Rotorflight dongle

What it does

• Wizards for a new helicopter and for tuning — each job in order, each step ticked off. À la carte goes straight to any one setting.
• Easy tuning: the sliders that matter, in one place, in plain words. All six banks (profiles), and a choice of how many you use.
• Copy a bank in one tap, read back and verified — and park a tune you are happy with in a spare bank before an experiment.
• Back up & restore the whole setup to the phone, see what is in a backup in plain lines, and share a backup file with a friend — his tune, not your servos.
• Read the black box and check the filters after a flight, with advice in English.
• Search any setting by the word you would use for it.
• Updates arrive by themselves: the app offers a new dongle firmware when one is out, and installs it over Bluetooth at the field or over WiFi at home. It will also tell you when a new Rotorflight is out, and to back up first.

One app, one firmware. The same board with a radio fitted is the LockDown receiver, which does all of this and flies the model. The dongle knows it is a dongle because it finds no radio when it starts.

What you need

• A dongle: a XIAO ESP32-S3, ready flashed, with its four-wire lead soldered on and ending in the plug that fits a Nexus-X port. I have loan dongles — ask me for one — or make your own: it is one small board, four wires, and five files to copy into it.
• A short USB-C cable, if you go the USB way. A Chenyang CY-UC-088-0.07M is ideal: 70 mm, flat, right-angled at both ends. Any short one will do, so long as it carries data as well as power — many short leads only charge.
• A Nexus-X port B or C — not port A: that is usually the receiver's, and the receiver that flies the model needs it. Both are printed RX TX + − and give 5 V. On other flight controllers, any spare UART with 5 V.
• The RXV2 app: Android from the Phone Apps page; iPhone and iPad from TestFlight (install Apple’s free TestFlight app first).

How to use it

Blades off, or motor unplugged, before you touch anything in the app. Setting up is done on the bench, never with the rotor turning.

1. Plug the lead into a spare UART on the flight controller — on a Nexus, port B or C, not A.
2. If you have one, add the USB cable from the flight controller's USB socket to the dongle's. Short, and one that carries data. By USB there is nothing to set, and the app can do everything the computer program can.
3. No USB cable? Most things still work by the lead alone; a few need USB (the black box, the command line, factory reset). Rotorflight has to be told where the dongle is: once, set that port to MSP 115200 — in the official configurator, or in the app's own Ports page if you can borrow a USB cable for the first visit.
4. Open the RXV2 app. It searches for dongles and receivers nearby and switched on; tap yours and follow what the screen says. If it finds none, it offers a demonstration, so you can see the whole thing before you buy a single part.
5. At home, give it your WiFi (Setup → WiFi). Everything is quicker over WiFi. At the field it uses Bluetooth alone, and needs nothing else.

On a Nexus the Ports page names the sockets as printed on the board: Port A, Port B, Port C, TELEM and S.BUS. Other boards show the socket numbers printed on them (UART 1, UART 2 and so on).

Simulator interface

Surprisingly perhaps, the same dongle is also the interface between one of your receivers and a simulator on the computer, Windows or Mac. Choose Simulator interface on the dongle’s Role card in the app, then use the same four-wire lead with the flight-controller end moved to the receiver: the receiver’s signal output to D5, its 5 V to the dongle’s 5V pin, ground to ground. A USB cable to the computer powers the whole rig — no battery, no flight controller — and the computer sees a joystick.

It recognises CRSF, SBUS, IBUS and PPM by itself; the app says which it found and how many channels arrived. Which stick drives which axis, reversing, the spool-up realism and the camera keys are all on the app’s Simulator pages.

A LockDown receiver needs none of this: it plugs into the computer on its own and flies the simulator from your own transmitter. This role is for someone else’s receiver — or for when all your own are in helicopters and the spare board is a dongle.

Like it? The receiver needs no dongle

The dongle is the little brother of the LockDown receiver: the same brain with the radio fitted. The receiver does everything on this page and flies the model, so with a LockDown receiver and transmitter there is no dongle to fit at all, and no spare flight-controller port used up. It also updates itself over the air, and the same receiver flies a PC simulator from your own transmitter. The whole system is free and open source — roughly £35 of parts for a receiver and £120 for a transmitter — and the V2 receiver works with the original V1 transmitter, which is on GitHub too.

The port and its pins

The dongle's lead carries the same four pins as the Nexus port, in the same order, so it plugs straight in. Building your own? This is the order, printed on the Nexus-X left to right with EXT-V on the left:

Easiest of all: use the four-way lead that came in the box with the Nexus. One end already carries the plug that fits the port, so there is nothing to make up — strip the four bare ends and solder them to the board: white to D6, green to D5, red to 5V, black to GND.

