AXP-340 Pressure Altitude Display Question
Printed From: Avidyne
Category: Avidyne General
Forum Name: AXP340/322 ADS-B Transponder
Forum Description: Topics on the Avidyne AXP340/322 ADS-B Transponder
URL: https://avidynelive.com/forum_posts.asp?TID=673
Printed Date: 26 Mar 2026 at 2:17pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com
Topic: AXP-340 Pressure Altitude Display Question
Posted By: pburger
Subject: AXP-340 Pressure Altitude Display Question
Date Posted: 18 May 2015 at 6:19am
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If I have only the RS-232 output of my encoder connected to the AXP-340, will the Pressure Altitude display on the AXP-340 show the pressure altitude in 10' increments? If the answer is yes, I would appreciate a picture if anyone has one.
(I know that it will display 100' increments with the parallel gray code input) Thanks in advance... |
Replies:
Posted By: roltman
Date Posted: 18 May 2015 at 7:43am
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Yes, it does show 10'. I don't have any photos handy showing the pressure altitude as all the photos I have show the flight timer. I'll try and snap some photos next time at the plane. |
Posted By: pburger
Date Posted: 18 May 2015 at 9:53am
No need to snap photos, I just wanted a credible source, and I hereby deem you to be credible http://www.freesmileys.org/smileys.php" rel="nofollow"> .I'm at odds with my installer, and want to make sure I have my facts straight. In this case the issue is that I specifically requested a new encoder with RS-232 output to have the better resolution (10' v. 100'). I specifically requested that the RS-232 output be wired to the transponder. I also requested that he wire the position output from the transponder to the IFD-540. He wired both the parallel and the RS-232 serial outputs from the encoder to the transponder which, per the manual, defaults to the parallel gray code data, which is NOT what I wanted. That is why I am seeing the 100' increments. He also wired the RS-232 output from the encoder to the IFD-540, and I do see the 10' increments there. I'm actually okay with going from the encoder straight to the IFD-540, but he needs to disconnect the parallel gray code lines to the transponder, so that the transponder will use the RS-232 serial altitude (which has the 10' increments). Now that I know that the altitude display on the transponder should be in 10' increments, I have a way to test it when he fixes it. Thanks!
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Posted By: roltman
Date Posted: 18 May 2015 at 10:42am
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No problem. I have the Sandia SAE 5-35 and during warm up on the ground I can watch it vary +/-10 on the transponder display. We sent one RS-232 to the IFD540 #1 and use the crosslink to IFD #2. The other RS-232 output we sent to the 340 and used it's "parrot" feature for the MLB100 input. Finally we sent the grey code to my STec Alt pre-selector. |
Posted By: pburger
Date Posted: 19 May 2015 at 5:05am
| Wait a minute -- You have an MLB-100? I didn't think they were out yet. And why does an ADS-B IN unit need pressure altitude? |
Posted By: n7ifr
Date Posted: 19 May 2015 at 5:42am
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Doesn't the ADS-B IN need to have "OwnShip" pressure altitude from some source (ES transponder/encoder) to show accurate Relative target altitudes? So, when will the MLB100 actually be available? Still "early to mid May"? Tom W.
