PE is usually a printer error. It would indicate that the printer cannot turn for some reason. Make sure there are no paper or ribbon jams in the printer and check that the gears can turn (if you can get to them).
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If anyone needs / looking for a Samsung ER-5100 manual, try here:
http://the-checkout-tech.com/manuals/cashregisters/samsung1.php
Registerman
July 2010