Reset mode on SM-T300 (printer)
To change the settings on the printer, you need to hold down the feed and power button at the same time until it enters setup mode (about 6 seconds). Once in setup mode, press the power button to go through the modes. The three (possible) modes are: Bluetooth, UART, and Bluetooth (iAP). Once you’re on the mode you want, (Bluetooth) press both buttons down again (about 6 seconds) to confirm the setting.
manual linked below
Setting the Power Down:
You may need to ensure your time out (power down) is set properly. It can be set as no power down in use, or power down anywhere between 1 minute to 5 minutes.
To check your setting, turn the printer on, then press the power button first and then keep it down while you press and hold the feed button until you see the screen change to the Settings mode and will be backlit. (about 5 seconds)
You can then step through the settings using the feed button. Step through until you see PWDN X min off (where X will be the number of minutes to power down) you can change the setting by pressing the power button to step through the different times.
Once you have selected your desired wait time before power down press both the power button as well as the feed button. The printer will print out a short summary of your settings and go back into standby mode.
Reference Pages & Links
Star Settings Application
Android SDK Reference