import serial

s = serial.Serial('/dev/ttyS0') 
s.setRTS(False)

def Ein(event):
    s.setRTS(1)

def Aus(event):
    s.setRTS(0)
        
from tkinter import *

v = Tk()
v.title("RTS - CTS")
v.geometry('200x100+300+300')
bTaster = Button(master=v, text="Taster")
bTaster.bind('<Button-1>',Ein)
bTaster.bind('<ButtonRelease>',Aus)
bTaster.place(x=30,y=37)
c = Canvas(master=v,width=60,height=60)
c.place(x=120,y=20)
led = c.create_oval(10,10,50,50,fill='#550000')

def Halt():
     s.close()
     v.quit()                             
     v.destroy()                          

v.protocol("WM_DELETE_WINDOW",Halt)     
               
def poll():
    if s.getCTS():
        c.itemconfig(led,fill='#ff0000')
    else:
        c.itemconfig(led,fill='#550000')
    v.after(20,poll)                  

v.after(0,poll)
v.mainloop()