What signals are used in a computer-radio interface for digital mode operation?

To operate digital modes like FT8, PSK31, or RTTY using a computer, the interface must handle three primary signals. First, "receive audio" must flow from the radio to the computer so the software can hear and decode the incoming data. Second, "transmit audio" must flow from the computer to the radio to send the encoded data. Finally, there needs to be a "transmitter keying" signal (often called Push-To-Talk or PTT). This tells the radio when to switch from receive to transmit mode so the computer's audio is actually broadcast over the air. Options mentioning RF power, GPS location, or general status indicators are incorrect because they are not the core signals required to pass audio data and control the transmit/receive cycle.