Giving serial devices meaningful names


This is a hack I’ve been using for ages, but I thought it deserved a write up.

I have USB serial devices. Lots of them. I use them for home automation things, as well as for talking to devices such as the console ports on switches and so forth. For the permanently installed serial devices one of the challenges is having them show up in predictable places so that the scripts which know how to drive each device are talking in the right place.

For the trivial case, this is pretty easy with udev:

$  cat /etc/udev/rules.d/60-local.rules
KERNEL=="ttyUSB*", \
    ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", \
    ATTRS{serial}=="A8003Ye7", \

This says for any USB serial device that is discovered (either inserted post boot, or at boot), if the USB vendor and product ID match the relevant values, to symlink the device to “/dev/radish”.

You find out the vendor and product ID from lsusb like this:

$ lsusb
Bus 003 Device 003: ID 0624:0201 Avocent Corp.
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 007 Device 002: ID 0665:5161 Cypress Semiconductor USB to Serial
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 002: ID 0403:6001 Future Technology Devices International, Ltd FT232 Serial (UART) IC
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 009 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 008 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

You can play with inserting and removing the device to determine which of these entries is the device you care about.

So that’s great, until you have more than one device with the same USB serial vendor and product id. Then things are a bit more… difficult.

It turns out that you can have udev execute a command on device insert to help you determine what symlink to create. So for example, I have this entry in the rules on one of my machines:

KERNEL=="ttyUSB*", \
    ATTRS{idVendor}=="067b", ATTRS{idProduct}=="2303", \
    PROGRAM="/usr/bin/usbtest /dev/%k", \

This results in /usr/bin/usbtest being run with the path of the device file on its command line for every device detection (of a matching device). The stdout of that program is then used as the name of a symlink in /dev.

So, that script attempts to talk to the device and determine what it is — in my case either a currentcost or a solar panel inverter.


Via M10000, video, and a Belkin wireless USB thing


I’ve had a Via Eden M10000 literally lying around since soon after I moved to the US two years ago, and I finally decided that it was time for a full blown home workstation the other day, having got fed up with the small screen on my laptops. Sometimes I really miss my 30 inch monitor at work, and the 24 inch I am using at home is a much better substitute than a laptop screen.

Anyway, getting the machine working was surprisingly difficult given its a three or four year old design. The problems:

  • The PCI 802.11g wireless card didn’t work. In fact, the machine wouldn’t boot with it installed. I suspect this was a PCI version problem, as I have had pain with this card in the past.
  • The PCI 802.11b wireless card I tried next wasn’t much better. The connection would drop out randomly, and the machine would occasionally lock up. This was the card I used as my first access point about six years ago (using hostap), so perhaps its just old. It got swapped out as well.
  • The Belkin USB 802.11g thingie didn’t work reliably. It would stay connected to the network for five minutes before something went wrong. This made me annoyed, especially when it turns out this is because the latest release of Ubuntu (gutsy) installs an old version of the rt73usb driver, which is known not to work with this card. Following these instructions from the canonical wiki which tell you to install drivers from here fixed the problem. Its annoying that Ubuntu ships with known broken drivers though.
  • Next, video. The video card built into the mother board sucks. I’m running a t24 inch LCD at 1920×1200, and there was significant ghosting on the monitor. Additionally, I couldn’t run at 24 bit, I had to use 16 bit because the video card was running out of RAM bandwidth.
  • Not to worry, I installed a Matrox G450 I had lying around, and now the monitor works nicely as well, with no ghosting.
  • Finally, the whole thing is much louder than I expected from an Eden machine (there is a fan on the board, a fan in the case, and the hard disk). Then again, given I built the entire machine for $90, I can’t complain too much.