Iec 61131-3 Second edition 2003-01 Programmable controllers – Part 3: Programming languages

2.4.3 Declaration
Each declaration of a program organization unit type (i.e., each declaration of a program, function, or
function block
, as defined in 2.5) shall contain at its beginning at least one declaration part which
specifies the types (and, if necessary, the physical or logical location) of the variables used in the
organization unit. This declaration part shall have the textual form of one of the keywords VAR,
VAR_INPUT, or VAR_OUTPUT as defined in table 16 a), followed in the case of VAR by zero or one
occurrence of the qualifiers RETAIN,NON_RETAIN or the qualifier CONSTANT, and in the case of
VAR_INPUT or VAR_OUTPUT by zero or one occurrence of the qualifier RETAIN or NON_RETAIN,
followed by one or more declarations separated by semicolons and terminated by the keyword
END_VAR. When a programmable controller supports the declaration by the user of initial values for
variables, this declaration shall be accomplished in the declaration part(s) as defined in this subclause.
Table 16 a) - Variable declaration keywords
Keyword
Variable usage
VAR
Internal to organization unit
VAR_INPUT
Externally supplied, not modifiable within organization unit
VAR_OUTPUT
Supplied by organization unit to external entities
VAR_IN_OUT
Supplied by external entities - can be modified within organization unit
VAR_EXTERNAL
Supplied by configuration via VAR_GLOBAL (2.7.1)
Can be modified within organization unit
VAR_GLOBAL
Global variable declaration (2.7.1)
VAR_ACCESS
Access path declaration (2.7.1)
VAR_TEMP
Temporary storage for variables in function blocks and programs (2.4.3)
VAR_CONFIG
Instance-specific initialization and location assignment.
RETAIN
b,c,d,e
Retentive variables (see preceding text)
NON_RETAIN
b,c,d,e
Non-retentive variables (see preceding text)
CONSTANT
a
Constant (variable cannot be modified)
AT
Location assignment (2.4.3.1)
NOTE 1 The usage of these keywords is a feature of the program organization unit or
configuration element in which they are used. Normative requirements for the use
of these keywords are given in 2.4.3.1, 2.4.3.2, 2.5 and 2.7.
NOTE 2 Examples of the use of VAR_IN_OUT variables are given in figures 11b and 12.
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61131-3 
 IEC:2003(E)