[ LADDER LOGIC SYMBOLS · STANDARD ]
IEC 61131-3 and ladder diagram symbols
IEC 61131-3 is the international standard for PLC programming languages. Ladder diagram (LD) is one of its graphical languages, alongside function block diagram (FBD), structured text (ST) and sequential function chart (SFC). It defines the contacts, coils and power rails, and a set of standard function blocks such as timers, counters and edge detectors.
- Full name
- IEC 61131-3, Programmable controllers, Part 3: Programming languages
- Current edition
- Edition 4, published May 2025
- Languages
- Ladder diagram (LD), function block diagram (FBD), structured text (ST), sequential function chart (SFC); instruction list (IL) was dropped in edition 4
- Standard blocks
- Timers TON, TOF, TP; counters CTU, CTD, CTUD; edge detectors R_TRIG, F_TRIG; bistables SR, RS
Contacts IEC 61131-3
About contactsCoils IEC 61131-3
About coilsRungs and branches IEC 61131-3
About rungs and branchesTimers and counters IEC 61131-3
About timers and countersCompare, move and logic blocks IEC 61131-3
About compare, move and logic blocksIEC names and Allen-Bradley names
Siemens, Schneider, Beckhoff, CODESYS-based PLCs and most others follow the IEC 61131-3 names. Rockwell Automation (Allen-Bradley) draws ladder logic the same way but uses its own instruction names:
| IEC 61131-3 | Allen-Bradley | What it does |
|---|---|---|
| Normally open contact | XIC | Passes power when the bit is 1 |
| Normally closed contact | XIO | Passes power when the bit is 0 |
| Coil | OTE | Writes the rung result to the bit |
| Set coil (S) | OTL | Latches the bit on |
| Reset coil (R) | OTU | Unlatches the bit |
| R_TRIG, P contact | ONS, OSR | One scan on a rising edge |
| TON, TOF | TON, TOF, RTO | Timers (RTO is retentive) |
| CTU, CTD | CTU, CTD | Counters |
| EQ, NE, GT, GE, LT, LE | EQU, NEQ, GRT, GEQ, LES, LEQ | Comparisons |
| MOVE | MOV | Copies a value |
How a ladder program runs
The PLC reads its inputs, solves the rungs from top to bottom and each rung from left to right, writes its outputs, and repeats. That cycle is the scan, typically a few milliseconds. See how to read ladder logic.
Frequently asked questions
What is IEC 61131-3?
The international standard for PLC programming languages, including ladder diagram, function block diagram, structured text and sequential function chart.
Is ladder logic part of IEC 61131-3?
Yes. Ladder diagram (LD) is one of its graphical languages, with standard contacts, coils and function blocks.
Do all PLCs use the same ladder symbols?
The graphics are almost the same everywhere. The names differ: Allen-Bradley uses XIC, XIO, OTE, OTL and OTU where IEC says contact, coil, set and reset.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016
The symbols on this page were drawn by Deskely and are free to use. Standards are revised and every project issues its own legend sheets, which take precedence.