KX3 Computer Interface for WSJT and other Digital Modes
The popular Elecraft KX3 is a very flexible, small footprint HF+6m SDR (Software Defined Radio) that can be used without a computer.  However, if one wishes to use the radio for digital modes such as WSJT, some interface between the computer and the radio must be used to isolate audio between them and provide for other computer communication functions.  There are a number of such interfaces made for ham radio transceivers, but at the time of the KX3 introduction, such interfaces with cables for the unique connectors on the KX3 were not commercially available.  

Many less expensive computer interfaces simply provide audio isolation, and do not utilize commands from computer COM ports for such functions as CW and PTT keying, firmware downloading and rig control.  Most of the reputable software programs for use in computer communication do provide for advance PTT keying ahead of the actual sending of any CW or digital mode signals, thereby assuring enough time to avoid "hot switching" any mechanical antenna relays (as would happen with simply "VOX" operation).  Since my primary application for the KX3 was to use it to drive an amplifier on JT65A mode during overseas 6m EME DXpeditions, it also was essential that I maintained the capability of advance PTT switching that is incorporated into all the WSJT software modes.

In summary, my goal was to arrive at an interface solution which provided:

1. Isolation of audio lines, particularly the one between the computer and the radio's microphone input
2. True CW keying from the DTR line of the computer serial port
3. Isloated PTT keying by the RTS line of the computer serial port
4. USB connection to the computer
5. As small a package as possible for portability

In the interest of convenience and time, a proven commercial interface was chosen as the basis for use with the KX3.   It is the RIGblaster NOMIC unit.  Measuring only  3" long x 2.25" wide x 1.375" high (excluding the connecting cables), and connecting to the computer via a USB cable, it met most of my requirements.  A shielded stereo audio cord was run between the computer MICROPHONE and the KX3 PHONES jack.  The computer HEADPHONES jack was connected to the AUDIO IN on the NOMIC.

The only things lacking from my rquirements list were CW keying via the DTR line, and inclusion of TXD, RXD and SIGNAL GROUND directly from the serial port for rig control and firmware upgrades.  Both these modifications were easily added inside the unit, and ported to the KX3 through the existing RJ45 connector, as shown in the photos below.

The second CNY17-3 was simply suspended in the air by rigid wires and mounted upside down "dead bug" style.  The connections were as follows:

PIN 1 - Attached to the 470 resistor connected to the DTR diode D1
PIN 2 - Attached to Signal Ground (RJ45 connector Pin 1)
PIN 5 - Attached to RJ45 connector Pin 4 for CW KEY line to KX3
PIN 4 - Attached to PTT Ground (connected to Pin 4 on the RTS CNY17-3 on the PC board)
As provided from the factory, DTR and RTS lines are combined after diodes D1 and D2, respectively.  That junction is then fed through a 680 ohm resistor to a CNY17-3 optical coupler to isolate the PTT line.  A diagram of the CNY17-3 is shown below.

To separate out the DTR line, the striped end of diode D1 was cut where it meets the printed circuit board, so the diode could be pulled upright and a 470 ohm resistor could be soldered to the end.   The other end of that resistor was soldered onto pin one of a second CNY17-3, as shown in the photo to the left.

The RX (DB9 pin 2), TX (DB9 pin 3) and SIGNAL GROUND (DB9 pin 5) lines from the DB9 connector were connected directly over to the first three RJ45 pins by soldering jumpers onto the bottom of the board, as shown in the photo above.

The NOMIC was designed to interface with transceivers having a standard 8 pin microphone connector via the 8 pin RJ45 connector on the unit.  Customization for various radios is made by connecting jumpers inside the unit to the desired pins of the RJ45 jack. The microphone cable with the RJ45 was cut and three shielded stereo cables were spliced into it, according the table to the right.  The standard 3.5mm stereo plug was Digi-Key part number CP-3502-ND.  The KX3 3.5 mm 4 conductor plug was Digi-Key part number  CP-35401SP-ND .  The splices were each wrapped in plastic tape then all were covered with a piece of heavy 5/8" ID  heat shrink tubing from the electrical department of a big box hardware store, as shown in the photo below:


RJ45     NOMIC INSIDE               CONNECTED      3.5 MM  CABLE
PIN #    CONNECTION                 TO CABLE          CONNECTOR

 1     Hard wired to DB9 Pin 5        ACC1              Shield
 2     Hard wired to DB9 Pin 3        ACC1              Tip    
 3     Hard wired to DB9 Pin 2        ACC1              Ring
 4     Hard wired from Pin 5,DTR chip CW KEY            Tip
 5     Jumpered from PTT GND          CW KEY            Shield
 6     Jumpered to MIC Out            MIC               Tip
 7     Jumpered to MIC GND            MIC               Shield
 8     Jumpered to PTT OUT            MIC               Ring 1

Detail of the MIC plug connections for the KX3. 
In order to permit use of a boom microphone with the KX3, and allow for the possibility of using a footswitch or other means to key the radio, an auxiliary MIC cable was prepared to accept a 2.5mm footswitch plug for PTT or a  standard stereo 3.5mm microphone plug.

An additional custom cable was made to key the amplifier.  Ring 2 of the 2.5 mm ACC2 jack on the KX3 was connected to a shielded cable that terminated in an RCA plug to interface with the PTT of my 6M-1000 amplifier.  The 2.5 mm stereo plug was Digi-Key part number CP-2501SP-ND.

The RIGblaster NOMIC provided a very easy way to interface the computer to the KX3 for digital modes, and with minor modifications  provided all the interface capability required for the KX3 to just about everything!. 

Revised 20 August, 2012