CITIZEN CT-S851 - Manual (Page 40)

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Press and hold the MENU button while the printer is running to be able to
change the memory switch settings.
Activate the key lock to prevent making changes by mistake.
Setting the key lock
To set the key lock, press and hold button 3 (for at least three seconds).
MENU disappears, and the LOCK icon appears.
In this condition, it is not possible to enter the memory switch setting mode
even if you press and hold the MENU button.
Releasing the key lock
To release the key lock, press and hold button 3 (for at least three seconds).
The LOCK icon disappears, and MENU appears.
4.5 Key Lock Function
FEED
Button 3
LOCK icon