Hi all

There has been some confusion about when temporary variables are automatically initialized, i.e. reset to their default values at the start of the block code.  Please note that this happens in only one scenario, which is:

• PLC is S7-1200 (FW 4+) or S7-1500
• The block is optimized
• The variable is elementary (e.g. BOOL, INT, REAL, DINT).  A PLC Data Type is an example of a complex (i.e. non-elementary) variable. 

In all other scenarios, variables are not automatically initialized and may contain junk data if not initialized in code. 

If you are using non-initialized temporary variables of a PLC Data Type to reset other variables, this will work until it doesn't.  It is safer to initialize them element-by-element, or with a stat variable containing the required default values.

Siemens' official recommendation is to always initialize temporary variables in code:

https://support.industry.siemens.com/cs/document/109478084/

Section 8, SE002 Rule: Initialize temporary variables:

Justification: As only elementary data types are initialized by the system, all others have an undefined value, which can cause unexpected program behavior