AXP340/322 ADS-B Transponder - Parallel vs Serial

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Parallel vs Serial - Can I just unplug?

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=1436
Printed Date: 26 Mar 2026 at 4:28pm
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: Parallel vs Serial - Can I just unplug?

Posted By: marktrav
Subject: Parallel vs Serial - Can I just unplug?
Date Posted: 18 Nov 2017 at 2:53pm

I had the transponder installed in 2015, same time as my Aspen install. I did not yet have the 240 or 540. At the time, the 340 was wired with my Aerosonic alt encoding altimeter (parallel). Later, when I had the 240 and 540 installed, I'm led to believe that everything was wired together. (240/340/540/Aspen/EDM900) Altitude is showing up in my EDM900 stream, so I'm pretty sure that is coming from my Aspen, though it may also be coming from my 540.

My Aerosonic is slowly creeping out of spec, and I found out they are no longer rebuilding them as they are obsolete, so I need to replace it with a United altimeter without an encoder.

How do I find out if the 340 was wired with the serial output from the Aspen? I unplugged the Aerosonic and verified failure of the 340, but I'm not sure how to unplug the parallel and instruct the 340 to pick up the serial instead. I was hoping the cutover would be automatic, but evidently, it's not. Do all of the parallel wires need to be cut from the 340 before it will switch over to the serial?

I'm doing research before I take it to the avionics shop. If it's as easy as unplugging the parallel harness and choosing a menu option for serial, I'd like to know that before I pay a shop some amount of dollars to "fix" the problem.

Replies:

Posted By: oskrypuch
Date Posted: 18 Nov 2017 at 6:23pm

IIRC, the 340 will NOT auto-switch between two encoding inputs, but it can be set up to accept Gray (parallel) and serial.

The ASPEN puts out 10' resolution serial encoding data. I have that, as well as a backup Nano parallel encoder hooked to my 540, which in turn is hooked to the remote AXP322. The 540 picks the best input, and does auto-switch. That allows for a fall back in a failure situation, although it does add the 540 to the failure chain for Mode ES output.

But, not sure if the AXP340 will work that way, but if it will, it may be of advantage to you. Otherwise you could just route the ASPEN serial directly to the 340, and call it a day.

* Orest

Posted By: marktrav
Date Posted: 19 Nov 2017 at 3:43am

From my reading of the installation instructions, it seems like I can access the programming menu by pressing and holding the "Func" key when powering on. Is this the way I would instruct the 340 to use parallel vs. serial? I haven't tried it yet. In another part of the instructions, it says that if the parallel feed is active (I assume powered), it will ALWAYS use the parallel feed, even if there is a serial feed present.

Posted By: PA20Pacer
Date Posted: 19 Nov 2017 at 4:35am

Hi Mark-

The installation manual for the AXP340 advises against connecting both serial and parallel inputs, so the chances are that just the parallel is connected. There does not seem to be a setting on the 340 to select serial or parallel input. It appears that it uses the parallel input if present, and if not reverts to the serial input. Connecting the serial input from the Aspen is just a single wire, and once you have that done you will know that the altitude you see on the Aspen is the altitude that ATC is seeing, as long as you are using the correct baro setting.

Regards,

Bob Siegfried, II

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Bob Siegfried, II
Brookeridge Airpark (LL22)
Downers Grove, IL