IF … THEN … ELSEIF … ELSE … ENDIF and attribute quality

When an attribute value is copied to another attribute of the same type, the attribute’s quality is also copied. This can be especially relevant when working with I/O attributes. For example, the following two statements copy both value and quality:

me.Attr2 = me.Attr1;

me.Attr2.value = me.Attr1.value;

If only the value needs to be copied and the attribute has the quality BAD, you can use a temporary variable to hold the value. For example:

Dim temp as Integer;

temp = me.Attr1;

me.Attr2 = temp;

If there is a comparison such as Attr1 <> Attr2 and one of the attributes has the quality BAD, then the statements within the IF control block are not executed. For example, assuming Attr1 has the quality BAD:

if me.Attr1<> me.Attr2 then

me.Attr2 = me.Attr1;

endif;

In this script, the statement me.Attr2 = me.Attr1 is not executed because Attr1 has the quality BAD and comparing a BAD quality value with a good quality value is not defined/not possible.

The recommended approach is to first verify the quality of Attr1, as shown in the following example:

if(IsBad(me.Attr1)) then

LogMessage("Attr1 quality is bad, its value is not copied to Attr2");

else

if me.Attr1<> me.Attr2 then

me.AttrA2 = me.Attr1;

endif;

endif;

An alternative method of verifying quality is to use the "==" operator:

if Me.Attr1 == TRUE then

Or, you can add the "value" property to the simplified IF THEN statement:

if Me.Attr1.value then

Your scripts will execute correctly if you verify the data quality using any of the above methods.