title: How do I fix the checksum error in modbus
body: <p>I have a script which should collect the concentration data from a sensor connected with MODBUS.
  When i run the script, this error occures:</p>
  
  <p>Traceback (most recent call last):
    File "C:\Users\AquaBattery.HP-LAPTOP\Desktop\H2-reading.py", line 33, in 
      val = H2.read_long(ADRH2,functioncode=4,byteorder=0)
    File "C:\Users\AquaBattery.HP-LAPTOP\Desktop\minimalmodbus.py", line 560, in read_long
      payloadformat=_PAYLOADFORMAT_LONG,
    File "C:\Users\AquaBattery.HP-LAPTOP\Desktop\minimalmodbus.py", line 1170, in _generic_command
      payload_from_slave = self._perform_command(functioncode, payload_to_slave)
    File "C:\Users\AquaBattery.HP-LAPTOP\Desktop\minimalmodbus.py", line 1244, in _perform_command
      response, self.address, self.mode, functioncode
    File "C:\Users\AquaBattery.HP-LAPTOP\Desktop\minimalmodbus.py", line 1756, in _extract_payload
      raise InvalidResponseError(text)
  minimalmodbus.InvalidResponseError: Checksum error in rtu mode: '\x00\x00' instead of 'c\x85' . The response is: '\x01\x04\x0e\x00\x00\x00\x00\x00\x00' (plain response: '\x01\x04\x0e\x00\x00\x00\x00\x00\x00')</p>
  
  <p>This is my code:</p>
  
  <pre><code>import time
  import os
  import serial
  import minimalmodbus
  
  
  
  delay = 1
  pdata = 1
  
  
  #-- H2 sensor read out  -----------------------------------
  #def getH2():
  res = 0      
  ADRH1 = 1 # read out register for H2
  ADRH2 = 0 # read out register for H2
  
  try:
          H2 = minimalmodbus.Instrument('COM6',ADRH1)  # open serial port
          H2.serial.port          # this is the serial port name
          H2.serial.baudrate = 9600   # Baud
          H2.serial.bytesize = 8
          H2.serial.parity   = serial.PARITY_NONE
          H2.serial.stopbits = 1
          H2.serial.timeout  = 0.25   # seconds
  
  
          val = H2.read_long(ADRH2,functioncode=4,byteorder=0)
          print("test")
          res = float(val)/10
          print(val)
          print("---")
          print(res)
  
  except ValueError:
          print("Failed to read H2")
  </code></pre>
  
  <p>How do I change the request to the sensor or the response from the sensor so that the checksum of the two data streams are the same?</p>
  
  <p>Sincerely,
  Lucas</p>