PTT keying circuitry, COM ports

Discussion in 'Working Different Modes' started by KC8QVO, Nov 15, 2019.

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  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    Since this is related to digital modes I figured this was an OK spot for it. If it should be in another forum I understand if it needs to be moved.

    I am trying to figure out PTT keying for digital modes. Note that I am aware of VOX and have found there are devices (Signalink appears to be a common one) that use audio to trigger a PTT circuit (pretty much an internal VOX to the interface to control external PTT). I want to physically control PTT and not emulate it based on detection of audio.

    For some background - I have used RigBlasters for years. In fact, the only one I've really used for digital modes is my trusty ancient RigBlaster Plus (RS232).

    My new laptop does not have conventional inputs for audio, nor an RS232 port. So I am using USB converters for everything.
    - USB to RS232 adapter
    - Focusrite Scarlett 6i6 USB audio interface (external sound card, but a whole lot more)

    The set up works fine, however it is a lot of "stuff". Add to it that this is all hooked to an Elecraft KX3 at the moment. I need a whole shack for the audio and interface stuff compared to how small the radio is.

    I got to digging in to interfaces today, partly because I was trying to get the wiring right for the KX3 4 pin TRRS mic plug, but also because I was curious on another solution to what I am trying to do as I don't really like using the Scarlett interface for digital modes. I have a "snapshot" saved of the programming so it is a simple plug in and pick the file to do the configuring now, but I just don't want that level of hardware.

    There are two things I am trying to accomplish:
    - Have a USB audio sound card with at least a mic and headphone jack, but if I could find one that has a mono line in and mono line out that would be better
    - Have a USB method of keying PTT

    The USB sound card seems like it is easy to come across - in many different forms. What is important for radio use (especially portable use), though, is to have optical isolation between the radio and computer = grounds separated. That leads to a lot fewer options - except homebrewing something or combining things.

    That brings me to PTT keying. RTS or DTR is apparently what can control PTT most readily = requires RS232, not USB directly, but rather through a converter.

    At the moment I have my Scarlett and USB-> serial adapter run through a multi-port USB hub to 1 USB port on the computer - it works fantastic. There are essentially 2 ports on the computer in use, however my devices list only shows 1 - COM6 - taken up by the USB->Serial adapter. The Scarlett shows up as a completely different device (2 actually - one is the Scarlett and the other is an "audio root", not sure why they are separated) - it is not listed as taking up a COM port.

    Do all USB sound card interfaces behave as the Scarlett does - where they are not showing as taking a COM port position even though it is communicating via a USB port/cable?

    I am using WSJT-X at the moment. I can set the Scarlett as the audio in and out in the computer's settings or the audio tab in WSJT-X. Then in the Radio tab (on WSJT-X) I can pick the COM port for the PTT keying (set to COM6 in my case).

    Is there any way to get PTT keying from USB without going to RS232 first?

    At the moment what I am looking at is trying to combine 3 devices in one box:
    1. External USB sound card
    2. USB->Serial adapter
    3. USB multi-port hub

    There are small enough sound cards (the size of a USB storage drive) that combining that with the internal circuitry of a USB->Serial converter and 2 port hub might still be a pretty small package compared to even a Signalink. The catch at this point that I see is isolating the audio and PTT circuitry - transformers on the audio paths and a T/R relay or pin diode.

    If the USB sound cards show the same way, and function the same way as a selectable audio device on my computer, the only COM Port would be the USB->serial converter as it is right now.

    Thoughts? Ideas?
  2. W5DXP

    W5DXP Ham Member QRZ Page

    How about the small Easydigi board which contains audio isolation and PTT opto-isolator?

    Last edited: Nov 15, 2019
    KB0MNM and KC8QVO like this.
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    That is a pretty slick circuit with all the parts.

    I found some really tiny USB->Serial adapters that might work.

    The next step is finding a USB splitter that will work (crack open a plastic case and pull out the bare circuit board, remove connectors, and solder the connections). As to how to package all of this - TBD - but judging by how small everything looks so far the biggest board would be the Easydigi.

    This blog post has some good info on the PTT keying - a couple different options, but still using RS232.
  4. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    I was going to suggest an arduino of some type. I can see mine on com21 from mmsstv, fldigi and wsjt-x.
    then I looked at the page KC8QVO suggested and that module is less than 8 loonies here plus its delivered to your door
    It has cts and dtr as outputs
    I've got to make a laptop i/f for an IC751A (used to do vox) and that circuit will be perfect for xmit control. not many laptops have a real 9pin D connector anymore
    TNX for that link KC8QVO, I just ordered 2
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Make sure you get a USB adapter that is Bi-Directional. Some are not able to control RTS or DTR.

    Have Fun.
    KB0MNM likes this.
  6. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    just an update.
    Got the usb2serial working good on my linux laptop with the FT757GX
    It does PTT on fldigi, qsstv, wsjtx and acts as the morse key on CWTYPE
    Interfacing CWTYPE was a small struggle, had to find out what its linux device name was then what com port wine had it on.
    I used the DTR line pulling an opto, serial resistor & led from 3.3V to drive the key jack on the rig.
    Should work easily on win7 too

    Thanks to KC8QVO Steve for the posting, saved a lot of time

  7. AG6QR

    AG6QR Premium Subscriber QRZ Page

    For the KX3, I believe the simplest solution is to key using CAT control via the serial cable that came with the radio (KXUSBa). That's what I'm using with WSJT-X on my KX3. It keys the radio, as well as setting the frequency.

    That mechanism for keying isn't so good for CW, because the timing isn't so great. But with the right software, you can send ASCII characters to the KX3 via the serial interface, and have the KX3 itself turn them into dits and dahs. Anyway, the original question was about WSJT-X, and CAT control of PTT works very well for that.

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