CODESYS and IEC 61131-3 cheat sheet

CODESYS and IEC 61131-3 addressing cheat sheet

IEC 61131-3 uses symbolic variables declared in sections at the top of each program. Direct addresses with a percent sign exist for located variables, but most CODESYS projects map I/O in the device tree and use names. This sheet lists the address forms, data types, literals and block pins you need to read IEC code.

CODESYS and IEC 61131-3 instruction reference

Direct addresses

Direct addresses
Item	Syntax	Meaning
Input bit	%IX0.0	Byte 0, bit 0 of the input area.
Output bit	%QX0.0	Byte 0, bit 0 of the output area.
Memory bit	%MX0.0	Byte 0, bit 0 of the memory area.
Byte, word, double word	%IB0 %IW0 %ID0	8, 16 and 32 bits starting at byte 0 of the input area. Q and M work the same.
Located variable	Start AT %IX0.0 : BOOL;	Gives a name to a direct address.

Elementary data types

Elementary data types
Item	Syntax	Meaning
Bit and bit strings	BOOL BYTE WORD DWORD LWORD	1, 8, 16, 32 and 64 bits.
Signed integers	SINT INT DINT LINT	8, 16, 32 and 64 bits.
Unsigned integers	USINT UINT UDINT ULINT	8, 16, 32 and 64 bits.
Floating point	REAL LREAL	32 and 64 bits.
Time	TIME	A duration.

Literals

Literals
Item	Syntax	Meaning
Time	T#3s T#500ms T#1h30m	Durations.
Based integers	16#FF 2#1010 8#17	Hexadecimal, binary and octal.
Booleans	TRUE FALSE	The two BOOL values.

Variable sections and unit types

Variable sections and unit types
Item	Syntax	Meaning
Sections	VAR VAR_INPUT VAR_OUTPUT VAR_IN_OUT VAR_GLOBAL	Where a variable lives and how it is passed.
Modifiers	VAR RETAIN VAR CONSTANT	Keep across a warm restart, and fixed values.
Unit types	PROGRAM FUNCTION FUNCTION_BLOCK	A task-run program, a memory-less function and a block with instances.

Standard block pins

Standard block pins
Item	Syntax	Meaning
Timers	TON TOF TP	Inputs IN and PT; outputs Q and ET.
Counters	CTU CTD CTUD	Count inputs CU and CD, reset or load, preset PV; outputs Q (QU and QD for CTUD) and count CV.
Edge blocks	R_TRIG F_TRIG	Input CLK; output Q is true for one call on a rising or falling edge.
Calling a block	T_PUMP(IN := PUMP_REQ, PT := T#3s);	Named inputs, then read T_PUMP.Q afterwards.

In this browser editor

In this browser editor
Item	Syntax	Meaning
Ladder text	\| (START OR MOTOR) AND NOT STOP \| := MOTOR ;	A rung: the condition between bars, := then the coil.
Reading a block output	\| T_PUMP.Q \| := PUMP_RUN ;	Block outputs are read in a ladder-text rung, not on the right of an assignment.
Functions as calls	IS_HIGH := GT(LEVEL, 70);	Comparison and arithmetic are written as function calls here, not infix operators.

What this sheet leaves out

The reset input name on counters, which differs between the standard text and library versions.
Structured data types, arrays and pointers.
Device tree I/O mapping, which replaces direct addresses in most projects.

Always confirm addresses and ranges against the manual for your exact controller and software version. Want to see these addresses used in logic? Read the CODESYS and IEC 61131-3 instruction reference.

Frequently asked questions

What does %IX0.0 mean?

The percent sign marks a direct address. I is the input area, X means a single bit, and 0.0 is byte 0, bit 0. %QX is the output area and %MX the memory area, and B, W and D select a byte, word or double word.

Should I use direct addresses in a CODESYS project?

Most projects map I/O in the device tree and use symbolic names, because it is easier to maintain. Direct addresses with AT still appear in older code and examples, so you need to be able to read them.

How do I write a time value?

Use a TIME literal such as T#3s for three seconds, T#500ms for half a second or T#1h30m for an hour and a half.

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.