Yes SFC20 BLKMOV doesn't accept Strings as valid inputs/outputs, I guess because strings are really arrays of characters (well arrays of character codes anyway).
The strings are in instance DBs and my FB is called more than once.
Actually it will be a pallet station FB called for each pallet conveyor, the relevant info passed in and written out. One part of that block will include copying a string from the previous station to this station as the pallet arrives. For simplicity here I called them MyString1 and MyString 2. So I cannot deal with the DB directly since it will be different depending on which instance is calling it.
The string are both a known fixed length.
"What error do you get?" - It won't accept #mystring1 as input to SFC20.
I'm afraid I come from the ControlLogix world where I would just use the COP or CPS functions which like simple and complex data types and structures.
Maybe I need to point to the start address of the source and destination strings and copy a number of bytes. I'm not sure how Siemens works in this regard.