Turn power off at wall.
Turn key to P mode.
Hold SUBTOTAL button down.
Turn power on at wall.
Let go of SUBTOTAL button.
Turn key to R (REG) mode.
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If anyone needs a Sam4s / Samsung ER-420M manual, try here:
http://the-checkout-tech.com/manuals/cashregisters/Samsung_ER-420M_operating_programming_manual.html
Registerman
November 2011