How do I fix a ram error on my ER-A771 cash register?

Hi Di. It sounds like you have corrupted data in the machine. This normally happens when the machine has been left unplugged for a long period or the internal backup battery has failed. It normally means you have lost all of the programming and it will need the battery checked. Best to take it to a dealer to be tested.
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If anyone needs more information, the Sharp ER-A771 manuals are here:

http://the-checkout-tech.com/manuals/cashregisters/Sharp9.php

Registerman
May 2012