This principle is the same for other digi-mode programs and other SDR programs.

Please note, although WSJT-X includes control of some SDRs in its CAT it has no IQ processing. A soundcard SDR will appear to work when the LO is set to the Dial frequency but it will receive and transmit on two frequencies simultaneously. To cancel the image an SDR programme MUST be used. (Here is a way to receive WSPR with WSJT-X and a soundcard SDR)

January 2017. The current WSJT-X suite does not have an IQ mode so a SDR program is required to interface with a SDR radio. Here I use a Softrock soundcard RXTX. WSJT-X v1.7.0, HDSDR v2.75
Note:- Use the WSJT-X default folder setting. Do not be clever and put it in any other place. And you may well need to"Run as Administrator" or settings may not be saved.

WSJT-X receives mono audio from a receiver. The receiver here is HDSDR with a Softrock front end.
This must be setup first to ensure that everything works correctly stand-alone.

HDSDR outputs mono audio which must be sent to WSJT-X, WSJT-X outputs mono audio which has to be sent to HDSDR to produce the IQ signals to drive the Softrock transmitter. WSJT-X controls the HDSDR/Softrock by sending CAT commands for frequency, mode and PTT.

Initial Requirements:-
Profiles. I suggest setting a profile for HDSDR https://sites.google.com/site/g4zfqradio/installing-and-using-hdsdr#Advanced  When it is working close it to save the settings.

To interface the audio connections an virtual audio cable is used https://sites.google.com/site/g4zfqradio/pipe_sound_between_sdr_and_decoder Items 2) to 5) are possible, here I illustrate the use of 2), VB Cable.
To send CAT signals Virtual Serial Ports are used, here I illustrate The K5FR Virtual Port Manager. If your SDR is listed in WSJT-X's CAT then here is an alternative to a COM port.

Get and install your virtual cable
You need two to start, this one gives a total of 3 once you have donated.

 [This one http://software.muzychenko.net/eng/vac.htm is more expensive but many more cables are available.] DO NOT USE THE DEMO VERSION FOR TRANSMITTING! A voice announcement is superimposed.

Get and install your virtual COM ports. Here are two possibilities https://sourceforge.net/projects/com0com/

Pin No.    Function   
1    DCD (Data Carrier Detect)
2    RX (Receive Data)   
3    TX (Transmit Data)   
4    DTR (Data Terminal Ready)   
5    GND (Signal Ground)
6    DSR (Data Set Ready)
7    RTS (Request To Send
8    CTS (Clear To Send)
9    RI (Ring Indicator)

 comOcom uses different names for it's virtual ports. You may rename them. There is normally no need to change from the default parameters.

November 2017, This one seemed to stop working on Windows 10 after an "upgrade".
http://k5fr.com/DDUtilV3wiki/index.php?title=VSP_Manager  http://k5fr.com/DDUtilV3wiki/index.php?title=VSPM  http://k5fr.com/binary/VSPMhelp.pdf 

Just one pair is required but more may be created.

When correctly installed the ports and cables appear in various places in Device Manager. In mine comOcom was in it's own section. Just one pair of serial ports needed. I also have 2 USB COM ports but these are not required.

Audio setup:-
I set HDSDR output and every other audio device to 16 bit 48000Hz to try to avoid any resampling problems.

The Softrock sends IQ signals to the stereo "Microphone" socket.
The RX audio output from HDSDR is fed by VB Audio Virtual C  to WSJT-X Input

 WSJT-X sends audio to HDSDR TX Input via VB-Audio Cable A [Cable B is not used.]
HDSDR sends IQ signals to the Softrock via the "Speaker/HP" output.

Typical Windows Sound Controls, Right Click the Speaker Icon. Select Playback (or Recording) Devices.
Click "Properties"
Set all associated audio devices to 16 bit 48000Hz Turn off all enhancements. Set for just 2 speakers.

