Catch IndexError

Author: christiansandberg

pymodbus/framer/rtu_framer.py

@@ -3,7 +3,7 @@
 import struct
 import time
 
-from pymodbus.exceptions import ModbusIOException, NotImplementedException
+from pymodbus.exceptions import ModbusIOException
 from pymodbus.exceptions import InvalidMessageReceivedException
 from pymodbus.utilities import checkCRC, computeCRC
 from pymodbus.utilities import hexlify_packets, ModbusTransactionState
@@ -328,14 +328,16 @@ def getRawFrame(self):  # pylint: disable=invalid-name
         _logger.debug(txt)
         return self._buffer
 
-    def get_expected_response_length(self, data=None):
-        """Get the expected response length if available in data, else None."""
-        data = data if data is not None else self._buffer
+    def get_expected_response_length(self, data):
+        """Get the expected response length if available in data.
+        
+        :param data: Message data read so far
+        :raises IndexError: If not enough data to read byte count
+        :raises NotImplementedException: If not supported by function code
+        :return: Total frame size
+        """
         func_code = int(data[1])
         pdu_class = self.decoder.lookupPduClass(func_code)
-        try:
-            return pdu_class.calculateRtuFrameSize(data)
-        except NotImplementedException:
-            return None
+        return pdu_class.calculateRtuFrameSize(data)
 
 # __END__

pymodbus/transaction.py

@@ -343,7 +343,7 @@ def _recv(self, expected_response_length, full):  # NOSONAR
             if isinstance(self.client.framer, ModbusSocketFramer):
                 min_size = 8
             elif isinstance(self.client.framer, ModbusRtuFramer):
-                min_size = 3
+                min_size = 4
             elif isinstance(self.client.framer, ModbusAsciiFramer):
                 min_size = 5
             elif isinstance(self.client.framer, ModbusBinaryFramer):
@@ -379,7 +379,11 @@ def _recv(self, expected_response_length, full):  # NOSONAR
                         length = struct.unpack(">H", read_min[4:6])[0] - 1
                         expected_response_length = h_size + length
                     elif expected_response_length is None and isinstance(self.client.framer, ModbusRtuFramer):
-                        expected_response_length = self.client.framer.get_expected_response_length(read_min)
+                        try:
+                            expected_response_length = self.client.framer.get_expected_response_length(read_min)
+                        except IndexError:
+                            # Could not determine response length with available bytes
+                            pass
                     if expected_response_length is not None:
                         expected_response_length -= min_size
                         total = expected_response_length + min_size