Combine PTT with xmt selector switch

OldBoldPilot

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OK, gang, it’s time for the dreaded noob-with-questions, so thanks in advance for any help you can offer. Before I start cementing configurations and buying components, I want to have a solid understanding of the overall architecture. I’ve learned a lot by reading existing threads, but I haven’t found any discussion addressing this particular wrinkle.

I’m planning a lot of HOTAS controls, some of which mimic the military aircraft I’ve flown. In an airplane with 2 radios, we used a single rocker switch on the throttle grip for PTT—rock one way to transmit on radio A, rock the other way to transmit on radio B. Looking at the schematic in the intercom install manual, it appears I can accomplish this dual function with a DPDT rocker.
  • One set of poles is the selector: pin 12 connects to the switch’s common terminal, and the other terminals connect to the PTT IN pin on each radio.
  • The other set of poles is the PTT, connecting pins 2 and 10 indirectly via the mic jack. These terminals would be jumpered so this circuit would close in either rocker position.
The trick here is that this switch would be remote from the intercom and therefore away from the shielding jackets of the 3-conductor wires: 4 ft of wire from intercom pin 12 to the rocker, then 2 wires 4 ft back to splice into the respective 3-conductor wires connected to pins 25 and 12. So I’m guessing that these wires need to be shielded, and their shields should connect to the shielding of the other wires, which ultimately connect to pin 1. The PTT wires would run independently from the rocker switch to the mic jack. My understanding is that it’s a good precaution to shield those wires also, but I’m not clear on where that shield should connect to avoid a ground loop. Maybe the jack housing itself, since it’s already connected to ground at pin 2?

MIXING

My intended installation is a SV-COM X25 as primary comm and a Garmin GTN650xi for nav and secondary comm. I haven’t been planning to use an audio panel or mixer because the SV intercom has dedicated inputs for everything: both COMMs, nav audio (pin 17), Skyview audio (pins 6 & 19), and I’m used to monitoring multiple radios so I don’t see a need. (If a radio gets annoying I can usually switch it to guard or a quiet freq, or just turn down the individual volume.) If the nav audio can’t be muted within the GTN650, I could add a simple SPST switch to (de)select it. What benefit(s) would a mixer provide?

INTERLOCK

I haven’t gotten access to the Garmin install manual yet, but my understanding is that the GTN650 doesn’t have an electrical transmit interlock connection, it uses software to mute itself when it magically senses another radio transmitting. All well and good when the SV-COM is transmitting, but what about the SV-COM when the GTN650 is transmitting? I’m planning to put one COM antenna on the turtledeck and the other on the belly, but I don’t know if that will be enough to rule out coupling. Anyone have experience with this?
 

Carl_Froehlich

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On the 650 “de-sense” interlock - this is a bandaid for Garmin to mitigate cross talk with the horrible GTR-200 (many use it as Comm #2). I can report that with the 650 as Comm #1 and the Dynon radio as Comm #2, and no connection to the 650 de-sense connection there is no cross talk problem when transmitting on either radio. Both of my comm antenna are mounted on the belly (RV-8) perhaps two foot separation.
 

OldBoldPilot

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On the 650 “de-sense” interlock - this is a bandaid for Garmin to mitigate cross talk with the horrible GTR-200 (many use it as Comm #2). I can report that with the 650 as Comm #1 and the Dynon radio as Comm #2, and no connection to the 650 de-sense connection there is no cross talk problem when transmitting on either radio. Both of my comm antenna are mounted on the belly (RV-8) perhaps two foot separation.
Sweet! 1 down, 2 to go!
 

Rhino

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Don't use a standard DPDT rocker switch to key the mic. It only has two positions (there is no "off" state). It will stay in one of the two positions, and always give you a hot mic on one radio or the other. What you want is a momentary On-Off-On. It's always off in the center (default) position, and the momentary function insures the switch returns to that position when you let go of it. That would be something like this:

I may not be understanding your install concept, so correct me if I have something wrong. Is this single stick or dual stick? Single stick makes things simpler. You may not even need pin 12. Dual stick introduces more variables, such as both mics always being hot whenever anyone keys a radio. Do you want one pilot to be able to use Com 1 while the other uses Com 2? That's even more complicated.

You don't actually have to route PTT through the intercom either. Already having wiring from the mic jacks, with PTT, to the intercom makes routing mic jack PTT fairly straightforward. But routing PTT from the stick can go straight to the radio, bypassing the intercom entirely, which simplifies wiring. You don't mention which connector you're referring to when you list pins, so I'm guessing you're referring to the DB25 connector on the SV-INTERCOM-2S. Radios are keyed with a ground input (both Dynon and Garmin). That is your PTT. So the common pin on the DPDT switch would be a ground. That can be chassis ground, but preferably it should be grounded to your intercom or wherever else you've established a common ground. The other two pins on that pole would route that ground to your radios, pin 15 of the DSUB connector on the X25, and pin 11 of the P1003 connector on the GTN 650.

