Ladder Logic Symbols – All PLC Ladder Diagram Symbols

Ladder logic symbols are the building blocks of ladder diagrams. This guide covers the symbols defined in IEC 61131-3 and includes downloads in several formats, including a PDF you can keep for reference.

Each ladder logic symbol corresponds to a specific instruction. Often called bit logic instructions, these symbols are used in ladder diagrams, a programming language for working with boolean signals.

Standard IEC Symbols for Ladder Diagrams

Ladder Logic Symbols

The PLCopen organization standardizes the ladder diagram graphical programming language and the symbols it uses. A PLC program written in ladder logic combines these symbols to form a graphical program.

The IEC 61131-3 standard contains all these symbols and defines ladder diagram programming. Buying the standard can be expensive, though, if you only need the symbols.

For that reason, I’ve made all the ladder logic symbols available to download.

The symbols come in these download formats:

• 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

All the ladder diagram symbols from IEC will also be available together in one file. Use it as a ladder logic cheat sheet while learning the basic symbols, or bit logic instructions.

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

IEC standards define ladder logic symbols, but the PLC programming software you use might display them slightly differently.

The symbols generally look much the same, with differences mainly in their appearance.

Some common differences in ladder logic symbols are described below:

SET/RESET or LATCH/UNLATCH
In a Siemens PLC the function is called SET/RESET, with symbols similar to those shown above. Other PLC brands use variations of the same function. Allen Bradley and some other brands call it LATCH/UNLATCH and use slightly different symbols.

The letters inside the coils are the main difference. SET/RESET uses S and R inside the function coils, while LATCH/UNLATCH can use L and U inside the coil symbols.

Positive/Negative transition-sensing symbols
The contact and coil symbols for positive and negative transition-sensing are also commonly called positive or negative edge contacts or coils. The name probably comes from the way these functions work in electrical engineering. Both names describe functions that register a positive or negative change.

These contacts detect changes in the input signal, known in electronics as positive or negative edges. Looking at the edges over time helps explain the name.

Variations in symbol dimensions
PLCopen, and therefore the IEC 61131-3 standard, does not specify exact symbol dimensions for ladder diagram PLC programming. Instead, the symbols are shown using ASCII characters, as in these examples:

NO Contact
–| |– or –! !–

NC Contact
–|/|– or –!/!–

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

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

The coil symbols follow the same approach:

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)–

Exact symbols are not published because dimensions and line thickness can differ slightly between brands. There is also a historical reason: ladder diagrams were once built line by line using ASCII characters.

Early graphical user interfaces grew out of the command line and were typically made up of characters. Each ladder logic symbol was a series of characters, as on this old Siemens SIMATIC S5 PG (programmer):

You can download and use any of the DWG, PNG and PDF files on this page whenever you need them. No rights reserved.