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VOXL 2 Connecting an External Flight Controller

Table of contents

  1. Summary
  2. UART Communication Options
  3. Hardware Setup
    1. Using USB/UART Addon Board (M0125)
    2. Using M.2 5G Modem Addon Board (M0090)
    3. Using J8 Camera Connector
  4. VOXL SDK Software Setup
    1. Component Overview
    2. VOXL2 SW Setup
      1. Disable PX4 Running on VOXL2
      2. MAVLink Server BETA RELEASE Installation
      3. Setup voxl-mavlink-server.conf
      4. Start voxl-mavlink-server
    3. PX4 SW Setup
  5. pymavlink Software Based Example


Although VOXL 2 has the capability to run PX4 in a real time DSP, some use cases call for an external flight controller.


UART Communication Options

You can communicate between a flight controller TELEM port and a VOXL 2 UART port. For example, Flight Core has a default telemetry port on J5 with a MavLINK instance running at 57600 baud.

Starting in System Image 1.3, there are 3 applications processor UARTs exposed. Additionally, there are 3 DSP UARTs used by PX4, that will be made available in future softwware releases via and API or similar.

UARTConnectorAdd-On / Adapter
/dev/ttyHS0J8, camera group 2M0076 with hand soldering to test points
/dev/ttyHS1J3, Board to BoardM0125 / Purchase Here
/dev/ttyHS2J5, HS Board to BoardM0090 / Purchase Here

Hardware Setup

Using USB/UART Addon Board (M0125)

In the following example, we’ll add the USB3/UART Add-On Board (M0125) to VOXL 2 (M0054) which gives us a 4 pin JST GH connector with a UART availabe at /dev/ttyHS1 from the applications processor.

Using MCBL-00067-1, connect from M0125’s J3 to Flight Core’s (v1/v2) (M0018/M0087) J5, TELEM1/TELEM2 (respectively).


M0125 J3M0018 J5
2 RX2 TX
3 TX3 RX

Using M.2 5G Modem Addon Board (M0090)

The VOXL2 (M0054) can have the M.2 5G Modem Board Addon attached, which exposes a UART on M0090 - J9 that is accessible in VOXL2’s application processor as /dev/ttyHS2.

Using MCBL-00066-1, connect from M0090’s J9 to Flight Core’s (v1/v2) (M0018/M0087) J5, TELEM1/TELEM2 (respectively).


M0090 J9M0018 J5
2 TX3 RX
3 RX2 TX
66 GND

Using J8 Camera Connector

You will need to use a level shifter likely, as the logic is 1.8V here. VOXL2’s camera group 3 connector can be used with M0076 interposer which has test points you can solder to.


  • UART_TX - TP7
  • UART_RX - TP8
  • VREF 1P8 - TP1
  • DGND - TP11

VOXL SDK Software Setup

Component Overview

The following image captures the main components in play when using VOXL2 with a USB/UART Addon Board and a Flight Core.


VOXL2 SW Setup

Disable PX4 Running on VOXL2

If you aren’t using PX4 on VOXL 2, you should likely disable it from running on bootup.

adb shell
systemctl disable voxl-px4

Currently, the test branch of voxl-mavlink-server supports the external flight controller use case.

To install from source:

adb shell
cd /home/
git clone -b eric-external-fc
cd voxl-mavlink-server/
voxl-configure-pkg-manager sdk-0.8
./ qrb5165 sdk-0.8
./ native

chmod 755 pkg/control/postinst
dpkg -i voxl-mavlink-server_0.1.4_arm64.deb

Setup the external flight controller options in voxl-mavlink-server.conf. For the px4_uart_bus field, these are current options:

  • /dev/ttyHS0 –> “px4_uart_bus”: 0
  • /dev/ttyHS1 –> “px4_uart_bus”: 1
  • /dev/ttyHS2 –> “px4_uart_bus”: 2

For px4_uart_baudrate, configure this the same as the MAVLink instance on Flight Core. In this example, we are using 921600.

For external_fc, set to true.

