how can i fix samsung ser-6500?

That means no one is logged on to make sales on the register. you need to use your clerk button to log on. Some versions have clerk letters, so if you have buttons just marked 'A' or 'B' or 'C' etc then they are one touch logons, so just press one to sign in. If you have a button marked CLERK or CLERK# or CLK# then you sign in by PIN number, so try the default: press 1 then CLERK

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If anyone needs more information, the Samsung SER-6500 manual is here:

http://the-checkout-tech.com/manuals/cashregisters/Samsung2.php

Registerman
February 2011