There are many brands of interface available to connect a PC to an amateur radio transceiver. Often home-brew interfaces are used. They all do essentially the same thing.
They key the radio to transmit, pass a signal based on the mode of operation from the PC to the radio, then un-key to go back into receive mode.
Some automatically bypass the microphone during transmission but allow the operator to override the computer simply by keying the mike. Links to suppliers of some interfaces are below. They generally work with any PC soundcard based software, digital or analogue.
Sites about soundcard interfacing
- Understanding soundcard interfacing By Ernie Mills WM2U
- Digimaster from ZLP Electronics. A variety of interfaces and connecting cables are available.
- Donner’s digital interfaces. Various models available for specific rig manufacturers.
- KK7UQ interface. Available in kit form.
- Microkeyer USB interface from Microham. Other models available.
- PCInterfaces In both kit and pre-constructed form.
- QDG Kit Construction project from the Queensland Digital Group 
- Rascal GLX from Bux CommCo. Available in kit or constructed form. A variety of cable options are also available from the site.
- Rigblaster from West Mountain Radio. Has interface models with both USB and RS232 DB9 connections. Includes inbuilt rig control capability.
- RigExpert USB interfaces and MixW software.
- SignaLink from Tigertronics. Two models available, one that used the computer soundcard and DB9 connection, the second has a built in soundcard and connects via USB
- W9KFB PSK Software, Interface, and cable specifically designed for operation with HamStationUltra software and a Kenwood TS-2000/TS-B2000 transceiver for a one-stop PSK solution.
- VA3TO linking interface. Operates under EchoLink software and doubles as a soundcard interface for various digital modes.
Interfaces can be fairly simple and thus are easily home-brewed. Here are 2 schematics. The first one is for PCs with a serial port being used for PTT, the second one for an Asus Eee netbook. It uses a VOX circuit for PTT.
Tips for using interfaces
Using the signaling USB with and ACER Aspire ONE for WSPR and MMSSTV / MMVARI File:Signalink.pdf