I was explaining this not detailed enough.
The IO-pin of an Arduino-Uno "sends" out 5V but the serial interface-IO-pins of the OpenCM is build for only 3.3V.
This means the Arduino TX-pin "feeds" a too high voltage into the OpenCM-Rx pin and this might have damaged the OpenCM
The microocntroller-world is not superstandardised like USB-devices.
In the micrcontroller-world you yourself have to take care of a lot of more things than "does the plug fit into the socket?"
best regards Stefan