Your stick PTT does not route through your mic jack. The mic jack has it's own ground used for PTT. You can connect those two together at just about any point along the line, but that isn't necessary, and could require more wiring. The radio doesn't care where ground comes from, the stick or the headset via the mic jack, so it could be counterproductive to try routing the same ground from both sources on a single wire.

In the case of your mic jack wiring, there's audio in there, so you definitely want shielded wire. Assuming you don't piggyback your stick PTT wiring in with your mic jack PTT wiring, shielding on the stick PTT wire is far less of a potential issue. But it's certainly a good idea to use shielded wires where feasible, so I recommend it. All wiring in your audio/radio system should ground to a common point, usually the audio panel. But since you won't be using one, the intercom or the GTN could be used. Using the GTN might require less wiring (more stuff comes together there), which is always good.

You likely don't need a mixer for what you envision. They're primarily used to combine multiple audio sources into a consolidated output (or vice versa), and to provide impedance matching. From what you stated, the intercom appears to fit your needs in that regard, and you can always mute nav audio with a switch.

I may be able to help you with a wiring diagram if you can give me more detail on your proposed system architecture and design concepts.

I started this post not understanding you were originally referring to the pins on the intercom. I think I edited everything sufficiently after I figured that out, but please forgive me if I missed something and said something that seems to make no sense.
 
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OldBoldPilot

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Rhino, thanks for the feedback. I thought the (on)-off-(on) rocker switch function was implicit in my description, but I guess not. It's a tandem cockpit, and for simplicity I'm planning for only partial functionalities in the RCP.

This weekend I took the time to do what I should've done up front: convert my scribbles into a presentable modification of Figure 6 in the intercom installation manual. Hope this helps.
  • You speculated that I might not need pin 12, but I don't see how I could avoid using it if I want to choose which radio I'm transmitting on.
  • For simplicity, this diagram focuses only on the transmit functions, omitting the headphones and other inputs which would all be unchanged. The "new" FCP wires are drawn in green, the RCP wires in red.
  • For clarity's sake, I didn't depict shielding on the "new" wires, but I guess they'd all be shielded with the braiding grounded to pin 1 like the rest.
  • I've got the PTTs connecting at the mic jacks, but I suppose they could connect at the appropriate intercom pins instead if that reduced the length of the wire.
I thought it would be simple to hard-wire the RCP PTT to one radio (in this drawing, the T25, using a DPST pushbutton), but once I got into it, it raised more questions.
1684156502126.png


I'm not intentionally giving each cockpit the ability to transmit simultaneously, but it appears that this architecture might allow FCP to transmit on the GTN650 and the RCP on the T25 simultaneously, depending on any interlock function. No idea what might happen if both tried to transmit on the T25 at the same time--maybe the whole system would explode. I suppose it wouldn't be much harder to duplicate the FCP setup in the RCP, giving both cockpits a combined selector/PTT switch, but again I have no idea what would happen if the PTTs were keyed simultaneously in different combinations.
 

Rhino

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Sorry. Haven't had time to play with it, but I'll look at it again soon.
 

Rhino

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Apologies again. I was in the Caribbean and didn't have access to all my resources. I probably should have waited until I returned before making my first reply, but I didn't want to leave you hanging.

The reason I said you may not need pin 12 on the intercom is because you can feed ground straight through from your PTT switch to your radios in some cases. Whether or not you want to do that, and how, depends on your cockpit setup and audio scheme. From what you posted later, you probably do want to use pin 12 to take advantage of the priority function the intercom will provide for keying the T25. However, you have pin 12 connected wrong in your diagram. It doesn't connect to the input from your PTT switches. It connects to the PTT output to the radios. That allows the intercom to prioritize which audio to use for the radios when both PTTs are pressed. So, if you both key the T25 simultaneously, there would be no issue (paragraph 4.9 of the install manual).

Assuming the digital interlock function on the 650 works as advertised, there shouldn't be any RF issue with two cockpits keying both radios at the same time. As long as the antennas aren't really close together, the worst problems you'd be likely to see is RF interference, and maybe noisy audio on the T25. Nothing should explode though.

