Rig control errors with USB isolator and extension cable

Using the digirig with a KX3 or a Kenwood TS-690, and connected to an RPi, the power level out from the radio varies, even in “tune” mode. I’m using the rig cables from the digirig store. Due to some likely source feedback I purchased the USB isolator. The power level is rock solid when using the USB isolator.

I added a short USB-A extender since the RPi USB ports are closely spaced. The USB isolator is bigger than the size of a USB-A connector.

However, after several CQs or other rounds of transmission, WSJT-X pops up with an error communicating with the rig (both rigs affected). I need to reconnect WSJT-X to get control over the radio again.

By removing the short USB-A extender, the occurrence of this error seems to go away. It’s intermittent, so I’m still testing that it really did go away.

Does anyone have a suggestion for a high-quality USB-A extender for the USB isolator? The digirig store does not offer one of these (USB-C and USB-A to USB-C are available).



I haven’t found a better cable. I did try adding snap-on ferrites which didn’t help.

I’m wondering if there is an issue with support of the USB Isolator and a Raspberry Pi 4B or if I have a bad USB Isolator. Can anyone confirm if they’ve successfully used it with an RPi 4B? Used it with WSJT-X?

I have done the following to eliminate ground loop and RF issues. I’ve unplugged the audio cables from the KX3 so no RF is generated. I’ve configured WSJT-X to use VOX so the KX3 never goes into transmit mode. I’ve put the Pi on a battery (PiJuice hat) so that there is no common ground (the ground would otherwise be through USB but should be isolated due to the isolator). With all the changes I still get USB reset errors.

The USB reset errors only seem to happen on “transmit”, in other words when the Pi is generating an audio signal to the KX3. I have never had the issue on receive only. The issue happens on Tune in WSJT-X.

The issue goes away if I configure WSJT-X to use radio “None” so that it does not send any control signals. Although the serial cable is still connected. It seems like there is some compatibility issue associated with sending audio from Pi to KX3 and having KX3 control signals enabled.

I’ve recreated the issue with a Kenwood TS-690 which is 4800 baud (much slower than KX3). Although I have not gone through all the reduction tests described above on the KX3. I’ve recreated it with an otherwise-normal configuration.

Maybe I have a faulty USB Isolator? Ideas and experiences appreciated.


The USB isolator is either works or it doesn’t. Sounds like yours does. There are no drivers required. The computer doesn’t even know the isolator is present. The issue you are describing is likely caused by RF in the shack. If you have a dummy load, try it instead of antenna and the issue should go away. Drop TX power to the minimum value for this test too.

I’m inclined to agree that any failure mode should be binary.

As I described in the last post the transmitter isn’t even turning on so there’s no RF. I set WSJT-X to use VOX and completely disconnected the audio cable. Only the serial cable is connected between the digirig and KX3. I can observe that the KX3 does not go into transmit (WSJT is using VOX with no audio).

Not mentioned, but what led me to disconnect the audio completely, I did try a dummy load and had the same issue.

So in summary, it only happens when the RPi 4 is sending audio out. The RPi is on battery so the only connection between the Pi and KX3 is the isolator/digirig path. The only connection to the KX3 is the serial cable. And the KX3 never goes into transmit.

Also, without the isolator, the USB error/reset does not happen.

It would be great to know if anyone has successfully used the USB Isolator with the RPi 4. Maybe there’s some signal level issue or something else that is causing the Pi to error and reset.

The CAT commands related to transmit would use the serial interface in exactly the same way as the VFO sync commands. Do you get errors when flipping bands from WJST-X or turning the VFO knob on the radio?

Now, you can also set PTT to RTS which will not use the serial data lines, but instead use a dedicated hardware PTT line in the audio cable. You can try that with the audio cable connected and not.

Agreed - the cat command would be exactly the same. No issues when exercising CAT and there is no outgoing audio. It seems to be related to outgoing audio and CAT commands together.

With everything wired correctly, but KX3 power turned down to 0, RTS does seem to do the trick. Let me try this with the power turned up and also try same on the TS-690 which also has the CAT-with-audio-out issue.

If this config works then it should be a good solution. More out of interest, and maybe this is just rhetorical, what about the USB Isolator is causing this issue. I’ve put USB voltage/current testers on the RPi side and the digirig side of the isolator. Things seem ok (I believe the isolator is limited to supplying 200 ma). On the digirig side it’s about 70 ma and on the RPI side it’s about 120 ma. Voltage is almost exactly 5V on each side within 10’s of mV. As you said before any failures should be binary, not intermittent.

I’ll follow up in a day or two after trying some contacts with TX power.

Thanks for the constructive ideas!

Sorry for the false alarm. Remember that this issue is intermittent although it usually happens within a few rounds of FT8 transmissions.

I turned up the power, had the issue and then dialed it back down to zero, disconnected the audio cables and still have the issue. This would make sense, though. I mentioned above that KX3 plus VOX mode had the symptom so one would expect that RTS should have it also. I suppose the good news is the symptoms continue to be consistent.

So back to the earlier question - does anyone have experience with the USB Isolator on an RPi 4? It seems there is nothing RF-related to the USB errors. There are no errors with a direct-connect to the radio. Insert the isolator and the issues occur.