I published previously Arduino LiXXX battery monitor & altimeter + vario for JETI DUPLEX with Arduino Pro Mini and MS5611. It would be perfect to have all telemetry data and alarms available directly in the transmitter, i.e. without JETIBOX device. One of possible solution is to use radio with OpenTX firmware. Unfortunately, new radios with ARM processor (9XR Pro) don’t have JETI telemetry as the option in Companion software (ver. 2.2). But when you are looking for telemetry types available, FrSky D system is definitively in the menu.
I would like to present here my code for Arduino Pro Mini as the converter from JETI DUPLEX (non EX) protocol to FrSky D telemetry system.

I was inspired by the autor jan68 here. But despite of more than one code version, I wasn’t able to run it without issues. Therefore I rewrote some parts of the code in order to offer you result for download here:

I built-in the JETI DUPLEX Tx module inside the box for JR Tx module and I added Arduino Pro mini PCB above the JETI Tx board (you can see it on the picture above).

Wires from / to Arduino board:
White = GND
Yellow = +12V
Green = JETI signal - you can see 2 resistors and diode, described in the ino file, directly connected to PCB
Blue = FrSky RX / TX - to pin 5 inside the rf module bay, as 9XR Pro radio has this signal connected already - from COM1. This pin should be 5 V tolerant, I identified internal connection:

The 4 pins from Arduino board are for future purpose, I can change firmware easily.

You have to set "FrSky D" type telemetry, not inverted (the check box is NOT TICKED) and you can select auto-discovering after that.

For successful compilation in Arduino 1.8.1 environment you will need modified serial port library (JETIBOX needs unusually serial port with 9 bits, odd parity and 2 stop bits).

And here you can see the result: radio’s telemetry screen with battery total voltage (calculated by OpenTX firmware from voltage of 3 cells), minimal cell voltage identified (derivated by the function of OpenTX firmware) and vertical speed on the left half of screen and voltage for each of 3 cells on the right half of display.