Python MODBUS Raspberry

import RPi.GPIO as GPIO
import time
import serial
import string
import binascii

GPIO.setmode(GPIO.BOARD)
GPIO.setup(11, GPIO.IN)
data = bytearray.fromhex("10 04 01 00 00 02 73 76")
#ser = serial.Serial('/dev/ttyUSB3', 19200, parity=serial.PARITY_NONE, stopbits=serial.STOPBITS_ONE, bytesize=serial.EIGHTBITS, timeout=10)
ser = serial.Serial('/dev/ttyUSB4', 19200, timeout=10)
if ser.isOpen():
   print(ser.name + ' is open...')
try:
        while 1:
                signal = GPIO.input(11)
                if(signal==0):
                  print ('Request: 1004010000027376')
                  try:
                     ser.write(data)
                     #time.sleep(2)
                     #print ('Wait Answer')
                     answer = ser.read(10)
                     b = bytearray()
                     b.extend(answer)
                     #answer = bytearray.fromhex("10 04 01 00 00 02 73 76")
                     #print (answer[0])
                     print ('Answer: '+binascii.hexlify(answer))
                     #answer = (bytes(answer))
                     if(b[3]==3):
                        num = ((b[7]<<24)+(b[6]<<16)+(b[5]<<8)+(b[4]))/float(1000)
                        print(num)
                        print('Weight: '+str(num)+' kg')
                     else:
                        print('Platform unstable')
                  except serial.serialutil.SerialTimeoutException:
                     print ('Error RS485')
                  ser.flush()
                time.sleep(0.2)
except KeyboardInterrupt:
        pass
GPIO.cleanup()