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FRStack and Remote PTT

Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

FRStack does support remote PTT since FRStack is SmartLink aware.

Mark provided the information required to use local foot switch.


For the FRStack feature to control MOX via a serial port:

MOX tab

  • MOX Com Port Selection and MOX mode; Toggle or Hold

You can choose to use

  1. DTR DSR pair or
  2. RTS CTS pair

You select which pair on the operations tab.

When operating SmartSDR Remotely you do not have access to the PPT hardware on the Radio. The MOX COM Port feature allows you to use a hand or foot switch to key the transmitter using your remote PC's serial port (or USB port using an inexpensive USB to RS232 Converter).

This feature has two modes;

Hold and Toggle. Hold mode keys the transmitter when the switch is closed and un-keys the transmitter when the switch opens. Toggle mode keys the transmitter on the switch's transition from open to closed and continues to transmit when the switch opens. A second switch transition from open to closed un-keys the transmitter.

Using a standard RS232 serial port; the switch contacts should be connected as follows:

Switch shield / ground to DB9 Ground Pin 5

  1. Switch PPT line to DB9 DSR Pin 6. Connect a 2K resistor between DB9 DSR Pin 6 and DTR Pin 4
  2. Switch PPT line to DB9 CTS Pin 8. Connect a 2K resistor between DB9 CTS Pin 8 and RTS Pin 7

*** I think you will find the resistor value (pull up) is not very critical. Anything from about 1K to 5K should work ***

Note: When you enable this feature (or start FRStack with this feature enabled) without the resistor in place between the DTR and DSR pins of the selected COM Port OR the hand or foot switch in the closed state; FRStack will report an error and disable the feature.

Here is the example using the CTS and RTS pair of pins.

To setup FRStack, go to View / Options.

Make sure you have plugged in your USB/RS232 adapter into your computer so that it is in Device Manager and you can find the correct number. In my example, it is COM13.

This is CTS Low and mode Hold for a normal foot switch. You can change it to Toggle if you want to click on and click off.

Now, if you are remote and are using SmartSDR on your PC you can just plug your USB/RS232 adapter into your USB port and operate with a normal foot switch or hand switch.




  • Dom wa2got
    Dom wa2got Member ✭✭
    edited May 2021

    Thanks for all the great info. I found 3 separate threads on the topic, but sounds like, for reliability this is the approach to use. Will this PTT option enable transmit in SSDR even when the SSDR window is not the active window?

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    @Dom wa2got

    The best I can say is go ahead and test it. If I understand what you are asking, the radio will transmit on the slice that has the transmitter assigned to it.


  • Dom wa2got
    Dom wa2got Member ✭✭

    @Mike-VA3MW Thanks Mike.

    Sorry, I did not describe the specifics in my original question. By window I meant Window O/S window. This scenario should make things clearer.

    Using Windows 10, Flex 6400, remote operation, laptop and radio are on the same subnet (my local home network). I'm in a room on the other side of the house from the Flex Radio.

    (1) Open SSDR (this is the 'active window')

    (2) I hear a CQ call and answer. Currently I am using my mouse and clicking on the MOX button in SSDR to transmit. To stop transmitting I again click the MOX button in SSDR. SSDR is the 'active window'.

    (3) During the response from the station who had called CQ, I flip to my browser window and lookup the call sign on QRZ.com. Browser is now the active window.

    (4) To transmit again, I must click somewhere within the SSDR window to make it active, followed by a 2nd click on the MOX button in SSDR.

    My question, will the device described in this thread allow me to continue to type and navigate in the browser window (browser window active window) as well as transmit. In short I can be typing in my browser window, depress the footswitch and transmit without having to click on the SSDR window to make it active? And when I release the footswitch and I am right back to typing in the browser window?

  • John KB4DU
    John KB4DU Member ✭✭✭✭


    i did a “conceptual” test. By that I mean I didn’t use frstack or the cable assembly. I used a wye able to connect the hand mic and a foot switch in parallel to the Pitt port on the 6400. With the windows focus on the logging window, both the mic putt and the foot switch will key the transmitter. Windows can only accept input on the “active” slice, which is why SmartSDR has to be selected to use the MOX button. I suspect that any solution that keys the radio hardware directly, not using SmartSDR, will work.

  • Dom wa2got
    Dom wa2got Member ✭✭

    @John KB4DU Thanks so much John!

    I'll give this a try with the cable and report back here. It may take a week or two to get the parts needed, built and test.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    @Dom wa2got

    The FRStack PTT will function regardless of where the 'focus' is. You should be able to still transmit via FRStack.

    It also has a 'hot key' feature and I have control-spacebar set for a MOX function and I can do that regardless of where my focus is.

    Give it a try.


  • Dom wa2got
    Dom wa2got Member ✭✭

    @Mike-VA3MW Thanks Mike! FRStack option is exactly what I was looking for. And no need to purchase anything!

  • VE6BUD
    VE6BUD Member
    Just thought I'd comment. On the Flexradio Enthusiasts group on Facebook, it was mentioned to me that using the FRStack MOX method for footswitch PTT is a workaround to the problem of SmartSDR not transmitting any audio intermittently when SmartCAT PTT is used with a footswitch. (MOX button in SmartSDR turning blue but with grey "MOX" text when there is no TX audio.)

    (I hope Google picks up on this and someone else sees this to save themselves from some frustration!)
  • K4XD
    K4XD Member ✭✭
    Reporting back - I built the Foot switch to USB adapter with a resistor between pins 7 & 8 and the footswitch between pins 7 and 5, set up FRStack to CTS Low and Hold, and it works! For contesting, I prefer having to hold the footswitch down for TX. In a ragchew mode, I would switch the option in FRStack to Toggle, so pressing the footswitch turns on MOX, then you can release the footswitch until you're done transmitting, at which point you press the footswitch one more time to toggle MOX off. Thanks for the contributions in this thread which got me past my last obstacle to remote operation of my Flex in all modes.
  • K9YK
    K9YK Member ✭✭

    @VE6BUD Thanks for your comment on this thread! I'm having exactly the same problem (grayed out MOX using Com port PTT in v3.2.39). FRstack seems to get around the issue. I wish Flex would hurry up and squash this bug.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin


    Can you post a screen shot please? The only way the MOX is greyed out is if your transmitter is set to something out of band or due to some lockout / interlock condition.

    I can't see how a Com port issue would grey out the MOX.

    The way the remote foot switch works is that when you push PTT via a SmartSDR CAT port or via FRStack, it actually sends a CAT command to the radio to put it into TX and then back into RX.

    Can you help me out and tell me how to recreate this?

  • alberto
    alberto Member
    hello there anyone may explain me how to operate SmartSDR remotely ( Flex radio in a different place/country than the PC running SmartSDR), using FRstack+SmartSDR+PC running windows10 AND the SPACEBAR as PTT? How can I configure DAX to obtain this? Many thanks for the help
  • John WA7UAR
    John WA7UAR Member ✭✭✭
    edited June 9

    @alberto — I hope this help article answers your question. This is how I run my remote station. SmartSDR on my local machine (PC or Mac and operating SSB) and all my digital programs on a PC that is co-located remotely with my Flex. I use TeamViewer or PC Anywhere to remotely monitor all digital software on the PC (which includes DAX and CAT on the remote PC).


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