The String tag in the Logix processor is actually two separate tags.
The .LEN tag is automatically updated buy the PLC and the Com Server as data is changed. It contains the number of characters (up to 82) that comprise the string.
The String maximum length is set for the tag but the data is dynamic length. To minimize the maintenance and communications overhead, not all the characters are overwritten
Data empty to start.
LEN is 0
Data has string 20 characters in length written so it now contains twenty ascii characters.
LEN is 20
Data has string 10haracters in length written but it still contains twenty ascii characters.
LEN is 10 and only the first 10 of the available 20 characters are sent in Data Tag
You do not write to .LEN, only the PLC and the Com server update it.
The .DATA tag is the actual variable length string data.
In your tags in Xpress or in InteractX only the tag name is imported and before you can use it in the application you need to add:
.DATA/82 if you have the max length set in the processor. Otherwise set it to the size limit in PLC 1-82 valid