Issue
The IEC61131-3 standard dictates that a function block should be able to both read from and write to VAR_IN_OUT class pins.
When I am creating a Function Block in EcoStruxure Machine Expert HVAC for the Modicon M172 PLC, and in my function block, I am writing to a VAR_IN_OUT pin, the value of the variable is not changed when I run the program.

Product Line
EcoStruxure Machine Expert - HVAC

Cause
By default, VAR_IN_OUT in EcoStruxure Machine Expert HVAC works by Value and not by Reference.

Resolution
In order to enable it to function by reference you have to enable the proper option in Application > Project Options menu.

In the Project options dialog popup, navigate to the Code generation tab.

Tick the checkbox row named "VAR_IN_OUT by reference (IEC standard=yes)" shown below.
(Note that it is not set to ticked by default)

Released for: Schneider Electric Hong Kong