4.4.3.4
Variable declarations
Of the variable types and attributes defined in IEC 61131-3 the following are suitable for creating safety-ori-
ented applications:
Keyword
Description
Suitable (Yes / To a limited extent /
No) (comment)
VAR
Local block variable
Yes
VAR_INPUT
Block input parameter
Yes
VAR_OUTPUT
Block output parameter
Yes
VAR_IN_OUT
Block reference parameter
To a limited extent (To illustrate the
side effect the parameter should be
identified with a prefix. Even better
would be to use an input and output
parameter instead.)
VAR_GLOBAL
Global variable
Yes (We strongly recommend identi-
fying global variables with a prefix
such as "G_" or "GS_" (for safety-ori-
ented variables).)
VAR_EXTERNAL
Declaration of global variables used in
the block
Yes/optional (not required in CoD-
eSys)
AT
Variable address allocation
Yes (see next chapter)
CONSTANT
Declaration as constant (no write
access possible)
Yes (We recommend to declare each
constant explicitly.)
RETAIN
Variable value is preserved after switch-
off
No, not supported
PERSISTENT
Variable value is preserved after
reloading
No, not supported
In the interest of better readability the following rules should be followed for the declaration of variables:
n
Only one block of declaration type (e.g., VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT,
VAR_GLOBAL and combindations with CONSTANT) per component
n
Only one variable declaration per line with informative comment
Bad:
VAR
A, B, C: BOOL; (* several variables *)
END_VAR
Good:
VAR
A: BOOL; (* first variable *)
B: BOOL; (* second variable *)
C: BOOL; (* third variable *)
END_VAR
n
Local variables (VAR) should always have a different name. Obscuring of global variables through local
variables must be avoided.
Configuration and programming
CoDeSys Safety programming guidelines > Language-specific programming guidelines
30.03.2017
AC500-S
213
