The setup is like this:
The first UNO, UNO1, is running its project and is powered by a USB power bank that cuts off due to low current consumption. To figure out how, when, why, another UNO is capturing data from UNO1 and prints them using Serial Monitor.
The second UNO, UNO2, is making measurements on outputs from UNO1.
Their GNDs are connected. One UNO1 pin acts as a strobe to UNO2. Another UNO1 pin carries the analog signal measuered by the UNO2.
They are both connected to the same Pc using different USB ports in order for program updates. I want to keep UNO2, the measuring one, to use Serial Plotter even if I need to make changes to the sketch in UNO1, the one being measured.
I don't really get hold of how the IDE Environment works. How to handle the IDE world and Serial Monitor/Plotter?
You can do that. The key is to open two instances of the Arduino IDE. You do this by starting the Arduino IDE twice. That allows you to have multiple Serial Monitor/Serial Plotter windows open to different ports at the same time. If you open two IDE windows (via File > New, File > Open, File > Sketchbook, File > Examples) then you will only be able to have one Serial Monitor/Serial Plotter open at once and any menu selection you make in one window will also affect the other window.
Thanks a lot pert! I did just as You wrote, started one IDE from desktop and the second IDE by File:new…. Then it calls for military disciplin to awoid loading the sketch into the wrong UNO etc.. Exactly, switching comport in one window changed the comport in the other window and then its Monitor was lost. Whow!