Garmin GTR-200 intercom
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Rhino
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I assume you mean adding the Dynon radio as a additional radio rather than as a replacement for the GTR 200 radio. Can it be done easily? No. But with a lot of creative wiring, it appears you can do it. The GTR 200 will accept another audio source via aux inputs, but it has no ability to control another radio. The Dynon radio can be controlled independently, and be heard via the GTR 200 aux input, but it keys and transmits over an entirely different route than the GTR 200 radio. At the very least, you'd have to install and wire a multi-gang switch or rotary knob that switches your headset inputs and outputs between the GTR and Dynon radio (that's what an audio panel does internally). There would be other custom wiring as well. It would be quite a project, but it's possible. Personally, I'd get an audio panel.
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Yes I have a GTR-200 with the integrated intercom tied into the HDX and it works really well. I am going to remove my Dynon Autopilot head and was hoping to pop in the Dynon radio head without the audio panel due to real estate availability. So given that I’ve wired the Garmin through a serial bus I’m assuming that if I add a Dynon radio, I’d have to rewire the Garmin to the audio panel?
Rhino
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There are two issues to consider here, data transfer and radio/audio control.
Data first. The HDX can display frequencies for two radios, but it only transfers frequencies for the primary radio. So your only decision here may be which radio you want to be the primary. You can try continuing to use the GTR 200 as the primary radio as you are now, and the new Dynon radio as the secondary. But I've never seen that done before, so I'm not sure how well it would work. For all I know, the HDX may default the Dynon radio to primary no matter what you do. In that case, the decision is made for you. But either way, you should still be able to leave the GTR 200 connected the way it is now.
Actual control of the radio transmitter and audio distribution are not done via serial bus. They're hard wired. Switching transmitter control and audio routing is something that's normally done via an audio panel, so you'd have to design in a way to do that switching with the switch or knob I mentioned (technically they're both switches). Fortunately you won't have to switch the receive audio because the GTR 200 provides an aux input you can use, so you won't need a mixer. However, you will have to route your transmit audio from your headset and PTT through that aforementioned switch. Basically it will switch three wires, audio hi, audio lo and PTT, back and forth to whatever radio you want to use.
Data first. The HDX can display frequencies for two radios, but it only transfers frequencies for the primary radio. So your only decision here may be which radio you want to be the primary. You can try continuing to use the GTR 200 as the primary radio as you are now, and the new Dynon radio as the secondary. But I've never seen that done before, so I'm not sure how well it would work. For all I know, the HDX may default the Dynon radio to primary no matter what you do. In that case, the decision is made for you. But either way, you should still be able to leave the GTR 200 connected the way it is now.
Actual control of the radio transmitter and audio distribution are not done via serial bus. They're hard wired. Switching transmitter control and audio routing is something that's normally done via an audio panel, so you'd have to design in a way to do that switching with the switch or knob I mentioned (technically they're both switches). Fortunately you won't have to switch the receive audio because the GTR 200 provides an aux input you can use, so you won't need a mixer. However, you will have to route your transmit audio from your headset and PTT through that aforementioned switch. Basically it will switch three wires, audio hi, audio lo and PTT, back and forth to whatever radio you want to use.
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Rhino
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Another thing to consider. When using two Dynon radios without an audio panel, the audio hi and audio lo lines are tied together. You could consider trying this with your setup, and only the PTT line would need to be wired with an SPDT switch. However, without knowing if there are impedance or other differences between the Dynon and GTR 200, I personally wouldn't risk that. It shouldn't physically hurt anything if there's a mismatch, but your audio and noise levels might not be good. Since you'd be wiring in a switch anyway, you might as well just switch all three lines with a 3PDT switch to be on the safe side.
Rhino
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This got interesting. This is a first draft for the T25 radio (you didn't specify which one). There's two things to keep in mind.
