Here is an overview of the Frsky R9M-Lite module, and we will test the latency and see how it compares to the TBS Crossfire. Both are great 900MHz RC systems for long range and many are hesitating which one to get. With latency being one of the main considerations we hope you find the test useful.
We previously reviewed the R9M module, which is for the radios with JR module bay such as the Taranis X9D-Plus, Q X7 and Horus X10S. The R9M-Lite module is the miniature version which is designed specifically for the Taranis X-Lite radio.
The R9M-Lite is an affordable long range option compared to the Crossfire. It’s easy to install on the X-lite without doing any mods, basically just plug and play! The X-Lite actually support Crossfire too but it requires special mounts.
900MHz radio system provides a much more solid and reliable link compared to the built-in 2.4GHz RF module in the X-Lite. You can fly around big fields and not have to worry about losing signal. You might even be able to go behind obstacles you could never dream of with 2.4GHz.
Radio receiver options are also pretty good, the R9-mini and R9MM receivers are even smaller than the 2.4GHz R-XSR, even better at half the price! But just like any 900MHz systems, antenna mounting is different than 2.4GHz. You need to mount them in a specific way to have the best possible signal.
The way we hold the transmitter will have the antenna pointing directly at the aircraft which gives a weak signal. You might want to get a 45 degree or even a right-angle RP-SMA adapter.
Flash the correct firmware on your radio and RF module (and receiver), and select the correct mode for your region. Using the correct frequency for your region is critical to signal reliability.
Here is a screenshot of the Model Setup in OpenTX.
The latency in our RC link can be divided into 4 parts:
Gimbals => TX Interface (OpenTX) => TX module => RX => FC
The latency is measured between the points before OpenTX and after RX (bold text).
The latency are measured with the TX modules installed in the Frsky X-Lite. Other gear used:
- R9M-Lite TX module with R9 Mini receiver
- Crossfire Micro TX module with Crossfire Micro V2 receiver
Further Reading: Does the Frsky X-Lite support Crossfire?
Note that there is a stick filtering (6-sample averaging) that exists in OpenTX, which introduces a varying delay. To properly measure latency a custom version of OpenTX was built to get rid of this filtering. This was explained in more detail in our previous latency test of the X-Lite.
First test consisted of tapping into the gimbal output line and generating a 260Hz PWM signal with a STM Discovery board, altering between 10% and 90% duty cycle every 128 milliseconds, while observing the resulting receiver output signal frame over the course of 60 seconds.
Latency is measured between the falling edge of gimbal PWM signal and the start of corresponding RX signal frame.
Note that this doesn’t take RX signal frame length into account. The frame length for SBUS is slightly less than 3 ms, that of CRSF is 0.7ms, which is in turn dictated by 100k vs. 420k baudrate, 2.97ms × 100/420 = 0.7ms.
The R9M-Lite is flash with FCC firmware.
Here is the average latency:
- R9M-Lite => SBUS – 14.07ms
- Crossfire Micro TX => CRSF – 13.9ms
- Crossfire Micro TX => SBUS – 20.23ms
The R9M-Lite actually performed surprisingly well, very close to the Crossfire in terms of latency. But note that SBUS protocol has a longer frame length than CRSF protocol – over 2ms.
Another surprising observation is that the Crossfire doesn’t seem to work as well with SBUS, there is an 8ms extra delay compared to when using CRSF protocol on the receiver. Our guess is that the Crossfire goes into 50Hz mode when SBUS is selected as output (instead of 150Hz). However R9M has 150Hz for SBUS.
Here is the full result:
|Latency (ms)||Mean||Stddev||Min||Max||25%||50%||75%||95%||Refresh rate (Hz)||Notes|
|R9M Lite – SBUS (R9 Mini)||14.066||2.501||7.515||20.205||12.140||14.524||16.016||17.843||150||add 2.97 ms for SBUS|
|XFIRE Micro – CRSF (Micro V2)||13.900||2.448||8.432||19.635||12.075||13.879||15.752||17.842||150||add 0.7 ms for CRSF|
|XFIRE Micro – SBUS (Micro V2)||20.231||6.233||7.680||32.831||14.828||20.606||25.346||29.756||50||add 2.97 ms for SBUS|
|OpenTX latency (switch)||5.626||1.478||2.046||8.937||4.628||5.630||6.574||7.558|
All captures for Saleae as well as Python script used to produce the results will be posted on Andrey’s GitHub.
- Jul 2018 – Article published
- Jan 2019 – Added overview of R9M-Lite and how to setup