Kind all, I'm really new to python and I'm facing a task which I can't completely grasp.
I've created an interface with Tkinter which should accomplish a couple of apparently easy feats.
By clicking a "Start" button two threads/processes will be started (each calling multiple subfunctions) which mainly read data from a serial port (one port per process, of course) and write them to file. 
The I/O actions are looped within a while loop with a very high counter to allow them to go onward almost indefinitely.
The "Stop" button should stop the acquisition and essentially it should:

Kill the read/write Thread
Close the file
Close the serial port

Unfortunately I still do not understand how to accomplish point 1, i.e.: how to create killable threads without killing the whole GUI. Is there any way of doing this?
Thank you all!
First you can use subprocess.Popen() to spawn child processes, then later you can use Popen.terminate() to terminate them.
Note that you could also do everything in a single Python thread, without subprocesses, if you want to.  It's perfectly possible to "multiplex" reading from multiple ports in a single event loop.
How to integrate killable processes/thread in Python GUI?