Lenze DDS v2.0 - Manual (Page 13)

Introduction to IEC 61131-3 programming
General language elements
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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(Output), 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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