Although it is almost certain that future generations (and the more expensive versions!) of radios will have rig control and digital mode keying built in with just a USB cable to go between radio and computer (such as Yaesu’s FT-991), for previous generations of radios, such as my FT-DX1200, the Yaesu SCU-17 Interface is required to act as that bridge between radio and computer.
There is nothing particularly special about the interface – you could make one yourself at home if you wanted. It consists of 2 USB to Serial ports, a sound card interface and a little bit of wizardry to wire it all together. As for me I’d rather just spend the money to save myself the hours of frustration of putting one of these together (and troubleshooting issues) and end up with a professionally made, compact unit.
However what I wanted to address was the distinct vagueness of documentation around that covers how they operate.