FlyDragon Pro: the GPS port, or RX2/TX2

A FlyDragon Pro gives you two good places, and either works — pick whichever suits your model. The six-pin GPS port on the end of the case has everything on one plug, so it is one cable to make up. Or the RX2 and TX2 pins along the side, which take ordinary DuPont plugs: their + pins measured 5 V on mine. Not the servo pins (CH1–3, TAIL): their + carries the servo voltage, up to 8.4 V, which would destroy the board. Meter whichever + you use before connecting.

The rule, whatever the colours: the controller’s RX goes to D6, its TX to D5. The plug is a 6-pin JST-GH (1.25 mm); buy it ready-made with leads and cut off SCL and SDA. Go by the pin names printed on the case, not by wire colours — and check the 5.0V pin reads 5 V on a meter first.

On RX2/TX2 it is the same rule — RX2 to D6, TX2 to D5, + to 5V, − to GND — and the port is UART2.

Then in Rotorflight, Ports tab, UART5 (or UART2): MSP at 115200 for a dongle; or Serial RX for an LDRC receiver, with the receiver type set to CRSF (the FlyDragon’s own receiver is on UART1).

Making your own: the firmware

The dongle is a bare Seeed XIAO ESP32-S3 and four wires — the pin table above is the whole of the wiring. This is how to put the software in.

The easy way: one click, in your browser

You do not have to touch a command line at all. Plug the chip into a computer, open the flasher and press one button — the browser writes the firmware straight into it. Nothing to install, nothing to download, nothing to keep in the right order. It erases as it goes, so the very same button also puts a board that is already in use back to exactly as it was when new.

Flash a chip from your browser →

Chrome, Edge or Opera, on a desktop or laptop. Safari and Firefox cannot do it, and nor can a phone.

By hand: five ready-made files

Only if you would rather do it yourself, or your browser cannot.

Nothing to compile, and no developer tools beyond one small program. Works on Mac, Windows and Linux.

1. Install esptool, Espressif's flashing utility. With Python already on the machine:

   pip install esptool

2. Download these five files into one folder, from messiter.com/rxv2/release/latest/ — they are always the current build:
   • bootloader.bin
   • partitions.bin
   • boot_app0.bin
   • firmware.bin
   • littlefs.bin — the app's own pages
3. Plug the board in and put it into flashing mode: hold B, tap R, let go of B. A brand-new chip will do it without the buttons, but a board that already has our firmware will not — the firmware takes the USB socket over for its own use, so no port appears at all. Doing the buttons every time always works — and you will know it has: the yellow light goes out (a red one may stay on — that is the charger, ignore it). It then shows as /dev/cu.usbmodem… on a Mac, COM3 or similar on Windows.
4. Write all five in one go, from that folder. Put your own port in place of the one here:

   esptool --chip esp32s3 --port /dev/cu.usbmodem101 --baud 921600 \
     write_flash -z --flash_mode dio --flash_freq 80m --flash_size 8MB \
     0x0 bootloader.bin \
     0x8000 partitions.bin \
     0xe000 boot_app0.bin \
     0x10000 firmware.bin \
     0x670000 littlefs.bin

   Every one of the five must report Hash of data verified. If one does not, run it again before going on.

Or build it yourself from the source

Everything is on GitHub under the GPL: github.com/Mmessiter/LDRC_V2. The receiver and dongle firmware is the RXV2 folder — one program does both jobs. With PlatformIO installed:

git clone https://github.com/Mmessiter/LDRC_V2.git
cd LDRC_V2/RXV2
pio run -e xiao_s3_ota -t upload      # the program
pio run -e xiao_s3_ota -t uploadfs    # the app's pages

Then: first switch-on

There is nothing to configure. A board with no radio chips on it works out for itself what it is: wire it to a flight controller and it becomes a dongle; feed it a receiver's signal on D5 instead and it becomes a simulator interface.

1. The USB port goes quiet after flashing. That is normal — it is not the sign of a failure.
2. The board raises its own WiFi network, called Untitled. Join it and open 192.168.4.1 to give it a name and your WiFi details, or simply connect to it from the phone app over Bluetooth.
3. After that it updates itself — over the air, from this site. You should not need a cable again.

What a dongle cannot do

It has no radio, so it does not fly or bind. Live channel bars and flight records belong to the receiver, not the dongle. Everything about setting up and tuning Rotorflight is there.