IEC 61131-3 Quick Reference
A handy one-page cheat sheet for the standard.
The Five Languages
| Language |
Acronym |
Style |
Typical use |
| Ladder Diagram |
LD |
Graphical (rungs) |
Discrete logic, interlocks |
| Function Block Diagram |
FBD |
Graphical (blocks) |
Process control, reusable functions |
| Instruction List |
IL |
Textual (low-level) |
Legacy — deprecated in 2013 edition |
| Structured Text |
ST |
Textual (Pascal-like) |
Math, loops, complex conditions |
| Sequential Function Chart |
SFC |
Graphical (steps) |
Sequences, batch, recipes |
Data Types
Elementary
| Type |
Bits |
Range |
BOOL |
1 |
TRUE / FALSE |
BYTE |
8 |
0 to 255 |
WORD |
16 |
0 to 65 535 |
DWORD |
32 |
0 to 4 294 967 295 |
LWORD |
64 |
unsigned 64-bit |
SINT |
8 |
–128 to +127 |
INT |
16 |
–32 768 to +32 767 |
DINT |
32 |
–2.1B to +2.1B |
LINT |
64 |
signed 64-bit |
USINT, UINT, UDINT, ULINT |
unsigned variants |
|
REAL |
32 |
IEEE 754 single |
LREAL |
64 |
IEEE 754 double |
TIME |
32 |
duration |
STRING |
variable |
character string |
Derived
ARRAY[1..10] OF INT — arraySTRUCT…END_STRUCT — record- User-defined types via
TYPE…END_TYPE
Program Organization Units (POUs)
| POU |
Has memory? |
Returns value? |
| PROGRAM |
Yes |
No (instance) |
| FUNCTION_BLOCK |
Yes |
No (instance) |
| FUNCTION |
No |
Yes (single value) |
ST Operators (precedence high → low)
() parentheses- Function call, exponentiation
**
NOT, unary -*, /, MOD+, -<, >, <=, >==, <>AND, &XOROR
ST Control Flow
IF cond THEN ... ELSIF cond THEN ... ELSE ... END_IF;
CASE expr OF
1: ...
2, 3: ...
ELSE ...
END_CASE;
FOR i := 1 TO 10 BY 1 DO ... END_FOR;
WHILE cond DO ... END_WHILE;
REPEAT ... UNTIL cond END_REPEAT;
Standard Function Blocks
| Block |
Purpose |
TON |
On-delay timer |
TOF |
Off-delay timer |
TP |
Pulse timer |
CTU |
Up counter |
CTD |
Down counter |
CTUD |
Up/down counter |
R_TRIG |
Rising-edge detector |
F_TRIG |
Falling-edge detector |
RS |
Reset-dominant flip-flop |
SR |
Set-dominant flip-flop |
SFC Action Qualifiers
| Qualifier |
Meaning |
N |
Non-stored — active while step is active |
S |
Set — latches on |
R |
Reset |
L |
Time-limited |
D |
Time-delayed |
P |
Pulse — single scan |
SD |
Stored and time-delayed |
DS |
Delayed and stored |
SL |
Stored and time-limited |