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Digital Interface (Homebrew) by KN4LF

Between Mark V Field rig "fixed" audio out and computer sound card in I have a Radio Shack 1:1 isolation transformer and a 47 k potentiometer in series. Alternately you could use a 10:1 voltage divider. Between sound card out and rig audio in on the Packet DIN jack in back, I also used a 1:1 isolation transformer and a 47 k potentiometer in series. For PTT I used an ECG-3084 optocoupler from Mouser between the computer DB-9 jack output and rig PTT RCA female jack in. All fits in one metal box with all three cable in and out. This interface is fully isolated and there is no audio feedback, hum, overload etc. possible. Your MP will produce 31 hz transmit audio at up to 100 watts with no ALC output.

The following information will allow you to use the "Packet" DIN jack on the back of the rig for "AF" PSK, MFSK16 and RTTY.

As far as wiring:
1.) Transceiver audio out to soundcard in is via pin #4 (Data Out).
2.) Transmit audio in from soundcard to pin #1 (Data In).
3.) If you want to use PTT actuation from your computer to transceiver then it's pin #3 (PTT). You could also skip the T/R switch and use VOX but I don't like to do that because the VOX settings for phone and digital are different.

As far as setting the transceiver up for use of the Packet DIN jack on back and the PKT mode button on front:
#1.) Push fast button and hold then the ENT button.
#2.) Select menu item 8-6.
#3.) Using the sub VFO tuning knob go to Easy Set.
#4.) Using the main VFO knob select PS31-u.
#5.) Hit ENT button.
#6.) Press PKT button once and the green LED on the LSB button lights up.
#7.) Press the PKT button a second time and hold for 1 second, you hear a beep and Voila you are in USB mode even though the LSB LED remains lit.

You can check that you are indeed in USB by going to 20 meters and tuning in a phone signal.

Thomas F. Giella, KN4LF