old_contents: #!/usr/bin/python 
  # -*- coding:utf-8 -*-
  
  import serial
  import time
  
  ser = serial.Serial("/dev/ttyS0", 115200)
  
  
  def main():
      while True:
          count = ser.inWaiting()
          if count != 0:
              recv = ser.read(count)
              ser.write(recv)
          ser.flushInput()
          time.sleep(0.1)
  
  
  if __name__ == '__main__':
      try:
          main()
      except KeyboardInterrupt:
          if ser != None:
              ser.close()
new_contents: #!/usr/bin/python 
  # -*- coding:utf-8 -*-
  
  import serial
  import time
  
  ser = serial.Serial("/dev/ttyS0", 9600)
  
  
  def main():
      while True:
          count = ser.inWaiting()
          if count != 0:
              recv = ser.read(count)
              ser.write(recv)
          ser.flushInput()
          time.sleep(0.1)
  
  
  if __name__ == '__main__':
      try:
          main()
      except KeyboardInterrupt:
          if ser != None:
              ser.close()
subject: change baud rate to 9600 to adapt the MCU part