PTT weirdness (and Mobilinkd TNC2)

KC7RBW
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PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Mon Mar 23, 2020 4:55 pm

I tried to use my Mobilinkd TNC2 with my GD-77 yesterday and it would not trigger the PTT. I emailed the developer who made their reply was that it just shorts the PTT to ground using a MOSFET. They asked me to try manually shorting the correct pins to see that it works. The weirdness is that it doesn't always work. Sometimes I can get it into a state where it never works. Other times I can get it to almost always work. I've also seen it work but be a bit slow to respond.

The GD-77 (or OpenGD77? I haven't tested the OEM firmware yet) seems to require something more than just shorting that pin to ground, or something else is getting in the way. Is it maybe flipping into USB mode?
  • The TNC2 cable (manually shorting the correct pins) and the TNC2 (tested using their Android app) both work perfectly on my Baofeng BF-F8HP.
  • A speaker-mic I have (BTECH QHM22) works perfectly on both the GD-77 and the BF-F8HP.
  • My new "Easy Digi" triggers PTT perfectly on both radios.
So I'm stumped. I don't think it can be a connector issue because it works on other radios and other devices work on the GD-77.
I'll try reverting to OEM firmware later today (US/Pacific timezone) but I'm wondering if there's something about the hardware design that could explain why it's behaving strangely.

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kd2lh
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by kd2lh » Mon Mar 23, 2020 5:59 pm

The device description I read from their website describes a simple "KISS" modem interface for the audio from a radio to and from an Internet connection. I did a quick read of their documentation.

I'm not completely clear on how it could be used with DMR as a hotspot, especially using only the audio connections to the handheld radio.

Note that OpenGD77 exposes layers of DMR connectivity implemented within the radio itself, and permits communication with the DMR portion of MMDVMHost directly via a USB connection. MMDVMHost digitally connects voice packets between the radio and an outside computer interface within PiStar or one of the other voice network interface platforms.

As a digital interface, what actually is sensed and manipulated by the device for switching T/R? Is it designed for VOX? Remember, Baofeng and Radioddity wire their speaker/mic connectors and expose USB differently (by reversing the polarity of the + - USB data lines).

KC7RBW
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Mon Mar 23, 2020 8:14 pm

kd2lh wrote:
Mon Mar 23, 2020 5:59 pm
I'm not completely clear on how it could be used with DMR as a hotspot, especially using only the audio connections to the handheld radio.
Not for DMR. Using it for AFSK AX.25 packet (APRS and Winlink) which the GD-77 doesn't do on its own.
kd2lh wrote:
Mon Mar 23, 2020 5:59 pm
As a digital interface, what actually is sensed and manipulated by the device for switching T/R? Is it designed for VOX?
It doesn't use VOX. The TNC2 is a modem and opens and closes PTT as needed to ensure that the data can be transmitted and received.
kd2lh wrote:
Mon Mar 23, 2020 5:59 pm
Remember, Baofeng and Radioddity wire their speaker/mic connectors and expose USB differently (by reversing the polarity of the + - USB data lines).
My Baofeng doesn't do USB at all, only speaker-mic and serial. The TNC2 also doesn't do USB - it only acts like a speaker-mic receiving and sending audio and opening and closing the PTT.

Do they do speaker-mic differently? I thought they were all standard Kenwood 2-pin for that.

KC7RBW
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Mon Mar 23, 2020 8:31 pm

I should clarify that the reason I brought up USB as a possibility is that this is a difference between my Baofeng and my GD-77.

I also have an MD380 that uses the same (AFAIK) USB interface which I just tested and it seems to work reliably with the TNC2. I also checked that that 3.5mm and 2.5mm connectors on the TNC2 cable are the same dimensions as the Radioddity USB cable (they are) and that it's sitting flush on the GD-77 when plugged in (it is).

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kd2lh
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by kd2lh » Mon Mar 23, 2020 10:56 pm

The physical connector is the same, but the wiring of the connector is different for USB. Baofeng uses one, and TYT and Radioddity use another (reversing polarity). I'm not sure if this reverses polarity of the audio as well, but it's a possibility, as is a different scheme for PTT.

That said, I know that speaker mics from Kenwood work in both radios.

Check out the schematics.