Mute Microphone and "PC Beep"
Right click gives the choice of a dB scale. Note! On the system I tested the dB scale was not calibrated, it was no better than the percentage scale, just a reference number.

Set 2 Channel 16 bit and sample rate.

NO boost.
On my system best settings are with the microphone input as above. -12dB still provides ample input from the Softrock.

There are four similar settings for all the VB Cables. [Muzychenko VAC has it's own panel.]

Setup HDSDR to receive CAT commands from WSJT-X

(Or, you may set PTT by the Virtual Serial port. Set WSJT-X "Rig" to None, "PTT Method" RTS, Port COM 7 in my example and HDSDR PTT to CTS on COM 6 as shown below. There will be no CAT tuning so WSJT-X and HDSDR will have to be set independently. DTR to DCD is the other alternative.) This may also be setup as an alternative PTT method using the same COM port while using CAT.

In my example I have sent the CAT commands from COM7 in WSJT-X to COM6 in HDSDR.

Set the tabs Port COM6, Baud rate, as WSJT_X, PTT pin, go back and click "Activate"

Set as above.

Make sure the CAT Baud Rates match, Either Stop Bit setting seems to work with HSDDR.
HDSDR responds to TS-2000 (and other Kenwood) commands.
Click "Test CAT" the button should go green.
Then "Test PTT" is active and should work giving no error message.
Here is another way to control some SDRs using WSJT-X CAT, no virtual port.

Check! I have seen ExtIO indicate a different frequency from that actually displayed in HDSDR. Usually stopping HDSDR and WSJT-X will resume normal operation.

Now WSJT-X should be working with HDSDR and the Softrock.
The usual TX checks must be made. Use WSJT-X's "Tune" button.
TX drive is affected by Pwr, Speaker, Mic gain (Set so it does not flash red) and Power Output.
Check with a receiver that IQ swap is correct, transmitting on the correct frequency.
It works like this but maybe when WSJT_X is set "Mono" then "Both Channels Added" might be best.  [BUT a more complex system might use right and left channels separately with two instances of WSJT-X and another real stereo soundcard for another IQ Softrock.]

And check Image cancelling with a receiver.  https://sites.google.com/site/g4zfqradio/hdsdr-iq-balance

Display RF Output Power. With regard to a Softrock but this should work with any radio.
This may be useful for those who like to use minimum power, often 1 watt is QRO!

There is a way of making HDSDR v2.75 display power output. I am not sure if this is the best way in regard to spurii, it needs checking, maybe when I suggest maximum settings for a control some intermediate position is best. But all controls need to be in a fixed preset position so the WSJT-X Pwr control retains the calibration I describe.

WSJT-X has a calibrated power control. Placing the mouse cursor on the "slider track" and using the wheel shows the 0.3dB calibration steps. First set this to maximum. Set "Speaker" output to maximum or to a value you remember.
Set HDSDR's "Output Power" control to maximum (or maybe some intermediate position you can remember as "standard"). When transmitting full power raise Mic Gain so that HDSDR automatically limits the gain.
Then reduce WSJT-X Pwr so that a power meter reads 1 watt into a dummy load. Many dual-meter SWR meters will indicate this level easily when connected in reverse, the reverse indication becomes forward power.
HDSDR when set to transmit displays the audio INPUT level below the meter. Go to the "S-Meter Calibration" tab when transmitting your measured 1 watt. The "Current Level" displayed under the meter will be shown. Enter 30dBm in "Correct Level" and click "Calculate". Then the indication below the meter should say 30dBm. If not click "Reset" and try again.

Now HDSDR will display the Softrock's output in dBm when you adjust WSJT-X's Pwr control. This should remain fairly accurate down to very low powers.

Subpage (1) Shows a way to use the WSJT-X CAT to control Softrocks and other SDRs without HDSDR CAT or a virtual port.
Subpages (1): Softrock_WSJT-X_CAT