UZ7HO Soundmodem and EXT PTT

I have been experimenting with Digirig for satellite operations, specially IO-117 Greencube, a Medium-Earth-Orbit cubesat orbiting at almost 6000km with a huge footprint.

My setup is a Yaesu FT-817ND paired to an UHF amplifier and an Alaskan Arrow 10 element yagi. The software includes a terminal built by OZ9AAR, PSTRotator for doppler and tracking correction and UZ7HO Soundmodem.

Since PSTRotator uses the CAT com port (COM15), I can’t use the same port from Soundmodem for PTT. My solution at this moment is to create a vitual port splitter using VSPE. COM15 is the source for virtual port in COM20, which then are used by both PSTRotaror and Soundmodem CAT PTT.

I have seen that Soundmodem is capable of using a CM108 chip pin to trigger PTT using the EXT in the drop-down menu. PTT.dll is in the same folder as Soundmodem and Digirig shows up as USB Audio Device. I have not been able to trigger the radio PTT using this method. Anybody else has? Although it is working with my solution, using PTT without a virtual port will save me processing power and time.


Were you able to achieve PTT with the sound modem when the serial port is used directly (no virtual splitter) and exclusively (rotator control is not involved)?

Yes, I’m able to trigger PTT from Soundmodem using the com port or by choosing CAT, without using a virtual port.

The problem with that is that the tracking software will need to run in order to use the satellite and it will occupy the port, preventing Soundmodem from accesing it to trigger the PTT.

I was hoping to use EXT+soundcard instead of using a virtual port to share CAT between the two programs.

I wonder if this could be an issue when using Winlink to control the radio frequency and Vara HF to trigger PTT.

The GPIO ports of the CM108 codec are broken out to the through hole test points so it would be possible to tap them for PTT. Depending on how you connect them to the existing PTT switching circuitry you can have that inverted or not:

Let me if this is something you are considering and I can point to the places on the board where you can make the mods.

1 Like

Ok, I see it now. Do not worry as my current setup is working fine for now. I will let you know if I decide to pursue that pin 13 route.

Stay tuned as I try some 440MHz Earth-Moon-Earth QRP contacts using the Digirig.


1 Like

I’m very interested in making the mod to use pin 13 to activate PTT. Would you give me some pointers?

Thanks and 73,


Depending on what level on CM108’s GPIO triggers PTT (low or high), there are two connection variants (red or blue).

Here’s how it will look like for Digirig 1.9 according to the schematic above:

Would that disable the native PTT methods (CAT and RTS)? If I understand correctly, that will allow to use the CM108 PTT like in Direwolf and FLDigi, right?

If it doesn’t disable the other methods, maybe adding it to a future revision would be great (although I know there is not much space available).

It will disable PTT by RTS, but keep PTT by CAT.
The support for PTT by RTS is more common than PTT by CM108’s GPIO so it makes sense to keep former.

1 Like

Agreed, that’s why I’ll keep the current configuration. I still want to use it with commonly available software.

Thanks Denis. The apps I use all support CM108 GPIO PTT, so I might give this a try. I’ve modified some of these CM108 USB sound cards to use GPIO PTT and they work great, but require very steady hands for soldering and some precision heat gun work to remove certain SMT components from the board. The modifications needed to the DigiRig to support this are much simpler, and no parts need to be removed or added.

1 Like

I would love to try it with the CM108 PTT method since that will allow me to get rid of an extra software. But, my soldering skills are as bad as they get. If Denis decides to release a CM108 PTT version, I would get one!

I’ll get that on the wish list. Need to confirm which GPIO level activates the PTT.
Is there a canonical schematic for homebrew PTT implementation with CM108 that I can refer to?

1 Like

This is what I’ve found. Instead of RTS, the trigger signal comes from PIN 13.

Screenshot 2024-01-20 112607

1 Like

Then it’s the blue option from the connection schematic above.

EDIT: this doesn’t look like connection for CM108’s PTT. The output from the codec is logic levels and this schematic shows RS-232 connection. That is also the reason for the diode because the signal can go as far as -15V negative.


I made the modifications you outlined earlier and it works great in soundmodem, VARA and Fldigi! PTT is triggered when the CM108 PTT GPIO 3 pin goes high, so I used the blue modifications in your diagrams above. The nice thing about this arrangement is that I can simultaneously run Flrig and soundmodem or VARA for remote control operation with just one cable between the PC and the DigiRig.

For your reference, here’s how to modify a cheap (~$2) CM108 sound card with PTT activation using the CM108’s GPIO 3 pin. I’ve made several of these (less the COS wire, which I didn’t need). I used a TRRS jack with the same pinouts as DigiRig so I can share cables. They work fine, but require a very steady hand to remove some of the SMT components as well as soldering. Modifying the DigiRig to do the same thing was easier, but still requires some careful soldering.

My home-brew sound cards:


Steve AG7GN


Thanks for reporting your custom configuration. I am always trying to reduce the number of cables, and I do not think I am alone!

73 Constrainted

Alright, prototypes fabrication ordered. Stand by for the updates.


I have a number of these available for testing. These work with Digirig audio/PTT cables and PTT by GPIO3 on CM108B. PM me if interested.

1 Like

Successfully used it with VaraFM on a Yaesu FT-530! PTT is triggered via the RA-Board command. Also able to use it with Soundmodem using EXT. Will try an IO-117 satellite pass later today

1 Like