For the PX4 side, on Flight Core v1 (M0018), check the SER_TEL1_BAUD setting if using the hardware setup above (on Flight Core v2 (M0087), check SER_TEL2_BAUD).

Here’s the setup to use if following allong and using M0125 like the picture at the top of this guide.

 * voxl-mavlink-server Configuration File
 * UART fields are for APQ8096 VOXL1 or QRB5165 VOXL2 with external fc
 * UDP fields are for PX4 on SDSP on QRB5165 only
 * External FC field is for QRB5165 only
	"px4_uart_bus":	1,
	"px4_uart_baudrate":	921600,
	"udp_port_to_px4":	14556,
	"udp_port_from_px4":	14557,
	"external_fc":	true

While setting things up, let’s manually start voxl-mavlink-server.

adb shell
voxl2:/$ voxl-mavlink-server
loading our own config file
Parameters as loaded from config file:
px4_uart_bus:               1
px4_uart_baudrate:          921600
udp_port_to_px4:            14556
udp_port_from_px4:          14557
external_fc:                1
Configuring for external flight controller
Successfully opened /dev/ttyHS1
starting receive thread
Init complete, entering main loop

To run this on startup, you can enable using systemctl enable voxl-mavlink-server.

PX4 SW Setup

You need to connect over the USB interface to configure the external flight controller, in this guide’s case the Flight Core v1.

  • Connect to QGroundControl over USB and open the parameters.
    • On Flight Core (M0018), J5 is mapped toTELEM1 in PX4 (/dev/ttyHS6). For this example to work out of the box, make sure to set SER_TEL1_BAUD to the default baud rate of 921600 if you’ve changed it.
    • On Flight Core (M0087), J5 is mapped toTELEM2 in PX4 (/dev/ttyHS4), so use SER_TEL2_BAUD instead.

Check the status of MAVLink instances by opening the MAVLink Console in QGC and running mavlink status:

On Flight Core v1, you should see something like this if using 921600 baud:

instance #1:
    mavlink chan: #1
    type:        GENERIC LINK OR RADIO
    flow control: ON
      tx: 81.8 B/s
      txerr: 0.0 B/s
      tx rate mult: 0.682
      tx rate max: 1200 B/s
      rx: 20.9 B/s
      rx loss: 0.0%
      received from sysid: 0 compid: 197: 43, lost: 0, last 343 ms ago
    FTP enabled: YES, TX enabled: YES
    mode: Normal
    MAVLink version: 2
    transport protocol: serial (/dev/ttyS6 @921600)

Above is showing you that the Flight Core is communicating with VOXL2 successfully, note tx: 81.8 B/s and rx: 20.9 B/s rates indicate bidirectional traffic.


This clashes with the example above, so ensure to disable the voxl-mavlink-server if you configured the VOXL SDK.

The SER_TEL1_BAUD/SER_TEL1_BAUD baud rate should be set to 57600 for thi example (for Flight Core v1/v2 respectively).


First, connect VOXL 2 to the internet so we can use apt to install pymavlink. You can connect an ethernet or WiFi dongle to the USB3 port.

Conect to the VOXL 2 over ADB:

adb shell

Now on target:

apt update
sudo apt-get install python3-pip
sudo apt-get install libxml2-dev libxslt-dev python-dev
sudo pip3 install pymavlink pyserial

Get the tool from PX4:

cd /home

Now, run the program and interact with the Flight Core.

python3 /dev/ttyHS1
Connecting to MAVLINK...

NuttShell (NSH)

nsh> ver all

HW type: V106
HW version: 0x00000000
HW revision: 0x00000006
FW git-hash: 7008425b466eba94e35304e502454208d578ba56
FW version: 1.11.3 0 (17498880)
FW git-branch: fcio-v2
OS: NuttX
OS version: Release 8.2.0 (134349055)
OS git-hash: ec20f2e6c5cc35b2b9bbe942dea55eabb81297b6
Build datetime: May 10 2022 18:38:31
Build uri: localhost
Toolchain: GNU GCC, 9.3.1 20200408 (release)
PX4GUID: 0002000000003930353534385109003a002e
MCU: STM32F76xxx, rev. Z