Two way serial communication with raspberry Pi (radio project)

[system]

Hi,

I tried the following to transmit the value "40" with no success:

echo 40 > /dev/ttyACM0
echo "40" > /dev/ttyACM0
echo 40\n > /dev/ttyACM0
echo "40\n" > /dev/ttyACM0
echo 40\n > /dev/ttyACM0
echo "40\n" > /dev/ttyACM0

Kind regards,

Schwabinger