G4VSZ and G8JAD have designed a sound card only interface between sound card of your computer and your rig to give data link and PTT.
The Sound card interface 2013.
The idea of this interface came because of the lack of RS232 ports on modern computers which now have USB ports.
The ideas were knocked about initially with John G6SXC and then later with John G8JAD with the introduction of the LM567 chip to extract the keying for FSK for rigs that has the need of such and input. Setting up the Demodulation of the FSK keying can be a bit tiresome and it has been found from experience that adjustment when transmitting into a dummy load and seeing the tone recovered on a separate Rx and interface to be the best route.
All rigs need a PTT switching facility and this started out as a transistor version but in the latest version is a LM358 comparator chip. This chip detects when there is audio on the output from the sound card and thus switches the transmitter into Tx. The circuit also provides for a display to indicate when the Tx line operates. So that you can still have control over the interface and when it keys the Tx line there is an inhibit switch to stop the Tx line being activated. This is also most important should the computer program and sound card lock up in Tx you can then switch off the Rig Tx line and stop the transmission.
The remainder of the circuit is to provide complete isolation between the rig and the computer. To achieve this two audio line transformers are used with level set presets of 10k in each line so that the rig and computer cannot be over driven.
The rig needs to provide audio output preferably not affected by the vol control on the front of the rig else the preset is of no use as one would have to set it for every change in vol level. Similarly the rig needs an audio input not through the mic socket but direct into the rig so again the mic gain control has no effect on the signal but is set but the audio from the sound card which can remain fixed and the 10k preset.
The interface can be used for FSK using the LM567 circuit and activating the switch which selects FSK or AFSK input to the rig. FSK is only used for keying RTTY if the rig is suitable equipped. All data modes can use the AFSK position when the audio tones from the sound card pass to the rig via the isolation transformer.
There is also a time delay selector switch which can delay the PTT line dropping out during data mode transmissions.
Power from the PCB can be often obtained from the rig's ACC socket but if not then from a 13.8V supply which is suitably regulated on board from either of the supply sources.