Turn the key to REG mode. Press clear (CL). Type some numbers on the numeric pad. can you see the numbers on the display. If not, it probably has a faulty keyboard. Otherwise, make sure you have logged on to the machine.
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If anyone needs a Sharp XE-A301 manual, c-tech collective has them here:
User and programming manual:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A301_operating_programming_manual.html
Service (repair) manual:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A301_service_manual.html
Registerman, January 2011