Rick_A

Rick_A

Well Known Member
When I bought my main avionics stack (GNS 480 & GTX 327) from Starck, I was surprised the way the Altitude Encoder was wired. The encoder output (RS232 from the EFIS) goes to the GNS480 then from the 480 to the GTX 327. This works fine as long as the 480 is powered on.

I'm getting ready to upgrade my 327 to a 330, so this gives me an opportunity to change the current configuration. My inclination is to simplify things and feed the encoder output directly to transponder. This will make it easier to install the 330 and remove the 480 as a potential failure point.

My questions is: What's the point of feeding the encoder data to the 480? Are 430's & 530's wired this way also?
 
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One advantage...

Rick_A said:
......

My questions is: What's the point of feeding the encoder data to the 480? Are 430's & 530's wired this way also?
Click to expand...

...is that if your GPS knows your altitude, it can give better airspace warnings - like being under a outer shelf of a Class B/C.

My GX-55 said I was in a restricted area up the Owens Valley in CA, but it was talking about an area over 18,000 ft...:rolleyes:
 
Your super duper GPS will want Altitude data or it may not ever activate/properly sequence certain approaches.....

It is also typically used as a cross check with the integrity monitoring system.

I would leave it connected...if not you may find yourself getting "no altitude data present" or something similar warnings all over the place...
 
I see that there may be some cross check & integrity monitoring value - to see if the GPS derived altitude is in the right ballpark. However, since the encoder is sending Pressure Altitude there may be a pretty big difference between True Altitude and and Pressure Altitude.

Since this is a WAAS GPS, I would hope that the GPS would be using the GPS derived altitude (which should be close to True Altitude) for both airspace alerts and approaches.
 
Answer from Garmin

I called Garmin support and the answer is that Altitude Input is an IFR Certification requirement.

I can wire the encoder output directly to transponder if I want but I have to leave the existing input to the 480 in place.
 
Or...

...you could connect the encoder to your transponder as you're thinking, and use the serial output from the transponder to the GPS. Either way you have a single point of failure in the mix ... Encoder -> GPS -> Transponder or Encoder -> Transponder -> GPS so you need to make a call as to which you're most (or least :) ) comfortable with.

I use the gray code (parallel) output from my Ameriking AK350 encoder as well as a serial output from my AF3500 to feed the parallel and serial inputs respectively on my GTX327, with the 2nd serial output on the GTX327 going to my GNS430W. According to the GTX327 installation manual, the parallel input has a higher priority than the serial, but if the encoder fails I'm assuming the GTX327 will get altitude info from the AF3500 and feed that in turn to the GNS430W...that's the plan anyways...
 
For the electrical gurus, could you just splice the encoder output so that each device gets it's data without having to daisy chain (as in Rick's example of his 480-327 wiring) thereby eliminating the single point of failure?
 
If you are using the serial formats, most likely yes similar to the way you can feed many input devices NMEA data with only one output device.

With the grey code formats, you might need diodes on the inputs of some devices as some of them have provisions for this and some don't. This prevents the lines from being pulled down if the unit fails or is turned off.
 
I ended up splicing the Serial Out from the GRT so I could send the encoder data to both the 330 and the 480. I'm using one Serial port on the GRT configured as FADC (Fuel Air Data Computer).

I like this much better since I will still have working Mode C/S if the 480 goes TU. I tested it today and it works great. Now all I have to do is go flying to test the TIS.
 
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