Stop time in Tkinter Stopwatch with a PushButton
In the pyserial docs it states this:


  Be careful when using readline(). Do specify a timeout when opening the serial port otherwise it could block forever if no newline character is received. Also note that readlines() only works with a timeout. readlines() depends on having a timeout and interprets that as EOF (end of file). It raises an exception if the port is not opened correctly.


You need to specify the timeout and the end of line string. Try using this  modified line:

if arduino.readline(eol='\n') == 'E\n':


Also, add the timeout option where you define arduino as a pyserial object. For example:

arduino = pyserial.serial(port,baud,timeout=t,etc...)