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Consistent device names with udev - M0YNG.uk

Created 2024-07-12

Tagged

Linux
Amateur Radio
Computer

I've often got multiple things plugged into my computer, for example a USB to serial cable for the TNC and a different USB to serial cable for CAT control of my HF radio. These usually show up as something like `/dev/ttyUSB0` and `/dev/ttyUSB1`.

The problem is that the order is not always the same. Sometimes a reboot results in the devices swapping number and suddenly your software can't talk to the radio anymore.

Let's fix that!

(I'm doing this on Debian 12)

Identify the devices

We need to know which device is connected to what, the easiest way to do this is use `lsusb` then plug or unplug the device and see what appears/disappears.

For example

lsusb
Bus 001 Device 006: ID 08bb:29c6 Texas Instruments PCM2906C Audio CODEC
Bus 001 Device 004: ID 067b:23a3 Prolific Technology, Inc. ATEN Serial Bridge
Bus 001 Device 003: ID 10c4:ea60 Silicon Labs CP210x UART Bridge

Then I unplug the CAT cable

lsusb
Bus 001 Device 006: ID 08bb:29c6 Texas Instruments PCM2906C Audio CODEC
Bus 001 Device 004: ID 067b:23a3 Prolific Technology, Inc. ATEN Serial Bridge

Now I know that `067b:23a3` is the TNC and `10c4:ea60` is the CAT.

Tell udev what to do.

Create or edit `/etc/udev/rules.d/99-usbtty.rules` and populate it with something like this:

SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="ttyUSBIcomCat"
SUBSYSTEM=="tty", ATTRS{idVendor}=="067b", ATTRS{idProduct}=="23a3", SYMLINK+="TNC"

We're tell it to look for a device with the vendor id `10c4` and product id `ea60` and then create a symbolic link called `ttyUSBIcomCat` to point to the actual device.

This way we can tell our software to always look for `/dev/ttyUSBIcomCat` and it doesn't matter if it's number 0, 1, or 6352528.

You'll need to reboot or kick udev somehow, then check and see if it's worked:

ls -lh /dev/
crw-rw---- 1 root dialout 188,  1 Jul 12 21:08 /dev/ttyUSB1
lrwxrwxrwx 1 root root          7 Jul 12 14:41 /dev/ttyUSBIcomCat -> ttyUSB1

You're looking for something with whatever name you specified (`ttyUSBIcomCat`) followed by `->` pointing to `ttyUSB`something. If it's there, it worked. If it didn't ... I don't know, check the IDs or something.

WSJT-X

WSJT-X didn't give the new symlink as an option for me. However you can just type the path in, you don't have to pick from the drop down list.

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