Epson GT-6500 EN - Manual (Page 162)

NAK Abnormal response (negative acknowledge)
Format: B1 B2 B3 B4 B5 A5
ASCII: NAK
Decimal: 21
Hexadecimal: 15
Explanation:
This code is used by the scanner for handshaking. This code indicates
that a command or a parameter was illegal and not accepted by the
scanner.
When the NAK code is returned from the scanner, it means a
command error has occurred in the scanner, and the command was
ignored. To recover from a command error, resend a legal command
or a command followed by a legal parameter.
3-68 Command Reference