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Ladder logic symbols
29 PLC ladder diagram symbols: contacts, coils, rungs and branches, timers and counters, and compare, move and logic blocks, drawn to IEC 61131-3 with the Allen-Bradley instruction names. Free to download as SVG, or as a printable chart.
How to read a ladder diagram
Read each rung from left to right: conditions on the left, the action on the right.
- Contacts: normally open (XIC) and normally closed (XIO), plus edge contacts that act for one scan.
- Logic: contacts in series are AND, parallel branches are OR.
- Coils: the output coil (OTE) follows the rung; set and reset coils latch and unlatch.
- Blocks: timers, counters, comparisons and moves sit on the rung like contacts or coils.
The full walkthrough, with a start-stop rung, is on how to read ladder logic.
Which standard ladder symbols follow
IEC 61131-3 defines ladder diagram and its standard blocks, and almost every PLC follows its graphics. For the wiring the PLC controls, see the electrical symbols; for interlock logic gates, the logic gate symbols.
Using these symbols
Every symbol on this page was drawn by Deskely. You are free to use them in your own drawings, documents and training material.
Frequently asked questions
What are ladder logic symbols?
The graphic elements of a PLC ladder diagram: contacts that read bits, coils that write them, the rails and rungs that connect them, and function blocks such as timers and counters.
What is the difference between XIC and XIO?
XIC (examine if closed) is a normally open contact: true when its bit is 1. XIO (examine if open) is a normally closed contact: true when its bit is 0. They are the Allen-Bradley names.
What does a latch coil do?
A set or latch coil (OTL) turns its bit on and leaves it on when the rung goes false, until a reset or unlatch coil (OTU) turns it off.
What is the difference between TON and TOF?
TON delays turning on: Q comes on after the preset once IN is on. TOF delays turning off: Q stays on for the preset after IN goes off.
Are ladder symbols the same in every PLC?
Almost. The graphics follow IEC 61131-3 in nearly every brand; Allen-Bradley uses its own instruction names, such as XIC, XIO, OTE and MOV.
Can I use these symbols in my own drawings?
Yes. The symbols on this page were drawn by Deskely and are free to use in your own drawings, documents and training material.
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