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 represents 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
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:
2D CAD files. Symbol as a block.
For use in documents and illustrations
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:
Ladder Diagram Coil Symbols:
Ladder Diagram Cheat Sheet with All Symbols:
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 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.
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:
–| |– or –! !–
–|/|– or –!/!–
Positive Transition-Sensing Contact
–|P|– or –!P!–
Negative Transition-Sensing Contact
–|N|– or –!N!–
… and the same with the coil symbols:
SET Latch Coil
–(S)– or –(L)–
RESET Latch Coil
–(R)– or –(U)–
Positive Transition-Sensing Coil
Negative Transition-Sensing Coil
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.