1. If you have a two place aircraft, you need two 3PDT switches instead of one.
2. Since the T25 is designed to use the SV-COM panel, it doesn't have pins for pilot and copilot headsets. So I've tied the pins together for that function. One thing to remember though, is that both headsets will be hot whenever a mic is keyed, so tell your passenger to keep quiet when you're on the radio. There is a way to get around that, but it's complicated, and I think you wanted to stay simple. It should work this way though.
No guarantees this is 100% correct, and I make no warranties.
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I very highly advise you use shielded wiring, and I've tied all the shields together.
1. If you have a two place aircraft, you need two 3PDT switches instead of one.
2. Since the T25 is designed to use the SV-COM panel, it doesn't have pins for pilot and copilot headsets. So I've tied the pins together for that function. One thing to remember though, is that both headsets will be hot whenever a mic is keyed, so tell your passenger to keep quiet when you're on the radio. There is a way to get around that, but it's complicated, and I think you wanted to stay simple. It should work this way though.
No guarantees this is 100% correct, and I make no warranties.
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I very highly advise you use shielded wiring, and I've tied all the shields together.
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jakej
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Just a small correction here Rhino 
HDX pin 30 should go to pin 31in the GTR ? I haven’t checked out the rest, just posted this obvious 1 
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HDX pin 30 should go to pin 31in the GTR ? I haven’t checked out the rest, just posted this obvious 1 .
Rhino
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Forgot to mention, you can use a 6PDT switch instead of two 3PDTs. Two 3PDTs gives you the ability to have the copilot use a different radio. A single 6PDT means you both must use whatever radio is selected on the single switch. I used two switches because it gives more flexibility.
Rhino
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I should add in the caveat that you may not be able to successfully tie in both headset microphones on the Dynon radio. Connecting microphones together is fraught with problems, and this just may not work. If you get any hum or unacceptable audio performance, take the copilot microphone out of the circuit (you can just unplug the headset to test this). And if both headset microphones don't have the same impedance, that could open up yet another ball of wax. You really need a mixer, which is one of the things an audio panel gives you (still recommend you get one). You can craft a custom mixer into the back of your panel somewhere to facilitate all this, but that gets more complicated. You'd normally get similar difficulties if you try to combine earphone (listen) audio, but fortunately the GTR 200 provides aux inputs that use an internal mixer to alleviate that problem.
You won't have the aforementioned problem with tied in microphones if both pilot and copilot do not have the Dynon radio selected. The connection between the microphones is severed if the two switches are set to different radios, or if both are set to the GTR 200. But it would be a major pain in the butt to have to make sure it always happens that way. Again, there is a way around that (actually a couple of ways), but yet again, that gets more complicated.
If tying the pilot and copilot microphone inputs together doesn't work well, you can still listen to both radios on the copilot headsets. You just won't be able to transmit on the Dynon radio with them. You would however, still be able to use the GTR 200 from both headsets. This is what that would look like;
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Oh, did I mention you really should get an audio panel?
You won't have the aforementioned problem with tied in microphones if both pilot and copilot do not have the Dynon radio selected. The connection between the microphones is severed if the two switches are set to different radios, or if both are set to the GTR 200. But it would be a major pain in the butt to have to make sure it always happens that way. Again, there is a way around that (actually a couple of ways), but yet again, that gets more complicated.
If tying the pilot and copilot microphone inputs together doesn't work well, you can still listen to both radios on the copilot headsets. You just won't be able to transmit on the Dynon radio with them. You would however, still be able to use the GTR 200 from both headsets. This is what that would look like;
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Oh, did I mention you really should get an audio panel?
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Rhino
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If you're a really ambitious techno-geek:
Rhino
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You posted in another thread where you asked if Dynon had a diagram for this. That's extremely unlikely. Dynon can't produce diagrams for every possible system configuration a builder might come up with, nor do they try. That's why many people come here or to other forums. You also need to realize that you're trying to use an intercom like an audio panel, something it isn't designed for, and Dynon isn't going to want to generate anything like that. It is possible to do this, and there are a couple of different ways of doing it, but none are perfect. That's why audio panels were invented, and why I keep recommending one.
