Companion Computer for Pixhawk class
Interfacing a companion computer (Raspberry Pi, Odroid, Tegra K1) to Pixhawk-family boards always works the same way: They are interfaced using a serial port to TELEM2, the port intended for this purpose. The message format on this link is MAVLink.
Pixhawk setup
Set the SYS_COMPANION parameter (in the System group) to one of these values.
Changing this parameter requires an autopilot reboot to become active.
0to disable MAVLink output on TELEM2 (default)921600to enable MAVLink output at 921600 baud, 8N1 (recommended)57600to enable MAVLink output at 57600 baud, 8N1157600to enable MAVLink in OSD mode at 57600 baud257600to enable MAVLink in listen-only mode at 57600 baud
Companion computer setup
In order to receive MAVLink, the companion computer needs to run some software talking to the serial port. The most common options are:
- MAVROS to communicate to ROS nodes
- C/C++ example code to connect custom code
- MAVProxy to route MAVLink between serial and UDP
Hardware setup
Wire the serial port according to the instructions below. All Pixhawk serial ports operate at 3.3V and are 5V level compatible.
Many modern companion computers only support 1.8V levels on their hardware UART and can be damaged by 3.3V levels. Use a level shifter. In most cases the accessible hardware serial ports already have some function (modem or console) associated with them and need to be reconfigured in Linux before they can be used.
The safe bet is to use an FTDI Chip USB-to-serial adapter board and the wiring below. This always works and is easy to set up.
| TELEM2 | FTDI | ||||
|---|---|---|---|---|---|
| 1 | +5V (red) | DO NOT CONNECT! | |||
| 2 | Tx (out) | 5 | FTDI RX (yellow) (in) | ||
| 3 | Rx (in) | 4 | FTDI TX (orange) (out) | ||
| 4 | CTS (in) | 6 | FTDI RTS (green) (out) | ||
| 5 | RTS (out) | 2 | FTDI CTS (brown) (in) | ||
| 6 | GND | 1 | FTDI GND (black) |
Software setup on Linux
On Linux the default name of a USB FTDI would be like \dev\ttyUSB0. If you have a second FTDI linked on the USB or an Arduino, it will registered as \dev\ttyUSB1. To avoid the confusion between the first plugged and the second plugged, we recommend you to create a symlink from ttyUSBx to a friendly name, depending on the Vendor and Product ID of the USB device.
Using lsusb we can get the vendor and product IDs.
$ lsusb
Bus 006 Device 002: ID 0bda:8153 Realtek Semiconductor Corp.
Bus 006 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 005 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 002: ID 05e3:0616 Genesys Logic, Inc.
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 004: ID 2341:0042 Arduino SA Mega 2560 R3 (CDC ACM)
Bus 003 Device 005: ID 26ac:0011
Bus 003 Device 002: ID 05e3:0610 Genesys Logic, Inc. 4-port hub
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 002: ID 0bda:8176 Realtek Semiconductor Corp. RTL8188CUS 802.11n WLAN Adapter
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
The Arduino is Bus 003 Device 004: ID 2341:0042 Arduino SA Mega 2560 R3 (CDC ACM)
The Pixhawk is Bus 003 Device 005: ID 26ac:0011
If you do not find your device, unplug it, execute
lsusb, plug it, executelsusbagain and see the added device.
Therefore, we can create a new UDEV rule in a file called /etc/udev/rules.d/99-pixhawk.rules with the following content, changing the idVendor and idProduct to yours.
SUBSYSTEM=="tty", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0042", SYMLINK+="ttyArduino"
SUBSYSTEM=="tty", ATTRS{idVendor}=="26ac", ATTRS{idProduct}=="0011", SYMLINK+="ttyPixhawk"
Finally, after a reboot you can be sure to know which device is what and put /dev/ttyPixhawk instead of /dev/ttyUSB0 in your scripts.
Be sure to add yourself in the
ttyanddialoutgroups viausermodto avoid to have to execute scripts as root.
usermod -a -G tty ros-user
usermod -a -G dialout ros-user