Ladder Logic Symbols – All PLC Ladder Diagram Symbols

Ladder logic symbols are the basic building blocks for ladder diagrams. Right here you will find all the ladder diagram symbols which are described in IEC 61131-3. The symbols are available for download in all formats and in a PDF-file.

It’s absolutely crucial to know about the symbols when you are working with ladder logic. Each symbol represent a certain ladder instruction. In ladder logic, these symbols are also known as bit logic instructions. The reason for this is that the programming language ladder diagram is designed for boolean signals.

 

Standard IEC Symbols for Ladder Diagrams

Ladder Logic Symbols

Ladder Logic Symbols

 

The ladder diagram graphical programming language is standardized by the PLCopen organization, and thereby the symbols used in ladder diagrams. Since ladder logic is a graphical programming language, the PLC programs written in ladder logic are a combination of ladder logic symbols.

All the symbols can be found in the standard defining ladder diagram programming: IEC 61131-3. But, those standards are quite expensive if you are just looking for the symbols used in ladder diagrams.

That’s why I have made all the ladder logic symbols available for download.

You can download the symbols as:

  • DWG-files
    2D CAD files. Symbol as a block.
  • PNG-files
    For use in documents and illustrations
  • PDF-file
    Printable cheat sheet with all symbols

 

Finally, all the symbols will be available as one file containing all the ladder diagram symbols from IEC. You can use this as a ladder logic cheat sheet to learn the basic symbols (bit logic instructions) of ladder logic.

 

IEC 61131-3 Ladder Diagram Symbols for Download

 

Ladder Diagram Contact Symbols:

NO Contact Ladder Diagram Symbol

NO Contact | DWG | PNG

NC Contact Ladder Diagram Symbol

NC Contact | DWG | PNG

Positive Transition-Sensing Contact

Positive Transition-Sensing Contact | DWG | PNG

Negative Transition-Sensing Contact

Negative Transition-Sensing Contact | DWG | PNG

 

 Ladder Diagram Coil Symbols:

Ladder Diagram Coil Symbol

Coil | DWG | PNG

Ladder Diagram Negated Coil Symbol

Negated Coil (NOT Coil) | DWG | PNG

SET Latch Coil Ladder Diagram Symbol

SET Latch Coil | DWG | PNG

RESET Latch Coil Ladder Diagram Symbol

RESET Latch Coil | DWG | PNG

Positive Transition-Sensing Coil

Positive Transition-Sensing Coil | DWG | PNG

Negative Transition-Sensing Coil

Negative Transition-Sensing Coil | DWG | PNG

 

Ladder Diagram Cheat Sheet with All Symbols:

IEC Ladder Logic Symbols

IEC Ladder Logic Symbols | DWG | PDF | PNG

 

Variations in Ladder Logic Symbols

Although the ladder logic symbols are standardized in the IEC standard, the symbols can vary. Depending on the PLC programming software you are using, you will be presented with variations of the symbols.

But generally speaking the symbols are very similar, and the variations are mostly superficial.

Here are some of the commonly known differences in ladder logic symbols:

SET/RESET or LATCH/UNLATCH
While in a Siemens PLC this is called the SET/RESET function and the symbols are similar to the above, other PLC brands can use another variation of this function. Allen Bradley and some others call the function LATCH/UNLATCH and use slightly different symbols.

The biggest difference is the letters in the coils. While the SET/RESET has an S and and R inside the function coils, the LATCH/UNLATCH can have an L and U inside the coil symbols.

Positive/Negative transition-sensing symbols
Both the contacts and the coil symbols for the positive and negative transition-sensing are often referred to as positive or negative edge contacts or coils. The name probably comes from the electrical engineering aspect of the function of these contacts and coils. As the name of the ladder logic symbols also tells, these functions are made to register either positive or negative changes.

The function of these contacts is to check for a positive or negative change in the input signal. In electronic signals that change is called a positive or negative edge. If you look at the two signals (negative and positive edge) plotted over time below, it is clear why the term edge is used to name those two.

Positive and negative edge signals

 

Variations in symbol dimensions
PLCopen and therefore the IEC 61131-3 standard doesn’t give precise symbols for ladder diagram PLC programming. In fact, they only show symbols made up with ASCII characters like this:

NO Contact
–| |– or –! !–

NC Contact
–|/|– or –!/!–

Positive Transition-Sensing Contact
–|P|– or –!P!–

Negative Transition-Sensing Contact
–|N|– or –!N!–

… and the same with the coil symbols:

Coil
–( )–

Negated Coil
–(/)–

SET Latch Coil
–(S)– or –(L)–

RESET Latch Coil
–(R)– or –(U)–

Positive Transition-Sensing Coil
–(P)–

Negative Transition-Sensing Coil
–(N)–

The reason why they don’t publish exact symbols is that small variations in dimensions and line thickness might occur from brand to brand. One other reason is that in the old days, ladder diagrams where made one line after another with ASCII characters as building blocks.

The early days of the graphical user-interface was just an extension of the command-line and would typically be made up of characters. All the ladder logic symbols where made out of s series of characters. Like this old SIMATIC S5 PG (programmer) from Siemens:

Simatic S5 PG

 

Feel free to use all the DWG, PNG and PDF files on this page. They are available for you to download and use whenever you want. No rights reserved.

9 Comments
Leave a response
  • homauoun
    September 5, 2015 at 7:46 am

    very brief and useful.

  • Ben
    October 6, 2015 at 11:13 am

    Well done. Thank you for this information.

  • Ebuka
    May 4, 2016 at 2:35 am

    Thanks for their information

  • SMB
    August 23, 2016 at 11:56 pm

    Even though it is very basic it is great to be able to simplify things like this again. Going back to basics is never a bad idea, the older I get the more it seems I forget!

  • seenu
    September 25, 2016 at 12:41 pm

    Thanks for posting

    keep on posting

  • U Kyaw Win
    November 21, 2016 at 12:51 pm

    Thanks for post. I wanna learn plc continuosly. Keep posting.

  • dave
    January 8, 2017 at 5:39 pm

    Hello sir,
    I just done my graduation with electronic and I have passion about plc programming especially Siemens .i am so Confused. How to start what software I use so on. Please give me advice so I can do something

  • Colin
    March 23, 2017 at 12:48 pm

    Hi Peter,

    Firstly, thanks for the post. I was wondering if you would allow me to use the IEC Ladder Logic Symbols png as part of an academic paper I’m writing? I will of course give full credit back here.

    Regards,
    Colin.

    • peter
      March 29, 2017 at 9:04 pm

      Hey Collin,

      Thanks for your comment! I’m glad that you find my material useful, and of course you are more than welcome to use the Ladder Logic Symbols in your academic paper. May I ask what the paper will be about?

      Cheers,
      Peter

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