Variable declarations

This is the web edition of the original ‎⮫ AC500-S safety user manual, version 1.3.2. This web edition is provided for quick reference only. The original safety user manual must be used to meet functional safety application requirements.

Of the variable types and attributes defined in IEC 61131-3 the following are suitable for creating safety 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 identifying global variables with a prefix such as "G_" or "GS_" (for safety variables).)
VAR_EXTERNAL	Declaration of global variables used in the block	Yes
AT	Variable address allocation	To a limited extent⮫ “Direct addresses”
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:

Only one block of declaration type (e.g., VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR_GLOBAL and combinations with CONSTANT) per component
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
Local variables (VAR) should always have a different name. Obscuring of global variables through local variables must be avoided.