1. A method of retrieving pen pressure for transmission, the method comprising: periodically sampling an electrical signal representing the pen pressure outputted from a pressure sensor disposed on a pen device when writing with the pen device, in response to an ON signal from a micro-switch disposed on the pen device when the pen device touches a writing surface; receiving digital data of pen pressure converted by an A/D converter, wherein a micro-controller compresses and encodes the digital data of pen pressure outputted from the A/D converter from a first number of bits of digital data to a second number of bits of compressed and encoded digital data that is less than the first number of bits; and outputting the compressed and encoded digital data of pen pressure via a plurality of light sources on the pen device each with a different color, wherein each of the second number of bits of compressed and encoded digital data corresponds to a different one of the plurality of light sources on the pen device each with the different color.