Your original post mentioned the COM _PANEL, but I didn't put that in these diagrams. Are you planning to use the SV-COM-PANEL? If so, I can add it to the diagram. It would only affect one wire that's on there now. The radio technically can function without the SV-COM-PANEL, but you need it to transfer frequencies from the EFIS. You also never specified which Dynon radio you plan to use, or which you want to be COM 1 and COM 2. Knowing that would be helpful as well.
By the way, you don't strictly need the volume knob Dynon mentioned in that other thread if you're controlling volume with the GTR 200, unless you want to control the volume of the Dynon radio independently. In that case, if you don't use the SV-COM-PANEL, you can install a panel mount knob to control Dynon radio volume, but it's yet more custom wiring.
MakerPlane sells several versions of an AMX mixer for aircraft that you could use to replicate most primary functions of an audio panel, without taking up any panel space. In fact, I believe it's designed and manufactured by vlittle, who is a member here. That should allow you to tie in both headsets without any problems. I would want to confirm with Vern (vlittle) that it's good for microphone impedance as well as earphone, but I'd bet it is. If you want to go that route, I can help you with the interconnect. But it's very unlikely Dynon could or would help you with that.
The bottom line is, there is just no simple or easy way to make an intercom function partially like an audio panel, and no possible way to make it function completely like one. You're either willing to get down and do the custom wiring and configuration necessary, or you bite the bullet and get an audio panel. You are by no means alone in that regard. What you want to do has been requested numerous times, here and elsewhere. Panel space may not be the most monetarily valuable real estate in the world, but it's hard to find, and it's very precious to those who build and fly airplanes.
Your original post mentioned the COM _PANEL, but I didn't put that in these diagrams. Are you planning to use the SV-COM-PANEL? If so, I can add it to the diagram. It would only affect one wire that's on there now. The radio technically can function without the SV-COM-PANEL, but you need it to transfer frequencies from the EFIS. You also never specified which Dynon radio you plan to use, or which you want to be COM 1 and COM 2. Knowing that would be helpful as well.
By the way, you don't strictly need the volume knob Dynon mentioned in that other thread if you're controlling volume with the GTR 200, unless you want to control the volume of the Dynon radio independently. In that case, if you don't use the SV-COM-PANEL, you can install a panel mount knob to control Dynon radio volume, but it's yet more custom wiring.
MakerPlane sells several versions of an AMX mixer for aircraft that you could use to replicate most primary functions of an audio panel, without taking up any panel space. In fact, I believe it's designed and manufactured by vlittle, who is a member here. That should allow you to tie in both headsets without any problems. I would want to confirm with Vern (vlittle) that it's good for microphone impedance as well as earphone, but I'd bet it is. If you want to go that route, I can help you with the interconnect. But it's very unlikely Dynon could or would help you with that.
The bottom line is, there is just no simple or easy way to make an intercom function partially like an audio panel, and no possible way to make it function completely like one. You're either willing to get down and do the custom wiring and configuration necessary, or you bite the bullet and get an audio panel. You are by no means alone in that regard. What you want to do has been requested numerous times, here and elsewhere. Panel space may not be the most monetarily valuable real estate in the world, but it's hard to find, and it's very precious to those who build and fly airplanes.
Rhino
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Okay, let's work on this. You can get mostly what you want without an audio panel, except there's a potential issue tying the pilot and copilot mic audio together. I'm going to make some assumptions and suggestions, and you say if any of them aren't applicable or wanted.
1. Use a Dynon T25 as Com 1, with the control head, because you get more functionality that way, as Dynon noted in the other thread.
2. The GTR 200 will be Com 2, and serve as your intercom. You won't need a mixer for earphone audio, because the GTR 200 has one built in.
