XenonFred:
I am trying to build an autonomous laser engraver, which does not receive data from the PC but instead from another Arduino or an SD card. Therefore, I do not want any external software like Universal GCode Sender involved because the laser engraver will not be connected to any PC.
I am even more confused now. In Reply #11 you said
From my understanding, GRBL occupies the serial monitor of the slave Arduino so there is no way to use the serial monitor.
and that would only make sense if the Arduino IS CONNECTED to the PC. The Serial Monitor is a PC program.
...R