I added an udev rule to generate a separate link to a special tty device. I took the vendor and product id from lsusb:

```
Bus 001 Device 016: ID abcd:1234 Foo Device

```

The `dmesg` output for the device is:

```
[  369.384850] usb 1-1.1: new full-speed USB device number 4 using xhci_hcd
[  369.470492] usb 1-1.1: New USB device found, idVendor=09d8, idProduct=0420
[  369.470506] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[  369.470515] usb 1-1.1: Product: TWN4/B1.06/CCL3.12/PRS1.04
[  369.470522] usb 1-1.1: Manufacturer: OEM
[  369.475188] cdc_acm 1-1.1:1.0: ttyACM0: USB ACM device

```

Normally the Debian system generates an ACM\* tty for this device:

```
$ ls /dev/ttyACM*
/dev/ttyACM0

```

So I created `/ect/udev/rules.d/99-bizrfid.rules`:

```
ACTION=="add", ATTRS{idVendor}=="09d8", ATTRS{idProduct}=="0420", SYMLINK+="ttyMyDevice"

```

But `ls /dev/tty*` does not show `ttyMyDevice`.

**What I tried so far without success:**

1. Running `udevadm trigger`. This should apply the new udev rule.
2. I did a reboot.
3. I had a look to `udevadm test /dev/ttyACM0`. My udevrule is shown at the end. So I am sure that it had not been overridden. But I wonder about the 'unable to open' hint. Nevertheless this hint is present for other ttys, such as tty0 or ttyS0, too.

> 
> 
> ```
> ...    
> Reading rules file: /lib/udev/rules.d/98-systemd.rules
> Reading rules file: /etc/udev/rules.d/99-bizrfid.rules
> rules contain 49152 bytes tokens (4096 * 12 bytes), 12883 bytes strings
> 4240 strings (40079 bytes), 3515 de-duplicated (27922 bytes), 726 trie nodes used
> unable to open device '/sys/dev/ttyACM0'
> Unload module index
> Unloaded link configuration context.
> 
> ```
> 
> 

4. I reconnected the USB device.
5. I tried another udev rule. This rule is 100% correct because I use this rule already fo other devices. But it does not work for the ttyACM0:

> 
> 
> ```
> ACTION=="add", KERNEL=="ttyACM*", MODE="0666"
> 
> ```
> 
> 

I wonder why this is not working.
**How to find reasons why an udev rule is not applied?**