After calibrating the printer, it will be put into "dump mode", where it sends the hex dump of the data sent to it, rather than translating that hex into the label. This will appear as random pairs of characters.

To exit from dump mode, either press the feed button twice, which has the added benefit of verifying the calibration has been done correctly and that the printer is lined up on the next label to print, or just power the printer off and then back on.