How to identify multiple USB-serial adapters under Ubuntu 10.1

Question

I am reading data from multiple identical USB-serial adapters under Ubuntu 10.1.

On occasion, their /dev/tty path changes (eg if other USB devices are connected on startup).

I need a way of repeatedly referring to the same adapter through any such changes.

The devices all have the same serial numbers, according to udevadm.

I think the most likely option is to identify an adapter by which port it is connected to (they don't get moved around).

I can find all sorts of interesting /dev paths that might work, but despite all the discussion about udev online, I can't locate a definitive statement about whether some of these paths are static if the device is plugged into a static port.

Answer 1

There is a solution. It's better late then never ;)

Use the following udev rule to map /dev/ttyUSB{?} devices into the /dev/usb-ports/%bus_id-%port_id link.

Here is my /etc/udev/rules.d/usb-parse-devpath.rules:

ACTION=="add", KERNEL=="ttyUSB[0-9]*", PROGRAM="/etc/udev/rules.d/usb-parse-devpath.pm %p", SYMLINK+="usb-ports/%c"

And the usb-parse-devpath.pm script:

#!/usr/bin/perl -w

@items = split("/", $ARGV[0]);
for ($i = 0; $i < @items; $i++) {
    if ($items[$i] =~ m/^usb[0-9]+$/) {
        print $items[$i + 1] . "\n";
        last;
    }
}

As you can see this helps us to create named links to /dev/ttyUSB{?} devices and place them at /dev/usb-ports in the following format: bus_id-port_id.

For example, the next command gives me the following:

$ udevadm info --query=path --name=/dev/ttyUSB0
/devices/pci0000:00/0000:00:1d.1/usb3/3-1/3-1:1.0/ttyUSB0/tty/ttyUSB0

So, the bus_id is 3 and port_id is 1 and now I have following in my /dev/usb-ports:

$ ls -al /dev/usb-ports
lrwxrwxrwx  1 root root   10 Май 12 00:26 3-1 -> ../ttyUSB0

Regards.

Answer 2

Much like Ilya Matvejchikov's answer, a good solution is to add udev rules to do what you want with the device. Like you, I was having a similar problem. I had a UPS on a USB-to-multi-serial adapter and occasionally the system would switch around the /dev/tty numbers.

My solution was to create a rule to match the type of device by driver and port, then create a symbolic link to the port to which my UPS was attached. I used NUT to monitor the UPS, which was always plugged into the same physical port.

# File contents of /etc/udev/rules.d/75-nut-ups.rules
# Create /dev/nut-ups0 to use as a persistent serial device that can be used
# reliably by nut to communicate with a UPS attached to the system.
# The UPS is attached to the first port of a MosSemiconductor dual USB
# serial adapter.
KERNELS=="ttyUSB*", SUBSYSTEMS=="usb-serial", DRIVERS=="moschip7720", ATTRS{port_number}=="0", SYMLINK+="nut-ups0"

Now I configure NUT to always use a constant /dev/nut-ups0, as the serial port and the rule takes care of mapping properly when the usb-serial device is recognized.

You can use the lsusb command to find out the actual device name to use in the rule when it's plugged in.

Answer 3

Look with $ udevadm info -n /dev/ttyUSB0 -a which port your USB device is plugged in. The variable KERNELS of one of the parent devices should be something like KERNELS=="1-1.2:1.0".

Create a udev rule:

SUBSYSTEM=="tty", KERNELS=="1-1.2:1.0", SYMLINK+="ttyUSB42"
SUBSYSTEM=="tty", KERNELS=="1-1.3:1.0", SYMLINK+="usb-serial"

and trigger udev

$ udevadm trigger