3. Your mic audio should be mixed (or switched as above), which is normally done by an audio panel. The Dynon radio and Garmin intercoms don't do that. However, you should be able to do that with an AMX-2A mixer by vx-aviation, available at maker plane.com for $129.95. It takes up no panel space, since it goes behind the panel. The AMX-2A is designed for earphone audio, but it has 600 ohm impedance inputs, which is what many headset mics use, and it outputs 150 ohms impedance audio, which is what the GTR 200 accepts. I'm pretty sure vx-aviation is owned by a user here, vlittle, so he can verify the suitability.
4. Although a mixer will alleviate the need for a switch to keep pilot and copilot mic audio being tied directly together, you'll still want to consider a switch to set who transmits on which radio. That's something else an audio panel does that the Dynon radio and GTR 200 don't do. They're both designed to handle only one radio, so they have no built in switching capability. You may be able to do without such a switch, but both microphones will be live over the radio when either person keys their mic. A switch would allow only one or the other to talk, based on where it's set. Since you had already planned to add the Dynon radio to your panel, this switch would be the only other thing taking up any real estate.
Assuming all that is acceptable, we can work on a wiring diagram.
1. Use a Dynon T25 as Com 1, with the control head, because you get more functionality that way, as Dynon noted in the other thread.
2. The GTR 200 will be Com 2, and serve as your intercom. You won't need a mixer for earphone audio, because the GTR 200 has one built in.
3. Your mic audio should be mixed (or switched as above), which is normally done by an audio panel. The Dynon radio and Garmin intercoms don't do that. However, you should be able to do that with an AMX-2A mixer by vx-aviation, available at maker plane.com for $129.95. It takes up no panel space, since it goes behind the panel. The AMX-2A is designed for earphone audio, but it has 600 ohm impedance inputs, which is what many headset mics use, and it outputs 150 ohms impedance audio, which is what the GTR 200 accepts. I'm pretty sure vx-aviation is owned by a user here, vlittle, so he can verify the suitability.
4. Although a mixer will alleviate the need for a switch to keep pilot and copilot mic audio being tied directly together, you'll still want to consider a switch to set who transmits on which radio. That's something else an audio panel does that the Dynon radio and GTR 200 don't do. They're both designed to handle only one radio, so they have no built in switching capability. You may be able to do without such a switch, but both microphones will be live over the radio when either person keys their mic. A switch would allow only one or the other to talk, based on where it's set. Since you had already planned to add the Dynon radio to your panel, this switch would be the only other thing taking up any real estate.
Assuming all that is acceptable, we can work on a wiring diagram.
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Wow, let me catch my breath. Can’t thank you enough for your input and experience on this. It sounds like a perfect set up. I’ll start pulling all the associated material available to get my head around the layout. Should be able to confirm impedance specs but if vlittle can offer some input all the better.
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Rhino
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The AMX-2A specs are in the install manual.
You already have the GTR 200 specs.
You already have the GTR 200 specs.
Rhino
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Different switch options. The simple way with one switch means both mics are hot when transmitting, and the switch selects which radio is being used. That means only one radio can be used at a time.
More complicated wiring and two switches mean both radios can be used at once. Pilot and copilot would have their own switches to select which radio they'll transmit on, but both mics are still live if both pilot and copilot have the same radio selected and one of them keys a mic (I think). I'm still mulling this concept over, and it's also possible another mixer may be required. It will definitely be more complicated than the simple first option, so I won't do much in this direction unless you really want that.
More complicated wiring and two switches mean both radios can be used at once. Pilot and copilot would have their own switches to select which radio they'll transmit on, but both mics are still live if both pilot and copilot have the same radio selected and one of them keys a mic (I think). I'm still mulling this concept over, and it's also possible another mixer may be required. It will definitely be more complicated than the simple first option, so I won't do much in this direction unless you really want that.