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Introduction to IEC 61131-3 programming

General language elements

l DDS-IEC61131 EN 2.0

4.6 Variables

IEC 61131-3 defines five different variable classes:

• Global variables

• Local variables

• Input variables

• Output variables

• Input and output variables

Local variables do not have a connection to the outside, i.e. they can only be addressed from within

a program part; global variables can be addressed from all POUs.

Input, output and input/output variables are related to a program, function or function block. They

can be changed by reading and writing within the assigned POU; outside the POU, the change must

be defined (input, output and input/output).

The variables are declared between the keywords VAR and END_VAR in the source text. In general,

every variable is initialized after a cold restart. The default value is usually 0 or FALSE. A user-specific

initialization with another value is possible with the sign ”:=” in the declaration.

Variable attributes

The following attributes can be used additionally when declaring a variable:

• RETAIN: These variables remain the same even after a power failure. The program continues

with the values saved when being restarted.

• CONSTANT: Variable values cannot be changed.

• AT: Variables have a fixed location in the memory map (fixed address).

Example: Declaration of an output variable with initialization value

VAR_OUTPUT

par_out1 : INT := 10; (* Output parameter 1 with start value 10

END_VAR

Fixedly addressed variables

Variables can be assigned to a physical memory location (PLC) by means of the keyword AT when

being declared.

The address is indicated as a special character sequence. The character sequence starts with a

percentage sign ”%” followed by a range prefix and a prefix (data type) for size and ends with a digit

sequence which indicates the memory location.

Range prefixes: I (Input), Q(Outp ut), M(Marker, internal memory range)

Size prefix: X (Single bit), B(Byte, 8 bits), W(Word, 16 bits), D(Double word, 32 bits)

Examples: %QX1.0.2 Output bit 2

%IW1.0.1 Input bit 1

%MB7 Marker byte 7

%MW1 Marker word 1

%MD3 Marker double word 3

%MX1.2 Third marker bit in marker word 1

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