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Radio interfacing help requested

Started by AAnnillo, November 14, 2021, 09:45:52 AM

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November 14, 2021, 09:45:52 AM Last Edit: November 14, 2021, 10:48:00 AM by AAnnillo
I'm attempting to interface OEM Gables Engineering VHF COMM/VHF NAV radios with X-Plane with the FlyJSim Boeing 727-200.

I do not have the OEM manuals for these two (2) radios but have successfully determined the pin outs for all three (3) jacks.

The dials, as I'm the last to discover, are rotary encoders.

On the COMM side, there are 18 places for the NNN.xx MHz frequencies (118.xx-135.xx) and 40 places for the xxx.NN kHz frequencies (.00, .02, .05, .07, .10, .12, .15, .17, etc.).

On the NAV side, there are 10 places for the NNN.xx MHz frequencies (108.xx-117.xx) and 20 places for the xxx.NN kHz frequencies (.00, .05, .10, .15, .20, .25, etc.)

I'm guessing that certain combinations of contacts of each disk of the encoders will 'trigger' the frequency dialed-in (18/40 for COMM and 10/20 for NAV).

Is there a way to determine what these connections/combinations are using a multimeter set for continuity reading (e.g., A3+A4=109.xx, B3+B9=120.xx, B12+C7=.97, etc.)?
I've tried every combination using the 18 places (pins) for the COMM NNN.xx as well as the NAV side and am not getting any 'hits' when connecting any of the NNN.xx or xxx.NN combinations.

If I can determine which pins/internal wiring 'hits' 118.5, for example, then I can try and interface directly into X-Plane.

I'm looking for assistance from the electronic wizards here!!

Thanks in advance for any direction.



Usually what I do is like you have, a pin discovery, and a ground discovery.

If you have a breadboard you can build a simple LED status array using 18 LEDs connected to ground with a 330-ohm resistor in series for each LED.

Once you have all that wired, You should be able to determine light patterns and what they mean.

Since you're using X-Plane you can use a Teensy 3.2 or 3.5/6 and interface it directly to X-Plane.

Ray Sotkiewicz


What a great idea and thank you very much for sharing this!

I've seen many different interfacing options (e.g., Mobiflight, RSC/SimVim, et. al.) and don't know which to use. 

On one of my radios, I've been able to determine some of the pin out assignments (3 jacks, mind you) and located a chassis connection which I suspect is a ground.  There are 3 individual wires attached to it and I was able to trace these wires to 3 different pins.  Still wasn't able to get a 'beep' for continuity on the multimeter, thus my original posting.   

I will try your suggestion.  Hopefully, it'll provide some clues.

Thank you again for taking the time to share your information and strategy.

All the best,



November 16, 2021, 06:42:49 PM #3 Last Edit: November 16, 2021, 07:47:12 PM by RayS
QuoteI've seen many different interfacing options (e.g., Mobiflight, RSC/SimVim, et. al.) and don't know which to use.

I don't use any of those. (I don't like middleware). I use a Teensy 3.2 loaded with PJRC Teensy-Duino.


I have about 15 Teensy 3.2 Microcontrollers running my sim. No middleware except for bigger things like the FlightIllusion Engine Stack and radio stacks. Aside from those major components, everything else uses the Teensy Microcontroller. It truly is plug and play for X-Plane.

 As luck would have it, I happen to have a Gables Engineering radio laying around collecting dust.

PN: G-6347

It's a COM radio, with 2 DB-25 connectors on the back. I suspect it is the same as yours. If you like, I can do a bit of my own research and get everything pinned out.

You could very easily program a Teensy to interface to your radios via USB to your main sim computer with no middleware for X-Plane.

Since I am pretty much along the same path as you, I would be happy to write the Teensy code for you, but only if our radios are pretty much the same make and model.
Ray Sotkiewicz



I had also seen PJRC/Teensy and am very intrigued and it's definitely something to consider moving forward. I appreciate your input and recommendation.  Among my considerations is the ability to use very old 'steam' instruments as opposed to the 'modern' ones.  My old steam instruments, I suspect, will require severe modifications in order to use; namely, inclusion of stepper motors to drive needles, etc. 

I've already gutted one of my engine instruments and fitted it with an automotive stepper and tested it with an Arduino script.  It works very well and I'm more than pleased.  I haven't yet attempted interfacing with X-Plane via any interfacing platform.  I really wanted to avoid doing any gutting of gauges/instruments and would prefer to try and utilize them as is but now am rethinking that credo.  I recently read something that said 'Do you want to simulate or stimulate your instruments' and that sunk-in and stuck with me.
Luckily, I have a voltage inverter (28VDC to 115VAC/400Hz, 26VAC/400Hz) as well as four (4) voltage transformers (115VAC/400Hz to variable 5-30VAC/400Hz) to power any of the synchro-resolver instruments I have.

I was fortunate to receive an email from Gables with the four (4) schematics I had requested for my transponder, radio switching box and COMM/NAV units.   Now comes the task of making 100% sense of them!  My amateur attempt at determining correct pin outs for the radios was spot-on when comparing the Gables schematics and this gave me some hope, albeit with a few broken wires/connections during my overzealous man-handling of these old and delicate wires.  No worries—I know how to solder.   BTW...my radio units aren't the part number you have and have dials with mechanical internal operations.  Nothing digital about them. Mine are the G-1728 and G-1728B.  VERY ancient.
Likewise, I have the wiring schematics for my old Sperry ADIs and am about 90% complete in my wiring pin outs...again, the trick is how to interface these beasts, operated via six (6) synchro-resolvers and varying voltage types and currents.  I figure if the developer of the B727-200 I'm attempting to replicate can do it, so can I.  Same for the Sperry HSIs I have...but without the wiring diagram/schematics.
Keep in mind:  All this is still very new to me.

This path in learning new things during my retirement has been fun and frustrating and overall, very worthwhile.  I'm more than grateful for the help provided to me by so many kind and considerate people, yourself included.

Keep in touch.


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