IEC 61131-3 languages and ladder logic basics — Industrial Automation — PLC, SCADA, Modbus and OPC UA

3. IEC 61131-3 languages and ladder logic basics

Intermediate★ Must knowLesson 3 of 10

Language	Style	Best for
LD — Ladder Diagram	Graphical relay logic	Discrete on/off logic, electricians can read it
FBD — Function Block Diagram	Graphical blocks	Process control, signal flow
ST — Structured Text	Pascal-like text	Calculations, loops, data handling
SFC — Sequential Function Chart	Steps and transitions	Sequences (batch, machine cycles)
IL — Instruction List	Assembly-like	Deprecated in the 3rd edition (2013)

Classic start/stop (seal-in) circuit:

text

 |  Start      Stop       Overload          Motor  |
 |---| |---+---|/|--------|/|---------------( )----|
 |         |                                       |
 |  Motor  |                                       |
 |---| |---+                                       |

| | normally open contact (true when the bit is 1), |/| normally closed contact (true when the bit is 0), ( ) output coil.
Pressing Start energises Motor; the parallel Motor contact "seals in" so it stays on after Start is released; Stop (wired NC in the field for fail-safety) breaks the rung.

Same logic in Structured Text:

pascal

Motor := (Start OR Motor) AND NOT Stop AND NOT Overload;

Other instructions: set/reset (latch/unlatch), rising/falling edge (R_TRIG, F_TRIG), compare, move, math, jump/subroutine calls.