Linux setup¶

Opening the arm's serial port on Linux requires membership in the group that owns the device. The brltty service can also hold the port. The commands below are for Ubuntu and Debian.

Port names¶

The arm appears as /dev/ttyUSB0 or /dev/ttyACM0, depending on the kernel driver.

Path	Driver	Adapter
`/dev/ttyUSB0`	`ch341`	WCH CH340 USB-to-serial adapter.
`/dev/ttyACM0`	`cdc_acm`	Built-in USB CDC ACM device.

Discovery probes every serial port, so either name works. Both drivers are part of the mainline kernel and load automatically. To list the ports and the driver in use:

ls /dev/ttyUSB* /dev/ttyACM* 2>/dev/null
udevadm info -q all -n /dev/ttyACM0 | grep -E 'ID_VENDOR|ID_MODEL|ID_USB_DRIVER'

If no port appears, check that the driver is loaded and what the kernel logged when the arm was connected:

lsmod | grep -E '^ch341|^cdc_acm'
sudo dmesg --human | tail -30 | grep -iE 'ch341|cdc_acm|tty(USB|ACM)'
sudo modprobe ch341    # or cdc_acm

Permissions¶

The device is owned by the dialout group (uucp on Arch Linux). Add the current user to the group, then log out and back in, because group membership is read at login:

sudo usermod -aG dialout $USER
groups | grep dialout     # after logging in again

Without membership, connect raises DobotPermissionError, and the panel shows the same message for the port:

Permission denied for /dev/ttyACM0: your user is not in the 'dialout' group. Run: sudo usermod -aG dialout $USER then log out and back in (or restart).

Busy port¶

A busy port is reported as port '/dev/ttyACM0' is busy. If no other program holds the port, brltty, the Braille display service installed with Ubuntu Desktop, may have claimed the device. Stop and disable it:

sudo systemctl stop brltty
sudo systemctl mask brltty

To restore it, run sudo systemctl unmask brltty && sudo systemctl start brltty.

To keep brltty running and exclude only the arm, add a udev rule in /etc/udev/rules.d/85-brltty-no-dobot.rules:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", ENV{BRLTTY_DEVICE}="-"

Then run sudo udevadm control --reload-rules && sudo udevadm trigger.

Selecting a port¶

from pydobotlab import Magician

Magician()  # automatic selection
Magician(port="/dev/ttyACM0")

The example scripts accept the port as an argument or with -p / --port:

python examples/smoke_test.py /dev/ttyACM0
python examples/smoke_test.py --port /dev/ttyACM0

Diagnosing connection problems¶

discover_with_diagnostics returns the arms found and the ports that were rejected, with a reason for each:

from pydobotlab import discover_with_diagnostics

found, failures = discover_with_diagnostics()
print("found:", found)
for failure in failures:
    print(failure.port, failure.reason, failure.detail)

The reason is one of permission, busy, no_reply, or other. See ProbeFailure.

To check each layer separately:

lsusb | grep -i -E '1a86|dobot|ch340'                                    # the OS sees the device
python3 -c "import serial; serial.Serial('/dev/ttyACM0', 115200).close()"  # the user can open it

Result	Cause
`lsusb` shows no device	Cable, power, or the arm.
`permission denied` when opening	The user is not in the `dialout` group.
`device or resource busy`	Another program or `brltty` holds the port.
Opens, but discovery finds nothing	The port belongs to another device, or the arm is not responding.

Qt libraries¶

On a minimal Ubuntu installation, import PySide6.QtWidgets may fail with a missing libEGL.so.1 or libOpenGL.so.0. Install the system libraries:

sudo apt install libegl1 libopengl0