If the whole machine blinks off then back on in the middle of a sale, it is usually a printer problem. See a dealer to get the printer checked. It happens quite a lot when used in greasy environments.
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If anyone needs more information, the Sharp XE-A102 manual is here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A102_operating_and_programming_manual.html
Registerman
February 2011