GTR 200 to Dynon HDX RS 232 Serial Interface Problem

I am working on a new install of a Dynon HDX display and Garmin GTR 200 VHF Com radio and am not able to send a new frequency via the Dynon touch screeen to the GTR 200.

The Dynon HDX displays the active frequency tuned manually on the GTR 200 at the top of the display and on the VHF COMM touch screen menu. When tuning the GTR manually the new active frequency of the GTR 200 is immediately displayed on the Dynon HDX display.

When trying to enter a new frequency via the Dynon touch screen, the change entered via the Dynon is immediately rejected and returns to the frequency that was previously manually set on the GTR 200.

Wiring has been confirmed multiple times to be correct. Different serial ports (1 and 3) of the Dynon that worked correctly when connected to other equipment have been tried with no success.

Have tried using GTR 200, SL 30 and SL 40 VHF Com radio options in the Dynon serial port setup configuration settings but all show the same behavior.

Have confirmed with Dynon tech support that the HDX is being configured correctly. As far as I can tell from reading the GTR install manual, the GTR 200 doesn't have any RS232 configuration settings.

The RS 232 status page on the GTR 200 never indicates that data is received even though the TX counter on the Dynon HDX increments each time a frequency change is entered.

All indication are pointing at a problem with the GTR 200 receiving tuning changes via its RS232 serial input port. Before sending the radio back to Garmin for bench testing, I wanted to see if anyone had experienced a similar problem or had any suggestions/comments or what I might be overlooking.

Hello James,

We can't speak for other manufacturers, but we have received information from other customers that "GARMIN / APOLLO SL40" is the correct format to select on the other system for use with the GTR 200.

The GTR 200 RS-232 interface is documented in the publicly-available installation manual, so the manufacturer of your other equipment should have no trouble confirming which format is correct.

- Matt

Did you double check that the transmit line from Dynon is connected to the correct receive pin on the GTR-200 (Pin 24).

Thx for the responses Matt and Tom. I have confirmed that the wiring between the HDX serial serial port #3 Tx (pin 8) on the HDX and the RS-232 Rx port on the GTR (pin 24) is good (low resistance less than .15 ohms). I am using mil spec D-sub pins on a twisted shielded pair wiring for the Rx/Tx connections between the Dynon and GTR. No sign of pushed back pin etc. on either end.

When speaking with Dynon tech support they were ok with using the GTR 200 setting. I did try selecting GARMIN / APOLLO SL40 but saw the same issue with either. In the Dynon config menu all the user is allowed to chose is the serial port receiver LRU and the HDX sets the correct TX/Rx baud rate etc. for both Tx and RX.

One thing I haven't checked is to make sure there isn't a short between the Tx and Rx wires so will do that next time I am at the hanger.

Thx for the responses Matt and Tom. I have confirmed that the wiring between the HDX serial serial port #3 Tx (pin 8) on the HDX and the RS-232 Rx port on the GTR (pin 24) is good (low resistance less than .15 ohms). I am using mil spec D-sub pins on a twisted shielded pair wiring for the Rx/Tx connections between the Dynon and GTR. No sign of pushed back pin etc. on either end.

When speaking with Dynon tech support they were ok with using the GTR 200 setting. I did try selecting GARMIN / APOLLO SL40 but saw the same issue with either. In the Dynon config menu all the user is allowed to chose is the serial port receiver LRU and the HDX sets the correct TX/Rx baud rate etc. for both Tx and RX.

One thing I haven't checked is to make sure there isn't a short between the Tx and Rx wires so will do that next time I am at the hanger.

A long shot

The Skyview and the Garmin GTR-200 must share a common power ground. Also, even if the continuity checks out good on the pins, make sure the pins are not recessed in the connector so they dont make a good connection. I would also double-check all of thre configuration settings.

More details:

Removed the HDX and GTR from the airplane and checked the wiring again and confirmed via DVM that the Tx an Rx are isolated from each other.

The HDX and GTR are sharing a common ground as the airplane is wired with all equipment grounded at a single point.

Hello James,

Do you have V2.90 software in your GTR 200?

If you have an Aera 5XX/660/79X or GPSMAP 69X portable with a bare wire cable, you could connect that serial output to the GTR 200 receive pin 24 and have another method to verify proper operation of the RS-232 input on the GTR 200.

You should also be able to verify that both the RS-232 transmitter and receiver are working properly by connecting pin 23 (Tx) to pin 24 (Rx) and observing the receiver activity on the RS-232 diagnostics page.

Thanks,
Steve

Hi Matt,

Yes, we upgraded the radio sw to 2.9 downloaded from the Garmin website a couple of days ago.

Your suggestion to test with an Aera 5XX/660/79X or GPSMAP 69X portable with a bare wire cable is a good one. I don't have either of these devices but will check to see if any one in the local RV crowd has one we could try.

Anyway, the plan is to come up with a way to test the GTR 200 RS232 input using different equipment. Will install the wire(s) to pin 23 and 24 of the GTR 200 directly at the radio tray to eliminate the aircraft wiring and see what can be learned. When this is accomplished I will post the findings here on VAF.

Thanks for the help!

Problem Resolved

The problem with the Dynon HDX and GTR 200 not communicating i.e. not being able to have the Dynon HDX send a frequency change to the GTR-200 has been resolved.

Skipping to the end of the story, the wire at pin 24 of the GTR 200 (Rx input) was pinched in the back shell of the GTR 200 connector, shorting to ground the RS232 transmission from the Dynon HDX. Even though the wire appeared to be fine i.e. continuity was good on the wire between the correct pins of the two units, no serial data was getting to the GTR 200.

When originally checking operation of this feature, it appeared that the Dynon HDX was not transmitting data to the GTR 200. I contacted Dynon tech support and it was suggested that the Dynon HDX serial port operation could be tested by placing a jumper between the Tx and Rx pins of the suspect serial port on the Dynon HDX 37 pin D-sub connector, configure the serial port input as "ADVANCED Control Module (ACM)" and check to see if the Tx and Rx counters in the serial port setup page were incrementing appropriately or not.

Rather than take the 37 pin connector on the Dynon HDX apart, remove pins and put in a jumper wire, I decided that the same thing could be accomplished by jumpering across pins 23 and 24 of the GTR 200 as they were all ready connected to the Dynon HDX serial port that did not appear to be working correctly. Checking with an ohm meter that the Tx and Rx pins on the Dynon 37 pin connector were jumper'd, I though I had implemented the desired wiring for the test, or so I thought. (Wrong) Because the wire was damaged in the GTR connector, the test failed.

Running out of things to try, I decided to go back and rerun the check exactly as recommended by Dynon tech support and wala the Tx/Rx ports of the serial port under test worked and the Tx and Rx counters incremented in lock step. Rerunning the test using my short cut method and the Tx counter incremented but the Rx was dead which lead to a "Huh" whats going on here ???

After removing the connector and backshell from the GTR 200 tray it was obvious that the wire in pin 24 was very slightly nicked and bare wire was present. After installing a new twisted shielded pair and taking care to carefully protect the wires in the GTR 200 backshell, everything is now working correctly.

Moral to the story: Trouble shooting 101 "Even though the basic connection looks good, don't forget to check that something isn't shorted to ground".