Ladder Logic Symbols – All PLC Ladder Diagram Symbols

Think of ladder logic symbols as the essential pieces of a puzzle in ladder diagrams. This guide covers all these crucial symbols, as defined in IEC 61131-3, and offers them for download in various formats, including a handy PDF.

Understanding ladder logic symbols is key—they each stand for a specific instruction, like a secret code. These symbols, often called bit logic instructions, are tailored for ladder diagrams, a programming language that speaks directly to boolean signals.

Standard IEC Symbols for Ladder Diagrams

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.

You can locate all these symbols in the IEC 61131-3 standard, the rulebook for ladder diagram programming. However, accessing this standard can be costly if you only need the symbols.

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 | DWG | PNG

NC Contact | DWG | PNG

Positive Transition-Sensing Contact | DWG | PNG

Negative Transition-Sensing Contact | DWG | PNG

 Ladder Diagram Coil Symbols:

Coil | DWG | PNG

Negated Coil (NOT Coil) | DWG | PNG

SET Latch Coil | DWG | PNG

RESET Latch Coil | DWG | PNG

Positive Transition-Sensing Coil | DWG | PNG

Negative Transition-Sensing Coil | DWG | PNG

Ladder Diagram Cheat Sheet with All Symbols:

IEC Ladder Logic Symbols | DWG | PDF | PNG

Variations in Ladder Logic Symbols

While IEC standards regularize ladder logic symbols, expect some variations. The PLC programming software you use might display these symbols a bit differently.

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 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.

These contacts serve to detect any shifts in the input signal, termed as positive or negative edges in electronics. By observing these edges over time, it’s evident why they’re aptly named.

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 of 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 were made one line after another with ASCII characters as building blocks.

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

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.