His custom circuit board takes that sound in through the headphone jack and demodulates it for use on the serial port of a PC. This connection ... he unplugs the audio cable for a few seconds ...

[Erik] enjoyed the gift, but since his friend hadn’t sent the necessary serial cable ... Apple MiniDin8 Male cable, he cut off one end and attached the wiring to a RJ45 connector.

A design is one any user is familiar with, including the familiar two square holes in its side. So what purpose do the holes ...