@adwsystems: you are right, there is only that one option to send data from PLX DAQ to Arduino. I haven't had any other use case ; yet. Attached you can find a special version of 2.11 with a new text field to send data to Arduino (please expand DirectDebugWindow to fully see the field + button + label). It only works while being connected (obviously 
). Give it a try and I might add the same (in beautiful) to the next release. I might add two or three text fields at the same time thus one can post different pre-defined commands more rapidly.
@Toinedrd: never had that kind of issue before. But: during connection establishment PLX DAQ resets the Arduino by default. This can be disabled by unchecking the box "Reset on Connect" on the UI. Keep in mind that you will have your Arduino start in the middle of the loop function, thus no "LABEL" or "CLEARSHEET" written in main function will be read. Also the first data received might be corrupted. But if it works you know your code might be buggy. Did you try with a really simple one like only posting numbers to Serial.println ?