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Lastly the more expensive ones do have a soundcard and radio control built in, but it still requires two cable from the interface to the radio - one for control, one for the audio in/out. Others have the radio control for keying, but no soundcard, so you need three cables from the PC to the interface - one for control, and two from the mic/headset connections.
#G4HFQ SOFTWARE TRANSFERRABLE SERIAL#
Some of the interfaces have a sound card built in, like the Signalink, but do not have the radio control (serial port) built in, so that means two cables from the PC to the radio - one USB for the serial port, one for the soundcard. Some kind of ground loop isolation is desirable between the radio and PC sound card, but not necessary in all cases.
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Without the PC control, it simply means you have to tune the radio and control the setups from the radio front panel. You do not have to have digital control of the radio to do digital modes. The things you need for digital modes are: USB uses 5V, but doesn't do serial data in the same way the radio does, so most of the USB/CAT cables have a chip to do the conversion, and then also the voltage converters to go back to 5V/0V data.
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Most of the radios use 5V/0V so the CAT cable has a small voltage change setup - often just a resistor network. With the older PC serial ports the voltage used was +/- 12V for the signals. The CAT cable is simply a serial port set up for the radio.