Task Cancellation and CRC Errors

Alternate solution for pymodbus-dev#356 and pymodbus-dev#360.

Changes the RTU to make the transaction ID as the unit ID instead of an ever incrementing number.

Previously this transaction ID was always 0 on the receiving end but was the unique transaction ID on sending.

As such the FIFO buffer made the most sense. By tying it to the unit ID, we can recover from failure modes such as: -
- Asyncio task cancellations (eg. timeouts) pymodbus-dev#360
- Skipped responses from slaves. (hangs on master pymodbus-dev#360)
- CRC Errors pymodbus-dev#356
- Busy response

pymodbus/client/sync.py

@@ -39,10 +39,7 @@ def __init__(self, framer, **kwargs):
         :param framer: The modbus framer implementation to use
         """
         self.framer = framer
-        if isinstance(self.framer, ModbusSocketFramer):
-            self.transaction = DictTransactionManager(self, **kwargs)
-        else:
-            self.transaction = FifoTransactionManager(self, **kwargs)
+        self.transaction = DictTransactionManager(self, **kwargs)
         self._debug = False
         self._debugfd = None
 

pymodbus/framer/rtu_framer.py

@@ -186,6 +186,7 @@ def populateResult(self, result):
         :param result: The response packet
         """
         result.unit_id = self._header['uid']
+        result.transaction_id = self._header['uid']
 
     # ----------------------------------------------------------------------- #
     # Public Member Functions
@@ -221,6 +222,9 @@ def processIncomingPacket(self, data, callback, unit, **kwargs):
                     _logger.debug("Not a valid unit id - {}, "
                                   "ignoring!!".format(self._header['uid']))
                     self.resetFrame()
+            else:
+                _logger.debug("Frame check failed, ignoring!!")
+                self.resetFrame()
         else:
             _logger.debug("Frame - [{}] not ready".format(data))
 
@@ -235,6 +239,7 @@ def buildPacket(self, message):
                              message.unit_id,
                              message.function_code) + data
         packet += struct.pack(">H", computeCRC(packet))
+        message.transaction_id = message.unit_id  # Ensure that transaction is actually the unit id for serial comms
         return packet
 
     def sendPacket(self, message):