This is a troubleshooting test used to check the connection between the printer and its host computer or device.
When the printer is in Diagnostics mode, it prints all data received from the host computer as straight ASCII characters, with the hex values printed below the ASCII text. The printer prints all the characters it receives. This includes control codes such as
CR
(carriage return). The test label prints upside-down as it exits the printer.
Typical Communications Diagnostics Test Label
To print this test label:
Ensure media is loaded and printer power is ON.
Set the print width equal to, or less than, the width of the label being used for the test.
Press and hold
PAUSE
() and
FEED
() simultaneously for two seconds.
When active, the Status indicator () alternately lights up Green and Yellow.
The printer enters Diagnostics mode and prints any data received from the host computer or management device on a test label.
Check the test label for error codes. If any errors are shown, verify if the communication parameters are correctly set.
Errors show on the test label as follows:
FE
Framing error
OE
Overrun error
PE
Parity error
NE
Noise
To exit the self-test and return to normal operations, press and hold
PAUSE
() and
FEED
() simultaneously for two seconds, or turn printer power OFF and back ON.
) and 
) simultaneously for two seconds. 
) alternately lights up Green and Yellow.