Use response byte length if available

Co-authored-by: jan Iversen <jancasacondor@gmail.com>

Author: christiansandberg

pymodbus/framer/rtu_framer.py

@@ -153,11 +153,9 @@ def populateHeader(self, data=None):  # pylint: disable=invalid-name
         `self._buffer` is not yet long enough.
         """
         data = data if data is not None else self._buffer
-        self._header["uid"] = int(data[0])
-        func_code = int(data[1])
-        pdu_class = self.decoder.lookupPduClass(func_code)
-        size = pdu_class.calculateRtuFrameSize(data)
-        self._header["len"] = size
+        self._header['uid'] = int(data[0])
+        size = self.get_expected_response_length(data)
+        self._header['len'] = size
 
         if len(data) < size:
             # crc yet not available
@@ -330,5 +328,15 @@ def getRawFrame(self):  # pylint: disable=invalid-name
         _logger.debug(txt)
         return self._buffer
 
+    def get_expected_response_length(self, data):
+        """Get the expected response length.
+
+        :param data: Message data read so far
+        :raises IndexError: If not enough data to read byte count
+        :return: Total frame size
+        """
+        func_code = int(data[1])
+        pdu_class = self.decoder.lookupPduClass(func_code)
+        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 = 2
+                min_size = 4
             elif isinstance(self.client.framer, ModbusAsciiFramer):
                 min_size = 5
             elif isinstance(self.client.framer, ModbusBinaryFramer):
@@ -362,7 +362,7 @@ def _recv(self, expected_response_length, full):  # NOSONAR
                 if isinstance(self.client.framer, ModbusSocketFramer):
                     func_code = int(read_min[-1])
                 elif isinstance(self.client.framer, ModbusRtuFramer):
-                    func_code = int(read_min[-1])
+                    func_code = int(read_min[1])
                 elif isinstance(self.client.framer, ModbusAsciiFramer):
                     func_code = int(read_min[3:5], 16)
                 elif isinstance(self.client.framer, ModbusBinaryFramer):
@@ -378,6 +378,12 @@ 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):
+                        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