brltty and Generic Serial Converter🔗
Distributions based on Ubuntu have
brltty enabled by default. Some braille
displays do not change the ids of those converters and therefore brltty claims
the serial device and no device file /dev/ttyUSBx is created. In the following
some solutions to this problem are presented.
In early versions of brltty several generic serial converters (e.g., cp210x)
were affected due to bug #1958224.
Since brltty version 6.4-4ubuntu3 this bug was fixed.
For other generic serial converter this problem still exists. The chipset
ch341 that can be found in the Ulanzi TC001
desktop clock can not be used. The issue is described in bug #1990189.
One solution is to disable the udev part of brltty with the following commands:
The main problem with this solution is that this disable all braille displays.
In the bug description above a better solution is described. This solution is already implemented in the Debian packages of brltty. In Ubuntu this patch is currently not available. The here described solution can be easily adopted to other generic serial converters that are disabled by brltty.
Check for existing serial converter:
In the kernel logs the following can be found:
$ sudo journalctl -b -t kernel
...
Feb 10 10:00:00 chumbucket kernel: ch341 1-3:1.0: device disconnected
Feb 10 10:00:00 chumbucket kernel: usb 1-3: new full-speed USB device number 37 using xhci_hcd
Feb 10 10:00:00 chumbucket kernel: usb 1-3: New USB device found, idVendor=1a86, idProduct=7523, bcdDevice=81.33
Feb 10 10:00:00 chumbucket kernel: usb 1-3: New USB device strings: Mfr=0, Product=2, SerialNumber=0
Feb 10 10:00:00 chumbucket kernel: usb 1-3: Product: USB Serial
Feb 10 10:00:00 chumbucket kernel: ch341 1-3:1.0: ch341-uart converter detected
Feb 10 10:00:00 chumbucket kernel: usb 1-3: ch341-uart converter now attached to ttyUSB0
...
If brltty is not interfere the /dev/ttyUSB0 device must be existing:
If brltty claims the serial device no device file is created and in the kernel logs the following lines show that brltty is catching the serial device:
$ sudo journalctl -b -t kernel
...
Feb 10 12:07:44 chumbucket kernel: usb 1-3: usbfs: interface 0 claimed by ch341 while 'brltty' sets config #1
Feb 10 12:07:44 chumbucket kernel: ch341-uart ttyUSB0: ch341-uart converter now disconnected from ttyUSB0
Feb 10 12:07:44 chumbucket kernel: ch341 1-3:1.0: device disconnected
...
The problem can be found in the rule file /usr/lib/udev/rules.d/85-brltty.rules:
# Device: 1A86:7523
# Baum [NLS eReader Zoomax (20 cells)]
ENV{PRODUCT}=="1a86/7523/*", ENV{BRLTTY_BRAILLE_DRIVER}="bm", GOTO="brltty_usb_run"
Here every chipset with vendor id 1a86 and product id 7523 will be handled
as a braille display. You can comment out these lines or adapt the patch of the
above bug description. In order to change the rules copy the file into
/etc/udev/rules.d:
Info
The directory /etc/udev/rules.d takes precedence over
/usr/lib/udev/rules.d and is not changed by brltty package updates.
And change the lines into:
# Device: 1A86:7523
# Generic Identifier
# Vendor: Jiangsu QinHeng, Ltd.
# Product: CH341 USB Bridge Controller
# Baum [NLS eReader Zoomax (20 cells)]
ENV{PRODUCT}=="1a86/7523/*", ATTRS{idVendor}=="1a40", ATTRS{idProduct}=="0101", ENV{BRLTTY_BRAILLE_DRIVER}="bm", GOTO="brltty_usb_run"
Now only a specific braille display will be claimed from brltty. All other
serial converter with chipset ch341 will be ignored.
The changed rule set must be loaded via udev:
Afterwards the brltty udev service must restarted: