I faced with the same bottle neck of UDT. I defined a struck tag which contains 16 boolean tags at the input of a FB. I wanted to pass the image of an input word and to the struck bits. I could not write source word address to the input of FB. 

It is same for the outputs. I defined a word/int output at the FB. I can not copy the content of a struck tag(contains 16 bits) to this output as a word.

I do not know whether is it a bottle neck or a bug at TiaPortal v17.

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Split from Physical Input through IN/OUT of the FB via UDT.

Closed cause question posted twice.

The second thread with the subject how to copy 6 word image on tags defined as struck type at static part of a DB is published within the conference STEP 7 (TIA Portal).