One option is the "standard" frky telemetry protocol (I guess), which you get when set SERIAL2_PROTOCOL to 4.
The other option is described as "Passthrough" protocol (set SERIAL2_PROTOCOL to 10), which is described at http://ardupilot.org/copter/docs/common ... rough.html. From that link:
When it comes to transmitters, most of the ardupilot documentation is opentx specific, and it's pretty clear that opentx supports this enhanced telemetry information. Does ersky9x? If it does, I would be happy to contribute documentation to Ardupilot that describes the configuration.Passthrough FrSky Telemetry is the latest protcol available in ArduPilot and it allows the transmission of raw data directly to OpenTX without any preprocessing by OpenTX and without the need for discovery. The passthrough telemetry protocol is optimized specifically for the FrSky datalink bandwidth and enhancements include having multiple data elements in a single telemetry packet and using floating point representation (e.g. we don’t really need centimeter precision altitude to be displayed on screen when flying at 100m). This results in an improved data refresh rate such that information such as attitude (roll, pitch, yaw) is displayed without any perceptible lag.
Compared to regular FrSky telemetry, passthrough telemetry contains ArduPilot specific information such as flight modes, error messages, warnings, and failsafes.