Reporting a bug

Please do not contact the author by email. The preferered method to report bugs and/or enhancement requests is through GitHub.

Please be sure to read the next sections before reporting a new issue.


FTDI uses the pyftdi logger.

It emits log messages with raw payload bytes at DEBUG level, and data loss at ERROR level.

Common error messages

“Error: No backend available”

libusb native library cannot be loaded. Try helping the dynamic loader:

  • On Linux: export LD_LIBRARY_PATH=<path>

    where <path> is the directory containing the libusb-1.*.so library file

  • On macOS: export DYLD_LIBRARY_PATH=.../lib

    where <path> is the directory containing the libusb-1.*.dylib library file

  • On Windows:

    Try to copy the USB dll where the Python executable is installed, along with the other Python DLLs.

    If this happens while using an exe created by pyinstaller: copy C:\Windows\System32\libusb0.dll <path>

    where <path> is the directory containing the executable created by pyinstaller. This assumes you have installed libusb (using a tool like Zadig) as referenced in the installation guide for Windows.

“Error: Access denied (insufficient permissions)”

The system may already be using the device.

  • On macOS: starting with 10.9 “Mavericks”, macOS ships with a native FTDI kernel extension that preempts access to the FTDI device.

    Up to 10.13 “High Sierra”, this driver can be unloaded this way:

    sudo kextunload [-v] -bundle

    You may want to use an alias or a tiny script such as pyftdi/bin/

    Please note that the system automatically reloads the driver, so it may be useful to move the kernel extension so that the system never loads it.


    From macOS 10.14 “Mojave”, the Apple kernel extension peacefully co-exists with libusb and PyFtdi, so you no longer need - and should not attempt - to unload the kernel extension. If you still experience this error, please verify you have not installed another driver from FTDI, such as FTDI’s D2XX.

  • On Linux: it may indicate a missing or invalid udev configuration. See the Installation section.

  • This error message may also be triggered whenever the communication port is already in use.

“Error: The device has no langid”

  • On Linux, it usually comes from the same installation issue as the Access denied error: the current user is not granted the permissions to access the FTDI device, therefore pyusb cannot read the FTDI registers. Check out the Installation section.

“Bus error / Access violation”

PyFtdi does not use any native library, but relies on PyUSB and libusb. The latter uses native code that may trigger OS error. Some early development versions of libusb, for example 1.0.22-b…, have been reported to trigger such issues. Please ensure you use a stable/final versions of libusb if you experience this kind of fatal error.

“serial.serialutil.SerialException: Unable to open USB port”

May be caused by a conflict with the FTDI virtual COM port (VCOM). Try uninstalling the driver. On macOS, refer to this FTDI macOS guide.

Slow initialisation on OS X El Capitan

It may take several seconds to open or enumerate FTDI devices.

If you run libusb <= v1.0.20, be sure to read the Libusb issue on macOS with OS X 10.11+.