Can't connect to serial device

Hi,
I'm trying to connect a MFB drum machine via serial on an USB port (putty on /dev/tty/ACM0) to check and maybe update the firmware.
My user is in the dialout group but even with sudo, I can't connect to it.
The cdc_acm module is loaded.

I tried this in /etc/inittab with no success:
s1:12345:respawn:/sbin/agetty -L ttyACM0 9600 vt100

I also tried to add a udev rule in /etc/udev/rules.d with no success too

What did I miss?

Thanks