Autopilot GPS 296 connection failure

[FELX]

Apologies for returning to a topic that has had previous appearances – but I’ve looked at as many threads as I can find but still not cracked the problem.

Problem – Autopilot works OK on Alt and Heading but will not ‘see’ the output from the Garmin 296 GPS. Pressing the Trk or Nav buttons on the AP74 does not light them up.

KIT    D100,    D120,   AP74    GPS Garmin 296

CONNECTIONS   Databus seems to be working OK
COM1 out on GPS connected to D120  37 way connector pin 19

SETTINGS
D100 has been set to SETUP>HSI>EFIS SERIAL    as manual
D120 has been set to SETUP>GENERAL>EMS SERIAL     INPUT = AVIATION     BAUD = 9600
GPS has been set COM1  AVIATION NMEA + VHF OUT  9600 BAUD  (ADVANCED) NORMAL

The system used to work OK but I had an intermittent databus problem which caused me to have to break it down and test the components. Since then I’ve not been able  to get the GPS connected.

Your advice would be really appreciated.
Thanks,
Terry

 

[Dynon]

On the Dynon side, your input should be NMEA, not AVIATION format. That should hopefully do it.

 

[FELX]

Hi
Thanks for the email.
I did change the D120 input to NMEA as suggested but I'm sorry to say that it did not solve the problem. Still no lights (or operation) when the TRK or NAV button pressed on the AP74.

At one point the D100 announcement said 'descend descend' for no valid reason... probably an unrelated glitch?

I have to admit that I've not had a chance to check the continuity of the wire from the GPS to the D120. I will do that when I can but meanwhile can you give me any other software/settings ideas please?

Are you saying that apart from the NMEA error the settings are correct?

Thanks
Terry

 

[Dynon]

The "descent" is part of the altitude alerter function. The most complete description is in the pilot's user guide - available at docs.dynonavionics.com, but an excerpt is below.

All of your other settings look good. That's basically all there is to it, once you have the connection. On the D100's HSI page, press NAVSRC until you have the magenta GPS page up. Does it show any information? Or does it say "waiting for GPS signal" (which it will say when there is either no GPS at all, or when it is connected but not locked on).


----


If the BUGS > ALM button is toggled to ALT, then turning the altitude bug on enables the altitude alerter function with the bug value as the target altitude. While climbing or descending toward the target altitude a level-off alert sounds when passing through 500 feet from the target altitude. The target altitude is considered captured when altitude is within 150 feet of the target. Flying more than 200 feet away from the target triggers a short audio alert and alternates the bug in red and yellow as a visual alert. When below the 200-foot window, a rising tone is sounded; when above the 200-foot window, a descending tone is sounded. The visual climb or descend alert clears after recapturing the target altitude or 30 seconds. Flying back inside the 150-foot capture window re-arms the alerter without any user interaction. If your system is configured to output voice via an HS34 or AP74 module, the voice alerts “climb” or “descend” are sounded, instead of tones. Additionally as you approach the altitude bug, the voice alert “altitude” is sounded when crossing 500 feet away from the bug.

 

[Robski]

The 296 and the D120 are connected to the 'same' ground, aren't they.
I know in theory there should only be one ground, but if the 2 connections are a long way apart could there be noise between them?

 

[Dynon]

That can indeed be an issue if there is a big difference in the two grounds. It's rare with RS-232 though. It's pretty resiliant.

 

[FELX]

Hi,
Thanks for those useful replies. I will do some electrical/continuity checks as soon as I can - a few days time.

I had not knowingly set the altitude function so that was helpful.

Thanks,
Terry

 

[FELX]

Hi
1.  D100 HSI page checked and found that there was no GPS connected (which was not a surprise)
2.  Continuity check of GPS to earth - good
3. Continuity check GPS COM1 out to D120 pin 19 - no connection.
4. I was about to pull out the wiring looms when I wondered if the COM2 had been used on the GPS . - YES = connection good!
5. Reset GPS for COM2 and the HSI page recognised a GPS present.
6. AP74 now lights up on HDG and TRK. No light on the NAV button but there was no course set on the GPS. No time for further checks.

Things are too obvious sometimes!!

Thanks for the advice.
Can I assume that as long as the D100 recognises a GPS present then if the TRK button lights there is no reason why the NAV feature should not work as well?

Looking forward to a flight test.
Kind regards,
Terry

 

[Dynon]

NAV only works when there's an active course being given by the GPS. So, you have to have an active direct-to or flight plan active. If you don't have an HSI on the GPS page, you won't seen one on the Dynon either (and won't be able to use NAV mode). You also need to have the GPS source selected on the HSI page on the D100, by the way.

 

[Robski]

It's rare with RS-232 though. It's pretty resilient.

Very true...

 

[FELX]

Hi,
Thanks everyone - AP seems to be working now.

Cheers,

Terry.