The AF/Key Interface is an audio frequency interface between a computer and a radio, AND a way to key that radio when it needs to transmit. There are a lot of options here. Over the years I have built a number of these interfaces that used GPIO pins or a serial port to key the radio. While these solutions work, they are often messy, convoluted, or need to be customized for every radio. I wanted to find a single cable solution, or as close as I could get to it.
To accomplish this I was hoping to find a reliable supply of USB soundcard modules with a built in serial port. There is a chipset that does this, but finding a devices that has that chipset, that the manufacturer hasn’t disabled the I/O, potted the chip, or is inordinately expensive, has been difficult.
Another factor for this project is the need for the solution that will age well and be easy enough to teach others to build and maintain.
Enter the compromise. I do not like VOX circuits. Especially for data modes. My dislike is mainly towards the switching capabilities of most radios built-in VOX circuits, but it does extend to external devices. The proliferation of the Signalink has proven that a VOX circuit will work and combined with the lack of reliable sources for a preferred option, I will acquiesce and use a VOX circuit in this project.