MK3 with 3.4.0 resets when I connect via USB  

MK3 with 3.4.0 resets when I connect via USB

I have a MK3 and I use Linux. I discovered with the 3.4.0-RC2 firmware, if it was USB connected, I could set the /dev/ttyACM0 USB device speed and then tail the file for information, and I could also echo g-code to it.

For example:
stty /dev/ttyACM0 115200 # set speed
tail -f /dev/ttyACM0 # watch output, such as bed/extruder temp and time-left-in-print
echo G1 Z150 > /dev/ttyACM0 # raise extruder
echo M104 S215 > /dev/ttyACM0 # set bed temp

This was very useful, as I could watch it while I worked, or quickly reset the extruder temp when a print finished, to stay warm for the next print.

Now, with the 3.4.0 final firmware, the MK3 resets when I connect, as if I've hit the reset button.

I do the "stty /dev/ttyACM0 115200" or "echo M104 S215 > /dev/ttyACM0" or whatever and it resets.

A workaround I've discovered is to connect with miniterm (e.g. miniterm -e /dev/ttyACM0 115200) and stay connected. It will reset once, but as long as it stays connected, further connections DO NOT cause a reset.

Is this a bug?

Or is this "well, that was only possible because it was RC firmware with debugging turned on" or something?