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Posted By: roltman
Date Posted: 19 May 2015 at 5:42am
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No I don't have a MLB-100, but I'm pre-wired and waiting patiently to just drop it in the plane, connect a couple of wires and I'm good to go. I think the pressure altitude is needed for relative traffic altitude computations. I know when we did the install I read up on ADS-B and the various data sent/received, but I honestly can't recall right this second. |
Posted By: bneub111
Date Posted: 19 May 2015 at 11:06am
| Why would you send the pressure altitude to the 540 in addition to the transponder? I don't have an ADS-B in or other traffic source - yet. |
Posted By: n7ifr
Date Posted: 19 May 2015 at 3:20pm
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It looks like some functions in the 540 require pressure altitude (from encoder/Air Data Computer data) to be automated: CAS (calibrated air speed) calculation, Density Altitude, VSR (vertical speed required) TOD (Time of descent), and Altitude Alert (on AXP340). Tom W. |
Posted By: bneub111
Date Posted: 19 May 2015 at 5:05pm
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Thanks n7ifr, that makes sense. One item I don't understand is why vertical speed required on the 540 needs baro input. Couldn't that be calculated with GPS altitude? |
Posted By: Gring
Date Posted: 20 May 2015 at 3:47am
| In 10.1.0.0 it is able to be computed using GPS altitude and position. 10.1.0.0 is the first release of significant enhancements and has a number of small ones like this that add usability to the box. |
Posted By: oskrypuch
Date Posted: 20 May 2015 at 3:49am
It can, and it will in 10.1. Using baro altitude more directly correlates to your other aircraft instruments, and I suspect that is why the Entegra R9 panel did it that way, obviously it always has access to baro altitude. I understand that that section of code was just moved over into the 540. * Orest |
bneub111 wrote:Posted By: n7ifr
Date Posted: 02 Jun 2015 at 12:30pm
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Orest, My 540 is near install completion this week (with pre-existing Aspen PFD/MFD), and since (I am told) the Arinc 429 line carries the input to 540 to accomplish both: . Hdg Bug synchronization PFD -->540 . ACU Baro Pressure --> 540 multiple calculations... If the '429' line will only carry one, I may have to choose - one vs the other. So, after the new Software Version 10.1 update with GPS Altitude integration, will this accomplish all of the 540 features otherwise related to the Pressure Alt, making the choice for HDG Bug the obvious way to go? Tom Wolf
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Posted By: oskrypuch
Date Posted: 02 Jun 2015 at 1:00pm
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The big question is, do you have an ASPEN ACU? (original or ACU2) * Orest |
Posted By: n7ifr
Date Posted: 02 Jun 2015 at 5:50pm
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ACU(1) only. I believe the lines from Aspen for ADC with raw data were wired to 540 without passing through the ACU (for filtering). Tom W.
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Posted By: oskrypuch
Date Posted: 03 Jun 2015 at 4:08am
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If 429 line is run straight to the 540, then you should have no trouble, all the data elements will be transmitted. You will have baro alt as well as the heading bug information available at the 540. * Orest |
Posted By: bellanca1730a
Date Posted: 03 Jun 2015 at 6:05am
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Tom, who's doing your install, if you don't mind sharing? ------------- Sean Andrews Bellanca Super Viking |
Posted By: ddgates
Date Posted: 03 Jun 2015 at 6:27am
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The problem has been the Aspen IM - it allows a 429 connection directly, UNLESS there is an ACU in the system. Avidyne (Jared) has been working with Aspen about a workaround. So far it hasn't happened. David ------------- David Gates |
Posted By: Royski
Date Posted: 01 Aug 2015 at 10:52am
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I just had a shop wire my AXP340 to receive altitude from my Aspen instead of the old encoder. Unfortunately on the flight home my mode C pressure altitude was way off, showing 500 feet instead of my altitude of 3000 feet. Any idea what might cause this? After landing I changed the AXP to show pressure altitude in feet instead of flight level, but I haven't had a chance to see if it makes a difference. I know I need to talk to the shop but if it's a simple software configuration issue I would like to save a trip. |
Posted By: n7ifr
Date Posted: 01 Aug 2015 at 10:59am
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Sean, Sorry about delayed response... Somehow I didn't get the email notification of forum action. Had my 540/340/240/MLB install work out in KTRM. I also note you are at KDVT. I hangar on North Side. Tom W.
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Posted By: bellanca1730a
Date Posted: 02 Aug 2015 at 3:25am
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Thanks, Tom. Much appreciated. Sent you a PM. ------------- Sean Andrews Bellanca Super Viking |
Posted By: jwjenks
Date Posted: 23 Oct 2015 at 5:21pm
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I have the same problem as Royski. I need to disable (off) the IFD and AXP Rs232 connection to get my 340 to send out the right altitude. Why? ------------- JWJ |