The 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 checked. Park a tune you like in a spare bank before an experiment.
• Back up and restore the tune and set-up, every bank, to the phone (not ESC, ports, LED or GPS). Share a backup with a friend: their tune, not your servos.
• Read the black box and check the filters after a flight, with advice in English.
• Search for any setting by the word you would use.
• Updates arrive by themselves. The app offers new dongle software when there is some, and installs it over Bluetooth at the field or over WiFi at home. It also tells you when a new Rotorflight is out, and to back up first.

One app, one software. The same board with radio modules fitted is the LockDownRadioControl 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 Seeed XIAO ESP32-S3, with the software in and its four-wire lead soldered on, ending in the plug that fits a Nexus-X port. I have loan dongles (ask me for one), or make your own: one small board, four wires, and the software from your browser.
• 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: iPhone and iPad from the App Store; Android from the Phone Apps page.

How to use it

1. Plug the lead into a spare UART

   On a Nexus, port B or C, not A.

2. Add the USB cable, if you have one

   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 does almost everything the Configurator does.

3. No USB cable? Set the port once

   Most things still work by the lead alone. A few need USB: the black box, the command line and a factory reset. Rotorflight has to be told where the dongle is: 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. 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

The same dongle connects one of your receivers to a simulator on the computer, Windows or Mac.

1. Choose the role

   In the app, on the dongle's Role card, choose Simulator interface.

2. Move the lead to the receiver

   The receiver's signal output to D5, its 5 V to the dongle's 5V pin, ground to ground.

3. Plug the dongle into the computer

   The USB cable powers the whole rig: no battery, no flight controller. 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. Stick-to-axis mapping, reversing, the spool-up realism and the camera keys are on the app's Simulator pages.

A LockDownRadioControl 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 yours are in helicopters.

Like it? The receiver needs no dongle

The dongle is the little brother of the LockDownRadioControl receiver: the same brain with the radio fitted. The receiver does everything on this page and flies the model, so there is no dongle to fit and no spare port used up. It updates itself over the air, and flies a simulator from your own transmitter.

The whole system is free and open source: about £35 of parts for a receiver, and from about £120 in parts for a transmitter (approximate). The Version 2 receiver works with the Version 1 and Version 2 transmitters.

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. Strip the four bare ends and solder them to the board: white to D6, green to D5, red to 5V, black to GND.

Check the colours on your own lead

Those four are right for the lead that came with my Nexus. Another may be wired differently, and leads from other suppliers certainly are. Read the pin labels beside the port, or the manual, and check with a meter before the first power-up. Getting + and − the wrong way round can destroy the flight controller.

Build it with four wires, not the circuit board

Do not order the dongle circuit board in the repository (RXV2/kicad/dongle). Its first batch had the plug wired wrongly: it put 5 V onto a signal pin and destroyed two chips. Four wires are all a dongle needs.

FlyDragon Pro: use the GPS port

On a FlyDragon Pro, use the six-pin GPS port on the end of the case: everything on one plug, and its 5.0V pin is regulated.

• 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.
• Using RX2/TX2 for the signals is the same rule (RX2 to D6, TX2 to D5), and the port is UART2.
• Then in Rotorflight, Ports tab, UART5 (or UART2): MSP at 115200 for a dongle; or Serial RX for a LockDownRadioControl receiver, with the receiver type set to CRSF. (The FlyDragon's own receiver is on UART1.)

Making your own: the software

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

Plug the chip into a computer, open the flasher and press one button. The browser writes the software straight in: nothing to install, nothing to download. It erases as it goes, so the same button also puts a used board back exactly as new.

Flash a chip from your browser

Chrome, Edge or Opera, on a desktop or laptop. Safari, Firefox and phones cannot do it.

By hand: five ready-made files

Only if you would rather do it yourself, or your browser cannot. Nothing to compile. Works on Mac, Windows and Linux.

1. Install esptool

   Espressif's flashing program. With Python already on the computer:

   pip install esptool

2. Download the five files

   Into one folder. They are always the current release:

   • bootloader.bin
   • partitions.bin
   • boot_app0.bin
   • firmware.bin
   • littlefs.bin (the app's own pages)

3. Put the board into flashing mode

   Plug it in. Hold B, tap R, let go of B. The yellow light goes out when it has worked. (A red light may stay on: that is the charger; ignore it.)

   Do it every time. A board that already has our software takes the USB socket for its own use, so no port appears without the buttons. The port then shows as /dev/cu.usbmodem… on a Mac, or 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

   Check: every one of the five reports "Hash of data verified". If one does not, run it again.

Or build it from the source

Everything is free on GitHub: github.com/Mmessiter/LDRC_V2. The receiver and dongle software (GPL-3.0-or-later) is the RXV2 folder: one program does both jobs. Install PlatformIO and Node.js. Do B and R before each of the two upload commands, and put your own port in place of the one here:

git clone https://github.com/Mmessiter/LDRC_V2.git
cd LDRC_V2/RXV2
pio run -e xiao_s3_usb -t upload --upload-port /dev/cu.usbmodem101
pio run -e xiao_s3_usb -t uploadfs --upload-port /dev/cu.usbmodem101

The first command writes the program, the second the app's pages. On Windows the port is COM3 or similar.

Then: first switch-on

1. Unplug it and plug it back in

   The USB port then goes quiet. That is normal, not a sign of failure.

2. Name it

   Open the phone app and tap Connect: the new board is in the list as Untitled. Rename it at once, because two boards with the same name cannot be told apart. Or join its own WiFi network, Untitled, from a computer and open 192.168.4.1.

3. Nothing else to configure

   A board with no radio chips works out what it is. Wired to a flight controller it is a dongle. Fed a receiver's signal on D5 it is a simulator interface. After this it updates itself over the air: you should not need a cable again.

What a dongle cannot do

It has no radio, so it does not fly or bind, and it keeps no flight records. View channels still shows the sticks, as the flight controller sees them. Almost everything for setting up and tuning Rotorflight is there.