A method of calibrating a digital data writing system (20), the digital data writing system comprising a plurality of wireless communication devices (30) configured to write digital data to a first plurality of wireless communication chips (7) provided in a printed sheet (10), the method comprising: providing, to the digital data writing system (20), configuration data relating to the locations of each of the first plurality of wireless communications chips (7); selecting each wireless communication device (30) in turn and identifying, for the selected wireless communication device (30), a corresponding one of the first plurality of wireless communication chips (7) that is located in closest proximity to the location of the wireless communication device (30) within the digital data writing apparatus (20); and determining, for the selected wireless communication device (30), the relative location of the device within the digital data writing system (20).