The stock module from FlySky is OK if you’re just controlling park fliers short-range (out to about 500 meters or so), but its Automatic Frequency Hopping Digital System (AFHDS) technology is fairly lacking if you want better range and robustness to interference. The AFHDS basically just searches for a free channel and uses that. Which usually works fine, but it could go disastrously wrong, for example if a new source of external interference arrives after the system has already started up and chosen its transmit channels.
The DJT module (Figure 1) by FrSky (pronounced “Free Sky”) is a true Frequency Hopping Spread Spectrum (FHSS) system that continuously jumps between channels so that it cannot be affected by interference on any particular set of channels. With this system and one of FrSky’s “full range” receivers you can expect to get about 1.5 km of range.
|Figure 1: The FrSky DJT two-way telemetry FHSS module|
The DJT is also a two-way telemetry system, which means it not only transmits an RC control signal to the aircraft, it also receives telemetry data back, as long as you are using one of FrSky’s telemetry receivers. The telemetry data would normally come from sensors connected through a sensor hub to your receiver, such as altitude, GPS position, compass, accelerometer, RPM, temperature, battery level, etc.
But if you get a FrSky receiver with Received Signal Strength Indication (RSSI), such the D8R-XP (Figure 2), you can make good use of the two-way telemetry feature even without any sensors attached: The DJT module on your Tx will start beeping to warn you when your radio signal is getting low. That way you’ll know when you’re starting to fly too far away, and it’s time to turn back!
|Figure 2: FrSky D8R-XP telemetry receiver|
Installation StepsThe FlySky Tx almost fits modules designed to be compatible with JR radios, but unfortunately not quite: in order to change to a non-stock module, you’ll first have to cut down the raised area of plastic that supports the five module pins. This supporting area will need to be cut back until it’s no more than 2.0 mm high (or just cut it away altogether, because once it’s cut back it won’t be doing anything to support the pins anyway).
1. Unplug and remove the battery, and remove the stock module. Once the stock module is out, you’ll see the five pins at the lower right of the module compartment, supported by the raised plastic area. This raised plastic is the only thing getting in the way of installing a new module.
2. Get the pins out of the way. When you’re cutting the raised area of plastic, you’ll want to move the pins themselves out of the way, so as not to damage them. First open the case of your Tx by removing the six screws at the back (Figure 3).
|Figure 3: Remove 6 screws to open case of FlySky TH9X radio|
|Figure 4: FlySky TH9X with case open|
|Figure 5: Remove 4 screws from FlySky TH9X back circuit board|
|Figure 6: Pull back circuit board to clear module pins away from area you'll be cutting.|
|Figure 7: Cut plastic support for module pins down to 2.0 mm high or less|
|Figure 8: Replace the back circuit board but don't tighten screws yet|
|Figure 9: FlySky Tx with FrSky DJT telemetry module installed|
5. Put the Tx case back together again, replacing the six screws. You’ll probably want to blow all the plastic dust you’ve generated out of the unit first though. Again, make sure both modules seat perfectly before putting the case back together.
Using the FrSky DJT SystemWe trust the FrSky DJT module and D8R-XP receiver will work great for you. You’ll get about 1.5 km range, telemetry, including RSSI, as well as a range check mode to make your pre-flight range check easier. I love the fact that the module beeps to tell you when the signal is getting weak, as it gives me great peace of mind to know I always have a strong signal while flying.
Using the DJT module and D8R-XP receiver is pretty straightforward. Binding, failsafe, range check, and setting for high-speed or regular servos is explained in the manual. But let us know if you have any questions.
Enjoy using your FlySky TH9X radio, now upgraded to the true FHSS system!