The file structure is probably different between the two as the 570 is more powerful. It still might work for PLUs and departments etc so it is probably worth a try if you don't mind wiping the data in the receiving register.
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If anyone needs manuals for the Sharp ER-A550 or ER-A570, c-tech has them here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp7.php
Registerman
January 2011