code: def write_single_register(slave_id, address, value):
      
      function = WriteSingleRegister()
      function.address = address
      function.value = value
  
      return _create_request_adu(slave_id, function.request_pdu)
docs: Return ADU for Modbus function code 06: Write Single Register.
  
      :param slave_id: Number of slave.
      :return: Byte array with ADU.