Hi. Try turning the key to P (program) mode. Turn the power off at the wall for 5 seconds and then back on. Turn key back to R (register) mode.
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If anyone needs a Geller SX-580 manual, one of the users has it here:
http://the-checkout-tech.com/manuals/cashregisters/Towa_SX-580_operating_programming_manual.html
Registerman, December 2013