Repeated PTT failure on DigiRig Mobile with Yaesu FT-840 (three separate units tested)

Hardware & Software

edited for clarity: I left out the fact that I was using the Silicon Labs driver

  • Radios: Yaesu FT-840, tr(uSDX), Baofeng UV-5R
  • Digirig Mobile units:
    • Unit #1 – purchased May 2024 (Amazon, fulfilled by Softcomplex LLC) Not used until late May 2025
    • Unit #2 – purchased early June 2025 (Amazon, fulfilled by Softcomplex LLC)
    • Unit #3 – purchased mid-June 2025 (Amazon exchange; currently on-hand)
  • Cables:
    • Yaesu GX16 Mic Cable for DigiRig
    • Dedicated DigiRig cables for tr(uSDX) and UV-5R
  • OS / software: Windows 10 Pro, WSJT-X 2.7.0
  • Driver:
  • Current - v11.4.0.393 Silicon Labs VCP USB Serial Bridge on Com9 sees the DigiRig and was utilized for all testing and troubleshooting of cables/DigiRig/radios. I started with the 6.7.6 version install that updated to current version noted above.
  • Guido’s single-USB CAT/Audio driver (latest release as of May 2025) Does properly control the tr(uSDX) as a control test with the computer.

Symptoms

  • Unit #1:
    • Receives audio but will not trigger PTT via RTS on any radio.
  • Unit #2:
    • Passes bench test (RTS toggles open/short on 3.5 mm PTT jack)
    • Once connected to FT-840 with GX16 cable, RTS line stays open; radio never keys.
    • Same non-keying behavior on tr(uSDX) and UV-5R.
  • Unit #3 (current):
    • Bench test and UV-5R transmit PASS.
    • Hesitant to connect to FT-840 until I understand why previous units failed.

Troubleshooting Performed

  1. Continuity checks on all cables (following DigiRig “Troubleshooting Digital Modes” video).
  • All cables short sleeve → first ring (PTT) correctly and put each radio into transmit when unplugged from DigiRig.
  1. RTS loopback test with WSJT-X:
  • Unit #1 & #2 – RTS line remains open (no continuity change).
  • Unit #3 – RTS line toggles as expected.
  1. Pin-by-pin mapping of GX16 cable:
edited for clarity
(previously noted 1st Ring instead of Ring 2)
(previously noted 2nd Ring instead of Ring 1):
DigiRig 3.5 mm TRRS → FT-840 MIC/ACC
  Sleeve   ↔ Pin 7 (PTT) & headphone sleeve  
  Ring 2 ↔ Pin 6 (PTT return)  
  Ring 1 ↔ Pin 8 (AF out)  
  Tip      ↔ Headphone tip (AF in)
  Pins 1-5 → no connection

Wiring appears correct and matches published schematic.
4. Verified CAT control via single USB is working on tr(uSDX) with Unit #3.


Request

  • Guidance on what may be damaging or disabling the RTS/PTT circuitry on the DigiRig when connected to the FT-840.
  • Recommended additional measurements (e.g., voltage on FT-840 MIC pins) before I risk connecting Unit #3.
  • RMA options for the first faulty unit if they are deemed defective.

Additional Notes / Questions

  • Could voltage present on FT-840 Pin 7 or Pin 6 exceed DigiRig’s safe limits?
  • Is there a firmware or hardware revision difference between early-2024 and mid-2025 DigiRig mobiles that might explain this behavior? My first DigiRig is a version 1.10. I didn’t look inside the second one that I returned. My current DigiRig is a version 1.11.
  • I did submit a Support Contact Form on this site after my first unit failed and after a week I ordered the second unit.

Thank you for any assistance. Please let me know if you need further details.

Regards,
Bill

Step 1 - use the standard SiliconLabs CP-210 driver for the serial port. Retest RTS functionality.

Unless there is some other radio issue, the voltage on pin 6 will not damage a Digirig. Pin 7 is ground, so again, unless there is something wrong with your radio, no issue.

Matt
AL0R

1 Like

Thanks for the reply @AL0R .

I started by installing the 6.7.6 version of the Silicon Labs VCP USB Serial Bridges and updated to the current v11.4.0.393 dated 11/13/2024. My computer running Windows 10 Pro recognizes the DigiRig on Com9 with that driver.

I noted the driver from Guido only to show that the tr(uSDX) is working and can be controlled from the same computer. In that configuration the tr(uSDX) is seen on Com7 and the custom driver is working on Com8. Thanks for pointing that out, it did confuse the issue and I’ll edit that original post to clarify.

Regards,
Bill