先看看帮助文件吧...
Copying Memory Area with SFC 20 "BLKMOV"

You use SFC 20 "BLKMOV" (block move) to copy the contents of a memory area (= source area) to another memory area (= destination area).

Permissible source areas are the following:

1.Parts of data blocks
2.Memory bits
3.Process-image partition (part process image) for inputs
4.Process-image partition (part process image) for outputs

（SRCBLK：INPUT，ANY类型）<---定长在这里设定

so,SFC20可以实现你要的功能。