EFIS audio to Garmin 340

I’m upgrading my old GRT WS EFIS units to the GRT Horizon EX units. I’ve had to run new wiring for the warning audio output from the EX units to the Garmin 340 intercom system (WS units didn’t have audio warning outputs).

I discovered during this process that there was already wiring to the Garmin 340 (pins 31 - Alt Warning In and & 32 Alt Warning Return). These are the two 340 pins recommended for this purpose. Unfortunately I have no clue what these two existing audio wires are connected to.

An airport friend ( who is an engineer and A&P/IA) suggested I just add my new audio warning input wiring to the existing two wires going to the Garmin 340 pins but in addition add 390 ohms resistors (covered in shrink wrap individually) to the two positive wires feeding pin 31. I asked GRT tech support about this but they never responded.

My questions:
1. Has anyone else attached two audio inputs to the same audio input pin?
2. Why are 390 ohm resistors required on both positive lead ins to the Garmin 340 pin 31?

Tankerpilot75 said:

My questions:
1. Has anyone else attached two audio inputs to the same audio input pin?
2. Why are 390 ohm resistors required on both positive lead ins to the Garmin 340 pin 31?

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1. Yes, and this should work okay. Each signal will be somewhat less loud than before, but, not too noticeable. With the GRT there is a software adjustment for the volume, anyway. If you had 4 or 5 connections to make you’d really start to notice the volume reduction.
2. Typical audio panels have input impedances around 600 ohms. However, various audio sources (radios, tone generators, CD players) have varying, and sometimes low, output impedances. So without the resistors, let’s say the unknown wire goes to a tone generator with an impedance, looking ‘backwards’, into the electronics, of 1 ohm. Your signal from the grt reaches the junction going to the audio panel input, and ‘sees’ 1 ohm going back down the unknown wire, and 600 ohms going into the audio panel. So 599/600 of the current goes into the unknown device, and only 1/600 goes into the audio panel. The unknown device may ‘short out’ the grt signal. Installing the resistors limits that problem to a tolerable level. Not critical, 220 ohms or 560 ohms will work.
3. Personally I’d want to know where that unknown wire is going!
4. These resistors should be strain relieved, they’re not designed to be pulled on. Same for solder joints. Cable tie them to a bundle for mechanical support.

Thanks Bob! I appreciate the clarification and suggestion. I will be sure to secure for mechanical support.

I’m thinking they might go to the EIS. I understand some older EIS’ had a tone generator for warning purposes but I’ve honestly never heard a warning tone. If I can, I’ll try to trace those two wires with my meter for connectivity.

Jim, Another option would be to route your grt warning tones to the com 3 input - assuming you don’t now, nor ever will, have three coms.
Disadvantage: you would have to remember to always have com 3 receive selected ‘on’. Because of this I would not recommend this. But, it is an option.
A third option: Bring the wires from the grt out to near the headphone jack; terminate the wires with an 1/8” plug. Plug the plug into the aux in (or music) jack on the battery module of your electronic noise reduction headset (I use Lightspeed). Now only you will hear the warnings. This is what I did, as a ‘temporary’ test. My temporary test has lasted many years now. I just haven’t gotten around to wiring it into my home-made audio panel/intercom. I like the idea that only I hear the traffic and obstacle warnings - no need to freak out my wife! Since I didn’t plan ahead for this option I’ll need to open up the intercom box and add some wiring. Not hard, just haven’t gotten to it.