Normally when I plug a CDC serial USB device, I get a device like /dev/cu.usbmodem1411. A remote user I'm supporting who's having difficulty with a serial device is reporting a device with FD in the name: /dev/cu.usbmodemFD211 (on a Macbook Air of unknown vintage running 10.13.4)?

What's the meaning of the FD? I've done a websearch but turned up nothing, though some people mention a lower-case fd instead. I know the digits just signify the controller, port, etc.

  • Which version of macOS (on the remote system), which USB device, which drivers are used?
    – nohillside
    Oct 24, 2018 at 17:31
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    Also, how does knowing the meaning of FD help you in solve the problem? Would it be better to focus on the actual issue instead?
    – nohillside
    Oct 24, 2018 at 17:33
  • Added MacOS info. We're supporting one of our devices which would normally work fine on a Mac. Can't reproduce the problem. It could be a USB hub issue or USB-C issue or something else. No special drivers. Knowing what "FD" means could help us narrow it down. Oct 24, 2018 at 17:51
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    Might "FD" mean full duplex? Modems are so 20th century ... :-)
    – IconDaemon
    Oct 24, 2018 at 18:07
  • I'm more interested in the fact that you plug in a CD serial device and get usbmodemXXX. Also, to echo @nohillside, why is the device name critical here?
    – Allan
    Oct 25, 2018 at 3:52

1 Answer 1


There's no technical consequences due to the device having "FD" in its name. It is "just a name", so the operating system doesn't treat it any differently because it says "FD".

The reason for the different namings is that the driver for the USB serial device decides the name of the device file. You will see different names depending on which driver the user has installed.

For these reasons, it is not possible to generally say what the "FD" stands for - as it depends entirely on which driver, the user has installed. Typically "FD" stands for "file descriptor" or "full duplex" in this context.

  • The presence of a non-builtin driver could be the problem in this case. I'll investigate further. Oct 25, 2018 at 2:03

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