV?

[@BAras-ug9zq]

Stop button is normally closed (so input is "1" when not pressed), so the "auto stop' contact on ladder diagram on 5.09 need to be NORMALLY OPEN!

[@realpars]

Hey! When you are using a normally closed switch as the stop switch, you always need to use a normally closed contact for that switch in the PLC program as well. Go ahead and test this with the simulator. :)

[@BAras-ug9zq]

I did it. When stop button is not pressed (so input module gets HIGH or 1), normally closed contact in PLC program blocks. When I press the button, input module gets LOW or 0, NC contact in PLC conducts. This logic needs to be reverse? Or is there something that I do not good?

[@cl6107]

@@BAras-ug9zq you are correct. The stop switch should be wired to a normally closed contact and will be open when the switch is pressed. Then the plc must use a normally open symbol (ie. XIC) which allows the logic to pass through when the contact is closed ("1") and breaks the circuit to stop the Auto Running when the stop is activated ("0").

[@vasanthkumar-zy4yg]

Please make video EMS/ BMS SYSTEM

[@realpars]

Great suggestion, Vasanth! I will happily pass it on to our course developers. Thanks for sharing and happy learning!

[@Cuong...nguyen]

Tuyệt vời!.

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Cảm ơn bạn

[@screwielouiepena5253]

Yea

[@AshutoshSharma-kz3lq]

Awesome

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Thanks for watching!

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Hello there sir do u have any branches here in Philippine?? U guys only give lessons online ?

[@realpars]

Hi Essa, thanks for your comment! We are a 100% online subscription-based learning platform. You can subscribe to our course library through the following link realpars.com/. If you have any other questions, please let me know. Happy learning!

[@23davidian96]

Would the AutoStop not be Normally Open? Because your actual AutoStop contact is normally closed?

[@realpars]

Yes, the AutoStop would typically be a normally-closed contact in the field, requiring a normally-open contact in the logic. For ease of explanation in the video, without getting into the field wiring, we chose to represent it by its logical meaning.

[@jessstuart7495]

There are many applications (power) that still require mechanical relays or contactors for switching. Solid State Relays become cost prohibitive for switching large currents/voltages.

[@realpars]

Thanks for sharing your knowledge and experiences with us, Jess! We appreciate that!

[@goldswaggamer4007]

Is it really this simple or is there something I'm missing? lol Anyways, great video! I'm taking Advanced Manufacturing classes in High School and this helped a LOT.

[@realpars]

Glad it was helpful!

[@albakhati5529]

That’s the core of industrial machinery

[@realpars]

Yes! That's where the magic happens.

[@mohanthomas9050]

SIMPLY SUPERB

[@realpars]

Thank you!

[@prickerbush]

Late to the party. How does the auto-running coil close the auto-running switch? Also, how does the auto-stop switch open the auto-running switch? Thank you.

[@realpars]

In ladder logic, a coil is used to set a memory bit, like Auto-Running (see 4:36 in the video). So when the Auto-Running bit is set ON by the coil, the memory location Auto-Running is true, so the contact Auto-Running is true as well. When the logic indicates that the Auto-Running bit should be OFF (as when the Auto-Stop pushbutton is pressed), the contact Auto-Running will also be OFF. The Auto-Stop bit is a normally closed contact, so when the Auto-Stop switch is pressed or energized, the NC contact goes OFF, or FALSE. This makes the downstream logic FALSE as well, including the coil.

[@prickerbush]

@@realpars great makes sense. Thank you.

[@joeblow1688]

The symbols ,themselves, the narrator shows ( at 4:06 etc.) are not physical "contacts". They are parts of a logical software program ( rungs) that read input values from contacts! look-up "Jim Pytel" videos for true clarity.

[@pologarcia516]

Jim Pytel is the real deal, him alone can rival this whole channel. Favorite channel by far in all of youtube

[@charliechimples]

Well explained, thanks. 🐵

[@realpars]

Thank you!

[@LydellAaron]

Thanks for explaining this so well, and the conventions. As a computer engineer, I'm finding myself going back to learn this more mature tech, in today's heavy robot automation. I see why these PLCs are still very useful, say, over writing a safety motor control program on scratch in Python or C, or creating some FPGA approach.

[@GRant16NCH]

Make vedeo STL language please

[@realpars]

We may consider creating videos on that in the future.

[@ibrahimgalih5163]

Please subtitle indo please thanks

[@realpars]

Hi Ibrahim, Thanks for your comment! We currently only provide our course videos in English, you are always able to turn on the English subtitles as that might make it a bit easier for you. Our apologies for any inconvenience! Happy learning!

[@ibrahimgalih5163]

Thanks you for your every videos im very very support , big thanks from me

[@user-xc4cn7og3s]

🤔 using ladder logic for an elevator, seems logical.

[@TomiBorchert]

make separate video about relays and another for ladder logic, no point making video title lagger logic and then mix all kinda stuff there. first you learn relays in a video, then another video is more advanced which expects viewer to know basics.

[@realpars]

Thanks for your topic suggestion, Tomi. I will happily pass your suggestion and feedback on to our course developers.

[@TomiBorchert]

@@realpars kzbin.info/www/bejne/iH7Th2iIbdyBndk here's a great example. I like this way. 1.xx is basic, 2.xx advanced and so on. 3.xx would be projects where we use those previously learned skills. :)

[@setiawannurli6112]

Please use Indonesian.

[@realpars]

Hi Setiawan, Thanks for your comment! We currently only provide our course videos in English, you are always able to turn on the English subtitles as that might make it a bit easier for you. Our apologies for any inconvenience! Happy learning!