KC7RBW
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Tue Mar 24, 2020 12:07 am

kd2lh wrote:
Mon Mar 23, 2020 10:56 pm
The physical connector is the same, but the wiring of the connector is different for USB. Baofeng uses one, and TYT and Radioddity use another (reversing polarity). I'm not sure if this reverses polarity of the audio as well, but it's a possibility, as is a different scheme for PTT.

That said, I know that speaker mics from Kenwood work in both radios.

Check out the schematics.
I will take a closer look at the schematics, but at this point I think it's more likely a firmware issue. Will have to dig deeper to find out.

The connectors being different in some ways seems unlikely to make a difference in the minimally simple case where I manually short the right pins and the PTT still doesn't trigger. Makes me think the firmware is looking for more than just those pins being shorted - something that's normally there, like a signal from the mic.

KC7RBW
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Tue Mar 24, 2020 5:47 am

I reverted my radio to the OEM firmware to see if it behaves differently. It's even less likely to trigger the PTT from the TNC2 or from manually shorting the two pins. Still works properly with a speaker-mic.

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DG1YFX
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by DG1YFX » Tue Mar 24, 2020 10:01 am

Sounds like a ground issue with the pinout.

Here it the correct pinout of the GD-77 connector:

3,5mm
Shield = PTT
Ring = Mike
Tip = USB Data (white)

2,5mm
Shield = GND (Mike, Speaker, PTT, USB)
Ring = USB Data (green)
Tip = Speaker

So, check the connections on the TNC2 if this corresponds with the pinout. Shield of the 2,5mm is GND. Shield of the 3,5mm is PTT, connected to GND for transmit. If even the manual shorting does not trigger the PTT relieable, there must be a fault within the wiring itself. Checking the wiring with a multimeter should pinpoint the fault.

Another issue can be different grounding for charger and GD-77 mic socket if TNC2 and charging bay is fed through the same power supply. Check it without charger. And always connect the mic connector first before putting the GD-77 into the charger. Otherwise you might fry the GD-77!

73s,
Jens

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DG1YFX
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by DG1YFX » Tue Mar 24, 2020 10:16 am

In addition to my last post, this should be the correct wiring from TNC2 to GD-77 after consulting the schematics of the TNC2: http://www.mobilinkd.com/wp-content/fil ... kdTNC2.pdf

TNC2 has a 3,5mm Socket with two (!) rings.

TNC2 shield => GD77 2,5mm shield (GND)
TNC2 ring 2 (next to shield) => GD77 3,5mm ring (Mike)
TNC2 ring 1 => GD77 3,5mm shield (PTT)
TNC2 tip => GD77 2,5mm tip (Speaker)

73s,
Jens

KC7RBW
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Re: PTT weirdness (and Mobilinkd TNC2)

Post by KC7RBW » Wed Mar 25, 2020 6:49 am

Thanks for the help so far, Jens!

Here's what I learned by poking around with a cheap multimeter:
This first part is interesting, but I don't think this is the problem...
  • Speaker mic
    • PTT not pressed
      • 2.5mm sleeve (GND) and 2.5mm tip (Speaker) are connected with a slight resistance
      • Nothing else is connected
    • PTT pressed
      • 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) are connected with almost no resistance
      • 2.5mm sleeve (GND) and 2.5mm tip (Speaker) are still connected with a slight resistance
      • 3.5mm sleeve (PTT) and 2.5mm tip (Speaker) are of course also connected the same
  • TNC2
    • Not transmitting (PTT not pressed)
      • Nothing is connected
    • Transmitting (PTT pressed)
      • 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) are connected with a slight resistance
      • Nothing else is connected
Here's where I think the problem really is...
  • GD-77
    • With generic cables
      • Shorting 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) triggers PTT
      • 3.22V measured between these two points
    • With TNC2 TRRS cable
      • Shorting TNC2 TRRS sleeve and Ring 1 does not trigger PTT
      • Around 0.35V measured between these two points
So I think what's happening is that the TNC2 cable is kicking the radio into USB mode, which lowers the voltage the radio puts between PTT and ground for that detection (presumably to avoid triggering PTT while connected to USB).

I don't know how the radio decides this. I can't figure how it's possible since in both cases, as far as I know, the wires are open-ended until they are connected to something. But I tested it many times while trying to figure out what was going on and the results above are completely consistent.

Is there maybe a capacitance being measured to detect if long wires are present?

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