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Serial Altitude Encoder Reading 2,000' Low Today

snopercod

Well Known Member
Last month I decided to "upgrade" my GTX-327 transponder by feeding it serial altitude information from my Dynon D-6 rather than from my ancient parallel encoder. It's been working fine and I love the fact that there's no longer a 10 minute warmup period for the pressure altitude. Today, the altitude readout on the GTX-327 started reading precisely 2,000' lower than my actual pressure altitude. I cycled the transponder power to no effect. When I got back on the ground, I de-selected the serial input on the transponder and it defaulted back to the parallel data from the encoder, which was still working. The pressure altitude reads correctly now. I'm thinking that the problem is with the Dynon, but is there any way to confirm this without sending the unit back?
 
You still have the encoder 'Grey Code' wiring hooked up to the xpdr? Did you try it with the encoder unplugged from it?
Yes, they're both hooked up. Are you thinking there is some kind of fight going on? I can certainly disconnect the Grey Code wiring, but it's been working with both hooked up for over a month. Here's the screen where I configure parallel vs. serial. The "Icarus" is for the Dynon:

bH3Tbc.jpg
 
Does the D6 show the correct altitude or is it 2000? low?
Any way the ouput format from the D6 could have changed?
 
Does the D6 show the correct altitude or is it 2000’ low?
I'm really embarrassed to report that I didn't even look. I only installed the D-6 for the directional gyro and fly by my steam gauge altimeter. Thank you for asking the obvious question :eek: I'll also recheck the Dynon output format.
 
NTF

So I went to the airport today to check the altitude tape on the D-6 as Bob suggested. Would you believe the serial data is working again? All I know to do now is have a checklist to switch the transponder back to parallel data should it fail again. I tried that today and it takes about 30 seconds or less if you know the right buttons to push.
 
This is getting weird. Apparently, the serial altitude fails when I turn on the avionics buss at a 1,000' MSL or lower airport. When I start out from a 2,000' MSL airport, it works OK all the way down to the lower airport. But it fails when I start up for the return trip. I guess I'm going to have to put a 'scope on the serial wire to see if there's noise or something. When it fails, I can switch to the parallel data in about 15 seconds now.
 
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