PuTTY can access serial port as /dev/ttyUSB0, but not as named udev device

Question

I have multiple USB-to-serial converters. I need to access one of them in particular. I'm using a udev rule to give it a special name. I have rebooted since I last modified it.

SUBSYSTEM=="usb", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", NAME="serial", MODE="0666"

That vendor/product combination is unique among all of my usb devices.

$ ls -l /dev/serial
crw-rw-rw-. 1 root root 189, 133 Feb  8 23:57 /dev/serial
$ ls -l /dev/ttyUSB0
crw-rw----. 1 root dialout 188, 0 Feb  8 23:58 /dev/ttyUSB0

I'm using PuTTY to read them, and it works on /dev/ttyUSB0, but not on /dev/serial . The error still appears when I am running PuTTY as root.

Unable to open connection to :
Unable to configure serial port

In case it matters, I'm running CentOS 6.

uname -a
Linux xxxxxx 2.6.32-279.22.1.el6.x86_64 #1 SMP Wed Feb 6 03:10:46 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

EDIT: WORKING

The following udev rule was what finally worked:

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="COM1", MODE="0666"

Note that SUBSYSTEM is tty, not usb, NAME has been changed to SYMLINK+, and serial has been changed to COM1 (to not interfere with /dev/serial, as a commenter pointed out.)

Thanks for your help, guys!

Answer 1

You want your rule to pay attention to the tty subsystem, not the usb one.

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="serial"

A USB device generates several udev events when you plug it in as the kernel recognizes more things about it. Since it's a USB device, it first engages the usb subsystem, which I think will create a raw USB device, which PuTTY can't use. A few steps later it will load the device's specific driver, and since this is a serial device, it will engage the tty subsystem, which creates a device file that PuTTY can use.

This rule will create a symlink to whichever /dev/ttyUSB* happens to be assigned to your device. Tested successfully with PuTTY on my own serial dongle.

Incidentally, for diagnostics I sometimes run the following rule, to get an idea of what the udev scripts are seeing:

RUN+="/home/me/bin/udev-diag .$kernel .$number .$devpath .$id .$parent .$root .$tempnode"

where udev-diag is essentially:

env >>/tmp/udev-events
echo "$@" >>/tmp/udev-events

For more general use, the udevmonitor program is also handy.

Answer 2

Using Jander's rule, I was able to access my USB serial adapter... but only if running PuTTY as root.

Modifying the rule to specify different permissions allowed me to access it by any user.

I.E.

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", MODE="0666", SYMLINK+="serial"

Answer 3

The following udev rule was what finally worked:

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="COM1", MODE="0666"

Note that SUBSYSTEM is tty, not usb, NAME has been changed to SYMLINK+, and serial has been changed to COM1 (to not interfere with /dev/serial, as a commenter pointed out.)

Answer 4

For some reason, I never posted an answer to this, even after I got this working.

The following udev rule was what finally worked:

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="COM1", MODE="0666"

Note that SUBSYSTEM is tty, not usb, NAME has been changed to SYMLINK+, and serial has been changed to COM1 (to not interfere with /dev/serial, as a commenter pointed out.)

Thanks for your help, guys!

Answer 5

Notice that the device is group-writable, by the group dialout.

$ ls -l /dev/ttyUSB0
**crw-rw----** 1 root **dialout** 188, 0 Feb  8 23:58 /dev/ttyUSB0

To fix this permissions issue, add yourself to the dialout group:

$ sudo adduser <username> dialout

Then restart the session