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Aviation Alerts and Advisories

Service Alert 1380 Rev A: GNC 255 installations that use the RS-232 NMEA output to display VOR information on external indicators or EFIS/PFD systems


GNC 255 series units with Display Software Version 2.01 that are connected to an external EFIS/PFD, excluding G500/G600 systems, are affected.


If the GNC 255 RS-232 Serial Port is configured to output ‘NMEA’ data, the CDI on an EFIS/PFD using this data will deflect in the wrong direction when the GNC is tuned to a VOR and the TO/FROM flag indicates FROM.

This issue is most prevalent in experimental amateur built (EAB) and light sport aircraft (LSA) aircraft using experimental EFIS/PFD systems since the RS-232 NMEA interface is typically used in these installations.

This issue does not affect the CDI deviation output on other interfaces (analog, composite, and ARINC 429). This issue does not affect the CDI deviation that is displayed on the GNC 255. This issue does not affect ILS or Localizer navigation. This issue does not affect G500/G600 installations that are configured to use the ‘SL30’RS-232 interface to communicate with the GNC 255.


For installations that do not have an EFIS/PFD system, no Pilot Action is needed.

For installations that have an EFIS/PFD system, pilots must determine the method of interface between the GNC 255 and EFIS/PFD display by referring to installation drawings or their Garmin dealer. If no determination is made or it is determined that the RS-232 NMEA interface is used, pilots must not use VOR navigation guidance that is displayed on the EFIS/PFD from the GNC 255.

Customers that have experimental EFIS/PFD systems connected to the GNC 255 using the RS-232 NMEA interface can contact Garmin Aviation Product Support for a copy of the Service Bulletin 1379. The service bulletin contains instructions for disabling the RS-232 NMEA interface.


This issue will be corrected in a future GNC 255 Display software release.

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