You may be able to take the mic jacks completely out of the PTT scheme for the rocker switch on your front stick. It's not a bad idea to use a shielded wire for your PTT ground, but it really isn't be necessary to shield a ground wire with a ground wire. It certainly isn't necessary if they ground at the same point, which they would in your diagram if your mic jack grounds to the intercom like it should. You can shield the ground wire out of an abundance of caution if you like, but there's no need to route it all the way from your mic jack if that's further away. You just choose whatever path is easiest and shortest to route that shield to your common ground point, presumably the intercom, which may be closer to the stick in your front cockpit. For the rear cockpit, the mic jack will probably be closer.

Are you planning to have the stick switches be the only PTT source, or do you also want the ability to key the radios via panel buttons? If you want to have panel buttons for PTT, the panel buttons will have to be tied in separately (you'll need one button for each radio since you don't have an audio panel). Your diagram seems to indicate you want to do that, since you have pins 10 and 16 connected. That's the primary reason why I said I'd need to know your PTT scenario, so I'd know exactly how it should be wired.
 

Rhino

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Wait a minute. I think I can see better what you're trying to do now. You want the front cockpit rocker to serve as both a com radio selector and PTT at the same time. That's why you have pin 12 wired that way, and why you used a double pole switch. I'll have to look at that more carefully, but not tonight.
 
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Rhino

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This is a first draft. I still want to look at it some more. I always go back and find mistakes I made.

1685222660808.png

Some things to consider:

Because the copilot has a direct PTT connection to the T25, that radio will always key when he presses his mic switch. However, because that same line also goes to the intercom, the intercom should use its' priority logic and not send copilot audio to the radio whenever the pilot simultaneously keys his PTT. So the net effect should be nothing at all.

I did not add panel PTT switches to this diagram.

The Dynon intercom has a master volume only. It cannot adjust volume on individual radios. Both pilots hear both radios at master volume, no matter which radio they're using. So, if you have panel space (knobs can be really small), I can add some panel knobs into this diagram to allow adjustment of each radio volume independently. The master knob on the intercom will still adjust both audio levels, but the panel knobs will allow adjustment of each radio volume independently. It would be similar to a PC. There's a master volume, and also a way to adjust the volume of individual media sources. If you go that route, I suggest panel volume knobs in the front cockpit only, to control volume to both cockpits. Adding them to the rear cockpit would get fairly complicated, and may require a mixer.
 
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Rhino

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This would be the panel audio volume control. A standard 2k ohm potentiometer should be about right. You would want the connections shielded.

1684797847975.png


CORRECTION: Don't use a standard potentiometer. I forgot that the human ear does not perceive audio in a linear fashion, so a standard potentiometer is not a good choice. You want an audio taper potentiometer.

 
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Rhino

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This diagram has panel PTT buttons depicted. Note that adding a panel PTT to the front cockpit will also require a radio select switch be installed. It only affects the function of the panel PTT switch in the pilots' cockpit. It does not interfere with radio selection using the stick mounted rocker switch.

1685249564563.png


I also removed the HDX from the diagram for the sake of simplicity.
 
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Rhino

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I've corrected some minor mistakes in both those diagrams. So, if anyone has copied or downloaded them, you might want to do so again.

I also forgot to mention, with this setup, the T25 will not transmit while the GTN 650 is transmitting. You can remove the transmit interlock connection on pin 18 of the T25 to change that, but there's an increased possibility of interference between radios if you do that, should they both be keyed simultaneously. It still shouldn't cause any damage unless the antennas are close together, though.
 
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OldBoldPilot

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Wow, this is way more in-depth help than I had a right to expect. Thanks so much!
Your first schematic seems to be exactly what I was going for--still comparing it to my original design to find any important differences. I'll probably go ahead and give the RCP the same dual-function rocker setup as the FCP, but I doubt I'll implement any panel PTTs. Additional volume pots to handle the mix seem redundant when the individual radios have built-in volume controls on their faceplates--maybe "nice to have" vs "need to have."
 

Rhino

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Yeah, the volume knob idea was from a project where the builder didn't have radio control heads for both radios, and was controlling them from the EFIS, so he didn't have the individual volume controls you do. He also had a mixer. And I forgot about the knob on the 650. Told you I always make mistakes.
 

Rhino

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Fixed a mistake in the diagram with the panel mount PTTs. Also simplified the shielding connections in all versions.
 

SBLNK

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These drawings show shields grounded at both ends. Is this correct? SV Intercom-2S Installation Guide Rev C Figure 7 of the SV COM -425 Diagram shows shield grounds at D25 connector Pin #1.
 
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Rhino

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The shields are all grounded at pin 1 on the intercom in these diagrams. I'm not sure why you think otherwise.
 
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