A perfect companion for your modded Turnigy 9X transmitter is the FrSky RC system with telemetry. FrSky system supports sending telemetry data back to the transmitter module from the actual model you are flying. By default the D8R-II plus receiver supports two ADC ports (A1 and A2) and a 9600bps serial connection (through which you can send pretty much anything). The A1 port is pre-configured to include a 1:4 voltage divider, and with the jumper it comes with from the factory it measure s the supply voltage for the receiver. The A2 port measures anything between 0 and 3.3V and has no built-in voltage divider. The er9x and open9x firmwares have versions which support the FrSky telemetry functions which enable you to display telemetry data on your transmitter and configure alarms.
In this how-to I’ll describe what you will have to do to get your transmitter to support FrSky telemetry data without any active components (no transistors, level converters or any external wires). The first thing we need to do is modify the transmitter:
Note: The yellow MOSI line points to the wire which goes to the MOSI pad under the MISO wire, not the the pad under the MOSI wire.
You just basically “remap” two switches (Ail and Thr switches I think) from the MOSI/MISO pins on the AVR to two unused pins. Notice the hot glue! I strongly suggest you somehow glue the wires or else they might fall off from vibration. Then we are going to use the MOSI/MISO pins for the serial communication between the FrSky module and the AVR. The MOSI/MISO pins are also shared with the ISP connection.
Next thing we need to do is cut some traces around the connector that goes to the radio module:
As you can (hopefully) see from the picture you need to cut the traces going to the second pin from the top (originally used for 6V supply voltage on the original JR system, unused and grounded in the turnigy 9X) and the last pin (used for the FM antenna). Notice that the second pin from the top is ALSO connected to the ground layer from the other side of the PCB! You must cut those traces as well. Use a multimeter to make SURE the pin is not connected to to ground.
Next you just need to connect the MOSI and MISO pins from the AVR to the pins on the connector for the radio module:
I personally soldered these wires to the ISP connector (D-sub) on the back of my transmitter to where the MOSI and MISO pins go from the AVR.
Then we need to modify the FrSky radio module. What we are doing is rerouting the serial lines from the FrSky main processor (an STM32 ARM) past the level converter chip (3232) and to the serial connector and the radio module connector. I personally have a v2 FrSky module which has a different PCB than the v1 model.
You can do the mod inside the FrSky radio module in couple of different ways. You can basically solder off the U1 chip (3232) and just connect the two vias to the serial connector pads on the right OR you can leave the chip there and cut the two traces going to R24 and R19. You can solder on the traces going to the vias by scratching the traces clean off the lacquer (flux helps when soldering). I personally soldered off the chip:
Then you also need to route the same serial lines to the actual radio module connector on the corner of the board:
Again, notice the hot glue on the fairly delicate solderings near the U1 chip!
Now you’re almost finished! You just need to assemble everything and flash a FrSky telemetry compatible firmware on your 9X. Without a FrSky telemetry compatible firmware your Ail and Thr switches will not work. Now just enjoy your new telemetry data:
The A1 channel is showing the supply voltage for the receiver and A2 is showing main battery voltage (3S). I had to make a voltage divider for the A2 channel so I could measure the battery voltage.