I have built a rig interface that allows an Icom IC-718 radio to be controlled by a PC. I have used this interface with a PC with the Debian Linux distribution installed. The hamlib library, along with the gmfsk application, implements a powerful and simple PSK31 mode radio tranceiver.
I've seen some other interfaces for the '718 that will apply excessive voltage to the ttl level CI-V input to the icom. Beware of this and double check that proper voltage levels are used in whatever radio interface you choose to use.
I made the design decision to wire all interface cables directly to the interface unit. Note that the interface is designed to use the line level inputs and outputs of the sound card. They interface perfectly with the signal levels available on the IC-718 ACC connector. My experience is that the many small plugs and jacks simply add more points of failure. Leaving the extra jacks off the unit also saves a few dollars since you no longer need both the jack on the interface box, as well as the plug on the cables.
Keeping the costs low, I used transformers salvaged from ISA modem cards.
The rig connectors are for the CW key, the "ACC" signals, an external speaker, and the CI/V data (left to right on the bottom).
Once the interface is built and connected, and the Debian Linux 4.0 installed, start the audio mixer application and set the capture device to line, and adjust the PCM and Volume controls to about 80-90%. Then, start up the gmfsk application and select the "use hamlib" option with PTT on RTS. Even better, set up the IC-718 for VOX. This works very well.
The current frequency set on the IC-718 should show up on the waterfall display. You can also have the mode and frequency sent in a QSO by using the macro send capability of the gmfsk software.
This is the audio signal schematic of the interface:
This is the control signal schematic: