DFC90/DFC100 Digital Autopilot - DFC90 and power source for Cirrus

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DFC90 and power source for Cirrus

Printed From: Avidyne
Category: Avidyne General
Forum Name: DFC90/DFC100 Digital Autopilot
Forum Description: Topics on the Avidyne DFC Digital Autopilots
URL: https://avidynelive.com/forum_posts.asp?TID=466
Printed Date: 26 Mar 2026 at 6:45am
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: DFC90 and power source for Cirrus

Posted By: Cirrusdude
Subject: DFC90 and power source for Cirrus
Date Posted: 21 Nov 2014 at 6:07am

I understand the DFC90 is connected to the main bus (Alt1/Batt1) whereas the current Stec autopilot is connected to the essential bus (Alt2/Batt2). Furthermore, if Alt 1 goes offline, and then you turn off Batt1, the DFC90 will go off line too. Why did Avidyne configure the DFC90 to connect to the main bus? I would have thought it should be connected to the essential Bus.

Thanks

Replies:

Posted By: JBAvidyne
Date Posted: 24 Nov 2014 at 10:07am

Avidyne has posted a Service Information Letter http://www.avidyne.com/publications/dfc90/606-00170-003.pdf" rel="nofollow - 606-00170-003 that addresses this very issue. I would recommend downloading and reading the SIL to have a better understanding of the behavior of the DFC90 in the Cirrus.

Avidyne has worked closely with flight instructors, owners, and pilots through Cirrus Owners and Pilots Association (COPA) to educate Cirrus pilot's of the DFC90 behavior during an Alt1 failure.

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Jared Butson (jbutson@avidyne.com)

Posted By: Cirrusdude
Date Posted: 25 Nov 2014 at 5:08am

Jared,

Can I buy the DFC 90 with Service Bulletin 1 installed but not Service Bulletin 2 installed?
Tim

Posted By: JBAvidyne
Date Posted: 25 Nov 2014 at 6:07am

For a variety of performance, safety, and feature set reasons, Avidyne does not make previous software releases available. This applies to all of Avidyne's equipment, including the DFC90.

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Jared Butson (jbutson@avidyne.com)

Posted By: zman520
Date Posted: 31 Mar 2015 at 8:59am

If the DFC-90 was a direct slide in replacement of the STEC, how is it getting its power form another bus?

Posted By: clydeps
Date Posted: 19 Jan 2016 at 3:17pm

Update on this issue - I had a simple wiring change done on my SR22 recently that solves the problem.

Pin 50 on autopilot connector P552 (labelled 14/28VDC TRIM) originally connected to the pitch trim power via wire CPT621-22. This has been reconnected to the autopilot power AACC629-22 which comes from the essential bus, and now the A/P works normally with the main bus off.

Posted By: ansond
Date Posted: 05 Dec 2016 at 12:43pm

Clyde - just wanted confirm that wiring change.. was pin 50 on the A/P an voltage input or output? I was wondering as the wiring diagram is not clear.... with the wiring change, is CPT621-22 now unconnected to the DFC?

Doug

Posted By: Melohn
Date Posted: 26 Dec 2025 at 6:17pm

I've been flying my SR20 with R1 of the DFC90 firmware since it was originally installed 12 years ago. The AP works just fine in an SR20 on the Essential bus, since the power for the roll trim and pitch trim servos runs through the DFC90, which is on the Essential bus.

With Vantage, I'm going to need to upgrade to R3. This will break my ability to use the DFC90 with only the Essential bus operating.

Would it be possible, in a future DFC90 firmware release, to consider removing the test for main bus voltage if the aircraft is an SR20? This would be a significant return in functionality for this model.

I understand that this may not apply to SR22s, where the pitch servo is connected to the Main bus.