Details on Serial Port Communications (updated January 2015)

[Evan Ludeman / T8]

CNv-XC communicates on the serial port according to a subset of the CAI Dataport Standard described in the attached user's guide.

Most users with 3rd party devices should be able to interface with the CNv-XC by selecting com options for the Cambridge 302.  Note that there are a few differences.  In particular, serial transfers of databases, tasks and flight logs are not supported. 

CNv-XC starts in "Pocket-Nav" (PNP) mode transmitting !w, $GPGGA and $GPRMC sentences.  It accepts only the !g command (MC, BUGS, BALLAST).

CTRL-C always resets the command port and puts CNv in command mode (the cmd> prompt).

From the cmd> prompt, the command "PNP" starts "Pocket-Nav" mode, as described above.

Upload, Download and Log modes are not implemented.  CNv-XC uses the USB drive for setup, waypoint, task and flight log transfer.


We use all of the functionality available in the !w sentence, here are the specifics:

·         wind direction in degrees

·         wind in 1/10 m/s

·         wind age in seconds

·         component wind 1/10 m/s + 500

·         altitude in m + 1000

·         kollsman in mb

·         TAS in 1/100 m/s

·         vario 1/10 kt + 200

·         average vario 1/10 kt + 200

·         relative vario 1/10 kt + 200

·         MacCready 1/10 kt

·         % ballast

·         % clean


Com port speed is currently fixed at 4800 baud, only because this is the speed that the 302 used and our immediate desire was for a "drop in replacement" for the 302 as far as serial comm goes.  There is no technical reason that the speed cannot be increased (a lot) and I expect that we will make this user selectable at some point.

[AM22]

Thank you, Evan!

[christophb]

Thanks for sharing the details. That helps to have the CNv supported by 3rd party software developers.

Some other recent flight loggers (i.e. PowerFLARM core) also do not support flight download via 3rd party devices. Flight download via USB drive is the only option. In day to day operations, this proved to be less convenient. Many pilots use their flight computers to upload flights directly to the internet. After an outlanding it's not easy to get the flight transferred from a USB drive to another mobile device with internet connection. On weaker competition days this ensures quick calculation of daily results when many pilots went to the cows.

Best,
Christoph

[Evan Ludeman / T8]

That's an interesting point.  Thanks.

[Evan Ludeman / T8]

Notes on CNvXC startup and changing settings:

On startup, CNvXC begins transmitting data in PNP mode within a couple of seconds of power on.  Prior to GPS lock, the location and time information will be missing, but the sentences are still transmitted.

!g may be used at any time to change bugs, ballast and MC.  No need to switch modes.