Handeling Multiple Serial Channels on a Single USB on Linux

I was plugging my Icom IC-705 into my laptop in the field and I sometimes find that the devices doesn’t always land on /dev/ttyACM0. which means configuring other tools like WSJTX and direwolf will need to point the next dev of /tty/ACM02. This can be a little tedious and it isn’t always convenient to debug technical issues in the middle of field.

To try and make this a little more painless I have setup a udev rule to pick out the IC-705 and give it a symbolic name such as /dev/ic705CAT.

So under /etc/udev/rules.d/98-icom705.rules I put the following lines:

ATTRS{idVendor}=="0c26",ATTRS{idProduct}=="0036", ENV{ID_USB_INTERFACE_NUM}=="00", SYMLINK+="ic705CAT"
ATTRS{idVendor}=="0c26",ATTRS{idProduct}=="0036", ENV{ID_USB_INTERFACE_NUM}=="02", SYMLINK+="ic705GPS"

The permissions are set under group dialout. so add your current user with: sudo usermod -a -G dialout $USER.

To make matters a little harder, the IC-705 has two serial devices on a single cable. One is for CAT control, the other I will need the GPS data. It is important that when I rename them, that they are labelled correctly!

So the trick I found here was the ENV{ID_USB_INTERFACE_NUM}=="00" bit, as I originally had it as: ATTRS{bInterfaceNumber}. The reason is that this attrivute doesn’t exist with idProduct or idVendor. As the udev system goes through each parent of the device.

This output can be seen by:

user@box:~$ udevadm info -a -p $(udevadm info -q path -n /dev/ttyACM0)

and you will see that as it iterates through the parent of the device level. They don’t co exist.

However a way around, I don’t fully understand is that environment variables get set. Presumable for shell/bash scripts to run and read. The environment vars that presumable are set can be seen with:

user@box:~$ sudo udevadm test $(udevadm info -q path -n /det/ttyACM0)

Finally, the IC-705 neesd to output the GPS on serial B. Go to: Menu -> Set -> Connectors -> USB (B) Function -> GPS Out -> On

As soon as you set that you will see the serial stream NMEA data out. This can be seen via:

user@box:~$  screen /dev/ic705GPS 9600

Now that is all done, need to setup gpsd. And that can serve GPS data to various software. And I will look into setting system clock for accurate time reference in FT8 etc…

I currently run Linux Mint 21.