MaxSONARtty.py and EZ2

# maxSonarTTY.py

import serial
import re

class MaxSonar( serial.Serial ):
    def __init__( self, port ):
        super().__init__( port=port, baudrate=9600, timeout=0.5 )

    def measure( self ):
        self.reset_input_buffer()

        data = self.read_until( b'\r', size=5 )

        if re.fullmatch( rb'\d*\r', data ):
            # partial line received, wait for next one
            data = self.read_until( b'\r', size=5 )

        if len( data ) < 5 and not data.endswith( b'\r' ):
            raise RuntimeError( "Timeout while waiting for data" )

        m = re.fullmatch( rb'R(0|[1-9]\d*)\r', data )
        if not m:
            raise RuntimeError( "Garbage data received: %r" % data )
        data = m.group( 1 )

        return int( data )  # measurement in inches (0-255)


# maxSonarTTY-example.py

from time import sleep
from maxSonarTTY import MaxSonar

sonar = MaxSonar( "/dev/ttyS2" )

try:
    while True:
        distance = sonar.measure()
        print( "distance =", distance, "inch" )

        sleep( 1 )

except KeyboardInterrupt:
    pass